Hey, guys. Some years ago I started building an MXR Gate (PCB from Tonepad (http://tonepad.com/getFile.asp?id=77)), and I never did get it to work. I just ran into it, and started looking it over. And while I normaly would do things different now (like socket the IC!), I cannot find why it is not working. I checked all the values of the resistors, caps, pot (A500K), transistors, etc. Diode is 1N4733, which is a 5.1V Zenner... I am using the 2N3904 for Q1 and Q2, NTE451 for Q3 (should work, as specs are almost identical; see here (https://www.diystompboxes.com/smfforum/index.php?topic=122660.0) - oh, and it will be cut down; legs are left long for testing), and solder joints seem good. Any ideas?
(https://i.postimg.cc/1thLk1VX/20190701-162526.jpg)
(https://i.postimg.cc/85RqDXLH/20190706-100116.jpg)
(https://i.postimg.cc/FsTX768C/20190706-100149.jpg)
(https://i.postimg.cc/Pq8gV5x0/20190706-100157.jpg)
(https://i.postimg.cc/MpX2RD86/20190706-100215.jpg)
(https://i.postimg.cc/9QS3B7WG/20190706-100230.jpg)
I am not sure what they mean by Ra in the resistors, but I used the same value it states in the build docs.
This noise gate can be a bit fiddly to get working but it is not too difficult. A quick look at your pictures doesn't show any thing wrong with construction that I can easily see. Good workmanship.
One thing to keep in mind is that the gate may actually be working correctly but it does not seem to. With the sensitivity control turned fully counter-clockwise (which one would would expect to be minimum ), the gate will be fully muted (closed ). A very large input signal would be needed to unmute (open ) the gate when set this way. Turn the sensitivity control fully clockwise and the gate should open, or open with a very small input signal. Basically the sensitivity control seems to work backwards to a volume control.
However, you may have a component problem or some error that I haven't noted yet, so some voltage measurements would be helpful - the IC, transistors and the voltage across the 5.1V zener (it won't be 5.1V, it will be less ).
Quote from: Slowpoke101 on July 06, 2019, 05:52:39 PM
This noise gate can be a bit fiddly to get working but it is not too difficult. A quick look at your pictures doesn't show any thing wrong with construction that I can easily see. Good workmanship.
One thing to keep in mind is that the gate may actually be working correctly but it does not seem to. With the sensitivity control turned fully counter-clockwise (which one would would expect to be minimum ), the gate will be fully muted (closed ). A very large input signal would be needed to unmute (open ) the gate when set this way. Turn the sensitivity control fully clockwise and the gate should open, or open with a very small input signal. Basically the sensitivity control seems to work backwards to a volume control.
Thanks for the workmanship comment! I did move the pot to different positions, so it is not that... In fact, most where around the middle, but I did go to both ends trying to get it to work.
Quote from: Slowpoke101 on July 06, 2019, 05:52:39 PMHowever, you may have a component problem or some error that I haven't noted yet, so some voltage measurements would be helpful - the IC, transistors and the voltage across the 5.1V zener (it won't be 5.1V, it will be less ).
OK, some voltages:
Battery (at the board 9V input): 9.3V
IC (TL072CP):
1: 3.91V
2: 3.94V
3: 3.90V
4: 0.00V
5: 4.71V
6: 4.63V
7: 4.63V
8: 9.28V
Q1 (2N3904):
E: 3.12V
B: 3.54V
C: 9.24V
Q2 (2N3904):
E: 0.00V
B: -3.88V (weird, right?)
C: 3.43V
Q3 (NTE451):
D: 3.92V
S: 3.92V
G: 3.49V
Zener:
Cathode: 3.93V
Anode: 0.00V
The voltages on pins 8&5 appear to be swapped. Pin 8 should be the 9volt. The pins from 1 to 8 wrap around in a U shape. If you got the pinout wrong when you took those measurements you're probably OK at the TL072.
Edit-He has corrected the posted voltages
QuoteThe voltages on pins 8&5 appear to be swapped.
I went to post that as well but I posted on the wrong thread.
One thing that's weird is most of your measurements show a your multimeter loading voltages but pin 5 doesn't show this effect! Are the resistors on pin 5 1M?
FYI, see this thread,
https://www.diystompboxes.com/smfforum/index.php?topic=116755.msg1157922;topicseen#msg1157922
This isn't the issue but, FYI, see updated schematic,
https://www.diystompboxes.com/smfforum/index.php?topic=122689.0
QuoteB: -3.88V (weird, right?)
The negative voltage could be due to rectification.
Maybe IC1 is oscillating. People have had this happen in the past. I saw this in my notes. One fix was to add a 22pF to 100pF cap across the 1M ohm resistor on IC1; see the notes on the updated schematic.
OK. I see from your voltages that you have mixed up the numbering of IC1's pins (5 to 8 ) and this has been pointed out previously. I also not that Rob has started a thread covering some errors that appear on some circuit diagrams of the MXR noise gate. Keep an eye on that thread as some interesting info is covered there.
The main issue at the moment is; Does the noise gate pass any audio at all or is it totally silent?
If you take the JFET out of the circuit, does the circuit now pass audio?
Things to note: On the Tonepad version the 10uF capacitor going to the drain of Q3 is shown with the correct polarity.
The 5.1V zener should be a 0.5W version and not a 1W version. Zener diodes do need some current to work correctly. Don't worry about this yet.
I see that you do have a small ceramic capacitor across pins 1 and 2 of IC1. This is good as it can help calm IC1 down a bit - not so inclined to oscillate.
There are some other modifications (component substitutions ) that would be a good idea when the circuit works correctly.
Rob does note that some people have experienced a sort of fizz noise with the circuit. I have experienced it myself with versions that I have built and some that I've had in for repair. It's nothing to worry about and can be controlled without too much stress.
QuoteThings to note: On the Tonepad version the 10uF capacitor going to the drain of Q3 is shown with the correct polarity.
Actually, I think tone-pad's cap is flipped as well.
Quote from: Rob Strand on July 07, 2019, 05:19:10 AM
QuoteThings to note: On the Tonepad version the 10uF capacitor going to the drain of Q3 is shown with the correct polarity.
Actually, I think tone-pad's cap is flipped as well.
Rob, you are correct when it comes to the circuit diagram, but the component overlay shows the correct polarity (square pad being positive ). This did annoy me a few years ago when I first built the Callate 2 and it didn't initially work.
QuoteRob, you are correct when it comes to the circuit diagram, but the component overlay shows the correct polarity (square pad being positive ). This did annoy me a few years ago when I first built the Callate 2 and it didn't initially work.
Ah, now I see, the layout is different to the schematic.
Thanks!
Another one of those simple circuits that always has a bug somewhere! So hard for the poor buggers building these things. (I'm not bagging Tonepad here. Francisco does a good job smoothing out a lot of bugs.)
Quote from: blackieNYC on July 06, 2019, 11:42:00 PM
The voltages on pins 8&5 appear to be swapped. Pin 8 should be the 9volt. The pins from 1 to 8 wrap around in a U shape. If you got the pinout wrong when you took those measurements you're probably OK at the TL072.
You are right: I did not go in the U shape when I measured. I started the second row from the top... Corrected the original post to show correct pins.
Quote from: Rob Strand on July 07, 2019, 12:01:36 AM
One thing that's weird is most of your measurements show a your multimeter loading voltages but pin 5 doesn't show this effect! Are the resistors on pin 5 1M?
Resistors are 1M. They are the one under and the one to the right of the IC in this pic:
(https://i.postimg.cc/Pq8gV5x0/20190706-100157.jpg)
Brown Black Green gold = 1M
You can just make out the trace to pin 8 and pin 4.
Quote from: Slowpoke101 on July 07, 2019, 04:49:37 AM
The main issue at the moment is; Does the noise gate pass any audio at all or is it totally silent?
Completely silent
Quote from: Slowpoke101 on July 07, 2019, 04:49:37 AMIf you take the JFET out of the circuit, does the circuit now pass audio?
No
Quote from: Slowpoke101 on July 07, 2019, 04:49:37 AM
Things to note: On the Tonepad version the 10uF capacitor going to the drain of Q3 is shown with the correct polarity.
The 5.1V zener should be a 0.5W version and not a 1W version. Zener diodes do need some current to work correctly. Don't worry about this yet.
I see that you do have a small ceramic capacitor across pins 1 and 2 of IC1. This is good as it can help calm IC1 down a bit - not so inclined to oscillate.
There are some other modifications (component substitutions ) that would be a good idea when the circuit works correctly.
My Zener is a 1N4733; I think those are 1.0~1.3W, right? Tried to check the spec sheet, but did not find it.
Quote from: Rob Strand on July 07, 2019, 05:19:10 AM
QuoteThings to note: On the Tonepad version the 10uF capacitor going to the drain of Q3 is shown with the correct polarity.
Actually, I think tone-pad's cap is flipped as well.
In my case, negative is going to ground, so I don't think that is the issue.
QuoteCompletely silent
I think this is the problem to address.
You should try to put a short across the collector and emitter of Q2. Just solder a wire.
If the JFET is working then the sound should come through.
(I can see you have the links across AB and CD which you do need if you don't have the mods.)
QuoteMy Zener is a 1N4733; I think those are 1.0~1.3W, right? Tried to check the spec sheet, but did not find it.
Don't worry at this point. I'm pretty sure the original used a 0.5W but the 1W should at least work.
The reason why the 0.5W is preferred is because zeners are only 5.1V at a particular current. That exact current varies but for a 5.1V zener it could be anything from 5mA to 50mA. The noise gate circuit operates the zener at *much* lower current, like 200uA. What this does is it makes the zener voltage somewhat lower than 5.1V. If you look at this pic there is an "upside down" curve on the left. Normal 5.1V is between the Imin and Imax points. If you operate at very low currents you might get below the knee point and the zener voltage is somewhat lower than 5.1V. The reason a 0.5W zener is preferred is it is more likely to have a knee point at a lower current than a 1W. Anyway this is all technical details. From your measurements we already know the zener operating at about 4V which is fine.
(https://narraaravind123.files.wordpress.com/2014/05/zener-diode1.gif)
QuoteIn my case, negative is going to ground, so I don't think that is the issue.
For the cap, don't worry about this also. The layout has the cap around the right way. The schematic shows the cap around the wrong way. It's not the cap you mentioned going to ground it's the cap going to the JFET.
In your pics, it's the 10uF to the left of the 1uF NP cap.
Rob's suggestions are very good. The first step is to make the circuit pass audio and then move onto getting the gating function to work.
Having had another look at your pictures I have noted some possible solder bridges. Shown in the picture below are three arrows pointing towards some possible solder bridges. Check them and remove them if they are bridges.
(https://i.postimg.cc/HV5VRM2P/Callate2-A.jpg) (https://postimg.cc/HV5VRM2P)
BTW, sorry for butting in on the thread.
Rob, there is no need to apologise for butting in...I welcome it and I am sure that everyone else does. The more info the better as far as I'm concerned.
Quote from: Rob Strand on July 08, 2019, 04:13:11 AM
BTW, sorry for butting in on the thread.
You are sorry for helping? No apologies needed, my friend!!!
Quote from: Slowpoke101 on July 08, 2019, 04:00:30 AM
Having had another look at your pictures I have noted some possible solder bridges. Shown in the picture below are three arrows pointing towards some possible solder bridges. Check them and remove them if they are bridges.
(https://i.postimg.cc/HV5VRM2P/Callate2-A.jpg) (https://postimg.cc/HV5VRM2P)
Just checked; they are reflections. No bridge (and checked with the MM also; no signal going through, except the middle one, but that signal is coming around the resistor and cap.
Quote from: Rob Strand on July 08, 2019, 01:39:04 AM
You should try to put a short across the collector and emitter of Q2. Just solder a wire.
If the JFET is working then the sound should come through.
Will try this today
Quote from: Rob Strand on July 08, 2019, 01:39:04 AM(I can see you have the links across AB and CD which you do need if you don't have the mods.)
So take them out then?
Leave the links across AB and CD (and leave the EF link in too ). You only remove them if you are wanting to use the attack and release modifications - don't bother with those mods at the moment.
I really did think that there was a definite solder bridge where the middle arrow was pointing. Oh well.
Try the short across Q2 and see if audio can get through. If it doesn't it is time to do some checks with an audio probe to see where it's getting lost.
Sorry guys, this a very basic circuit...
first, lift the damn 10uF and see if signal passes.
There are just 2 stages in signal path, Q1's buffer and IC1b's buffer, what can go wrong?
Quotefirst, lift the damn 10uF and see if signal passes
Actually his JFET is socketed so pulling it out would also work.
I think removing the JFET and shorting the CE terminals are both worth trying.
The CE short needs a bit more circuit working and checks the JFET.
If the CE short still doesn't pass the signal but removing the JFET does then that narrows the problem to around the JFET.
Quote from: Slowpoke101 on July 08, 2019, 05:07:50 PM
Leave the links across AB and CD (and leave the EF link in too ). You only remove them if you are wanting to use the attack and release modifications - don't bother with those mods at the moment.
I really did think that there was a definite solder bridge where the middle arrow was pointing. Oh well.
Try the short across Q2 and see if audio can get through. If it doesn't it is time to do some checks with an audio probe to see where it's getting lost.
OK, made sure to clean up the suspected bridges. The middle one is not an issue because it is traced anyway... Also bridged the Emiter and collector of Q2. Still no sound coming through (regardless of pot position).
Solder joints today (including bridge) - NOTE, IMAGES ARE 180* FROM PREVIOUS!:
(https://i.postimg.cc/sxq98m89/20190709-192733.jpg)
And with backlight to see the traces:
(https://i.postimg.cc/CMQNzM1B/20190709-192820.jpg)
QuoteStill no sound coming through (regardless of pot position).
So try pulling the JFET out.
try resoldering your pot legs - there is what looks like a cracked solder in your backlight pic. test the resistance across the pots legs ON THE BOARD, see if there is connection or not.
Quote from: Rob Strand on July 09, 2019, 07:00:29 PM
I think removing the JFET and shorting the CE terminals are both worth trying.
The CE short needs a bit more circuit working and checks the JFET.
Removing the FET should have been the first thing to do... 2 days are passed...
I know it may be anything when a builder has no experience, but a circuit analysis should be explained...
There are 2 buffers in series and in between them there's 1 cap shorting signal to GND through the FET switching job.
Assuming input, output and footswitch are correctly connected, no signal whatsoever means one or both the buffers are failing their duty...
Lift the cap or remove the FET and check, possibly with a signal probe.
OK, hold up; I shorted Q2 as stated, but keep in mind Q2 is the 2N3904 (same as Q1) and not the NTE451! The one that is socketed is the NTE451.
Now, I did try to play it with the NTE451 taken out, and it is not playing. Tonight I will break out the audio probe and try to follow the signal and see where it is disappearing. I will also re-solder the pot, just to rule that out.
Quote from: jfrabat on July 10, 2019, 12:49:08 PM
Now, I did try to play it with the NTE451 taken out, and it is not playing.
Pot has nothing to do with signal path. Don't look for minor issues 'till you don't sort out why signal does not pass trough 2 basic buffers...
If it does not work, check the 2 buffers, assuming your solder bridges quest is actually ended and assuming input, output and footswitch are correctly connected.
QuoteNow, I did try to play it with the NTE451 taken out, and it is not playing. Tonight I will break out the audio probe and try to follow the signal and see where it is disappearing. I will also re-solder the pot, just to rule that out.
Don't bother putting the JFET back until you get the signal to pass.
Something odd is going on as your DC voltages seemed OK. Although I did note this before,
QuoteOne thing that's weird is most of your measurements show a your multimeter loading voltages but pin 5 doesn't show this effect! Are the resistors on pin 5 1M?
I know your resistors are 1M but maybe the 10nF cap to the 1M has an issue. That however is unlikely to be the cause of no signal!
Audio probe would be helpful especially to see if you are getting signal from the output of the first opamp.
I will check it with the audio probe in Saturday (just got home today and I got a client coming in tomorrow, so no pdals until the weekend!).
Quote from: jfrabat on July 11, 2019, 12:04:01 AM
I will check it with the audio probe in Saturday (just got home today and I got a client coming in tomorrow, so no pdals until the weekend!).
Well, client cancelled. Brought out the audio probe. Traced the input signal. This is where I could hear the audio signal (I used a clean tone, for this purpose, 500Hz):
(https://i.postimg.cc/m2w7RR7d/Signal.jpg)
This means the signal is making it into the board, going down the 1K resistor, up the 0.047 cap, into Q1, out of Q1, past the 0.01 cap and into the pot, out of the pot and onto pin 3 of the 072, Out Pin 1, across the 0.047 cap, and nowhere else. Am I tracing it right?
Now, is it me, or should I not have sound coming out of pins 6 and 7 of the 072?
What you have done so far is good.
There's another audio path to check - see below.
QuoteNow, is it me, or should I not have sound coming out of pins 6 and 7 of the 072?
yes.
So now follow the part from the emitter of Q1.
Q1.emitter ---> 22k ---> 10nF ---> IC2 pin 3 ---> IC2 pin 6,7
Also,
(Q1.emitter ---> 22k) ---> 10uF ---> 1M
See where the sound stops in each case.
I tested all traces. Those are the only ones with signal.
QuoteI tested all traces. Those are the only ones with signal.
Hmmm. Was that with the JFET in or out?
One thing that's weird is you are getting signal on the emitter of Q1 but not on the base.
I'm assuming you just forgot to color those traces in red?
The key problem is why the signal is on one side of the 22k but not the other.
This part,
Q1.emitter ---> 22k ---> 10nF ---> IC2 pin 3 ---> IC2 pin 6,7
EDIT:
OK, as a short cut perhaps try resoldering that 22k, the horizontal one just above Q1.
Quote from: Rob Strand on July 12, 2019, 12:42:17 AM
QuoteI tested all traces. Those are the only ones with signal.
Hmmm. Was that with the JFET in or out?
In
QuoteIn
Leave it out for now it makes it harder to debug.
There could even be two problems: the one stopping the audio (when the JFET is out), then another related to the JFET.
You need to get it to pass audio with the JFET out.
With the JFET out, use the audio probe to find at what point in this chain the signal stops:
Q1.emitter ---> 22k ---> 10nF ---> IC2 pin 3 ---> IC2 pin 6,7
OK, with JFET out, the signal is going through (note: I still have a jumper from collector to emitter of Q2). But as soon as I put the JFET in, the signal dies... If I plug it in backwards, it lets the signal through, but no effect (no gating).
Here is where I found signal (not I mirrored the image to have it in the same direction as the build doc):
(https://i.postimg.cc/7ZDN48wK/Signal-2.jpg)
Orange is regular signal, red is amplified signal, yellow is somewhat muted signal.
Light blue is where I should have signal according to build docs, but I do not due to the jumpers (expected loss of signal)
Purple is where I cannot understand why I do not have signal.
Here is the original image from the build doc:
(https://i.postimg.cc/kXWyLX3V/Build-doc.jpg)
QuoteOK, with JFET out, the signal is going through (note: I still have a jumper from collector to emitter of Q2). But as soon as I put the JFET in, the signal dies... If I plug it in backwards, it lets the signal through, but no effect (no gating).
Despite not working those measurements look a *lot* better. Putting the JFET in backwards is probably looking similar to like it's not there. So the good thing is the audio is passing through.
Don't worry about your purple traces (yet). I would have expected some signal on a few of those points as well. Perhaps it's because the 1M divides the signal down by a large factor. The good thing is signal is coming out on pin 1 of the IC. Just put that result on the shelf for now until we need it.
The point we are at is the JFET is shorting out the signal despite the fact Q2 has its CE terminals shorted.
So next steps are is to check the DC level on the gate with the JFET removed. I'd expect about 2V (depending on you meter impedance it could be a bit lower).
Beyond that I'd start thinking the JFET is not turning off because the VP (Vgs_off) for that JFET is too high.
So if the DC on the gate looks OK you should:
- lift one end of the 100k located near the Output connection
- Remeasure the DC on the gate (with the JFET removed).
Now you should see about 3.9V (perhaps a little lower).
If that new voltage looks OK, put the JFET back in and confirm the unit it passes signal.
Also try listening to it with your guitar and amp to judge is your getting any signal drop.
If you get this far the problem is the VP (Vgs_off) for the JFET is too high to work in the circuit.
If it were me at this point I would try to measure the VP of the JFET to confirm that.
Wait a sec I will find the thread on how to do it:
So, in this thread, see reply 1 and the test circuit is the left hand side. I recommend adding a 220 ohm resistor in series with the power to prevent damage if you connect the JFET incorrectly.
https://www.diystompboxes.com/smfforum/index.php?topic=122138.0
Quote from: Rob Strand on July 13, 2019, 07:25:16 PM
So next steps are is to check the DC level on the gate with the JFET removed. I'd expect about 2V (depending on you meter impedance it could be a bit lower).
1.767V I am reading at the gate. I am guessing that is within the OK range, right?
Quote from: Rob Strand on July 13, 2019, 07:25:16 PM
So if the DC on the gate looks OK you should:
- lift one end of the 100k located near the Output connection
- Remeasure the DC on the gate (with the JFET removed).
Now you should see about 3.9V (perhaps a little lower).
You mean this one?
(https://i.postimg.cc/SQfTypgn/100k.jpg)
If so... Actually, a whole LOT lower... 0V at the gate. Source 3.88V, D 3.82V the drops to 3.53V.
Quote1.767V I am reading at the gate. I am guessing that is within the OK range, right?
Yes it look OK.
QuoteYou mean this one?
If so... Actually, a whole LOT lower... 0V at the gate. Source 3.88V, D 3.82V the drops to 3.53V.
Yes that resistor.
0V is correct.
Sorry, I screwed up with the 4V number. 4V is what the JFET sees (gate to source voltage) but what you would actually measure is 0V.
So the important question, with the 100k removed does it pass signal? and is it full level or down in level?
With JFET and 100K out, signal does go through. Actually, even with JFET in, signal goes through, but no gating effect (regardless of pot position, sound is the same).
QuoteWith JFET and 100K out, signal does go through. Actually, even with JFET in, signal goes through, but no gating effect (regardless of pot position, sound is the same).
FYI, pulling the 100k out is diagnostic step not a fix. Also note there will be no gating as we have bridged out the collector and emitter of Q2 - another diagnostic step.
So just to be clear, we have to following results:
- with the JFET in and 100k in the unit doesn't pass audio
- with the JFET in and 100k out the unit does pass audio
Correct?
So that result would imply the JFET VP (Vgs_off) is too high for the circuit. The circuit needs a JFET with a VP less than 2V.
So where to go from here:
It would be very useful to measure the VP (Vgs_off) of the JFET to *know* for certain what your particular JFET needs. Do you think you could do that measurement? (the link at the end of my last post). The problem is even though the unit is passing signal, ie. we have made an improvement, we still don't know if the JFET is fully off - even with the 100k pulled out.
As far as reaching a solution goes. The problem is the VP (Vgs_off) parameter of your particular JFET isn't suitable for that circuit. The reason is JFETs have tolerances and if you look at the datasheet it states VGS_off can be from 0.5 to 4V. That circuit needs VGS_off less than 2V so you were unlucky enough to get one with a high VGS_off. Now, while I didn't intend on pulling the 100k out to be a solution it can actually be used as a valid solution. Pulling the 100k lets the gate voltage swing twice as much and that lets it handle VGS_off's upto about 4V.
Quote from: Rob Strand on July 14, 2019, 06:59:06 PM
So just to be clear, we have to following results:
- with the JFET in and 100k in the unit doesn't pass audio
- with the JFET in and 100k out the unit does pass audio
Correct?
Yes
Quote from: Rob Strand on July 14, 2019, 06:59:06 PM
So that result would imply the JFET VP (Vgs_off) is too high for the circuit. The circuit needs a JFET with a VP less than 2V.
So where to go from here:
It would be very useful to measure the VP (Vgs_off) of the JFET to *know* for certain what your particular JFET needs. Do you think you could do that measurement? (the link at the end of my last post). The problem is even though the unit is passing signal, ie. we have made an improvement, we still don't know if the JFET is fully off - even with the 100k pulled out.
As far as reaching a solution goes. The problem is the VP (Vgs_off) parameter of your particular JFET isn't suitable for that circuit. The reason is JFETs have tolerances and if you look at the datasheet it states VGS_off can be from 0.5 to 4V. That circuit needs VGS_off less than 2V so you were unlucky enough to get one with a high VGS_off. Now, while I didn't intend on pulling the 100k out to be a solution it can actually be used as a valid solution. Pulling the 100k lets the gate voltage swing twice as much and that lets it handle VGS_off's upto about 4V.
OK, the issue is my MM does not have a way to measure transistors/MOSFET/JFET/etc, so I would probably need a new one that has that feature (knowing the local market in Panama, it is highly likely it will be a Chinese brand, so quality is far from assured!). But that I can fix by getting a new one (been thinking about it anyway).
Now since the JFET is actually socketed, I could reproduce the circuit on the breadboard and we can try different things. Now, I am a noob, so I need guidance on the fix, but reproducing the circuit should be simple enough. Would that help?
Have a quick look to the image below;
(https://i.postimg.cc/dhHr04Hm/Callate2-B.jpg) (https://postimg.cc/dhHr04Hm)
Some MXR units came out with those component values. Changing the 1M resistor going from the JFET gate to Q2 collector can help with a JFET with a very low Vgss(off) characteristics. Also changing the zener to 6v2 can help.
Edit: Have now uploaded the correct picture.
QuoteYes
OK cool. I think we are on the right track.
QuoteOK, the issue is my MM does not have a way to measure transistors/MOSFET/JFET/etc,
No multimeters will test JFETs. To check the VP (Vgs_off) all you need is a volt-meter.
Wire-up the test circuit on the left. I recommend adding a 220 ohm resistor in series with the power to prevent damage if you connect the JFET incorrectly. Connect your multimeter (set to Volts) where it shows the "V".
(https://i.imgur.com/bbrz3o3.jpg)
QuoteChanging the 1M resistor going from the JFET gate to Q2 collector can help with a JFET with a very low Vgss(off) characteristics. Also changing the zener to 6v2 can help.
Changing the zener is OK we should get 1V or 1.5V extra drive that way. Changing the 1M fixes one problem and creates another smaller problem. The 1M helps linearize the JFET.
Pulling the 100k doesn't affect the linearization however it could have an effect on the dynamics. It's possible the cap needs to be decreased but making the cap too small also negates the linearization.
OK, been busy with work and stuff (had a business trip to Costa Rica, and since I had to stay Friday night, I took advantage and played with my old band there on a gig they had).
Battery: 9.22V
Vgs Measure: 3.60V
Quote from: Rob Strand on July 15, 2019, 12:55:53 AM
I recommend adding a 220 ohm resistor in series with the power to prevent damage if you connect the JFET incorrectly.
Now I wanted to add this for safety, but I have NO IDEA what you meant, so I just double checked all the pinouts. If you could educate me for the future, it would be greatly appreciated!
QuoteOK, been busy with work and stuff (had a business trip to Costa Rica, and since I had to stay Friday night, I took advantage and played with my old band there on a gig they had).
No problem.
Quote
Battery: 9.22V
Vgs Measure: 3.60V
OK great. So clearly your VP value is high and that's the problem. To add to the problem your zener voltage is 3.93V, which is a little low.
So if you replaced the 5.1V zener with a 6.2V zener, as SlowPoke suggested, it would certainly help. In fact that change alone will probably get it going. See if you can get 0.5W zeners not 1W zeners. If you still aren't happy after the zener change, the next mod would be to replace the 100k I mentioned before with say 1M. You probably don't need to do that until you try the zener, it's another trick up our sleeve we can use if we have to.
So ideally I want a 1N5234? If unavailable, 1N4735 would be the second choice? Is this correct?
Just to double check (I do not think any of these will work), let me tell you what I have in my stash as far as diodes go:
Germanium
- DJ025 (I understand they are interchangeable to the 270 below)
- 1N270
- 1N914
Silicon
Zenner
Schottky
QuoteSo ideally I want a 1N5234? If unavailable, 1N4735 would be the second choice? Is this correct?
Yes, we can get the 1N4735 to work.
QuoteJust to double check (I do not think any of these will work), let me tell you what I have in my stash as far as diodes go:
No can't use any of those.
I have a feeling we might *need* to change that 100k to at least 680k also (the one connecting to the 1M and the 47n, near IC1b pin 6). More drastic changes would be to change the 1M to 1k8 or 2k2, as in Slowpokes picture.
We *can* get it to work.
QuoteNow I wanted to add this for safety, but I have NO IDEA what you meant, so I just double checked all the pinouts. If you could educate me for the future, it would be greatly appreciated!
Sorry, I didn't get back to you question. In the diagram I posted the 9V rail connects to the JFET drain. You "cut" that wire and place a 220 ohm resistor between two points. The idea is if you connect the JFET incorrectly the current is limited to about 40mA (9 V / 220 = 40mA), which is going to cause a lot less damage than "shorting" the battery.
OK, I got some news, and some decisions to make; I called a local electronic store, and asked them if the had any 2N5485 and 1N5234 and they said yes. I asked my wife to get me 5 of each (you know, to keep in stock). The diodes are in fact 1N5234 but the JFET I got actually reads K161 on top and GR 6K on the bottom. I am thinking this is a 2SK161, which is NOT the 2N5485 I asked for...
Now, the question is, what to do. Do I try the K161, or do I keep the NTE and change the diode?
It is a 2SK161-GR which should work but so should the NTE device.
Measure the Vgs(off ) voltage as you did for the NTE device - If it is lower than 2.8V it is worth trying.
We know that the NTE JFET has a Vgs(off ) of about 4V which is too high and will not work without modifications.
Measured Vgs is 2.41, so I figured I try it. Checked pin-outs, and it is the same also.
Re-soldered the resistor in, de-soldered the jumper, and tried it. WE HAVE LIFT OFF!!!! (obviously, this is AFTER I remembered to take out the jumper! Because, you know it, I tried it first without taking out the jumper and signal just went through! LOL!)
Thanks to everyone for their help!
QuoteRe-soldered the resistor in, de-soldered the jumper, and tried it. WE HAVE LIFT OFF!!!! (obviously, this is AFTER I remembered to take out the jumper! Because, you know it, I tried it first without taking out the jumper and signal just went through! LOL!)
Good stuff.
You also changed the zener to a 1N5234 (6.2V 500mW), yes?
No, kept the one that was there.
QuoteNo, kept the one that was there.
With your zener at an actual voltage of 3.9V it might not be passing the signal at full level.
For VP = 2.4V you want the zener at more than 2*2.4V = 4.8V. At a zener voltage of 3.9V
the JFET won't cut-off fully and you will get a bit of attenuation.
Let me test it this weekend and lets see how it goes. I have the other zenner on hand, so it would be a quick replacement if I do need to replace....
Swapped out the Zener for the 1N5234. Pedal works (gate function), but there is a slight "hum" when I play the B and high E strings (unless the gate is less than halfway). It could well be the JFET (it is Chinese, and just looking at it, it looks a bit shady), so I tried the NTE, and that one is not gating; it lets signal through, but does not gate (at least not noticeably).
Quoteso I tried the NTE, and that one is not gating; it lets signal through, but does not gate (at least not noticeably).
OK for this. Don't worry too much. I had a feeling the 6.2V zener wouldn't be quite enough to get that JFET working. To use that JFET we would need to do further mods. So the reason for that is understandable.
QuoteSwapped out the Zener for the 1N5234. Pedal works (gate function), but there is a slight "hum" when I play the B and high E strings (unless the gate is less than halfway).
So that's more of a problem.
Why the hum with 6.2V zener (V=?) and not with 5.1V zener (3.93V)?
We know:
1) A 6.2V zener makes the JFET turn off more and hence the gate is open more.
2) The "hum" reduces when the Threshold is turned down.
This will cause the JFET to stay on more often.
3) From the previous result, the 2SK161-GR & 5.1V Zener (3.93V) didn't have a
a hum problem. Here the is JFET probably not turning off completely.
All three cases are consistent from the JFET's perspective. It seems the fully turning off the JFET is a factor.
However, it might be a symptom more than a cause!
So possible causes might be:
- Hum or noise is getting into the Threshold pot. This circuit is high gain could pick-up hum/buzz. The thing that add difficulty is you don't hear that signal directly you only hear it's affect on modulating the gain. That's not really helped by the high value 500k pot.
- Oscillation of the Threshold amp. That could cause also sorts of weird behaviour.
- When the Threshold amp is saturating it might be putting junk throughout the ckt. Perhaps a 100uF on the power rail will fix it.
I guess one question I have for you is, are you sick of debugging this or are you happy to see it through?
Some things I would try at this point would be:
- Try to make the wiring to the sensitivity pot neatly as possible, in particular the wire going to the pot wiper.
Try twisting a grounded wire around the wiper wire.
- 100uF on the power rail.
- See what happens when you lift the 1uF cap. Possibly replace it with 10n.
- Try replacing the 680 ohm resistor. Try say 2.2k.
The aim is to see what fixes the "hum" problem.
Quote from: Rob Strand on July 27, 2019, 11:11:42 PM
Why the hum with 6.2V zener (V=?) and not with 5.1V zener (3.93V)?.
Important note; when I tried the pedal with the other zener, the amp was VERY low (kids were asleep) so I am not so sure the hum was not 5here before. I just noticed it now...
QuoteImportant note; when I tried the pedal with the other zener, the amp was VERY low (kids were asleep) so I am not so sure the hum was not 5here before. I just noticed it now...
Ah OK. I guess the important thing is it is there. You can get weird stuff like that happening. If you want to check the previous configuration again it's up to you.
Quote from: Rob Strand on July 28, 2019, 12:58:40 AM
QuoteImportant note; when I tried the pedal with the other zener, the amp was VERY low (kids were asleep) so I am not so sure the hum was not 5here before. I just noticed it now...
Ah OK. I guess the important thing is it is there. You can get weird stuff like that happening. If you want to check the previous configuration again it's up to you.
What do you recommend? You know more than me, so I will follow your lead... It's not like the soldering iron is not out (working on a Super8 looper anywaY!).
QuoteWhat do you recommend? You know more than me, so I will follow your lead... It's not like the soldering iron is not out (working on a Super8 looper anywaY!).
BTW what are you powering the effect from?
The idea is to find out where how the hum is getting in. One important thing try to get an idea in you mind how loud the hum is now before we change anything. Some of the tests might reduce the hum but not remove it but that might help work out what is wrong.
Try this sequence of tests:
Short the collector and emitter of Q2 again like we did before.
Set the sensitivity pot in different positions min, 12 O'clock, max
and see if it changes or removes the hum.
Leaving the Q2 collector-emitter short in place.
Change the 680 ohm to something like 3.3k (anything from 2k2 to 4k7).
See if there is any change in the hum with different settings of the
Sensitivity pot.
If you didn't get any hum at all in the above tests. Remove the short on
the collector and emitter of Q2. Then again see if there is any change
in hum with different settings of the Sensitivity pot.
You could also try putting a 100uF cap across the power rails.
See how you go. What to do next depends the above results.
Quote from: Rob Strand on July 28, 2019, 07:03:57 PM
BTW what are you powering the effect from?
The idea is to find out where how the hum is getting in. One important thing try to get an idea in you mind how loud the hum is now before we change anything. Some of the tests might reduce the hum but not remove it but that might help work out what is wrong.
You know, now that you mention it, it could be a noise power source. Mine is a regular 9V source (not for pedal, but regular electronic one) with a long cable (I spliced 2 together to make it about 8 feet or so in length).
Currently, my pedal box uses this (https://www.amazon.com/dp/B00WHLLDWO/ref=nav_timeline_asin?_encoding=UTF8&psc=1) power source, but I just ordered this one (https://www.walmart.com/ip/Truetone-Cs12-One-Spot-Pro-Power-Brick/107476648) yesterday because I am getting a bit of hum from the first one, and I have read it is not really isolated (and when I run the looper, all pedals will be on at once).
Quote from: Rob Strand on July 28, 2019, 07:03:57 PM
Try this sequence of tests:
Short the collector and emitter of Q2 again like we did before.
Set the sensitivity pot in different positions min, 12 O'clock, max
and see if it changes or removes the hum.
Leaving the Q2 collector-emitter short in place.
Change the 680 ohm to something like 3.3k (anything from 2k2 to 4k7).
See if there is any change in the hum with different settings of the
Sensitivity pot.
If you didn't get any hum at all in the above tests. Remove the short on
the collector and emitter of Q2. Then again see if there is any change
in hum with different settings of the Sensitivity pot.
You could also try putting a 100uF cap across the power rails.
See how you go. What to do next depends the above results.
OK, will try to do this tonight (at least advance some). Will let you know how it goes...
QuoteYou know, now that you mention it, it could be a noise power source. Mine is a regular 9V source (not for pedal, but regular electronic one) with a long cable (I spliced 2 together to make it about 8 feet or so in length).
The MXR noise gate has some design issues that lets hum through from the power rails.
The main one is the 1M + 1M divider which feeds the last opamp (pin 5 on Tone-pad's schematic). FWIW, the cap here actually stuffs-up the linearization of the JFET. The second is the zener is running at low currents.
A fix for power supply hum is to put a 100uF cap in place of the 10uF cap across the power rails. Then add say a 47 ohm in series with the +9V lead between the power inlet +9V and the PCB +9V.
If that's the problem it's much easier to solve than some obscure circuit behaviour.
Quote from: Rob Strand on July 28, 2019, 09:13:07 PM
QuoteYou know, now that you mention it, it could be a noise power source. Mine is a regular 9V source (not for pedal, but regular electronic one) with a long cable (I spliced 2 together to make it about 8 feet or so in length).
The MXR noise gate has some design issues that lets hum through from the power rails.
The main one is the 1M + 1M divider which feeds the last opamp (pin 5 on Tone-pad's schematic). FWIW, the cap here actually stuffs-up the linearization of the JFET. The second is the zener is running at low currents.
A fix for power supply hum is to put a 100uF cap in place of the 10uF cap across the power rails. Then add say a 47 ohm in series with the +9V lead between the power inlet +9V and the PCB +9V.
If that's the problem it's much easier to solve than some obscure circuit behaviour.
Then let me start with that, as it is a known issue (and even if I solve the other one, I will need to look into this one!). So if I understand you correctly, this means replacing the 10 uF electrolytic cap on the top right (that's the one that connects the power rails, right?).
(https://i.postimg.cc/kXWyLX3V/Build-doc.jpg)
with a 100 uF one? And add a 47 Ohm resistor somewhere between the power jack and the board? If this is so, it is simple to do (the pedal does not yet have a board for it, so I can easily add the resistor and heat shrink wrap it).
Quotewith a 100 uF one? And add a 47 Ohm resistor somewhere between the power jack and the board? If this is so, it is simple to do (the pedal does not yet have a board for it, so I can easily add the resistor and heat shrink wrap it).
Yes that's it. Should be easy to do.
Quote from: Rob Strand on July 28, 2019, 09:59:10 PM
Quotewith a 100 uF one? And add a 47 Ohm resistor somewhere between the power jack and the board? If this is so, it is simple to do (the pedal does not yet have a board for it, so I can easily add the resistor and heat shrink wrap it).
Yes that's it. Should be easy to do.
OK, that's done. I messed up in my description, though; I remembered using the power supply, but I used it on the Green Russian I connected with the gate to try the gate function (that pedal is noisy!). But the gate I actually powered through a 9V battery. Here is the issue I have:
Especially at the end of the chord, you can clearly hear the hiss I mentioned...
PS: Pardon my narrative; English is not my first language... and I have not practiced it in some time!
QuoteEspecially at the end of the chord, you can clearly hear the hiss I mentioned...
Ok that's helps a lot.
I am fairly certain it is coming from IC1a (on tonepad's schematic). So when you have the Sensitvity pot full IC1a is clipping very hard somehow the clipped signal is finding its way back into the audio. These problems are tricky to identify and solve.
The question is how is is getting in!
So here's the possibilities:
- Through the power supply. Lucky you have the 100uF on the power supply because the cap should help reduce the effect. For now we will assume this is not the cause.
- Coupling between the tracks. When I look at the layout the clipped signal from IC1a passes through the 47nF capacitor then that track runs right next to all the audio signals. So that is asking for trouble. Without re-laying out the PCB there's a some things to try. Put a 220ohm resistor in series with the 47nF at the output of IC1.
- Oscillation of IC1a. The input of IC1a (pin 3) runs right next to the output on pin 1. That's also asking for trouble. The thing is through, you problem gets worse when the sensitivity pot is full. In this case, I'd probably expect oscillation to get worse when the pot was at the center. The oscillation can be from loading the output with a capacitor (the 47n+1uF in the circuit) and the 220 ohm should fix that.
So try the 220 ohm resistor. See what happens then we will take it from there. Don't take it out because we might need it in the next steps.
QuotePS: Pardon my narrative; English is not my first language... and I have not practiced it in some time!
Your English is great. Really clear.
Quote from: Rob Strand on July 29, 2019, 12:00:36 AM
So try the 220 ohm resistor. See what happens then we will take it from there. Don't take it out because we might need it in the next steps.
OK, will try it tomorrow and see how it goes... Just to be clear, I would be taking one leg out of the cap and the resistor would be going from the newly freed leg to the old pad, right?
Quote from: Rob Strand on July 29, 2019, 12:00:36 AM
I am fairly certain it is coming from IC1a (on tonepad's schematic). So when you have the Sensitvity pot full IC1a is clipping very hard somehow the clipped signal is finding its way back into the audio. These problems are tricky to identify and solve.
The question is how is is getting in!
So here's the possibilities:
- Through the power supply. Lucky you have the 100uF on the power supply because the cap should help reduce the effect. For now we will assume this is not the cause.
- Coupling between the tracks. When I look at the layout the clipped signal from IC1a passes through the 47nF capacitor then that track runs right next to all the audio signals. So that is asking for trouble. Without re-laying out the PCB there's a some things to try. Put a 220ohm resistor in series with the 47nF at the output of IC1.
- Oscillation of IC1a. The input of IC1a (pin 3) runs right next to the output on pin 1. That's also asking for trouble. The thing is through, you problem gets worse when the sensitivity pot is full. In this case, I'd probably expect oscillation to get worse when the pot was at the center. The oscillation can be from loading the output with a capacitor (the 47n+1uF in the circuit) and the 220 ohm should fix that.
One of these days you will need to point me to some text to study a bit more. That explanation is still over my head! I kind of follow you, but I still don't get the logic of how it all works (or why). But I CAN follow instructions! LOL!
QuoteOK, will try it tomorrow and see how it goes... Just to be clear, I would be taking one leg out of the cap and the resistor would be going from the newly freed leg to the old pad, right?
Yes, that's correct.
QuoteOne of these days you will need to point me to some text to study a bit more. That explanation is still over my head! I kind of follow you, but I still don't get the logic of how it all works (or why). But I CAN follow instructions! LOL!
To tell you the truth some of these pedal problems are very tricky. At this point I don't know what the problem is, I only know where it might be (and still could be wrong). The idea is to try a few things to narrow down the possibilities. Debugging via forums takes a bit of patience and perseverance.
Resistor is in. No change in hiss.
QuoteResistor is in. No change in hiss.
OK, no problem, just leave it we might need it later.
So the next step would be to increase the 47pF to 100pF. It's the ceramic cap next to the IC. You could solder a 47pF in parallel with the existing 47pF it that's easier. It's not actually shown on the tonepad schematic but it goes in parallel with the 1M resistor on IC1a.
You may add a bigger cap @ Vb also....
actually the issue is the dual op-amp: IC1a has a huge amount of gain... MXR used 2 singles back then, and kept them far apart each other
Quote from: Fender3D on July 31, 2019, 06:22:40 AM
You may add a bigger cap @ Vb also....
OK, so whats is Vb again? Sorry, I am a newb and do not yet understand electronics...
Vb is Vb on schematic
Quote from: Fender3D on July 31, 2019, 12:26:37 PM
Vb is Vb on schematic
Ah, that makes sense... So you mean the 10uF electrolytic that is in the same line as the diode? If so, that would be the one marked below, right? (trace marked in yellow)
(https://i.postimg.cc/SQ11Mwbr/Cap-Change.jpg)
(https://i.postimg.cc/Wzy9cJwK/Schematic-cap-change.jpg)
If that is correct, how much bigger? I have 22uF, 33uF, 47uF, 100uF, 220uF...
Quote from: Rob Strand on July 31, 2019, 02:08:43 AM
So the next step would be to increase the 47pF to 100pF.
Done. Still there is hiss...
QuoteYou may add a bigger cap @ Vb also....
Certainly worth trying.
Another thing worth trying in this area is to reduce the 22k on the zener. As a test try 2k2.
QuoteDone. Still there is hiss...
In your video you increase the sensitivity to full, then play the chord you hear a hiss at the ends. There's two things that is not clear to me about that test:
- Does the hiss stay there forever after the note dies down? For example if you hold the strings or turn down the guitar volume to minimum.
- When you increase the sensitivity to full does the gate stay open without any notes being played?
So one test worth doing at this point is to lift one end of the 10uF going to the JFET. Then see if you hear noise with the sensitivity on full. The gate will not longer work but it will pass audio at full level. The idea behind this test is to see the noise is getting into the audio path via the JFET (actually the JFET modulation) or via the power rails or PCB tracks.
I tried to determine the nature of the noise from the audio in your video. I took the tail part of the audio, heavily high-pass filtered it at 1kHz and amplifier it. I head a constant noise which has an unnatural character.
So the bigger Vb cap and the stuff I mentioned in my last post should help narrow down:
- oscillation
- modulation from the JFET. It's possible that noise from the guitar or input is triggering the gate circuit and modulating the audio.
- noise from the zener.
Something I wasn't clear on as well, it's important to know if the noise *suddenly* appears at a certain pot setting, and, if it gradually gets worse as you rotate the pot to full.
Quote from: Rob Strand on July 31, 2019, 06:54:29 PM
QuoteYou may add a bigger cap @ Vb also....
Certainly worth trying.
How much bigger? I have 22uF, 33uF, 47uF, 100uF, 220uF...
Quote from: Rob Strand on July 31, 2019, 08:15:03 PM
I tried to determine the nature of the noise from the audio in your video. I took the tail part of the audio, heavily high-pass filtered it at 1kHz and amplifier it. I head a constant noise which has an unnatural character.
I am not sure you are not hearing the ceiling fan there... The "hiss" I hear is more of a sort of clipping or something. Maybe this vid will help:
By the way, this is where we are at now:
(https://i.postimg.cc/MTQhWQDh/20190731-230033.jpg)
220 Ohm Resistor added in series with the with 47 uF cap and added a 47 Ohm resistor to the +9V line. 100 pF replaces 47 pF ceramic capacitor.
QuoteHow much bigger? I have 22uF, 33uF, 47uF, 100uF, 220uF...
100uF should be enough to know if it has an effect or not.
QuoteI am not sure you are not hearing the ceiling fan there... The "hiss" I hear is more of a sort of clipping or something. Maybe this vid will help:
OK it's much clearer now. I would call that a "fizz". It seems to be there when the notes are present. Also it kinds of gets worse as the Sensitivity pot is increased.
There's a couple of places in the video where it seems to cut-out. Once at 2:12 (1/4 pot setting) and 2:19 (0 pot setting). It that the pedal doing that?
QuoteSo one test worth doing at this point is to lift one end of the 10uF going to the JFET.
You should definitely try this test as it will help narrow down the search. The gating function is disabled in this test. The test will tell us if the noise is getting into the audio from the Sensitivity circuit or if the process of gating is causing it. So with that change repeat the test in the second video with the different pot settings.
Yes, the lower the pot, 5he less "fizz" there is. Actually, now with all the changes, i cannot fully close the gate (I could before) and totally block out the guitar (not that I woukd want to, though).
I will try the test tomorrow (cant today) and report back.
QuoteActually, now with all the changes, i cannot fully close the gate (I could before) and totally block out the guitar (not that I woukd want to, though).
That piece of information might be useful. If you can't full close the gate with the pot set to zero that means noise is getting into the circuit around IC1a, or, IC1a is oscillating. Think of it this way: If you have the pot set to zero you would expect the output of IC1a to be zero that means the gate should have no signal to turn on and the gate should stay closed.
Another thing you should try is connecting the metal body of the sensitivity pot to the circuit ground. That can help stop noise getting into the pot and IC1a.
Quote from: Rob Strand on August 02, 2019, 01:00:35 AM
If you can't full close the gate with the pot set to zero that means noise is getting into the circuit around IC1a, or, IC1a is oscillating.
Quote from: Fender3D on July 31, 2019, 06:22:40 AM
actually the issue is the dual op-amp: IC1a has a huge amount of gain... MXR used 2 singles back then, and kept them far apart each other
UPDATE: Here is what we have done so far:
- JFET is a K161 (2SK161?)
- Zener changed to 1N5234
- Ceramic cap replaced from 47pF to 100pF
- 220 Ohm resistor added in series with 47uF cap
- 47 Ohm resistor added to power line
- Changed power rail cap from 10uF to 100uF
- Changed the Vb cap from 10 uF to 100 uF
- Grounded the rear pot casing
Issues still present:
- Fizz inserted with guitar sound (dies away as the guitar sounds dies)
- Fizz highly dependent on pot position (the more I close the gate, the less fizz there is)
- Gate is not fully closing
Voltages (as of today):
Battery: 9.1V
IC (TL072CP):
1: 6.31V
2: 6.30V
3: 6.30V
4: 0.00V
5: 4.12V
6: 4.32V
7: 4.32V
8: 8.67V
Q1 (2N3904):
E: 5.31V
B: 5.66V
C: 0.00~190mV
Q2 (2N3904):
E: 0.00V
B: 0.501V
C: 4.63V
Q3 (K161):
D: 6.30V
S: 6.30V
G: 5.16V
Zener (1N5234):
Cathode: 6.30V
Anode: 0.00V
Sound clip (at youtube standards!):
NOTE: After gate halfway open, I turned the pot before the sound completely died out. Sorry about that!
So what's the next step (other than giving this thing a name, 'cause there is lots of changes from the original!)?
QuoteSo what's the next step (other than giving this thing a name, 'cause there is lots of changes from the original!)?
Don't worry about the many changes. When debugging it is common to modify the circuit to understand the problem. Many of those changes can be removed later on. At this point it is OK to leave them in. If you had an oscilloscope you could check stuff without making so many changes. It's always more difficult to debug circuits with limited equipment - I did that a lot when I was young.
As far as progressing is concerned there's still *many* things to try.
Try doing these one at a time in this order:
- Change to 47pF (which is now 100pF) to 220pF
- Remove the 1uF cap (the one across the 100k resistor, brown color) and replace it with a 10nF cap
- Replace the 6.2V zener with the original 5.1V zener in series with two silicon diodes.
So in place of the zener use three diodes like this. Note the silicon diodes point
in the opposite direction to the zener diode.
Vb ---- >|------ >|------|<------ 0V
Si Si 5.1V zener
With that connection you should get about 5.2V on Vb.
Don't give up!
Quote from: Rob Strand on August 03, 2019, 08:48:33 PM
Change to 47pF (which is now 100pF) to 220pF
The biggest I got is 180 pF; is that enough?
QuoteThe biggest I got is 180 pF; is that enough?
It would be fine. In fact I'd just put that across the 100pF that's already there giving 100pF+180pF=280pF.
For this test a bigger is better.
The reason why we change this part is to filter the high frequency noise more. It can help reduce oscillations also. In the original circuit the opamp had a low "gain bandwidth" and a high gain (gain = 1MEG / 680 ohm) which creates a filter by itself.
Quote from: Rob Strand on August 04, 2019, 06:35:00 PM
QuoteThe biggest I got is 180 pF; is that enough?
It would be fine. In fact I'd just put that across the 100pF that's already there giving 100pF+180pF=280pF.
For this test a bigger is better.
The reason why we change this part is to filter the high frequency noise more. It can help reduce oscillations also. In the original circuit the opamp had a low "gain bandwidth" and a high gain (gain = 1MEG / 680 ohm) which creates a filter by itself.
OK, something is odd here; I added the 180 pF cap, and used my capacitance multimeter to be sure the solder was true and had joined both caps together. Turns out, the MM reads 42 pF with both caps on. I measured my stock caps, and they are reading more or less OK (168 pF for the 180 pF and 90.6 pF for the 100 pF caps). Unless something in the board is playing games with me, I cannot understand why it would only read 42 pF...
As far as sound, same issue persists. Gate closes better now (not 100%, but at least 95%, meaning that if I strum lightly, the sound does not get through; if I strum strongly, then about 1~2 seconds goes through, then it closes).
Quote from: Rob Strand on August 03, 2019, 08:48:33 PM
QuoteSo what's the next step (other than giving this thing a name, 'cause there is lots of changes from the original!)?
Try doing these one at a time in this order:
- Change to 47pF (which is now 100pF) to 220pF
- Remove the 1uF cap (the one across the 100k resistor, brown color) and replace it with a 10nF cap
- Replace the 6.2V zener with the original 5.1V zener in series with two silicon diodes.
So in place of the zener use three diodes like this. Note the silicon diodes point
in the opposite direction to the zener diode.
Vb ---- >|------ >|------|<------ 0V
Si Si 5.1V zener
So, do I move to the 1uF cap and replace with 10uF, or do we examine this weird cap issue?
QuoteOK, something is odd here; I added the 180 pF cap, and used my capacitance multimeter to be sure the solder was true and had joined both caps together. Turns out, the MM reads 42 pF with both caps on. I measured my stock caps, and they are reading more or less OK (168 pF for the 180 pF and 90.6 pF for the 100 pF caps). Unless something in the board is playing games with me, I cannot understand why it would only read 42 pF...
Don't worry at all. When you measure capacitors in-circuit the other parts in the circuit affect the measurement. Some meters work better than others depending on the method that meter uses to measure the capacitance. The way to interpret in circuit measurements is if it is close to what you expected then the measurement work. If it didn't work then it's likely the other parts in the circuit are affecting the measurement. You can get the same problem with resistance measurements.
QuoteAs far as sound, same issue persists. Gate closes better now (not 100%, but at least 95%, meaning that if I strum lightly, the sound does not get through; if I strum strongly, then about 1~2 seconds goes through, then it closes).
The extra filtering from the cap is probably keeping the noise under the gate threshold.
Hi guys
I had exactly the same problem but it is easily solvable.
This is my first post here. I registered here only for this issue.
I work as an analog design engineer and as a hobby I'm a bass player and build some stompboxes for me and my friends.
Yesterday I built this popular MXR noise gate for me and made some minor modifications because I couldn't find the original JFET and Bipolars. Everything was ready but just before closing the box I noticed strange bizz when I increase the sensitivity of the gate. I couldn't sleep... Today I read this topic and decided to debug it. I spent few hours trying to isolate detection circuitry but the bizz was there... I almost gave up but after few glasses of wine I've got the answer. The bizz comes through the input of the high gain amplifier (OA1a). The input buffer is very weak - around 200uA and its output impedance is relatively high. This allows the the bizz to penetrate between the input and output buffer. The problem is easily solvable by increasing the current through the first buffer.
jfrabat, please change the resistor on Q1 emitter from 22k to 4.7k. This significantly will decrease the buzz to levels that you can't hear it.
(https://i.postimg.cc/VdRnc0bD/2019-08-27-03-06-39-MXR-noise-gate-cdr-get-File-asp.png) (https://postimg.cc/VdRnc0bD)
Good luck!
Greetings from Bulgaria
Quote from: aspangev on August 26, 2019, 08:10:09 PM
Hi guys
I had exactly the same problem but it is easily solvable.
This is my first post here. I registered here only for this issue.
I work as an analog design engineer and as a hobby I'm a bass player and build some stompboxes for me and my friends.
Yesterday I built this popular MXR noise gate for me and made some minor modifications because I couldn't find the original JFET and Bipolars. Everything was ready but just before closing the box I noticed strange bizz when I increase the sensitivity of the gate. I couldn't sleep... Today I read this topic and decided to debug it. I spent few hours trying to isolate detection circuitry but the bizz was there... I almost gave up but after few glasses of wine I've got the answer. The bizz comes through the input of the high gain amplifier (OA1a). The input buffer is very weak - around 200uA and its output impedance is relatively high. This allows the the bizz to penetrate between the input and output buffer. The problem is easily solvable by increasing the current through the first buffer.
jfrabat, please change the resistor on Q1 emitter from 22k to 4.7k. This significantly will decrease the buzz to levels that you can't hear it.
(https://i.postimg.cc/VdRnc0bD/2019-08-27-03-06-39-MXR-noise-gate-cdr-get-File-asp.png) (https://postimg.cc/VdRnc0bD)
Good luck!
Greetings from Bulgaria
Great! I had this just laying around and really moved to something else, but I will give this a go! THANKS!
QuoteI had exactly the same problem but it is easily solvable.
This is my first post here. I registered here only for this issue.
I work as an analog design engineer and as a hobby I'm a bass player and build some stompboxes for me and my friends.
Excellent work, thanks for posting! That problem really needed someone to get in there a fix it.
I noticed that high emitter resistor as well but I couldn't see a reason why it would cause a problem since all the loads off the buffer are relative high impedance. However now, after looking at the Waza buffer thread, I can see the 22k load resistor to the JFET is going to make the buffer stage (with a 22k emitter) clip at a lower voltage than Vcc/2.
So maybe the real problem is when the gate shuts off and the JFET starts to turn on, the 22k series resistor then loads down the buffer, then that reduces the swing of the buffer and the buffer clips prematurely.
I wonder why a lot more people aren't seeing that? It seems like it should only occur then the signal is strong but jfrabat's videos show the effect when the guitar notes are decaying and guitar voltage will be low! Raising the zener voltage should have helped a bit too.
Quote from: Rob Strand on August 26, 2019, 09:01:27 PM
So maybe the real problem is when the gate shuts off and the JFET starts to turn on, the 22k series resistor then loads down the buffer, then that reduces the swing of the buffer and the buffer clips prematurely.
I wonder why a lot more people aren't seeing that? It seems like it should only occur then the signal is strong but jfrabat's videos show the effect when the guitar notes are decaying and guitar voltage will be low! Raising the zener voltage should have helped a bit too.
No, the problem is not overloading the buffer and clipping the signal. The real problem is that pulses from OpAmp's output (IC1a) occur also on IN- because the OpAmp is saturated and then, probably through parasitic capacitance or just electrically inside the OpAmp, the pulses occur on IN+. Once appeared on IN+ the pulses penetrate the signal chain through the cap 0.01uF. A lower impedance buffer would not allow this penetration. This was proven by some experiments - cutting the JFET circuitry - no any impact, the pulses are there. This means that the only way for back penetration is through the OpAmp's input or supply but we know that putting bigger supply cap doesn't help, so the supply is not the problem.
The other experiment I did is limiting the OpAmp's output - I put 2 parallel diodes on the OpAmp feedback. That way the output voltage swing is about 2*0.7V and the OpAmp is not saturated - the bizz disappeared completely but the circuit was not sensitive enough.
The only solution I see is making the input buffer stronger or completely redesign the schematic. Happily making stronger buffer is not tough task - just replace the emitter resistor. The result is incredible - I can't hear any bizz!
Please share this mod on the most popular topic about this MXR noise gate.
Best regards
Pangev
QuoteNo, the problem is not overloading the buffer and clipping the signal. The real problem is that pulses from OpAmp's output (IC1a) occur also on IN- because the OpAmp is saturated and then, probably through parasitic capacitance or just electrically inside the OpAmp, the pulses occur on IN+. Once appeared on IN+ the pulses penetrate the signal chain through the cap 0.01uF. A lower impedance buffer would not allow this penetration. This was proven by some experiments - cutting the JFET circuitry - no any impact, the pulses are there. This means that the only way for back penetration is through the OpAmp's input or supply but we know that putting bigger supply cap doesn't help, so the supply is not the problem.
The other experiment I did is limiting the OpAmp's output - I put 2 parallel diodes on the OpAmp feedback. That way the output voltage swing is about 2*0.7V and the OpAmp is not saturated - the bizz disappeared completely but the circuit was not sensitive enough.
The only solution I see is making the input buffer stronger or completely redesign the schematic. Happily making stronger buffer is not tough task - just replace the emitter resistor. The result is incredible - I can't hear any bizz!
Excellent experiments. I was thinking of that diode trick as well. 2x Red LEDs might be better but beyond that you run the risk of the problem coming back when the battery runs down and the opamp clips before the LEDs.
I'm not 100% convinced the fizz is getting back through the opamp inputs since jfrabat still got fizz with sensitivity pot backed off from full (so the impedance from the opamp input back to the emitter is fairly high). Capacitive coupling sounds like a good cause. The tone-pad layout has quite a few "noisy" tracks passing near the audio signals around the opamp. The PCB layout is really asking for trouble.
Were you using the tonepad PCB?
The circuit could be helped with some re-design. I noticed the Boss NF-1 Noise gate is pretty much an improvement on the MXR.
Anyway, you did a great job working all that out! I felt like building that circuit but I can't afford the time to build every circuit that has weird problems.
To jfrabat, you can remove all the mods we added. I would keep the 220pF on the opamp since I believe that addition makes the circuit behave more like the original. Obviously you need to keep the 6.2V zener.
Quote from: Rob Strand on August 27, 2019, 03:26:23 AM
I'm not 100% convinced the fizz is getting back through the opamp inputs since jfrabat still got fizz with sensitivity pot backed off from full (so the impedance from the opamp input back to the emitter is fairly high).
Hey Rob
I'm happy you mentioned that. I had impression that this problem occurs only in my project because I don't use the original JFET and bipolars. This actually is another problem that is also easily fixed.
The popping sound that is heard at the end of the last clip is caused by the sharp switch-off of the JFET. In order to cure this, I connected a cap of 4.7nF between the JFET's gate and the ground. This way the gate voltage is 'soften' and the popping sound disappears. Proven!
I use universal PCB but similar layout as from tonepad. I don't believe the parasitics are from the PCB layout but from IC itself. Maybe it's possible to find some OA (different type, different producer) that don't have this issue. I use TL072, Texas Instruments.
Anyway, I will draw later today all the mods I did.
Here are the changes in my project:
(https://i.postimg.cc/K10bPn94/2019-08-27-13-41-37-MXR-noise-gate-mods.png) (https://postimg.cc/K10bPn94)
1. I don't see benefit of using the 3 knobs: ATTENUATION, ATTACK and RELEASE so I removed them. I just increased the 'release' resistor to 680k. I prefer slower release. It's actually less than a second.
2. Increased all signal capacitors because I use it for a bass guitar.
3. Connected 4.7nF cap on the JFET gate - removes the popping sound around the threshold of the gate.
4. Decreased 22k -> 4.7k resistor connected to the Q1 emitter - removes the hizz we discussed earlier.
5. Added LED for visualization the gate. Used NMOS BS170 but can be any small power NMOS with Vth ~ 2V
6. This is the mod I'm going to try when we get back from the tour. I don't see benefit of using such big cap (10uF) in JFET circuitry. I face a problem, when I switch-on power supply the output is muted for about 10 seconds before I could start playing. This probably is caused by charging this big cap through 1M resistor. Calculated, RC = 10 sec. I believe a 1uF cap will be enough. The cut-off freq will be 1/(2Pi*1u*22k) = 7.2Hz which for me is enough.
@jfrabat, I saw you have the same problem with the popping sound around the gate threshold. Please, make the mod No.3 also. This will almost remove the popping and at the same time the attack will be enough not to have delay in deactivating the gate. Please share info when you fix your project.
Generally I'm very happy with the gate behavior with all the changes I did.
I'll be grateful if someone else prove and like the mods
BTW you can support our 'young' band (I hope is not forbidden from the forum rules):
Best regards
Pangev
Quote from: Rob Strand on August 27, 2019, 03:29:34 AM
To jfrabat, you can remove all the mods we added. I would keep the 220pF on the opamp since I believe that addition makes the circuit behave more like the original. Obviously you need to keep the 6.2V zener.
OK, understood.
Quote from: aspangev on August 27, 2019, 07:39:16 AM
@jfrabat, I saw you have the same problem with the popping sound around the gate threshold. Please, make the mod No.3 also. This will almost remove the popping and at the same time the attack will be enough not to have delay in deactivating the gate. Please share info when you fix your project.
Will do!
Changes will.most likely be updated in 2 weeks, as this week is a little hecktic, and next one I am taking my family on vacation...
By the way, I like your music... will download it and take it with me on my vacation!
QuoteI'm happy you mentioned that. I had impression that this problem occurs only in my project because I don't use the original JFET and bipolars. This actually is another problem that is also easily fixed.
The popping sound that is heard at the end of the last clip is caused by the sharp switch-off of the JFET. In order to cure this, I connected a cap of 4.7nF between the JFET's gate and the ground. This way the gate voltage is 'soften' and the popping sound disappears. Proven!
Excellent. So the last problem solved!.
QuoteHere are the changes in my project
Looks good. Finally all the the bugs are out of that thing.
FYI, at the start of this thread I re-traced the schematic of the MXR original unit. The final schematic is at the end of this thread.
https://www.diystompboxes.com/smfforum/index.php?topic=122689.msg1157934#msg1157934
The 1uF cap on the base of the transistor was actually 10n on the original unit. The 1uF came from an old (DIY) schematic which had a few bugs. I don't know if you want to see what effect that has.
QuoteBTW you can support our 'young' band (I hope is not forbidden from the forum rules):
You guys sound great.
QuoteFYI, at the start of this thread I re-traced the schematic of the MXR original unit. The final schematic is at the end of this thread.
https://www.diystompboxes.com/smfforum/index.php?topic=122689.msg1157934#msg1157934
The 1uF cap on the base of the transistor was actually 10n on the original unit. The 1uF came from an old (DIY) schematic which had a few bugs. I don't know if you want to see what effect that has.
Well done!
I'll check it this evening (now it's 6AM here). I'm very curious. When I was debugging, once I removed this cap and the gate doesn't work at all Looking now the schematic seems that 100k is on the edge of open/close the gate. Seems that it strongly depends on beta of Q2. The base current is max ~90uA. If Q2's beta is 100, then collector current is 9mA. This will make voltage drop over 150k resistor of about 1.3V but this is valid if there is no 1u cap. This 1.3V is not enough to close the Q3 gate. If beta is 300, then it will work... My opinion is that this 100k should be chosen depending on the used Q2. Strange, it is the first time I see a schematic that so strongly depends on beta.
Cap of 10n together with 100k makes tau of about 1ms. This will help open/close Q2 at the beginning. Reducing 1u -> 10n could help removing popping sound around the threshold. I'll check it.
BTW I've read some opinions in this forum and it seems that this circuit is very sensitive on the right choice of components - Q1 - Q3 and probably the right choice on OpAmp's. This means that the design is not the best ever made. I'm pretty sure I can make it more robust, starting with the JFET - replacing with MOSFET.
QuoteI'll check it this evening (now it's 6AM here). I'm very curious. When I was debugging, once I removed this cap and the gate doesn't work at all Looking now the schematic seems that 100k is on the edge of open/close the gate. Seems that it strongly depends on beta of Q2. The base current is max ~90uA. If Q2's beta is 100, then collector current is 9mA. This will make voltage drop over 150k resistor of about 1.3V but this is valid if there is no 1u cap. This 1.3V is not enough to close the Q3 gate. If beta is 300, then it will work... My opinion is that this 100k should be chosen depending on the used Q2. Strange, it is the first time I see a schematic that so strongly depends on beta.
Cap of 10n together with 100k makes tau of about 1ms. This will help open/close Q2 at the beginning. Reducing 1u -> 10n could help removing popping sound around the threshold. I'll check it.
Yes the Beta has an effect on the strength of the collector current pulses. The way I look at it is if the transistor gain is low the collector current pulses will be small and the attack time will become longer. The MXR Dynacomp compressor has a similar arrangement for the rectifier, it uses 10nF caps.
The circuit acts like a peak detector but the details are no so easy to understand. The base-emitter diode charges the base cap and the 100k provides the discharge time constant (like you mentioned). When the base cap is large it will discharge more slowly. At the time of the next peak the cap voltage will not drop much so you might expect the current pulse to be more narrow. However, because the cap is large it takes longer to charge. A smaller cap will discharge to a lower voltage but it will charge-up quicker. On the simulator the width of the base current pulses stays almost the same. What changes is the base current. The base current is higher with the larger cap. So perhaps the cap can compensate for a low beta. To me it's a little weird have a 47nF coupling cap feeding a 1uF peak detector. I thought the 47nF cap might charge up but it doesn't seem to charge because the current pulses are narrow.
Hey Rob
I spent few hours playing with the circuit. First I checked behavior with 10nF instead of 1uF. Honestly I don't see huge difference. With 10nF it seems the noise-gate to be more sensitive meaning that it is opening easier but closes harder. For me open gate means you hear the sound, closed gate means there is no sound and noise. I liked the behavior with 1uF but to be closer to the original schematic I choose 100nF.
I changed also 10uF cap in JFET circuitry to 1uF (mod 6 from my list) and for me it's working better - no impact on the sound but the noise-gate starts faster after power-on.
I also decreased the gain of OA1 stage by replacing resistor 680 -> 2.2k. It makes the sensitivity pot more sensible in the whole range of the pot.
Changed also the output cap to 22uF - doubled the original cap because of using with a bass guitar.
List of all my changes:
1. I don't see benefit of using the 3 knobs: ATTENUATION, ATTACK and RELEASE so I removed them. I just increased the 'release' resistor to 680k. I prefer slower release. It's actually less than a second.
2. Increased all signal capacitors because I use it for a bass guitar.
3. Connected 4.7nF cap on the JFET gate - removes the popping sound around the threshold of the gate.
4. Decreased 22k -> 4.7k resistor connected to the Q1 emitter - removes the hizz we discussed earlier.
5. Added LED for visualization the gate. Used NMOS BS170 but can be any small power NMOS with Vth ~ 2V
6. I don't see benefit of using such big cap (10uF) in JFET circuitry. I face a problem, when I switch-on power supply the output is muted for about 10 seconds before I could start playing. This probably is caused by charging this big cap through 1M resistor. Calculated, RC = 10 sec. I believe a 1uF cap will be enough. The cut-off freq will be 1/(2Pi*1u*22k) = 7.2Hz which for me is enough.
7. Changed 1uF/NP -> 100nF - closer to the original value
8. Changed 680 ->2.2k in OA1 stage
Used devices: TL072, 2xBC549C, JFET - PN4393
The final choice:
(https://i.postimg.cc/8s3dpRJk/MXR-noise-gate-mods.png) (https://postimg.cc/8s3dpRJk)
I also played little bit with mods 3 and 4 in order to show what is the difference with/without them:
No 3 and 4 mods:
https://www.aronnelson.com/DIYFiles/up/No_Mods.mp3 (https://www.aronnelson.com/DIYFiles/up/No_Mods.mp3)
Only mod 3:
https://www.aronnelson.com/DIYFiles/up/Mod3.mp3 (https://www.aronnelson.com/DIYFiles/up/Mod3.mp3)
Mod 3 and 4:
https://www.aronnelson.com/DIYFiles/up/Mod3and4.mp3 (https://www.aronnelson.com/DIYFiles/up/Mod3and4.mp3)
Here the first 'chord' is at MIN sensitivity while the second is at MAX sensitivity.
You can still hear popping sound with mod 3 at MIN sensitivity but it is softer. If your sensitivity is in reasonable level, then this popping almost disappear.
I'm thinking about inserting hysteresis on the open/close thresholds. I've got this idea by looking at the MXR D80 noise gate - there are 2 different circuits for both thresholds but for me it is easier to do it here by adding an additional MOSFET which changes the gain of the first OA stage.
Best regards
Atanas Pangev
Excellent documentation and much improved behaviour.
QuoteI'm thinking about inserting hysteresis on the open/close thresholds. I've got this idea by looking at the MXR D80 noise gate - there are 2 different circuits for both thresholds but for me it is easier to do it here by adding an additional MOSFET which changes the gain of the first OA stage.
A Schmitt trigger might help but what I've found is it's better to do some filtering before a Schmitt trigger.
What about adding a cap across the 1M resistor on the opamp. The original circuit used an LM741 with a very high gain, so high that the opamp starts to operate like a low-pass filter. Using a TL07x loses that effect and adding a cap across the 1M restores the filtering. To match the LM714 you need about 150pF to 180pF but there's no reason not to use larger values if it works better. frabat noticed some improvement with the cap.
You should check out the Boss noise gate schematic. It's like a better version of the MXR. They use a full-wave rectifier. They also have specs for the behaviour in the tail and added trimpots to set-up the tail behaviour. I can only guess they did this for consistent behaviour. Maybe it avoids some of that weird behaviour.
Quote from: aspangev on August 26, 2019, 08:10:09 PM
jfrabat, please change the resistor on Q1 emitter from 22k to 4.7k. This significantly will decrease the buzz to levels that you can't hear it.
Left everything else as was; changed this, and, while I cannot say the fizz is 100% gone, I can say that it almost is! Level has gone down dramatically, and goes down even more as you start closing the gate (and since there is no sense in turning on a gate with the gate fully open, this helps!). Also, now the gate fully closes. Only if I strum REALLY hard, does it open (and for a fraction of a second) when it is closed.
Thanks @aspangev for this! And everyone else for also lending me a hand!
I will also ad the mod 3 to mine (not done it yet).
Thank you too, jfrabat
Honestly if hadn't read about it I wouldn't dig into the problem ;)
I had the same problem. An annoying bizz with the gate open or semi open.
Did all the mods in the universe. No luck. Only two things worked:
1) Reducing the gain of IC1a. 10k instead of 680r
2) Keeping the high gain, but increasing the 47pf cap across the 1M. From 330pf to 1nF
What's doing that cap? At 1nf there's no noise/harsh clipping at all. And this is with 1M and 2k2 resistor in IC1a.
I tried tons of ICs, bipolars & FETs. They are not the problem.
Cheers!
Quote from: nonost on May 18, 2023, 05:03:11 AM
Keeping the high gain, but increasing the 47pf cap across the 1M. From 330pf to 1nF
What's doing that cap?
Although it isn't shown in the vast majority of respective schematics, that cap forms a Low-Pass filter with 1M resistor..
(frequencies above -3dB cut-off point are attenuated or, more presicely, aren't amplified following the (1+1M/2k2) gain formula..)
The impedance Z
C (capacitive reactance = 1/2π*
f*C) of that cap is set in parallel with 1M resistor, resulting into [1 + 1M*Z
C / 2k2*(1M+Z
C)] frequency dependent gain formula.. :icon_wink:
A more crude approximation could be:
The higher the frequency (or the capacitor value or both) the lower the capacitor resistance hence the more the "leaking" out of the 1M resistor current resulting into lower negative feedback closed loop gain..
Quote from: nonost on May 18, 2023, 05:03:11 AM
I had the same problem. An annoying bizz with the gate open or semi open.
Did all the mods in the universe. No luck. Only two things worked:
1) Reducing the gain of IC1a. 10k instead of 680r
2) Keeping the high gain, but increasing the 47pf cap across the 1M. From 330pf to 1nF
What's doing that cap? At 1nf there's no noise/harsh clipping at all. And this is with 1M and 2k2 resistor in IC1a.
I tried tons of ICs, bipolars & FETs. They are not the problem.
Cheers!
For (1), the original circuit used an LM741 with a lot of gain. So much that the opamp runs out of gain and roll-off the high frequencies. So if you use a wider bandwidth opamp you need to add that cap to roll off the high frequencies. If you don't the high gain makes that part of the ckt prone to oscillate due to layout. I've posted some stuff in the past about choosing that cap value.
Given that opamp clips it's also possible that if you don't have a good layout or bypass caps on the supply that noise could get from the power rail back into the audio - sort of like how LFO clicks get into the audio on phasers. You could try an RC filter on the supply to that opamp.
Another issue I found is the particular JFET could cause issues. Many JFETs people source these days have low VP and sometimes low Rds on. Low Rds On JFETs are likely to cause problems on that circuit. What can happen with low VP JFETs is the ripple on the gate can modulate the JFET a lot more. This is especially true in the region where the gate is just closing.
In this thread I saw a lot of potential for problems in that circuit. jfrabat's post plugged up some but there's others and maybe your case has problems in the other areas.
Don't forget the most of the schematics are wrong, please checkout my retrace thread, which is linked a few posts back in this thread.
Ey Antonis! I see...Well, the detector doesn't seem to bother the LP filter. It looks quite aggressive with the 1nf value, but it's working.
Hi Rob. Yes, I followed your schematic. I tried plenty of FETs, bipolars and ICs, and that nasty clipping only goes away with the cap across the 1M resistor.
With single coils, a low value as 100pf might be sufficient. But with a PAF in the bridge position, you get that bizz. That's only at low settings, which is a good thing...
I used some vero layout from tagboardeffects. After a bit of tweaking with the rows regarding OP1a, the clipping got a bit better...Next time I'll make a new layout. So far, the 1nF cap did the trick.
Pretty interesting the LM741 stuff, makes sense. I tried all the mods around...But I didn't see people increasing the cap over let's say 220pf. As I said, this clipping "appears" with hot or medium output pickups. With single coils you can get a away with 100pf and a good layout.
Yes, I have an RC filter and I even put a 100nf cap also right into pin 8.
Right now it's working great. Only complaining is output being a little bit below the bypass. It's subtle but noticeable.
Cheers!
It seems your problem is definitely around that opamp.
Back in my mind I think 330pF is about right to match the original roll-off but if you have problems you don't have have a choice but to increase it.
As far as the signal loss is concerned you might be able to shave off some loss:
- decrease the 22k (between the BJT and JFET) a bit; try 15k
- increase the 1M (across the S and D of the JFET); try 2M2
- raise the two 1M resistors which set the VCC/2 voltage pin 3 of the second opamp.
At the moment there is effectively 3x1M resistors in parallel, at pin 3 of the second opamp, which appear after the 22k resistor. 3x1M in parallel is 333k so that forms a divider with the 22k of 333k / (22k + 333k) = 0.94 = -0.6dB. Which you have to add onto the loss from the BJT buffer.
If you have changed the 22k emitter resistor to 4k7, from the jfrabat mods, you will reduce the loss if you use a high gain transistor for the buffer; especially when the guitar is connected.
Each individual change is probably small but the sum of all of them might shave off some attenuation.
330pf is good for classic PAF type bridge humbuckers. There's only a tiny bit of clipping and it's in a very narrow sweep of the pot. I put the 1nf because it might be used with hot pickups in the near future, so just to be safe. For low-medium output pickups I would throw a 330pf and call it a day.
I tried 200hfe to 300hfe bipolars. Maybe a 500hfe one could help for that tiny loss in volume. I thinks it's a better idea than changing plenty of stuff around. Yes, I'm using the 4k7 at input buffer.
The gate responds pretty well. It opens and closes quite smoothly. I find the release pot a good feature. 500k pot + 68k yield around 650ms. Below 68k the gate gets kind of stuck/bugged releasing.
BTW, if I increase the voltage supplied to FET source&drain to let's say 6v, would I get more attenuation? Or does it need further modifications?
Thanks a lot Rob!
Quote from: nonost on May 19, 2023, 06:02:55 AM
330pf is good for classic PAF type bridge humbuckers. There's only a tiny bit of clipping and it's in a very narrow sweep of the pot. I put the 1nf because it might be used with hot pickups in the near future, so just to be safe. For low-medium output pickups I would throw a 330pf and call it a day.
Regarding the buzz. Something that really bugged me about the MXR noise gate, especially with the incorrect 1uF base cap, is the threshold opamp is driving straight into the base of the transistor. The caps are there but they might cause other issue.
Firstly driving into the base makes the opamp output a lot of current. And secondly the capacitor could form a capacitive load for half a cycle which might cause the opamp to go unstable.
A mod to get around that would be say a 470 (to 1k or more) resistor in series with the 47nF cap at the output of the detector opamp. That resistor would fix both those problems. I don't know if it is *your* problem but it's not good.
FWIW, increasing the feedback cap can sometime help avoid oscillations but there is a point where it isn't a solution.
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I tried 200hfe to 300hfe bipolars. Maybe a 500hfe one could help for that tiny loss in volume. I thinks it's a better idea than changing plenty of stuff around. Yes, I'm using the 4k7 at input buffer.
Yes, maybe best to leave good alone.
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The gate responds pretty well. It opens and closes quite smoothly. I find the release pot a good feature. 500k pot + 68k yield around 650ms. Below 68k the gate gets kind of stuck/bugged releasing.
That's a tricky one. The 68k obviously requires the transistor to pull more current to open the gate. Perhaps the need to back-off the signal gain at the opamp has cause the drive the transistor to be so marginal that when you get to 68k the circuit no longer works.
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BTW, if I increase the voltage supplied to FET source&drain to let's say 6v, would I get more attenuation? Or does it need further modifications?
More attenuation would require a lower resistance from the JFET. There's two options:
- a lower rds_on JFET
- make sure the gate voltage is rising high enough (gate-source voltage getting near 0V => JFET on)
For the attenuation the closer you get the gate-source voltage to zero the more attenuation. It's possible to play around with the part values a bit.
A higher zener voltage would help the gate open on high VP JFETs, but you can lose signal/opamp headroom.
I've a added a few notes on attenuation in the comments:
(https://i.postimg.cc/bGQ7Nh4j/MXR-Noise-gate-gate-closed.png) (https://postimg.cc/bGQ7Nh4j)
For the gate open case:
(https://i.postimg.cc/c6DVJTk6/MXR-Noise-gate-gate-open.png) (https://postimg.cc/c6DVJTk6)
Varying Rc:
(https://i.postimg.cc/zLSQYkRz/MXR-Noise-gate-gate-open-vary-RC.png) (https://postimg.cc/zLSQYkRz)
Now Varying Rc with the sensitivity dialed back:
(https://i.postimg.cc/PvLKJbN8/MXR-Noise-gate-gate-open-vary-RC-low-input.png) (https://postimg.cc/PvLKJbN8)
In the last case you can see if the sensitivity is marginal varying the Rc value can prevent the gate opening. This is a contrived example but it could be happening.