Author Topic: Callate 2 (MXR Gate)  (Read 11176 times)

Rob Strand

Re: Callate 2 (MXR Gate)
« Reply #100 on: August 27, 2019, 06:55:32 PM »
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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!
Excellent.   So the last problem solved!.

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Here 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.

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BTW you can support our 'young' band (I hope is not forbidden from the forum rules):
You guys sound great.
The internet:  answers without the need for understanding.

aspangev

Re: Callate 2 (MXR Gate)
« Reply #101 on: August 28, 2019, 12:02:53 AM »
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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.
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.
« Last Edit: August 28, 2019, 03:11:17 AM by aspangev »
"Digital!? Every idiot can count to one..." - Bob Widlar

Rob Strand

Re: Callate 2 (MXR Gate)
« Reply #102 on: August 28, 2019, 06:55:37 PM »
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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.
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.

The internet:  answers without the need for understanding.

aspangev

Re: Callate 2 (MXR Gate)
« Reply #103 on: August 29, 2019, 02:12:29 AM »
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:



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
Only mod 3:
https://www.aronnelson.com/DIYFiles/up/Mod3.mp3
Mod 3 and 4:
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
"Digital!? Every idiot can count to one..." - Bob Widlar

Rob Strand

Re: Callate 2 (MXR Gate)
« Reply #104 on: August 29, 2019, 07:50:19 PM »
Excellent documentation and much improved behaviour.

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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.
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.
« Last Edit: August 29, 2019, 07:56:03 PM by Rob Strand »
The internet:  answers without the need for understanding.

jfrabat

Re: Callate 2 (MXR Gate)
« Reply #105 on: August 31, 2019, 03:58:08 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).
« Last Edit: August 31, 2019, 04:02:00 PM by jfrabat »
I build.  I fix.  I fix again.  And again.  And yet again.  (sometimes again once more).  Then I have something that works! (Most of the time!).

aspangev

Re: Callate 2 (MXR Gate)
« Reply #106 on: August 31, 2019, 04:34:55 PM »
Thank you too, jfrabat
Honestly if hadn't read about it I wouldn't dig into the problem ;)
"Digital!? Every idiot can count to one..." - Bob Widlar