So I've just finished a phase 90 clone, General guitar Gadgets and I have a problem that hopefully someone can assist with. I think i know what the issue is, but just want clarification. The issue i'm getting is no effect at all.
The phaser has been built with 2N5457 Jfets, and a 2N4125 Transistor. All the IC's are LM741's
The voltages are as follows, and were taken with the rate knob in the fully counterclockwise position:
All JFets Q1 thru Q4 are reading
Gate = 5.05 to 5.06
Source = 5.16
Drain = 5.16
IC1 pins are reading
1 = 0.002
2 = 5.15
3 = 3.52
4 = 0
5 = 0.002
6 = 5.15
7 = 9.25
8 = 0
IC2 thru to IC5 pins are reading
1 = 0.002
2 = 5.16
3 = 5.06
4 = 0
5 = 0.002
6 = 5.15 to 5.17
7 = 9.25
8 = 0
The issue comes when we get to IC6.
1 = 0
2 = 5.14
3 = changes between 3.92 to 5.38 constantly
4 = 0
5 = 0
6 = Changes from 1.85 to 8.61 constantly
7 = 9.25
8 = 0
I suspect IC6 is at fault as according to GGG's info the voltages on pins 3 & 6 should be around 4.7 & 4 volts respectively and shouldn't move around on their own. Would that all sound correct?.... I believe so, but just wanted it clarified.
Quote from: bettsaj on July 07, 2019, 05:19:41 AM
I suspect IC6 is at fault as according to GGG's info the voltages on pins 3 & 6 should be around 4.7 & 4 volts respectively and shouldn't move around on their own. Would that all sound correct?.... I believe so, but just wanted it clarified.
IC6 is the LFO, so I
would expect the output to vary. I don't know why JD would suggest otherwise? He
does say that the JFET gates should vary (otherwise there would be no phasing) and they're connected to the LFO... ???
He says on the pedal info page that the voltages will vary on IC6 when you turn the rate knob, but the voltages on pins 3 and 6 are changing by themselves when you hold the multimeter on the pins.
All the other voltages on the other IC's are constant and don't change.
Also, before anyone asks... The Jfets were bought as a matched quad set from www.stompville.co.uk
The rest of the components were purchased separately... i only bought the PCB from GGG
QuoteIC1 pins are reading
6 = 5.15
IC2 thru to IC5 pins are reading
6 = 5.15 to 5.17
What you will find is the zener voltage might vary (say 4V to 5.1V) and that shifts many of the other voltages up or down. The pin 6 voltages on all but IC6 are indicators of the zener voltage and the fact the opamps are probably working. So the 5.15V vs ggg 4.4V (or whatever) is simply caused by variations in the zener voltage. That should not cause the device to misbehave.
It is normal that IC6 doesn't follow the pin6 of the others since it is operating in a completely different way. The pin 6 voltage should swing say roughly 1V to 8V. Off-hand 1.85V looks a bit high but it's not unreasonable.
Quote3 = changes between 3.92 to 5.38 constantly
So when I bashed some numbers on my calculator those voltages look a little lower than expected but again not-unreasonable and perhaps your meter is loading the voltage a bit. The span 5.38-3.98 looks reasonable.
Perhaps you should check the range of voltages on your trimpot.
Also can you find a setting on the trimpot that sounds good at all?
If you can find such a setting but it sounds weak it could be a JFET matching issue.
I'll check the trim pot at both extremes and post the voltages, I'll rig it up again and report what I'm getting (which isn't much in the way of phasing) but i'll still triple check.
I'll do all this in an hour of so as i have other things need to be getting with....
the ggg circuit I looked at, and the board I looked at, were layed out for 2N5952 jfets. if you are using 2N5457, from any supplier, you would fit them reverse to the board silkscreen, as revealed by the datasheets.
might we see photos of your built board?
I thought i had checked the pinout of the Jfets!! i'm sure they're correct, looking at the end with the pins with the flat uppermost they should be D S G. So looking at the photo reading the pins from left to right they are G S D.............. I think you're correct, they're back to front
(https://i.postimg.cc/hQ9dmmpH/20190707-160259.jpg) (https://postimg.cc/hQ9dmmpH)
Swapped them around.... Sorted!
I'm an idiot.
Quote from: bettsaj on July 07, 2019, 12:47:45 PM
Swapped them around.... Sorted!
I'm an idiot.
Actually, less an idiot than many of us ;) As many times as we see the same build error - we'll still make the same one ourselves, ha ha!!!
Quote from: bettsaj on July 07, 2019, 12:47:45 PM
Swapped them around.... Sorted!
I'm an idiot.
easy. I like these ones. and as someone once said:
datasheets.
This phaser doesn't sound like I expected it to.... It's not a smooth phase at all. I'll try and upload a video of it so you can hear what I mean. When you set the trimmer so that you get a "full phase" sound there is an audible noise at the top of the phase.. It's hard to explain, but when compared to a real Phase 90 it's totally different. It's like there's a bump at the top of the phase.
When I set the phase to the slowest on the speed, instead of being smooth and flowing, it's quite jarring to listen to.
As I said, I'll try and upload a video or audio clip tonight so you can hear it.
Yes, there's something wrong with the LFO. The duty cycle isn't 50%. In the sound, there's a slow rise and the sharp fall.
Perhaps measure the voltage across the zener. I think the original was around 4.8V.
OK I see you already have measurements above. Are you sure the 9V rail and the zener voltage are still those values?
Beyond that check the parts around the LFO and the polarity of the polarized cap.
BTW, that video is very helpful.
I checked your photo against the info on the ggg site and I couldn't see any part problems.
I think your 15uF is around the right way but I can't quite see the markings.
QuoteThe issue comes when we get to IC6
6 = Changes from 1.85 to 8.61 constantly
The wonky LFO could be related to this. When I listen to the clip the duty cycle extremely skewed. Perhaps even more than those voltages alone would imply.
Check the soldering on pin 4 of IC6.
As a diagnostic check you could try temporarily soldering a 2k2 to 4k7 resistor from pin 6 of IC6 to ground.
See if that improves the situation by listening to the unit. Then perhaps measure that pin 6 voltage again to see what it is.
Ok.... When I get home tonight i'll remeasure all the voltages as before and post what they are now.
I'll also re-check the soldering on all the IC pins and re-flow as necessary. I'll also recheck the orientation of the polarised tant capacitors
Ok, here are the latest voltage readings.
IC1 pins are reading
1 = 0.002
2 = 5.15 (According to GGG this should be less than 0.01v)
3 = 3.52
4 = 0
5 = 0.002
6 = 5.15
7 = 9.25
8 = 0
IC2 thru to IC5 pins are reading
1 = 0.002
2 = 5.15 to 5.16
3 = 5.07 to 5.15
4 = 0
5 = 0.002
6 = 5.09 to 5.16
7 = 9.25
8 = 0
IC6.
1 = 0
2 = Changes between 4.58 to 5.82 constantly
3 = changes between 4.80 to 5.25 constantly
4 = 0
5 = 0
6 = Changes from 1.89 to 8.20 constantly
7 = 9.25
8 = 0
I've rechecked the soldering on IC6, especially pin 4 and all seems OK..... I'll fire up the soldering iron though and re-flow all the pins just in case there's a dry solder joint I can't see.
Tant caps orientated correctly, and as far as I can see all the pins on IC6 are soldered OK..... I'm at a loss.
I've actually bought another PCB, a Fuzzdog PCB for his Faze90 clone.... I started soldering resistors onto it tonight. if I can't get this Phase 90 clone working i can just swap out the PCB's. I'll at least then have one working effect pedal, and I can tinker around on this one that's not working right.
QuoteTant caps orientated correctly, and as far as I can see all the pins on IC6 are soldered OK..... I'm at a loss.
Yes, I couldn't see anything wrong. Like I mentioned before the swing on the opamp isn't enough to cause the problem.
However, I analysed your audio file, and it is fairly obvious now that your JFET's VP (or Vgs_off) parameter is on the low side and the LFO is driving them too hard. You have adjusted the trimpot by ear to get the best result and so it sounds *nearly* right, *but*, there's no way to get it 100% right by just using the trimpot.
What you need to do is reduce the value of R17 (1M). I'd try soldering another 1M on the back of the board across R17. Keep in mind that's only a guess. Too small a value will make the phasing weak too large will get the problem you are seeing now. When you change R17 you *must* tweak the trimpot for best sound. If you want you could put 1M trimpot in place of R17, adjust until you get it right then use fixed resistor later with the same value.
Since you bought the JFETs you won't have any VP (Vgs_off) measurements. If you did measure them then we would have a good idea what to set R17 to suit your JFET's VP (Vgs_off).
What people don't realize is there is only a narrow range of VP (Vgs_off) values that actually work like the real units. If the VP (Vgs_off) is quite different from the normal 2N5952 values then it will sound different. In many cases it's different to the original but doesn't sound bad after tweaking the trimpot. In extreme cases like yours there is a side-effect which cannot be fixed without tailoring the circuit to suit the actual JFETs.
I thought of another possibility, maybe your trimpot is actually way off. Try to find another completely different trimpot setting that sounds good.
As a matter of interest I've bought another set of Jfets from the same supplier... Likely from the same batch. I could measure those to get the vp measurements. They might be in the same ball park. Actually they have measurements on the bag.
Fairchild 2N5457 Batch 1922C
Vgs (10M) -0.496 V
Vgs (10k) -0.152 V
Idss 1.80 mA
Vgs (10M) -0.496 V
Vgs (10k) -0.151 V
Idss 1.80 mA
Vgs (10M) -0.496 V
Vgs (10k) -0.152 V
Idss 1.80 mA
Vgs (10M) -0.495 V
Vgs (10k) -0.150 V
Idss 1.77 mA
We can assume that the Jfets in the pedal are in the same ball park as they are from the same batch. Unfortunately I don't have the packaging the original one's came in.
Quote from: Rob Strand on July 16, 2019, 11:07:23 PM
I thought of another possibility, maybe your trimpot is actually way off. Try to find another completely different trimpot setting that sounds good.
The trimmer that is in it has phasing at top dead centre... Either way of that there is no phasing, If i changed it to another value maybe?
QuoteAs a matter of interest I've bought another set of Jfets from the same supplier... Likely from the same batch. I could measure those to get the vp measurements. They might be in the same ball park. Actually they have measurements on the bag.
Awesome!
I don't know what JFET measurement jig they used (I suspect maybe Run-off-groove, ROG) but it doesn't matter much as the 10M reading should be very close to the Vgs_off for the JFET.
So that does confirm my suspicion that the Vgs_off values are low. The 2N5952's are normally around 2.25V but your values are around 0.5V. FYI: You might see values around 1.6V posted. These are done on RG's (GeoFX) JFET matcher which gives lower readings.
Quotehe trimmer that is in it has phasing at top dead centre... Either way of that there is no phasing, If i changed it to another value maybe?
No don't change it. It is fine as is.
To start, try changing R17 (1M) to 150k. You might even have to go down to 47k!
Quote from: Rob Strand on July 17, 2019, 04:50:34 AMThe 2N5952's are normally around 2.25V but your values are around 0.5V. FYI: You might see values around 1.6V posted. These are done on RG's (GeoFX) JFET matcher which gives lower readings.
To start, try changing R17 (1M) to 150k. You might even have to go down to 47k!
Mine are 2N5457... make any difference?
I'll change the 1M resistor as soon as I get home..... I know I have 150k.....
Hi rob,
Here's the latest... i changed R17 from 1M to 150k, it made a big difference, but still not perfect. So i swapped it for a 100k, and this was the result
Do you think this is about where it should be?
QuoteMine are 2N5457... make any difference?
Yes they are different. The VP parameter is lower. In your case the VP values are low even for 2N5457's. There seems to be a pattern of low VP values these days. One way to look at it is the circuit was designed for 2N5952's so you should use JFETs close to that. The reality is these days JFETs are harder to get. So what needs to be done is to tweak the part values in the circuit so it operates very close to the original. That's pretty much what we did. From a technical perspective higher VP JFETs keep distortion down.
QuoteHere's the latest... i changed R17 from 1M to 150k, it made a big difference, but still not perfect. So i swapped it for a 100k, and this was the result.
That's a vast improvement. The video is was really helpful.
QuoteDo you think this is about where it should be?
It's pretty darn close certainly in the range of variations in normal units.
My feeling is the depth of the sweep could be a little more (but it could simply be the things you played biasing my opinion). Maybe worth trying 120k? Perhaps try adding a 22k in series with the 100k you have in there. You should tweak the trimpot in each case. When I analyse the audio it seems that way too but I can't see things very accurately from that clip. (BTW, the trimpot is harder to adjust accurately for the lower VP JFETs. If I were building these things in production I'd probably makes some circuit tweaks around the trimpot to make life easier.)
There's still something wrong. Your slowest sweep seems like about 1.5 seconds when it should be in the 7sec region. Is your C7 value 15uF or 4.7uF?.
Some more details.
A side effect of reducing R17 is C10 should be increase from 47nF in order to match the behaviour of the original. At high sweep speeds the original reduces the sweep range and smooths the sweep. The effect is noticeable for speeds faster than about 2Hz.
With R17 at 100k you probably need to increase C10 to about 220nF. With the cap in the sweep is reduced and you might find you like a larger R17 value.
The very early scripts used a 10nF cap for C10 instead of 50n and that tends to keep the sweep even over the speed range. So perhaps you should increase C10 to at least 100nF.
Quote from: Rob Strand on July 17, 2019, 07:24:04 PM
My feeling is the depth of the sweep could be a little more (but it could simply be the things you played biasing my opinion). Maybe worth trying 120k? Perhaps try adding a 22k in series with the 100k you have in there. You should tweak the trimpot in each case. When I analyse the audio it seems that way too but I can't see things very accurately from that clip. (BTW, the trimpot is harder to adjust accurately for the lower VP JFETs. If I were building these things in production I'd probably makes some circuit tweaks around the trimpot to make life easier.)
I'll swap it for a 120k if I have one.....
QuoteThere's still something wrong. Your slowest sweep seems like about 1.5 seconds when it should be in the 7sec region. Is your C7 value 15uF or 4.7uF?.
My cap is 15uf
QuoteA side effect of reducing R17 is C10 should be increase from 47nF in order to match the behaviour of the original. At high sweep speeds the original reduces the sweep range and smooths the sweep. The effect is noticeable for speeds faster than about 2Hz.
Ok, I'll look at increasing that too
QuoteWith R17 at 100k you probably need to increase C10 to about 220nF. With the cap in the sweep is reduced and you might find you like a larger R17 value.
Good call..... I'll change it
QuoteThe very early scripts used a 10nF cap for C10 instead of 50n and that tends to keep the sweep even over the speed range. So perhaps you should increase C10 to at least 100nF.
As a matter of interest, i've just started to build another phase 90 clone, this time from Fuzzdog pedal Parts in the UK. He uses the same FET's as me as they're readily available so I've started to compare the GGG schematic and the Fuzzdog schematic and unless i'm seeing things I think the values on some of the components are different, likely to compensate for the FET's having a low vp. You'll notice that the component numbers are different between the two schematics, however it's not hard to see that on the GGG schematic C10 is a polarised cap, and on the fuzzdog schematic C10 is C7 and is a non polarised cap at 47n
I've included the Fuzzdog schematic and BoM below
R1 10K
R2 470K
R3 150K
R4 150K
R5 56K
R6 150K
R7 10K
R8 10K
R9 22K
R10 10K
R11 10K
R12 22K
R13 10K
R14 10K
R15 22K
R16 10K
R17 10K
R18 22K
R19 22K*
R20 150K
R21 3M9
R22 150K
R23 4K7
R24 470K
R25 150K
R26 47R
R27 10K
R28 1M
R29 CLR**
R30 1M
R31 1K
R32 390R
C1 10n
C2 47n
C3 47n
C4 47n
C5 47n
C6 47n
C7 47n
C8 10n
C9 15u elec
C10 100u elec
C11 100n
C12 22u
C13 10u
Q1 2N5087***
Q2-4 Matched FETs
D1 5.1v zener
D2 1N4001
D3 LED
IC1 TL072
IC2 TL074
SPEED 500KC
T2‡ 20KB/22K TRIM
T1 470K TRIM
(https://i.postimg.cc/V56PtQMM/schematic.png) (https://postimg.cc/V56PtQMM)
QuoteQuote
There's still something wrong. Your slowest sweep seems like about 1.5 seconds when it should be in the 7sec region. Is your C7 value 15uF or 4.7uF?.
My cap is 15uf
Hmmm, 1.5sec is a long way off from 7sec. There's something wrong around the LFO. Unless there's something wrong with that cap! In your vid there was something shorted for while maybe that was the cause. You should be able to check the slowest speed with your watch. Another possibility is the pot value 500k, yes? Maybe measure it with you meter.
QuoteI'll swap it for a 120k if I have one.....
Ok, I'll look at increasing that too
Good call..... I'll change it
I think it's worth tinkering around a bit.
QuoteAs a matter of interest, i've just started to build another phase 90 clone, this time from Fuzzdog pedal Parts in the UK.
I went over the schematic and I could only see changes related to the feedback (known as Script vs Block differences). I also downloaded their build doc. No changes related to the JFETs on the Fuzzdog. The ggg schematic shows the feedback for the Block but the built unit is for the Script, as it leaves out the feedback resistor. What could be the case is the 2N5457's Fuzzdog are using have higher VPs; they are usually 1V or 1.2V. Your VP value of 0.5V is very low even for 2N5457, in fact it's on the edge of the spec, something you never really see on "real" 2N5457's.
Quote from: Rob Strand on July 18, 2019, 02:09:14 AMHmmm, 1.5sec is a long way off from 7sec. There's something wrong around the LFO. Unless there's something wrong with that cap! In your vid there was something shorted for while maybe that was the cause. You should be able to check the slowest speed with your watch. Another possibility is the pot value 500k, yes? Maybe measure it with you meter.
I'm using a 100k linear pot..... I did have a 500k rev log in it at the beginning but swapped it as it failed and was shorting out internally. The sweep was intermittent and when I isolated the pot, discovered it was the pot that was failing.
I'm also going to be cheeky and email Lee Callaghan at Fuzzdog to find out what VP values his FETs are. His FETs are all tested by him on a working phaser test rig so he knows when he supplies them they will work on his circuit. Unfortunately he doesn't sell the FETs separately, only with a full kit.
Quote'm using a 100k linear pot..... I did have a 500k rev log in it at the beginning but swapped it as it failed and was shorting out internally. The sweep was intermittent and when I isolated the pot, discovered it was the pot that was failing.
Ah, that makes sense now.
QuoteI'm also going to be cheeky and email Lee Callaghan at Fuzzdog to find out what VP values his FETs are. His FETs are all tested by him on a working phaser test rig so he knows when he supplies them they will work on his circuit. Unfortunately he doesn't sell the FETs separately, only with a full kit.
It can't hurt to see what JFETs he is getting.
Here's a stack of 2N5457 measurements. Note these are *real* ones, before JFETs started to go bung.
The measurements are at low currents so they are real VP values.
https://viva-analog.com/characterizing-and-matching-2n5457-jfet-transistors/
The 2n5457s I used to get here were mostly around 1.2V to 1.5V. (I think I said 1V to 1.2V before, sorry).
Quote from: Rob Strand on July 18, 2019, 06:17:41 AMIt can't hurt to see what JFETs he is getting
So Lee has got back to me and said basically what you've said.... He normally sorts them and matches them to within 1.2 to 1.6. Anything lower normally goes back in the bag. He's also said that he can't remember ever measuring anything that low.
I've also now emailed the supplier so see if he has any that measure higher than 0.5 or 1.0v
QuoteSo Lee has got back to me and said basically what you've said.... He normally sorts them and matches them to within 1.2 to 1.6. Anything lower normally goes back in the bag. He's also said that he can't remember ever measuring anything that low.
Those values look more normal for 2N5457s to me.
QuoteI've also now emailed the supplier so see if he has any that measure higher than 0.5 or 1.0v
Actually, he might have a record of the VP values for the JFETs in your unit. You can only ask. Your pic shows 2N5457 with batch code BC27.
Quote from: Rob Strand on July 18, 2019, 07:37:26 AM
QuoteSo Lee has got back to me and said basically what you've said.... He normally sorts them and matches them to within 1.2 to 1.6. Anything lower normally goes back in the bag. He's also said that he can't remember ever measuring anything that low.
Those values look more normal for 2N5457s to me.
QuoteI've also now emailed the supplier so see if he has any that measure higher than 0.5 or 1.0v
Actually, he might have a record of the VP values for the JFETs in your unit. You can only ask. Your pic shows 2N5457 with batch code BC27.
I've just checked his website, and it looks like he uses this rig to test Jfets...... He even sells kits of this rig http://stompville.co.uk/?p=601
Here's the page where he sells them http://www.stompville.co.uk/shop/2-2n5457-matched-quad.html
Just also ordered a set of 4 matched jfets from GGG
The latest...
R17 is still at 100k
I've changed C10 to 220n and this is the end result
QuoteI've just checked his website, and it looks like he uses this rig to test Jfets...... He even sells kits of this rig http://stompville.co.uk/?p=601
OK so it's sup'd-up version of RG's tester with the option of using a 10M resistor (using the multimeter impedance).
QuoteHere's the page where he sells them http://www.stompville.co.uk/shop/2-2n5457-matched-quad.html
OK. He must update the data for each set he sells or something since the JFETs on offer now have a different batch code to the JFETs used in your phaser. Yours have batch code Fairchild BC27.
QuoteThe latest...
R17 is still at 100k
I've changed C10 to 220n and this is the end result
You might have misunderstood the motivation for changing C10. The primary reason is to make the sweep on the modded unit (with R17 around 100k) behave like the original unit (with R17 1M). C10 makes the sweep smoother at higher speeds (and avoids the risk of weird pulsations at high speeds as well). It can't change the sweep speed.
If you really wanted to you could tweak the sweep circuit to get a 7 second sweep with your 100k pot but that would be a separate set of mods which addresses only the sweep speed.
So as I said in the video... changing C10 has smoothed the phase out, and achieved it's intention.
How would I go about changing the sweep length? I intend to change the pot back to a 500k rev log as soon as I can as I need the 100k for something else.
QuoteSo as I said in the video... changing C10 has smoothed the phase out, and achieved it's intention.
Something you said at the end made me think you expected to fix it. No worries, as long as you know.
QuoteHow would I go about changing the sweep length? I intend to change the pot back to a 500k rev log as soon as I can as I need the 100k for something else.
Basically you have reduced the resistance by 1/5 and that means you need to increase the 15uF cap by a factor of 5, so 5 x 15uF = 75uF. So to do that you will need 47uF in parallel with 33uF (or 25uF). The other thing is if we make the cap bigger we need to make the 4.7k resistor (R20) five times smaller, so about 1k; that ensures the maximum speed is correct. It's a bit of a pain but OK for a temporary fix.
You could also do a "nearly there" fix by changing the 15uF cap to 47uF and R20 to 1.5k. That will give you upto about 4.5 seconds, which is a lot better than 1.5 seconds.
Quote from: Rob Strand on July 19, 2019, 01:51:27 AM
QuoteSo as I said in the video... changing C10 has smoothed the phase out, and achieved it's intention.
Something you said at the end made me think you expected to fix it. No worries, as long as you know.
QuoteHow would I go about changing the sweep length? I intend to change the pot back to a 500k rev log as soon as I can as I need the 100k for something else.
Basically you have reduced the resistance by 1/5 and that means you need to increase the 15uF cap by a factor of 5, so 5 x 15uF = 75uF. So to do that you will need 47uF in parallel with 33uF (or 25uF). The other thing is if we make the cap bigger we need to make the 4.7k resistor (R20) five times smaller, so about 1k; that ensures the maximum speed is correct. It's a bit of a pain but OK for a temporary fix.
You could also do a "nearly there" fix by changing the 15uF cap to 47uF and R20 to 1.5k. That will give you upto about 4.5 seconds, which is a lot better than 1.5 seconds.
When you say the 15uf cap, are you talking about C7, the tant cap?? DOH!!! Of course you are, just looked at the schematic and realised that's the only 15uf cap on the circuit.
I've just ordered some 47uf tant caps... will be about 3 to 4 days I guess so I'll not post anything until then. In the mean time have ordered 4 JFets from GGG for the Fuzzdog build and i'm about 3/4 of the way through that build. I just need to buy a couple of extra components that I don't have in my stock... As soon as I get those Jfets I'll let you know how the Fuzzdog build goes. I suspect the GGG JFets will be better than those that I bought from the UK with a higher VP value, so i'm fully expecting that build to be a straight "plug and play" affair (I hope).
Quoteso i'm fully expecting that build to be a straight "plug and play" affair (I hope).
More likely. There's still room for a lot of variation in the results with a 1V and a 2V JFET. Normally people adjust the pot for best sound and that kind of hides the differences.
The funny thing is once you tweak the LFO to suit the JFET, like you did, the sound difference between low VP JFETs and high VP JFET is vastly reduced. Basically what I'm saying is don't expect large improvements in the sound by changing the JFETs. IMHO you would probably gain more by adding mods to make the trimpot easier to adjust. (Some of the 80's and 90's Phase 90s actually did just that!).
Rob,
The latest... And likely the last instalment. I changed the 15uf tant cap as suggested to a 47uf cap, and also R20. I didn't have a 1k5 resistor so i used a 1k8 resistor... and this is the end result.... i'm quite pleased, and can't thank you enough Rob
Quote from: Rob Strand on July 19, 2019, 06:44:21 PM
Quoteso i'm fully expecting that build to be a straight "plug and play" affair (I hope).
More likely. There's still room for a lot of variation in the results with a 1V and a 2V JFET. Normally people adjust the pot for best sound and that kind of hides the differences.
The funny thing is once you tweak the LFO to suit the JFET, like you did, the sound difference between low VP JFETs and high VP JFET is vastly reduced. Basically what I'm saying is don't expect large improvements in the sound by changing the JFETs. IMHO you would probably gain more by adding mods to make the trimpot easier to adjust. (Some of the 80's and 90's Phase 90s actually did just that!).
What sort of mods are you talking about?
QuoteThe latest... And likely the last instalment. I changed the 15uf tant cap as suggested to a 47uf cap, and also R20. I didn't have a 1k5 resistor so i used a 1k8 resistor.
Yes it's much more usable like that. No prob with 1k8.
QuoteWhat sort of mods are you talking about?
If I explained all the options it's probably going to get confusing. So I'll explain what I think is best in your case.
So going back to the start. The sweep sounded bad so I suspected your JFETs had low VP. Then I suggested a mod to see if we were on the right track. And all-in-all things worked out pretty well. Which is what you have now. So you could just leave it there.
Now phasers are hard to adjust accurately at the best of times but in your case it's made harder. When I look at the audio from each of the demos I can see the phaser adjustment has moved around quite a bit between the video. That's because it's hard to adjust. You can get away with some misadjustment but if you compared a misadjusted unit side by side with a factory adjusted unit you will hear a difference.
I'll try to explain the degree of the problem. Suppose you have a trimpot that adjusts a filter frequency. Suppose the trimpot "knob" goes from 0 to 10. On your unit one octave change in the filter frequency corresponds to only 0.057 change on the 0 to 10 scale. That's nuts, so it's hard to adjust.
So an improved mod would be to make that adjustment less sensitive. An improved mod achieves what we have so far but allows the pot to be adjusted more easily. So for the improved mod I'm suggesting has one octave change in the filter frequency for 0.44 change on the 0 to 10 pot scale. So that's 8 times easier to adjust.
So the downside of this mod is you have to add a resistor. The previous mod didn't change add anything it only changed values. Note also at the moment your trimpot is probably at setting position 9 but with this mod you it will be to around 1.5; no big deal I'm just mentioning it.
I just want to be clear that 150k on the Improved Mod is *the same* as 100k on the the First Mod. The reason is the 1M is present.
(https://i.postimg.cc/JshVHfzQ/phase90-low-VP-mods-V1-0.png) (https://postimg.cc/JshVHfzQ)
Hi Rob,
That's brilliant.... Really helpful. So, to do the improved mod I need to replace the 1M resistor back to where it was at R17, and graft in the 150k as indicated.
When you say "for same sweep as original perhaps use slightly higher value than those indicated", what resistor values, both 150k and 1M, or just 150k?
This is all going to be really helpful on my second phase 90 build....
QuoteThat's brilliant.... Really helpful. So, to do the improved mod I need to replace the 1M resistor back to where it was at R17, and graft in the 150k as indicated.
Yes. So you have to find a home for the added 150k.
QuoteWhen you say "for same sweep as original perhaps use slightly higher value than those indicated", what resistor values, both 150k and 1M, or just 150k?
For the "Improved mod", up the 150k a tad and leave the 1M as is.
The originals have a bit wider sweep than yours does at the moment.
If you like how sounds now just leave it.
QuoteThis is all going to be really helpful on my second phase 90 build....
Whether to tweak or not depends on the VP values of your JFETs. Should be no worries anyway just tweak that added resistor until it sounds right. You will need to tweak the pot each time you try a resistor.
So.... my new supply of 500k Rev log pots arrive, I change the pot on the phase 90 and i'm now left with a sweep that lasts about 10 seconds on its slowest setting :icon_eek: :icon_eek:
QuoteSo.... my new supply of 500k Rev log pots arrive, I change the pot on the phase 90 and i'm now left with a sweep that lasts about 10 seconds on its slowest setting
Normally you end-up with about 7.5secs. I've seen a few real units go out to about 10 secs though, which I put down to extreme tolerances on the 500k pot and 15uF cap.
With 100k you were getting about 1.5 secs. So from that you would predict a 500k pot would be 5 times longer 5 x 1.5secs = 7.5s. If you used the same cap I wouldn't expect it to be too far off that. Can you measure the pot resistance?
Did you change the cap back to 15uF and the resistor back to 4k7? (I expect you did, otherwise it would be more like 20 seconds.)
At 12 O'Clock it should be about 1 sec but that can easily vary 20% because pot tapers aren't that accurate.
I haven't changed anything.... Only the pot. so I still have the same cap, and R17 is still 100k.
Maybe my timing isn't that accurate, but the longest sweep is loooooong
QuoteI haven't changed anything.... Only the pot. so I still have the same cap, and R17 is still 100k.
Maybe my timing isn't that accurate, but the longest sweep is loooooong
Ah, OK that' explains it.
The slowest setting is so long I can't make out the whole sweep either.
I think it's probably around 20 to 30 seconds!
In the video the 12 O'clock setting with 47uF + 500k pot is about 3.5 seconds. With the 47uF cap in there instead of 15uF we would expect the sweep to be (47/15) = 3.1 times longer. Also I expect around 1sec with 15uF at the mid setting so that means we should expect 3.1sec with the 47uF. So it's all working as expected.
If you change the 47uF back to 15uF and the 1k8 back to 4k7 the sweep will be how it is supposed to be.
All changed, and now working like a charm.... Many thanks Rob
QuoteAll changed, and now working like a charm.... Many thanks Rob
Good stuff. No worries.
Quote from: Rob Strand on July 24, 2019, 02:23:07 AM
QuoteAll changed, and now working like a charm.... Many thanks Rob
Good stuff. No worries.
Rob, now I have this phase shifter in my guitar rig it's now quite noticeable there's a slight volume drop and loss of bottom end when I engage the effect. If I can I'll try and record it going through my rack, but even on the small practice amp i use for testing pedals there's a slight drop in volume.
The effect chain in my switcher is:
- Tuner/Mute
- Dyna comp
- Electric Mistress
- Uni-Vibe
- Phase 90
- Big Muff
- Power boost
- Empty loop
I have tried placing the phaser directly after the guitar then going into my pedalboard, but even then there's still a slight drop of volume and more concerning a loss of bottom end...... Any suggestions?
QuoteI have tried placing the phaser directly after the guitar then going into my pedalboard, but even then there's still a slight drop of volume and more concerning a loss of bottom end...... Any suggestions?
For the LF issue try increasing the 10n cap input C1 to 47n, also increase the C9 to say 100nF (you could go higher).
For the level try increasing R26 by one step 150k to 180k. *OR* decrease both R27 & R24 by one step 150k to 120k. One might be better than the other. I'll need to check.
I'm thinking the loss of volume might just be the lack of bottom end.... i'll adjust the LF caps and see what it's like after that
QuoteI'm thinking the loss of volume might just be the lack of bottom end.... i'll adjust the LF caps and see what it's like after that
The same thing crossed my mind about 2 mins ago. The input cap is the first place to start.
FWIW, if you need more volume I'd go with the feedback resistor R26. It has slightly less boost than the input resistors R27 & R24 but it should be enough. If you really want to fine tune it you can use smaller steps like 160k.
So i changed the LF caps... And it's made a significant difference.... I was right, it is just the lack of bottom end that gives the impression of volume drop.... I may stuff a slightly higher cap in C9 and see what that's like
Changed C9 to 150nf and it's perfect.