Help me troubleshoot DJ Thomas White Lopass Gate

From circuitbending to homebrew stompboxes & synths, keep the DIY spirit alive!

Moderators: Joe., lisa, luketeaford, Kent

Synthiq
Veteran Wiggler
Posts: 513
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Sun Mar 22, 2020 9:47 pm

These signals still looks reasonable so I don't think there is a problem with the 4.7nF capacitor so I'm not really sure what the problem is. Did you go back to VCF mode and listened to the signal after lowering the amplitude? You may have to vary the offset pot as well to find the best frequency for the resonance. If you still don't hear any difference when you vary the resonance pot, there are two more tests you can do.

The first is to set the mode to VCA+VCF (center position) and apply a +/-2V square wave to the audio input and set the offset to max to maximize the filter bandwidth. Then take pictures of the signal at pin 14 of the TL084, first with resonance set to minimum and then at maximum. In the first case you should see a square wave with the same +/-2V amplitude as the input signal and in the second case +/-4V amplitude since the gain should now be 2 in that amplifier.

The second test I can think of is to set the mode to VCF and apply the same +/-2V square wave to the audio input and connect the scope to the out jack. Adjust the offset pot until the rise and fall times of the signal is roughly 25% of the half period of the signal when the resonance pot is set to minimum. There may be some overshoot even at the lowest setting but when increasing the resonance pot you should start to see increased ringing after each transition of the square wave if the circuit works correctly.

User avatar
wackelpeter
Wiggling with Experience
Posts: 454
Joined: Fri Apr 17, 2015 5:56 pm
Location: Germany, MD

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wackelpeter » Mon Mar 23, 2020 8:26 am

Also perhaps check the resistor from the reso pot to gnd (51K) for the correct value... As far as i remember the smaller the value the quicker the resonance gets in and the bigger the more tamed the resonance is above a certain value mostly no resonance.

User avatar
wendallsan
Wiggling with Experience
Posts: 339
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Tue Mar 31, 2020 8:17 pm

Synthiq wrote:
Sun Mar 22, 2020 9:47 pm
These signals still looks reasonable so I don't think there is a problem with the 4.7nF capacitor so I'm not really sure what the problem is. Did you go back to VCF mode and listened to the signal after lowering the amplitude? You may have to vary the offset pot as well to find the best frequency for the resonance. If you still don't hear any difference when you vary the resonance pot, there are two more tests you can do.

The first is to set the mode to VCA+VCF (center position) and apply a +/-2V square wave to the audio input and set the offset to max to maximize the filter bandwidth. Then take pictures of the signal at pin 14 of the TL084, first with resonance set to minimum and then at maximum. In the first case you should see a square wave with the same +/-2V amplitude as the input signal and in the second case +/-4V amplitude since the gain should now be 2 in that amplifier.

The second test I can think of is to set the mode to VCF and apply the same +/-2V square wave to the audio input and connect the scope to the out jack. Adjust the offset pot until the rise and fall times of the signal is roughly 25% of the half period of the signal when the resonance pot is set to minimum. There may be some overshoot even at the lowest setting but when increasing the resonance pot you should start to see increased ringing after each transition of the square wave if the circuit works correctly.
Hi and thanks again for the assistance, my apologies for the delay in response, I have been on vacation for a week or so here and am resuming work on my electronics projects now. Great to hear that the 4.7nF cap seems to be doing what you expect and is not the culprit. I set up another test with a +-2V signal in VCF mode and listened, and am still not hearing (or seeing on a scope) any difference in the waveform as I sweep the RESO pot from one end to the other.

For your first test, I set up the module to run in VCA+VCF mode and applied a +-2V square wave and set the OFFSET to max as you suggested. I then hooked up my scope to pin 14 of the TL084, but there doesn't seem to be any signal at pin 14 in my test, regardless of the position of the RESO pot.

For your second test, I may have your instruction confused, but here is what I did and my results. I used the same square wave input used in test 1, set the module to VCF mode, and adjusted the offset so the square wave's sharp edges got 'sloped' so they were about 25% rising slope and 25% falling slope. I then increased the RESO pot from 0 to 10, but there is no difference in the output tone or the output on the scope.
Also perhaps check the resistor from the reso pot to gnd (51K) for the correct value... As far as i remember the smaller the value the quicker the resonance gets in and the bigger the more tamed the resonance is above a certain value mostly no resonance.
Finally, testing the resistance to ground on the 3 lugs on the RESO pot show one side having 96.1k resistance, the other side having 49.4k resistance, and the wiper ranging between these two values as it is adjusted. Let me know if these values seem suspect to you.

Many thanks once again!
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 513
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Tue Mar 31, 2020 11:19 pm

In the first test, the input signal should be amplified by one in the first opamp and you should see it on pin 1 of the TL084. If the vactrols are turned on fully you should also see the same signal on pin 10 as well as pin 9 and 8 as this opamp is also configured as a unity gain buffer. If the jumper between JP1B and JP2B is in place the same signal you see on pin 8 should also be seen on pin 12 (and 13.) This amplifier is a non-inverting amplifier with a gain between 1 and 2 depending on the potentiometer setting so you should see a +/-2V to +/-4V signal but it looks like you don't have a signal on pin 14. The resistance you measured between the pot and ground looks correct so measure the resistance between pin 8 and 12 as well as between the pot wiper and pin 13 to make sure everything is connected even if you listed 0V on pin 12-14 in your first post which would indicate that the amplifier is indeed correctly connected.

User avatar
makhho
Common Wiggler
Posts: 157
Joined: Mon Feb 05, 2018 11:53 am

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by makhho » Wed Apr 01, 2020 3:19 am

If I recall correct, while the thomas white build calls for all pots to be linear, the BoM mouser/digikey # were linked for audio.
If you blindly plugged them in and added them to your cart like I did, then perhaps you may also be experiencing weird throw behavior.

User avatar
wendallsan
Wiggling with Experience
Posts: 339
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Thu Apr 02, 2020 9:49 am

Synthiq wrote:
Tue Mar 31, 2020 11:19 pm
In the first test, the input signal should be amplified by one in the first opamp and you should see it on pin 1 of the TL084. If the vactrols are turned on fully you should also see the same signal on pin 10 as well as pin 9 and 8 as this opamp is also configured as a unity gain buffer. If the jumper between JP1B and JP2B is in place the same signal you see on pin 8 should also be seen on pin 12 (and 13.) This amplifier is a non-inverting amplifier with a gain between 1 and 2 depending on the potentiometer setting so you should see a +/-2V to +/-4V signal but it looks like you don't have a signal on pin 14. The resistance you measured between the pot and ground looks correct so measure the resistance between pin 8 and 12 as well as between the pot wiper and pin 13 to make sure everything is connected even if you listed 0V on pin 12-14 in your first post which would indicate that the amplifier is indeed correctly connected.
Hi and thanks again for the response.

I may have performed the previous test wrong, as I tested again to confirm that the input signal could be seen at pins 8, 9, 10, 12, and 13 as you suggested. Since I had things hooked up and I WAS able to see the signal at all of those pins, I then tested pin 14 again and am now seeing the expected 'double strength' waveform there when I didn't see it there in my previous test. My apologies for providing what I'm guessing was a false result previously. I do now see my ±2V signal stepping up to a ±4V signal at pin 14, as you suggested.

I have tested the resistance between the RESO pot wiper and pin 13 and there is no resistance between these two points.

I have tested the resistance between pin 8 and 12 of the TL084 and there is also no resintance between these two points.

So the good news here is that I think the circuit passed all these tests this time around. :banana:

This lead me to carefully scrutinize what I was looking at on my scope, and I'm left with a possible conclusion that maybe the resonance circuit is very subtle and maybe I'm 'missing' it even though its there ... Maybe you all can confirm or deny this with a couple shots from my scope.

These pics were taken with the LPG in VCF mode with the OFSSET wide open. The waveform at the top of the screen is the signal being fed into the LPG, and the bottom waveform is the output of the module.

Here is my waveform with the RESO pot turned all the way down:

Image

And here it is with the RESO pot turned all the way up:

Image

I'm seeing a clear doubling of the level of the signal, PLUS just a little bit of 'filter wiggle' at the edges of those squares, which is what I expect to see for a filtered signal. The effect is very subtle, but it is there. Is this what you would expect to see from a properly working resonance circuit? I was under the impression that there would be a less subtle effect, but I could be expecting the wrong thing here. Please let me know if this signal looks like it ought to under these circumstance, and maybe we'll all agree that it really is working after all!
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 513
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Thu Apr 02, 2020 11:11 am

Your latest tests confirm that all amplifiers work as expected, so that's good. And in the VCA+VCF mode the resonance pot has no effect on the sound since the signal is not fed back via the 4.7nF capacitor so that explains why you don't see or hear any change at the output when you adjust the resonance potentiometer.

The next step is to switch to VCF only mode to activate resonance and look at the signal when the resonance pot is changed. You should see more ringing when you increase the resonance pot. It may be necessary to adjust the offset pot for best effect as this will change the frequency of the resonance.

User avatar
wendallsan
Wiggling with Experience
Posts: 339
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Thu Apr 02, 2020 7:02 pm

Unfortunately, that IS the VCF mode. :sadbanana:

When I use VCF+VCA mode with the Offset all the way open, I see an exact copy of the input (with no wiggles on the edges of the squarewaves) at the output, and the Resonance knob does not seem to have any effect, which I think is what its supposed to do in this mode(?)
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 513
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Thu Apr 02, 2020 9:47 pm

I ran your circuit in my circuit simulator again. I could reproduce what you see only if I reduced the resistance in the vectrols to 100ohm or less. According to the specs the vactrols can be as low as 5ohm with a 10mA control current so this isn't unreasonable at maximum offset. So the first test would be to reduce the offset potentiometer to see if there will be more resonance or not.

The two vactrols and the 1nF capacitor at pin 10 of the TL084 and the 4.7nF capacitor connected to the node between the two vactrols are part of a Sallen-Key lowpass filter. The resonance is a function of the ratio of 4.7nF times the gain of the amplifier driving it and the 1nF capacitor. So it is also possible that this ratio is off because the the 4.7nF capacitor is actually smaller or the 1nF is too large. If I reduce the first by a factor of 10 or increase the second by a factor of 10 I also see almost no resonance even with the resonance pot at max. You have already traced the signal from pin 14 of the TL084 to the 4.7nF capacitor so this is probably fine but set resonance to max and check if the +/-4V signal can be seen directly at the capacitor just for peace of mind. Also check if the 1nF capacitor really is 1nF and not larger.

User avatar
wendallsan
Wiggling with Experience
Posts: 339
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Sat Apr 04, 2020 11:25 am

Hi and thanks again for the help. I've visually inspected both caps in question and they are printed with the expected values; I don't own a meter that can test capacitance, so I don't have a way to test the caps directly to confirm that they are providing the capacitance advertised on their labels.

I did check the 4.7nF cap and here is the signal I see at it, it looks quite different than all others, in that it looks resonant-y and not squarewave-y. Again, the top waveform is what is being input to the LPG module, and the bottom waveform is what I see at the 4.7nF cap.

Image

I had a thought based on other questions I've been asking about caps in this forum. The 4.7nF cap is a Poly Film cap but the 100n cap is an MLCC cap, and I've been told that MLCC's may not work well in an audio signal, and that is why Film caps are used for this purpose instead, and that I should check the board before placing a cap to see whether it is in an "MLCC" or a "Film cap" form factor on the board to determine which type to use if it is not clear in the BOM. Looking at the PCB for this module, it looks like there is room in the 100n cap spot for a film cap, so could it be that I have an MLCC in this spot where I should be using a film cap, and would this possibly be enough to cause problems in this resonant circuit?
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 513
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Sat Apr 04, 2020 12:30 pm

The waveform at the bottom of your scope is consistent with a squarewave passing through a highpass filter, in this case the 4.7nF capacitor and the input resistance of the scope itself. This is correct if you are in VCA or VCA+VCF mode where on side of the 4.7nF cap is floating but not in VCF mode where it should be driven by opamp output at pin 14 of the TL084. If you were not in VCF mode, test again in that mode with resonance at maximum. If you were in VCF mode, you have to check the connection between the capacitor and pin 14 again.

As for capacitor types, I'm not sure which 100nF capacitor you are referring to. There are two filtering the supplies where MLCC are fine. From the photo of the pcb, the 1uF, 4.7nF and 1nF capacitors are clearly film capacitors and fine. The 33pF between pin 13 and 14 of the TL084 is just for stability and a MLCC is fine here, too. This leaves the 220pF capacitor which I can't determine the type of but since it kind of is in parallel with the much larger 4.7nF capacitor I don't think it will have much impact on the resonance in VCF mode.

Post Reply

Return to “Music Tech DIY”