CDXL Response

benny_profane

Well-known member
My CDXL build (corrected board) is having a number of issues.
  1. With ‘Level’ maxed out, I get my clean signal with a fizz mixed in, but no autowah.
  2. The response is sometimes extremely slow, and only kicks it on the decay of the note. This happens at various points of the sweeps.
  3. The wah doesn’t fall smoothly. It peters out in a very wobbly (think super fast phaser) way.
  4. Overall, it’s very weak (when it’s catching the input signal). Not at all as extreme as a wah sweep should be.
I used the Xvive VTL5C4/2 from SBE. I’ve tried various values at R9 (i.e., 2k2, 4k7, and 6k8). 4k7 seemed to get the closest. I’ve also tried various dual op amps (i.e., TL07, TL062, and TL022). The TL072 has the worst response.

I’ve tested the circuit both with and without a buffer before it.

I’m wondering if others have had good luck with the Xvive reissues or not. I know there’s one successful build report, but that’s it. In any case, any ideas?
 

benny_profane

Well-known member
Pictures attached. The socket is empty in the shots but various dual op amps have been used with correct readings. Pots are wired up with leads because I haven't drilled an enclosure or finished off-board wiring. I'm still working through the verification phase with a breadboard and testing rig. Thanks for taking a look.

CDXL - Top (Small).jpeg
CDXL - Bottom (Small).jpeg
 

Chuck D. Bones

Well-known member
Build quality looks good, no obvious shorts or bad solder joints. You can see the board from different angles, so you have a better chance of finding a bad solder joint than we do.
SBE carries quality parts, so I doubt that the VTL5C4/2 is the cause.
Are all the caps the correct values?
Changing opamps should make little or no difference.
Can you verify that the opamp outputs (pins 1 & 7) are at 4.5V with no input signal present?
 

benny_profane

Well-known member
Thanks for the compliment about the build quality. Caps are indeed correct. I was pretty slow with this one to make sure I didn’t mix anything up accidentally. The Vc, Vref, and GND values are all good with the IC.

I’m a little concerned about the vactrol since it is a relatively new reissue. I don’t think SBE would’ve been at fault—just that the part may not be exact to the original part.

Beyond that, I’m not sure what to think here. Any other thoughts?
 

Chuck D. Bones

Well-known member
Do you have any more VTL5C4/2's? It would be pretty simple to bench test one so it can be ruled in or out as the culprit. The most important device parameter in this circuit is the resistance tracking of the two LDRs inside.
 

benny_profane

Well-known member
I don’t, unfortunately. That’s my main reason for asking for others with successful builds to chime in. It’s a pretty expensive reissue part—and not one that’s used in too many projects. You’re right that the entire circuit depends on this part. I could remove it from the circuit this evening and test resistances. That’s probably the only step in order to move forward.
 

Chuck D. Bones

Well-known member
You can test the Vactrol in-circuit, no need to remove it. Make sure IC1 is out of the socket. Temp install a jumper across R10 and a 1K resistor across R11. Set LEVEL to zero. Apply power. Record resistance pin 1 to 2 and 2 to 3 on the Vactrol. Set LEVEL to 1 and repeat all the way up to 10. If tracking is within ±20% you should be fine. Some of the measurements at the low end of LEVEL will be more-or-less the same.

Can you better describe this "fizz"? Are we talking about noise or distortion?

How are you powering this thing?

What is C5's voltage rating?
 
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benny_profane

Well-known member
LEVEL Position (A100k)Vactrol 1 to 2Vactrol 2 to 3
0 – 5
220k​
220k​
6
151k​
135k​
7
8.7k​
8.6k​
8
2.15k​
2.2k​
9
446R​
465R​
10
288R​
299R​


Earlier tests were powered via leads to breadboard rails powered from a wall wart. These tests were powered via leads to breadboard rails powered from a 9v battery.

C5's voltage rating is 35v.

The fizz is an underlying high timbre distortion of the guitar signal when the level is maxed out.
 

Chuck D. Bones

Well-known member
Those readings all look good. I wanted to check C5's voltage rating because the rectifier that charges it (C4, D1 & D2) is a voltage doubler. 35V is plenty.

I'm out of ideas for the moment.
 

benny_profane

Well-known member
Those readings all look good. I wanted to check C5's voltage rating because the rectifier that charges it (C4, D1 & D2) is a voltage doubler. 35V is plenty.

I'm out of ideas for the moment.
Thanks for helping out. Let me know if you have any other thoughts here. I’m stumped too.
 
I'm having the exact same issues, using the replacement board. I bought (2) original boards, both turned out bad, got a couple replacement batch ones and while they "work," in some sense, the fizz/distortion you speak of is ever-present, the downward settling of the wah is stuttery and nonsmooth, the auto envelope opening is hyper sensitive to level and guitar dynamic control, and frankly... I'm just giving up. At some point the cost of one's time outweighs the rest.
 
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Chuck D. Bones

Well-known member
I've got another idea. I recall troubleshooting a Mutron III a while back. Turned out one of the opamps was oscillating at high freq. If this board was on my bench, I'd put a 'scope on the opamp outputs. Since you don't have one, let's see if some band-aids will fix things. Tack solder a 470pF cap across R7 and another one across R5. See if that makes any difference.

There really should be a small resistor in series with C4, something around 100R would work. I've seen this time and again. Whenever D1 or D2 conduct, C4 and C5 load the opamp's output and that is a recipe for oscillation. It's sloppy circuit design and completely avoidable with one resistor. Before adding a resistor in series with C4, just remove C4 and see if you can tune the filter back & forth with the LEVEL control. If the sound is good with C4 removed, then put it back in with 100R (or something close to that) in series. You will have to get creative with how you accomplish that, but desperate times call for desperate measures.
 

benny_profane

Well-known member
I've got another idea. I recall troubleshooting a Mutron III a while back. Turned out one of the opamps was oscillating at high freq. If this board was on my bench, I'd put a 'scope on the opamp outputs. Since you don't have one, let's see if some band-aids will fix things. Tack solder a 470pF cap across R7 and another one across R5. See if that makes any difference.

There really should be a small resistor in series with C4, something around 100R would work. I've seen this time and again. Whenever D1 or D2 conduct, C4 and C5 load the opamp's output and that is a recipe for oscillation. It's sloppy circuit design and completely avoidable with one resistor. Before adding a resistor in series with C4, just remove C4 and see if you can tune the filter back & forth with the LEVEL control. If the sound is good with C4 removed, then put it back in with 100R (or something close to that) in series. You will have to get creative with how you accomplish that, but desperate times call for desperate measures.
Cheers. I’ll give that a shot tonight and post my results.
 

music6000

Well-known member
Chuck, Should the 100r be in series with - side of C4 disconnected to right side of 1N4148's OR
+ side of C4 disconnected to 2m2 & Pin 1 of Opamp
 
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