Thumb Sucker Compressor "scrapy" sound

Matopotato

Active member
Hi,
I just "finished" building a Thumb Sucker Compressor. Sounds very cool, more impressed than I had expected.
But..
1. There is an element of distorted scratchy sound. More with humbuckers than single coils. It is not distortion as you would like from a distortion pedal. More scratchy. It is all in the box so I doubt there is some loose cable or joint (famous last...). I am not very used to the controls yet, but it seems to vary with very low setting on release but maybe not. More so with (higher) ratio above 12 and (lower) threshold below 10-12...
(2. The attack and release knobs. Checked connection and that seems to be all good at least in that vicinity of the board. But they do not change things much. Is it because they are that subtle? Or do they only come in play with other settings? (CORRECTION: Asked this here as well: https://forum.pedalpcb.com/threads/thumb-sucker.6769/ post #6 I think. sorry for double posting))
 
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That's bizarre. I wish I could help, but I've never heard anything like it. It sounds like something is rattling acoustically.

Are there any knob settings that make it go away?
 
Yes, I agree. As if something was loose, not any "normal distortion".
It is most prominent with high Rate and low Threshold. The others don't affect it as far as I can make out.
 
I'm hoping someone more knowledgeable than me can jump in cuz I'm at a loss. Visually everything looks correct.

If the LM13700 came from Tayda or Futurlec you might try a known good one. But that's just a stab in the dark. Good luck.
 
I'm hoping someone more knowledgeable than me can jump in cuz I'm at a loss. Visually everything looks correct.

If the LM13700 came from Tayda or Futurlec you might try a known good one. But that's just a stab in the dark. Good luck.
Thanks, Well honestly I am just happy someone reacted to my post. Feels better :)
I got it from Das Musikding, but not sure where they source from.
I tried with (Trio +) looper and audio probe, but it gets very high pitchy noisy, so I went for guitar instead. Downside is that your hands are no longer very free...
But for sure the audio at the in jack is clean, and on the out jack it has the crackly thing. So something happens "in between", just a matter of tracing until I can figure out where the problem begins and then either re-solder or swap that component. I tried swapping the TL072s but could not say if that mattered. Also the 2N3906 felt a bit wobbly in its socket, but I could actually pull it out of the socket with no clear impact.
So thanks for cheering me on, and I hope someone who might know this build will find this thread eventually.
 
Not sure if this means anything, but I probed to the green point which still sounds "clean". I assume this makes sense since this seams to be the clean signal route.
The red junction is where it sounds sh*t. But this does not have to mean that this is the problem. There is signal, but much more scrapy than the out result. Still I will try to swap the TL072CP for a spare one, and resolder the socket legs and D1 and D2. And yes, I also doublechecked that the diodes are facing according to pcb layout.

1644336646044.png
 
Thanks @JamieJ . It traces to the out. I did not unbox it just yet, so I couldn't verify each side of C4, but since the signal passes through all the way to the output, I take that as the red path checks out.
I have been swapping the two TL072CPs with each other and at some point it felt as if that made a difference, but now I am kind of back on square one. Actually pin 1 on IC2.1 has the stronger signal and it sounds really broken.
 
Thanks
One problem is that the audio is at one level coming in and until pin3 on IC1.1 TCL072 then on pin1 it gets amplified (of course) making it hard to distinguish between any scraping vs just very loud, it is not until Level knob I can get back to some more unity level, and then for sure the scraping is there.
I would say the noise is about the same on R8 (1) going in as on R7 (2) down side, so I doubt the C4 is dodgy.
Considering it is loud I would still say C2 (1) and IC1.1 pin 1 junction is noisy, but pin 3 seems fine. But again not easy to separate the variation in volume there...
But would that be pointing at the TL072? Or more likely a badly soldered socket leg?
I can always re-solder the legs of that socket, or re-wet them at least.
Now I switched the TL072s and there is no change so I would rule out them being faulty.
Listened on pin 5 on the LM13700 and pin2 on TL072 and it is pretty noisy there...
I guess resolder the socket is about what I can come up with. Unless of course this is some sort of built in aspect: High ratio, low threshold, will be noisy...
 
OK, I went through and re-heated and re-soldered as needed the sockets, including the 16-pin one for good measure. Q1 was a bit wobbly in its socket, so I soldered it to another socket and stacked them. I reheated and reflowed the Release and Attack.
It still has that scraping noise. Right now it sounds perhaps slightly worse than before. More with humbuckers than SCs, and more on Marshall DSL40 than solid state practice amp.
Chatted with Origin Effects and it might be an inherent thing with compressors on full Ratio and very low threshold that you get distortion, so that might be a factor in this case as well.
But the noise I get does not sound like any distortion-y sound I get from other pedals. It is just scrapy as if something was loose (as per sound example).
 
It's supposed to sound like crap at the red node, that's the peak detector. If it's good at the green node and dirty at the output, that narrows the problem down to IC1's socket, C4, R8, the LEVEL control, the switch or the output jack. And all of the associated solder joints. INSPECT the solder side before reflowing.
 
Thanks a lot! New hope...
I might have placed the green in the wrong spot (pin 1), it is more like pin 3, the plus going in to IC1.
At 1 and 2 pins it is noisy.
I will inspect as you suggested anyway.

Is there a way to verify that ATTACK and RELEASE does something? If they do it is very subtle.
 
Yes, I agree. As if something was loose, not any "normal distortion".
It is most prominent with high Rate and low Threshold. The others don't affect it as far as I can make out.
I assume you meant "RATIO."
RATIO sets the max gain. Turning RATIO up increases the maximum available gain.
THRESHOLD sets the sensitivity of the peak detector. The peak detector measures the loudness and uses that information to turn down the gain. Turning THRESHOLD down makes the peak detector less sensitive and it is therefore not going to reduce the gain as much.

Bottom line: setting THRESHOLD low and RATIO high is not a good combination. You are causing the pedal to overdrive itself. Your pedal is working as it should.

Another effect of setting THRESHOLD low is it causes the ATTACK and RELEASE knobs to have little effect.
 
Does the distortion go away if you back off on the humbucker volume? CDB's suggestion that you may be setting it to overdrive its circuit would show up more with higher input levels.
 
As luck would have it, I breadboarded a Thumbsucker yesterday. This is the input (green) & output (blue) with these settings:

RATIO 2:00
THRESHOLD 8:00
ATTACK 9:00
RELEASE 9:00
VOLUME 5:00

IC1-1 is being driven to the rails. The input is from a sig-gen. 350mVp-p is a moderate Humbucker output.

Engineer's Middle Finger 01.png
 
Thanks a lot. Yes, I meant RATIO. My bad. Too many recent builds and this my first compressor.
I tried to ask early on if low THRESHOLD and high RATIO were inherent (?) to cause distortion. Now I know. Thanks.
Humbuckers pronounce the "problem" more, but with RATIO at 03:00 and THRESHOLD at 09:00 a Single Coil is much affected as well.
I have many distortion pedals and feel that they have some commonality in how they sound. I mean, I think I can pick out distortion among other sounds.
And the sound sample I send at the beginning of the thread does not sound as I expected it. IMHO sounds more like something almost mechanical broke.

I will try with higher THRESHOLD to see it ATTACK and RELEASE will yield som effect. I think I already did, but will try again.

@zgrav : I didn't try around much with guitar volume on either one in an organized fashion, but as far as I remember, the noise trails off a little bit with instrument volume down, but less effect than the knobs give.

I now understand that my settings is pushing it too much in this case.
Still when I yesterday had both RATIO and THRESHOLD at noon, there is still noise.
So my thought was that it is a bit waste of knob turn to have around half of each as useful. I understand compression is a bit sublte, and I am more "all in" than subtle, but as I learn my pedals, subtle is making progress.
I had always shied away from compression before due to this, but decided to try out a diy to see what it is like. And at some more pushed settings got my Strat to sound much more thick and alive(?) So I naturally wanted more of the same. And then I soon reach disappointment when the distortion got so prominent so soon.
So a huge thanks for setting me straight and taking your time in doing so.
If you have time over, please confirm that my sound clip indeed is due to what you have explained. I would appreciate it.
2 questions remaining from me:
1. Do you know of any other build where the RATIO/THRESHOLD "wrong way" distortion is much less pronounced?
2. Or any pedal on the maket known to have that quality? (Cali76, Keeley plus, ThorpyFX,....)
Many thanks in advance
 
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