Engineer's thumb compressor question

eh là bas ma

Well-known member
Hello,

I would like to have your opinion on something.

I built this version from a musikding's kit :


It distorts at some settings unless i turn Threshold above 12', so there is obviously something wrong.

As I looked for anything suspicious I noticed something real strange...

Both R10 pads are showing continuity with E, B and C. One R10 side reads 4.57 V, the other 1.117 V From the schematic I didn't expect both R10 pads to be connected with all transistor's pads...

I have also continuity between D4 negative side, one R10 pad (the one at 4.57 V) and both R11 pads, I wouldn't expect that either.

But this is what puzzles me the most : these connections happen only when the DC power jack is inserted.

If I unplugg the power jack, connections are as shown in schematics, no more strange continuities...

I can't see any explanation for this...of course I reflowed, checked for shorts, nothing I can find, and without the BC237 transistors, the same behavior happens with sockets too.

Here are my readings :

DC 9V+ = 9.50 V

BC327

E= 4.60 V
B= 4.19 V
c= 1.122 V

TL74 ACN

1= 4.62 V 14 = 4.61 V
2= 4.61 V 13 = 4.62 V
3= 4.36 V 12= 4.15 V
4= 9.23 V 11= 0 V
5= 4.37 V 10= 4.42 V this pin 10 follows the voltage at C3/R5 junction, depending on the Release setting.
6= 4.62 V 9= 4.62 V
7= 4.63 V 8= 4.19 V

LM13700N

1= 1.120 V 16= 0 V
2= 0.6 V 15= 0.8 V
3= 4.62 V 14= 0.5 V
4= 4.62 V 13= 0.6 V
5= 4.62 V 12= 0.7 V
6= 0 V 11= 9.23 V
7= 0 V 10= 0 V
8= 0.5 V 9= 0.650 V

My question is : What do you think about these connections happening only when DC 9V power jack is plugged ?

Could it be correct by any chance ?

Thanks for your help !
 
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First of all, there is no good reason to set THRESHOLD below noon. It is normal to get distortion when THRESHOLD is set too low because the circuit runs out of headroom. I think your Schalt is working as it should.

Did you say you're measuring continuity with the power on?

Most of your voltage readings make sense. What brand DMM are you using?
 
Thanks for your reply !

My DMM is a VICI VC97A

The continuity between these points happens when the power DC jack is plugged, pedal switched off. When unplugged it's not ringing anymore.

Powered and switched on, i don't have continuity between R10, B and E. Only with C, but D4 negative side rings with R10 on one side and both R11 pads.

Here is a sound sample :


first, clean guitar.
Then settings as shown on the picture ( Ratio is at 3').
Finally, Threshold (Seuil) fully CCW, Ratio fully CW, Release (Sortie) fully CCW . Settings with the worse distorted signal possible with my build.
 
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Let me correct : I just tried again a continuity test powered and switched on, and now I do have continuity between R10 pads and E, B and C.

I didn't a moment ago, like it is irregular...

Edit: I also have continuity between E and C sockets , only when power DC jack is plugged, both on or off. No longer when unplugged...

I wonder how is it possible ?
 
You can't make a meaningful continuity measurement with the power on. It's hard enough to make sense of in-circuit continuity measurements with the power off.

That's a good DMM, it has 10M input impedance. The cheaper ones are 1M and they load the circuit down.

I think your pedal is working as it should. When THRESHOLD is at 0, there is no compression and the circuit runs at max gain. It's running out of headroom. Don't set THRESHOLD below 5 (noon). Not all combinations of control settings are useful.
 
Ok, i am relieved in a way.

On the other hand, if this compressor is working as it should, I find it quite deceiving. It's weak and easily distorting the signal.
I have a hard time understanding why people think it's good and even among the best compressors... I'd say it's among the worst, honestly. Edited : among the hardest to use and to understand.

Be that as it may, thank you very much for your help !
 
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Try some other control settings, you may find something you like. Keep THRESHOLD full CW. Keep RELEASE above noon for minimum distortion. Don't set RATIO any higher than necessary.

I was not all that thrilled with it either. The design is clever, I wanted to like it, but that never happened.
 
My experience is that with the threshold at the minimum, there is no compression, and with the threshold at the maximum, we have the maximum compression. Just the opposite of what you'd expect.
 
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