Soft distortion if i play all 6 voices

Here i found something in the Vol output. If i play 3.4 Voices , all sounds good, but if i play all 6 Voices i become a distortion.
The problem was the SUM AMP TL072 before the Volume. I reduce R 13 from 10 K to 5 K / 2x 10 K parallel and the distortion was gone.
Maybe some others have the same problem and it is helpfull.



yeah - I have noticed this also and figured I should probably look into that IC’s passives, thanks for confirming!

What voice cards are you using, if I may ask?

my completed build currently has SVFs in it, and they exhibit this behavior

I don’t recall this occurring with the SMR4 cards, and I haven’t tried all 6 of the 4P cards yet - maybe since they have less juice coming off their lm13700 VCAs than the 2164 VCA used in the SVF they’re unable to clip the summing node in the same way

I have a 6 SVF voicecards Ambika and have the same behaviors on several patches when playing polyphonically. Here is the Olivier’s answer and possible solutions.

In fact, I did not change the resistor as he advised because I’m planning to use the separated outputs and will mix them with 6 of my Kurzweil KPS8 FX processor (and will still have 2 inputs left … this “old” unit is still very powerful :-).

Hédi K.

Hi, i have made a test with a second Ambika. The sum amp amplification is a little to high.This gives distortion in the Opamp. IC1B. R13 is 10 k, you can reduce it to 5 k or 7 k. 5 k is 6dB volume decrease. I simply solder a second 10 k parallel.
The other way, Olivier told me , is to reduce the patch volume. But this decrease the VCA Envelope digital.
With 5 k it is also loud enough.
I use here 6 x the normal SMR4 Voicecards.


@defenestration, the distorton dosn’t come on each sound, but sometimes if the Sound is a little louder and had more deep frequencies. , and if you play 5…6 Voices simultan
On 3 …4 Voices all sounds good.


I have the same problem. But which one of the resistors is R13? They are not labelled. I guess it is the one next to RN1, right?

Yes, the one just above the resistor network.

Ah, nice tip. Been having this issue myself when holding down big chords (with 6xSMR4); though it might have been a headroom issue or something.

How do you solder a second resistor in parallell? I’m working on the mobo and have already soldered the 10k resistor. Do I need to desolder it first?

Solder the 2nd resistor on top of the 1st.

Please, no filthy ReSister jokes now…

2 sisters have 1 138ohm resistor each that they want to put on top of the other…

@qp But how do I fit the second resistor? Where do I place it? Solder it on the same side as the first one, or on the flipside?

Its in parallel so it doesn’t matter. I’d guess it would be easier to solder it to the flip side. Either way will work.

Just think of steps on a ladder. All of the steps are stacked but are connected by the sides.

Maybe this issue should be mentioned in the assembly instructions and change the resistor in the BOM to a better value (7k)?

I solved the problem in which I have swapped the opamp tl072 against to one LME49860

This is not a fantastic solution since it’s only giving you 1V or so of extra headroom.