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MFOS VCA Feedthrough
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author MFOS VCA Feedthrough
Hi everyone!
I've just finished to build this Dual Log/Lin MFOS VCA module: VCA/DLLVCA001.html&VPW=1910&VPH=841

The VCA 1 is working properly and the calibration was smooth.
On the other hand the VCA 2 shows a significant Feedthrough:
if I connec just a signal at the CV input (In this test a 10VPP [+/-5V Triangle]), I can measure that same signal at the output with an amplitude of 1VPP (-/+0.5V), even with the trimmer R41 turned all the way to minimize this effect.

Can someone please help me to cancel this feedthrough ?

It's quite annoying for me to have a VCA that changhes the harmonic content of the input signal !!!

Thanks in advance
the problem is with the design. if you want to eliminate the feedthrough, you will need to cut some traces and add some components. with the way its setup now, its pure chance as to whether it can null it (whether its a positive or negative offset), and it is not a consistent null over amplitude, and it modifies the gain of the circuit to null.

remove trimmer R18 and R20. short out R18 and replace R20 with a 1k 1% resistor. cut the trace from U2-pin4 to ground, and replace with a 1k 1% resistor. replicate the "bias adjust A" circuit, connecting the 2M resistor to U2-pin4.

short the input to ground, apply a 5Vpp signal to the CV in, and listen to the output while you trim the new bias circuit you added. you should be able to null it out.
First of all, Thanks for the help!

I am going to dig into this problem and find some more insights on how the circuit works and why I am getting this issue. I'm going to share those insights if 'll manage to sort this thing out.

I have one doubt now: I'm facing this problem only with VCA2 (VCA1 is giving me absolutely no feedthough), but you suggested me to modify the components in the VCA1 part of the circuit so....

a) Fixing VCA1 will make both work properly
b)I have to do your modifications on both VCA1 and VCA2
c)You were saying to modify those components, but you intended the VCA2's ones

Which of these options is the right one ? I'm confused confused


If only I had three or four extra rounds on the trimmer I would probably had obtained a nice Feedthrough compensation on VCA2, since I was getting there, but I couldn't sweep the trimmer any more.
Would it be a nice solution to simply replace the 2K trimmer with something higher or just try to slighly increase R42 with something like 220R ?
sorry, i gave the schematic part numbers for VCA1 as that was the schematic i was looking at. you can swap the identical parts for VCA2.

one thing i might suggest, is to apply a fixed CV of 5V or so, and a fixed audio input (anything but square should work), and turn the offset trimmer from one end to the other. listen to the output, or look at it on a scope. how much does the amplitude change? from the schematic, it looks like the trimmer would adjust this over a 1:10 range, which is a really bad design. im guessing this trimmer configuration is a mistake, as the typicall LM13700 datasheet shows offset trimming by using a 2k trimpot with the wiper going to ground, and the 2 ends going to the 2 inputs. which is almost what is here, but not quite. you could reconfigure the trimpot to that configuration, which would be easier than what i previously described, but not quite as good. it will be good enough, though.
Great! thanks for you help!
I will work on these on Monday evening, and I will take the measures you told me, and let you know!
I'm already liking the easier solution of grounding the wiper of the trimpot, I'll see if that is good enough (which is a great engineers' way of doing things )
check out figure 17 of the LM13700 datasheet if you want to see a schematic of what im talking about. considering that the rest of the setup is almost the same thing, id guess it was just a mistake to wire it the way it is.

to do the fix, cut the trace from pin4 to ground, and the trace between pin1 and pin2 of the trimpot. remove R20 and run a wire from pin4 to pin3 of the trimpot. then connect pin2 of the trimpot to ground.
I checked fig. 17 on the datasheet, and had no problem understaning the modifications I should do, your explaination is very clear.

Yesterday I took some measurements, in order to investigate more on what could be causing this feedthrough.
Those measures were taken while I was following the calibration suggested by Ray in the same page linked in the first post

It's clear that the feedthrough on VCA2 is way bigger!
Listening to it (only CV input connected and headphones at the output), on the VCA1 is barely audible, but It gets more consistent in the VCA2.

Even though, I think the response of the VCA2 is acceptable (it's significantly quieter in respect to input signals foe both VCA1 and VCA2, but I guess it's normal)

With a CV in ranging from 0V to 7 V (from MFOS 1V/Oct Calibrator)

If someone has ever built this in the past, is it normal that the output is no longer a 10VPP signal, as one can see from the pictures I posted earlier?
again, the feedthrough trimpot adjust the gain, so its most likely due to that same error in the circuit. it should give a pretty close to 1:1 output with a 5V CV applied.
Ok I got it,
That trimpot is used for two purposes:
- cancel the feedthrough
- limiting the input signal

These two factors are strongly related in some obscure manner.

The trimpot in this design can adjust the input signal's amplitude from almost 0V to 100mV (the LM13700 datasheet suggests not to feed the input with signals bigger than 60mV in order to have a good S/N ratio), which is fine.
It also affects the feedthrough, which is fine!

The problem is the slightly different implementation from the datasheet, that causes this behaviour, and it also limits the output signal's amplitude!

I'm surprised that, even if this is such a popular module in the SDIY world, no one seems to have reported these limitiations, in particular the very low output amplitude!
well, it works, just not perfectly well. i think most people are happy to see the thing they built doing something, and digging down to the level of reducing feedthrough is a bit more complicated. you need a scope, etc. people are also more likely to suspect themselves over the designer for having made a mistake, so they just figure thats the way its supposed to be.
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