Yusynth VCLFO2 problem

yoopee · Joined: Jan 24, 2014 Posts: 3 Location: czech

hello,
thanks for this wonderful modular diy project.

I'm building few of this modules, now trying to trim the VC LFO2 module.
The pulse and sawtooth waveforms are ok, but triangle and sinus are distorted as you can see on attached screenshots.

I tried to change U3, checked R19,R20,R21,R22,R23,R24, diodes D1,D2
and everything looks ok...

any suggestions what else to check?

thank you

yoopee · Joined: Jan 24, 2014 Posts: 3 Location: czech

uh, I've attached wrong screenshots,these are correct

triangle.PNG

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sinus.PNG

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marfoski · Joined: Jan 26, 2011 Posts: 36 Location: Italy

It's a known quirk of this circuit, something you have to live with, in a sense.
Take a look at this : http://electro-music.com/forum/viewtopic.php?highlight=lfo&t=44023

yoopee · Joined: Jan 24, 2014 Posts: 3 Location: czech

thank you! So my VCLFO is trimmed okay...

kupfer_m · Joined: Oct 14, 2010 Posts: 48 Location: Belgium

gibrancurtiss · Joined: Mar 16, 2016 Posts: 22 Location: brazil

PLease, check this:
http://electro-music.com/forum/topic-44023.html&postorder=asc&highlight=lfo&start=25

kkissinger · Posted: Mon Apr 06, 2020 10:48 am Post subject: Spikes on Yusynth VCLFO solved!

The problem turned out to be confusion about the value of C8.

C8 is in the feedback loop of IC3c and, as such, implements a low-pass filter.

Increasing the value of C8 decreases the voltage spike. If C8 is too large, part of the desired waveform gets filtered* and if C8 is too small, the spike passes through.

(* -- if C8 is way too large, the amplitude of the triangle and sawtooth outputs are reduced. All this is is a fixed low pass filter -- with a low enough cutoff frequency, nothing will pass!)

When I replaced C8 with a 4700p (4n7 or .0047u) Polyester Film Capacitor, the spike was filtered out. On the very highest frequency the corners of the triangle wave are slightly rounded -- shouldn't really be an audible issue, though.

The confusion is that C8 is specified as a:

100nf on the parts list,
47p on the parts layout
1n on the schematic

I originally used a 47p for C8 which was too small a value. I experimented with different values and arrived at using the 4700p cap.

The current stay-at-home order (due to the Coronavirus) has given me time to revisit some old problems that had remained unsolved.

I know others have had this problem and hope that this solution helps.