On 30 Jul 2015, at 03:03, Richard Brewster pugix@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:
I think I've solved my problem. The input level was too high. I put a pot between the VCO output I am using and the input to the gain cell. Lowering the input level made the parasitic oscillation disappear.
Richard
On 7/29/15 7:37 PM, neil.a.Baldwin@... [cgs_synth] wrote:
Exactly what I'm getting Richard though oddly only in one of two I'm using on the same build.
A solution to this would be an enormous relief: this has spent about 3 solid days on my bench and I don't seem to be able to do anything to alleviate the noise.
I just built six of these for two Serge VCFQ modules. I am getting approximately unity gain with the test circuit with a 5K6 to ground on the CV input and the CV pot disconnected. My problem is a high frequency parasitic oscillation on a portion of the negative excursion of the output signal. All six behave the same. Thinking it has to do with my test breadboard, I added .1uf bypass caps near the power pins to the Gain Cell board. That didn't make any difference. When I added an external 47pf cap from OUT to FB in parallel with the 5pf on the board, I saw a reduction in the width of the parasitic, but it was still there. I also swapped in a TL072ACP for the LF353. Looked the same. Any ideas? I'm running on +/- 15V. Should I try a feedback cap larger than 47pf?
Thanks,
Richard Brewster
http://pugix.com
On 7/24/15 11:22 AM, Neil Baldwin neil.a.Baldwin@... [cgs_synth] wrote:
And a short video with audio. You can see/hear as I increase the gain from 0 up to (nearly) max and back down again.
On 24 July 2015 at 16:15, Neil Baldwin <neil.a.baldwin@...> wrote:
I hooked up the scope again and this time turned up the gain to max. The interference is a square-ish wave that decreases in pitch as you increase the gain (and vice versa). Seems to kick in at the crossover point between -ve and +gain
On 24 July 2015 at 15:57, Neil Baldwin <neil.a.baldwin@...> wrote:
Thanks for the tip. Unfortunately it has little to no effect.
I've practically rebuilt that cell from scratch now, even matched the other transistor pair. I'm wondering if it's being adversely affected by other circuits around it though the fact that I've got two identical gain cells built onto a piece of strip board in the same M-class module would suggest that's not the case.
For reference, the M-class module is a custom one that I made for a client (his scheme, my design) which includes:
CGS96 Triple ComparatorCGS84 Peaks and TroughsCGS90 PhaserCGS60 Stompbox Adapter
The gain cells are to replace the input parts of the CGS60 to give more gain and CV control so I'm only using the output side of that circuit.
On 23 July 2015 at 19:50, ghpicard@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:
It looks like an HF oscillation. You could try enhancing the decoupling of the power supply input. Ceramic 10nF capacitors between the power legs of the ICs (+ and -) and ground can help. They must be soldered directly on the pins to avoid spurious inductance. Also, try not to route the output wires near the input section.
HTH
