On Thu, Jul 23, 2015 at 4:24 AM, Neil Baldwin neil.a.Baldwin@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:Thanks Ken.On one of the gain cells, the output breaks up quicker than the other. I matched the NPNs as suggested but it doesn't make any difference. If I turn the gain pot up, at a certain point (just past midway if it matters) suddenly there is an inaudible load on the output. Here's a photo of it hooked up to my oscilloscope. If I then turn the pot further both the frequency and amplitude changes (of course). The initial appearance of this is not gradual but jumps in quite alarmingly. The other gain cell has a much more gradual breakdown at high gain.Any ideas?Here's a Dropbox link to a picture from the scope:NeilOn 20 July 2015 at 23:29, Ken Stone otherunicorn@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:To decrease the available gain, reduce the value of the CV pull-down, or reduce the value of the feedback resistor between pins 5 and 6 of the cell.KenOn Tue, Jul 21, 2015 at 1:12 AM, Neil Baldwin neil.a.Baldwin@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:Getting somewhere now, cheers Ken.Incidentally, if you wanted to decrease the amount of available gain, would it just be a relatively simple mod?On 20 July 2015 at 11:44, Ken Stone otherunicorn@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:Leave the 10n there, and add a couple 100n on the main board (it's stripboard, isn't it?)Adding a 100k trimmer wired b/w the rails via 100k (or greater) to the CV input will let you trim it to unity. It seems to vary a little, dependent on the transistors used.On Mon, Jul 20, 2015 at 8:40 PM, Neil Baldwin neil.a.Baldwin@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:So try swapping out the two 10n caps for something else?Also I've notice that with the CV disconnected and, for example, a 5V sine wave connected to the input, the gain at the output stage is massive. Should you see unity gain with no CV applied?On 20 July 2015 at 10:47, Ken Stone otherunicorn@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:You probably need more decoupling on the power pins to the cells. These cells have huge gain.On Mon, Jul 20, 2015 at 7:00 PM, Neil Baldwin neil.a.Baldwin@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:Thanks KenWith +/- 12V voltage to the CV voltage divider part I'm seeing about -0.6 to +0.6 V at the CV pin which is correct.However, with no input signal to Input, turning up the gain pot eventually results in horrendous feedback-type output.NeilOn 19 July 2015 at 11:34, Ken Stone otherunicorn@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:4.7 instead of 5 pf can be ignored. 100n instead of 220n will cut your bass response.It is ESSENTIAL that there be a pulldown resistor on the CV input e.g.: a 100k to 5k6 voltage divider is needed between your CV and the gain cells CV input.On Sun, Jul 19, 2015 at 8:16 PM, neil.a.Baldwin@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:http://www.cgs.synth.net/modules/cgs108_vca.html
I've built a couple of these to add CV amps to an existing project but am having trouble getting them to behave. Some things to note:
- they're running off 15V so I'm wondering if there are some resistor changes required?
- I had to substitue a couple of capacitors as I didn't have exactly the right values and they didn't appear to be that critical based on my loose understanding of this circuit: the 5pF on the board I changed for a 4.7pF and the 220n required on the input when wiring up I substituted with a 100n
With a 5V peak sine wave input I'm getting horrible distortion. My latest idea was to install a 50K trimmer instead of the 100K fixed resistor on the CV input wiring to see if I could dial in a resistance value that would work better but that's no been particularly successful.
Neil
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Message
Re: Gain Cell
2015-07-22 by Ken Stone
Sorry, I have no idea what's causing that. Maybe read up on Blackmer cells.
