VCO-1 range and distorted core issue - Solved!

Gingoism · Joined: Feb 01, 2017 Posts: 23 Location: Singapore

Hi guys,

Merry Xmas and Happy New Year

So here is some context before diving in. I have previously built Thomas's designs with limited issues albeit without test equipment and quality components. Fast forward few years, I have acquired proper test equipment and have been nonchalantly prototyping circuits but now the issues abound.

I first noticed the problems when prototyping iterations of the vco-1 which I redesigned to work on +/-12V. The freq. range had a ceiling of 10-12khz and the core started distorting at 1-5khz. I used bcm857bs for the pnp pair and I accepted the quirks since 10khz is more than enough for musical use.
Then I tried to build the electronotes through zero vco and this time everything was falling apart incredibly, from the expo circuit to the osc core.
troubleshooting it drove me mad.
So I reverted to building the original vco-1 exactly as is and the same problems as mentioned above persist. The freq. this time maxed out at 12-15khz and started distorting around 1-5khz. I have attached below 3 scope shots for comparison, the first being vco-1 built many moons ago and you can see it is pretty stable all the way to 28khz. The second is a slightly improved version of vco-1 with a matched pnp pair and a transistor array to replace the discrete schmitt trigger. Finally just for ref. the madness of the thru-0 core.

the few things i have tried but to no avail are,
1. swap ic(s) all over the circuit exhausting my inventory
2. trying different power supplies
3. thoroughly checking my bare breaboard for any anomalies.

im at a loss to how to proceed?!
thanks in advance guys for any inputs.

Guna

vco-1 tri_out(old)(max_freq).jpg

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vco-1 core(new)(max_freq).jpg

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EN-129 Thru-0 core(max freq).jpg

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Gingoism · Joined: Feb 01, 2017 Posts: 23 Location: Singapore

ok think i might have sussed out the culprit(s). in the older circuits i used a opa2134 for the integrator so i swapped the tl072 in my breadboard circuit and problem solved. no limited range or degradation of the core! so i replaced all my other circuits where i have used a tl071 or tl072 as an integrator with opas2134 and ca3140 and all work without issues.