Darkglass/Demonfx Alfa Omicron

skizoide · Post by **skizoide** » 12 Nov 2024, 10:29

Hello.
I traced a DemonFX Omicron Darkglass that appears to be a copy of the Omicron Darkglass Alfa. I ordered a few PCBs with the schematic I traced, and it works just like the original (Demonfx). The only thing I have changed is the switching bypass, which I have made truebypass (the original used a CD4003 and JFETS). The rest, as is. Here is the schematic, in case you are interested.

ABtheBassPlayer · Post by **ABtheBassPlayer** » 30 Jan 2025, 02:53

Thank you! Any chance you could trace the switching with the JFETs?

temol · Post by **temol** » 30 Jan 2025, 20:34

Did you use 33178 and 33179 or substitutes?

karter2000 · Post by **karter2000** » 31 Jan 2025, 20:19

Would you be interested in sharing the gerber files?

skizoide · Post by **skizoide** » 01 Feb 2025, 16:54

karter2000 wrote: ↑31 Jan 2025, 20:19 Would you be interested in sharing the gerber files?

I have a first version, but it takes up a 1590BB case, and to be honest, I didn't put much effort into designing the layout either.

temol wrote: ↑30 Jan 2025, 20:34 Did you use 33178 and 33179 or substitutes?

No, I used TL072 and TL074.

ABtheBassPlayer wrote: ↑30 Jan 2025, 02:53 Thank you! Any chance you could trace the switching with the JFETs?

I'm not really interested in that part of the circuit, but I'll take a look at it.

temol · Post by **temol** » 02 Feb 2025, 18:34

I'm slowly putting this together on a breadboard but I quickly simulated the circuit in ltspice. Replacing three capacitors values with standard ones is practically unnoticeable in terms of frequency response C17 - 72n -> 68n. C20 7.2n -> 6.8n. C6 - 135n -> 150n (or 120n). It is quite possible that these strange values result from measurement errors, deviation within the tolerance limits of the values, measurement without desoldering, etc.

aion · Post by **aion** » 03 Feb 2025, 14:36

temol wrote: ↑02 Feb 2025, 18:34 I'm slowly putting this together on a breadboard but I quickly simulated the circuit in ltspice. Replacing three capacitors values with standard ones is practically unnoticeable in terms of frequency response C17 - 72n -> 68n. C20 7.2n -> 6.8n. C6 - 135n -> 150n (or 120n). It is quite possible that these strange values result from measurement errors, deviation within the tolerance limits of the values, measurement without desoldering, etc.

SMD capacitors don't have the values printed, so you have to measure them directly to know the value, but then all you have is the actual capacitance and not the nominal value. On my traces I'll usually replace it with the closest standard value, but if it's more than ~10% different then I'll also include the measured value in the schematic in case there is ambiguity. For example a capacitor that measures 240n could be a 270n/20% or it could be a 220n/10%.

skizoide · Post by **skizoide** » 05 Feb 2025, 08:51

temol wrote: ↑02 Feb 2025, 18:34 I'm slowly putting this together on a breadboard but I quickly simulated the circuit in ltspice. Replacing three capacitors values with standard ones is practically unnoticeable in terms of frequency response C17 - 72n -> 68n. C20 7.2n -> 6.8n. C6 - 135n -> 150n (or 120n). It is quite possible that these strange values result from measurement errors, deviation within the tolerance limits of the values, measurement without desoldering, etc.

Hello. All the capacitors, except for the electrolytic ones, which do have their values marked, I have unsoldered and measured with a capacitance tester. It is true that these values were a bit out of the ordinary, and in fact, in the version I built, I used closer values, as you say.