Dual FX power decoupling, simplification

Putting two effects in one box almost looks too easy, but you will save a lot of time reading the frequent questions before starting such a project...

Dual FX power decoupling, simplification

Postby jwyse » 23 Oct 2014, 20:37

I'm planning to build two pedals in one enclosure, and have a question about simplifying the power decoupling.

This is going to be a dual-Timmy, but I don't think the specific circuit makes much difference for my question; it's a dual-opamp overdrive.

The Timmy power supply is something like this:
Image
V+ (9V) goes to the dual opamp; Vref (~4.95V) is used elsewhere in the circuit.

I want to add the 100-ohm + 220uF filtering as described by bajaman here: http://www.freestompboxes.org/viewtopic.php?f=49&t=1392

I know I could add that completely separate from the Timmy boards, but since I'm making the layout anyway, could I simplify it by putting the filtering after/within the power supply section, like this?
Image
This is using the existing 47uF cap (x2) instead of the 220uF -- is that enough? Should I replace the 47uF with 220uF?

This filters the ~4.95V supply, and doesn't require me to duplicate the whole power supply section for each circuit. I'm hoping the opamps themselves don't really need the decoupling/isolation -- but I could be dead wrong. Anyone know?

Thanks!
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Re: Dual FX power decoupling, simplification

Postby jwyse » 25 Oct 2014, 15:38

I think what I'm after is basically similar to the AnalogMan King of Tone: one enclosure, two independent and identical low/med-gain overdrives (each using a dual opamp) connected in series, with +9V referenced only by the opamps, and Vref (4.5~5V) referenced elsewhere throughout the circuit.

Going by Madbean's schematic for his Aristocrat (based on the King of Tone), there is no power isolation between the two halves. So I think I should be safe keeping the power supply section pretty simple.

Is there a rule of thumb for when you would really want to isolate/decouple power to multiple circuits? I'm sure it could never hurt, but how do you weigh the benefit vs the extra time/parts? Is it mostly recommended for very-different and/or higher-gain circuits?

Any advice would be appreciated. Thanks!
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Re: Dual FX power decoupling, simplification

Postby Nocentelli » 25 Oct 2014, 16:41

If you have an LFO in one circuit (e.g. as part of a modulation effect) and another circuit that carried the audio, you would want separate filtering and separate vref circuits to avoid tick bleeding into the audio output. Two low-ish gain overdrive circuits probably won't present much of a problem, you might even be ok with them sharing vref as well as ground.
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