Vary internal voltage on a Boss (AC adapter) pedal?

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Vary internal voltage on a Boss (AC adapter) pedal?

Postby Cableaddict » 14 Oct 2011, 05:55

I just found my old Boss HM-2 in a dusty corner of my studio. I forgot how much I used to love it. (Yes, an aquired taste.) Like many Boss pedals, it requires a 12v AC adapter, though it runs internally on a 9v battery.

Many OD pedals sound MUCH better with 12vdc or even 18v. Some fuzzes sound subjectively better with less voltage (old carbon batteries, whatever.) I'm dying to experiment, but I don't understand how the AC input works. I assume the pedal converts to dc internally, so how exactly would you vary the rails on this thing? Would raising / lowering the AC supply do anything, or would it have to be adjusted internally, after the DC conversion?

Suppose I just wired the internal battery wires to a jack, and use a DC psu? I assume there's some reason why this won't work, but what is it?
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Re: Vary internal voltage on a Boss (AC adapter) pedal?

Postby Dirk_Hendrik » 14 Oct 2011, 06:54

Although the 12 volt adapter for a Boss is called ACA it does output a DC voltage already. Therefore the pedal does not do internal DC conversion.

Do not increase the voltage before verifying that all components in the pedal are properly dimensoined to handle that voltage. Furthermore the HM, as well as many dist pedals do not have their distortuion generating based on the supply voltage. Therefore operformance when decreasing voltage will give results as noise, lo-fi, oscillations or less. That said by someone (me) who does have little understanding for the hobby of lowering voltages anjd considering the poor sound it gives as "better".
Sorry. Plain out of planes.

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Re: Vary internal voltage on a Boss (AC adapter) pedal?

Postby Cableaddict » 14 Oct 2011, 09:10

Thanks, Dirk. I didn't have an ACA adapter to test. Dunno' why I assumed it was dc volts!

As for your opinion about the sound, what would you say is different in this circuit that makes it NOT sound better with more volts, vs, say, My OCD, GT-500, and Timmy, which both sound noticebly better to my ears with a small voltage bump?

Is it an easy thing to explain?
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Re: Vary internal voltage on a Boss (AC adapter) pedal?

Postby blackbunny » 24 Oct 2011, 18:18

Most Boss pedals can run on regulated 12VDC safely, as they have 16V or 25V electrolytic capacitors for power supply filtering and inter-stage coupling where appropriate. Many have 6.3V bias supply (1/2 supply voltage) electro capacitors, so it is definitely wise to check the capacitor voltage ratings before increasing the supply voltage.

Circuits containing higher slew rate / low noise / high performance op amps usually benefit from increased supply voltage. An added benefit for TL071 / 072 / 074 op amps is that the higher supply voltage helps to keep them from latching to the supply rails, which sounds bad and should be avoided.

Your HM-2 would have an M5216L, M5218L or NJM4558S SIL op amp, which are not high slew rate op amps, and there is a 6.3VDC bias filter capacitor on the PCB, so I think bumping the supply voltage up to 11-12VDC REGULATED would be the safest way to go here.
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