Voltage Mod Question

[methodofcontrol]

i'm working on cloning a pedal for a friend. the pedal has an AC cord going to a 36vac transformer, then a rectifier. both -18v and +18v are routed to 15v regulators (- and + respectively) and the 4558 chips have +15v on pin 8 and -15v on pin 4.
what i've been asked to look into is whether or not the clone can be built to work on a standard 9v battery or adapter. i can use a doubling circuit to coax it up to 18vdc, but i still wouldn't have any negative voltage to use. any ideas?
thanks!

[Steve Conner]

The usual voltage doubler circuits don't work on DC. If you had 9V AC, you could get positive and negative voltages easily, but you don't.

You can:

Use a DC-DC converter like the ones from Traco, C&D Technologies, Recom, etc. They have models that will accept 9V DC and give you +/-15. They cost about $20 but if you ever tried to make one yourself, you will pay it gladly.

Use two 9v batteries to give you +/-9V. You're just a ground reference change away from running off +18V.

Use four 9v batteries to give +/-18.

[Mark Hammer]

Depends on the pedal. Some pedals are designed around a presumed supply voltage, while others can work with a variety of voltages and a higher supply voltage simply provides some insurance.

As well, many voltage doubler circuits have built in assumptions about maximum current, and we don't know anything about the current requirements of the circuit in question.

A fairly simple solution, if the pedal in question is one of the more flexible ones, is to simply use a 9-10vAC wallwart, and regulate that to +/-12vDC on board.

[methodofcontrol]

hey mark...yeah, that's what i was thinking. just running off an AC wall-wart instead of a DC one. but can i get a (+) and a (-) 9vac off of that? i thought i would have to run an 18vac adapter to get that.
also...i'm theorizing that the +/- 15v in the original pedal will allow more headroom than dropping that to +/- 9v.

[Mark Hammer]

yeah, you can get +/-9v or whatever off 9vAC. Remember, as AC, it swings both positive and negative. The rectification and regulation would simply separate the positive and negative half cycles from each other. Rectification, depending on the type, can yield as much as 1.414 x original AC voltage, such that 9vAC could give you over +/-12VDC.

For instance, in this example from a PAiA mic preamp, you can see that a plain vanilla 12VAC wallwart is used to derive +/-15VDC via half-wave rectification. D1 and D2 separate one half-cycle from the other, and C1/C2 and C3/C4 serve to smooth out the ripple.

BTW, what are you trying to fix/alter? It may well be one of those pedals that doesn't really benefit much from +/-15v over 12 or even 9.

[Steve Conner]

Thanks, Mark! That was some great advice and just the circuit I was thinking of.

[methodofcontrol]

should i copy the diode and cap layout for that posted schem? or can i just use a 4 diode config as would be found in most amps? thanks for the help!

[Mark Hammer]

I'm going to say you need to identify the circuit it is going into. You can PM me if you are uncomfortably with saying it publicly.

I say this because there may well be other regulation in the circuit that makes it reasonable to just duplicate what I've shown, or there may be a reason to aim for something a little more complex. Different circuits can require different degrees of regulation and smoothing at the power supply.

[methodofcontrol]

thanks for the offer, mark. i'll have to scan in my schematic and get back to you.