Polarity in vintage Muff Fuzz

[Vizz]

Hello sirs/madams

I have acquired a 70s op amp muff fuzz that sounds wonderful and eats batteries. So I Switched batteries For a battery snap & 9v supply. Estupendo!

However , I now want to daisy chain with another pedal and  I suspect that they have opposite polarities. At this point I realise that I probably sound like I don't know what I'm talking about- which is partially true.

Anyway- question is: should I worry about damaging my vintage pedal by using the inverted polarity to what is required? Or is positive always positive on a battery snap, regardless of how the power supply is set.

Eternally yours,
Vizz

[Gebrey]

Hi
indeed polarity could be a problem. If you post model and brand we can probably find out a scheme and have a confirm.

But there is a way to know that empirically.
You just need a diode like 1N4148

Put diode reverse between positive and negative (or put it in series in the positive, but I prefered the first method).
If you connect it wrong, reverse polarity, the diode short circuit the PSU and the pedal is protected. Otherwise it should work.

Google "diode protection circuit scheme" for more hints.

You can also build/buy a female to male PSU jack with build in diode that you will use everytime you have a doubt.

Last method less reliable is to measure the potential of the case's ground. If its positive it is a reverse polarity pedal.

In my experience, with good PSU unit (probably safe diodes build inside) you will never burn a pedal. Ive connected a +-18V power amp to a reverse pedal in daisy chain to ther normal pedals. The PSU leds went off together with pedals.
But no damage happened.

Have a nice year

[antonis]

Have a nice year

You too..

And thanx for your valuable info about reverse polarity protection..
(never mentioned before in this forum..)

[FSFX]

You just need a diode like 1N4148

Put diode reverse between positive and negative (or put it in series in the positive, but I prefered the first method).
If you connect it wrong, reverse polarity, the diode short circuit the PSU and the pedal is protected. Otherwise it should work.

I would not recommend the diode between positive and negative at all.

It is a legacy of early battery powered pedals and is totally unsuitable for use with modern pedal power supplies
  - with reverse polarity, the diode short circuits the PSU and burns out and is no longer any protection for the pedal, so the pedal still gets exposed to reverse polarity.

That method only works with power diodes like 1N400x, not small signal like 1N4148, and with batteries or power supplies with current limiting.

Think about it - why on earth would you want to short out your power supply? It is plain stupid.

Series diodes (usually Schottky like 1N5817) or a P-Channel MOSFET (for negative ground and an N-Channel MOSFET for positive ground) are the only proper way of doing it. If you use a shunt diode like you suggest then its best to use a Zener diode and add series resistor or a PTC resettable fuse. That will give overvoltage and reverse voltage protection.