Converting -12v circuit to standalone positive charge?

[jaycobie]

Short version: I have a circuit that runs on -12v potential, with earth as the positive, and would like to make the circuit stand alone and run on normal +12v but I'm not sure how to achieve this.
Here is the schematic from the service manual
https://imgur.com/a/5pDd7Fg

Longer version
I recently gutted a Yamaha Electone B-20R electronic organ from the 70's, and have been working on taking the speakers and amplifier sections and making it into a standalone guitar amplifier.
I've hooked it up in a test jig and it works well, but I decided to try and use whatever parts of the pre-amplifier boards as was possible and found a small circuit that really sounds good to me when inserted into the signal path. I'd like to be able to take this small circuit and make it into a standalone pedal, to see how it performs by itself, but since the whole amplifier runs on a negative voltage potential as referrenced to earth, I'm not sure how to run this circuit on a normal +12v power supply.

Would I just treat the -12v as positive and earth as negative?
Or would I treat the earth on the schematic as positive and the -12v as negative?
Or would more modifications be neccessary?

[mandu]

The circuit is a npn pre amplifier. Just disconnect the 15K resistor from +12v (existing ground) and connect to 0 volts. Supply +12v and 0v to the circuit. The 0V is the new ground. Good luck.

[eschertron]

Since the signal is capacitively de-coupled from the power rails, you can simply level-shift the -12vdc and 0vdc to 0vdc and +12vdc without problems. It looks like it would work on 9vdc (typical pedal supply voltage) without trouble, too.

If you're curious, google (or search here on MEF) for transistor preamps - or single transistor preamps - to see other designs similar to yours. Have fun!

[J M Fahey]

And since "an image is worth 1000 words" here´s the schematic:



You must also invert electrolytics; G means Ground/0V

Extra: to simplify your life, that circuit will work fine with +9V so you can use the same supply feeding other pedals (if any)

2) or straight build the Electro Harmonix LPB1 , same thing but built specifically for guitar.
The Yamaha circuit is designed for Organ use and your guitar will have some trouble driving its 15k input impedance (which is actually lower, less than 10k)

[Mick Bailey]

Yest - the 15K resistor should be changed to 1M for guitar use to improve overall response (especially treble and upper mids) unless you specifically like the circuit exactly like it is.

[Helmholtz]

the 15K resistor should be changed to 1M for guitar use

Input impedance of the stage will still be in the low k range as the transistor is wired as inverting amplifier with voltage feedback between collector and base.

[jaycobie]

You guys are amazing, thanks! I'll get to breadboarding this when I get home and tweak it from there.