Wednesday, 8 November 2017

Linear Power Booster 1 - Electro-Harmonix

This is the "pedal" that started it all for Electro-harmonix.
Note the chicken head knob.
This pedal boosts input signals making the output stand out more. Most of the examples I've seen have a jack at the amp output allowing you to plug this directly into your guitar amp.  You could just sit this one on the floor or use it earlier in your signal chain, between effect pedals rather than at the end. I'm guessing this is from around 1969 to 1971 ??? The pot markings are obscured with solder making this difficult to date.

I've read this sold for about $20USD in 1970 which in today's money (2017) equals $128USD
What a shock. No PCB.

Good Old Eddie

The schematic is uber basic.
Just 4 resistors, 2 caps , a transistor and a pot.

 


R2 = 1M
R3 = 100K
R4 = 10K
R5 = 390R
R6 = A100K (volume pot)

C1 = 100nF/0.1uF
C2 = 100nF/0.1uF

Transistor 2N5088

It's interesting that C2 is ceramic & C1 a polyester.
I think they are just coupling caps to pass the wanted audio (AC) signals, while blocking any DC.

The first part of the circuit (C1 & R3) works a bit like a high pass filter.
Increasing the value of C1 should let in more bass. If you decrease it, more treble should appear.
With 0.1uF, most of the input signal goes through.

R2 & R2 forms a voltage divider. It determines the bias at the base of the transistor
The ratio between R4 (collector resistor) and R3 (emitter resistor) controls the gain of the transistor.

The trannie is a 2N5088. Its a NPN. They are cheap as chips on Ebay.



It's a nice little amplifier circuit. Could easily be used in a modular synth

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