Monday, March 25, 2019

Emerson Em Drive - DIY build - Old Turkey Head - Layout and Schematic

I started with IvIark's layout, it "worked"

The problem is crackly decay on the distortion, and too much distortion.

I have seen at least 3 different value resistors for the collector resistor in various internal pictures of original Emerson pedals posted around the web.  The 2Megaohm resistor seems to be constant.

The obvious answer and solution is that they are selecting resistors for each transistor that they test.

The easiest way to do this in a DIY build like this one is to simply remove the bias resistor and replace it with a 10K trim pot.

When I biased my 2N5088 to approximately 4.6 VDC on the collector, I was given a smooth, pretty gain sound with a gradual transition to heavy clipping with the Pico pot on full rotation.

This is a wonderful sounding pedal, and I recommend it.  I've had the privilege to play a "real" one and the DIY version is every bit as satisfying.  I'm currently using it in front of a Vox AC30HW but I'm sure it would work with almost any amp.

This is a very easy build.  There is some mojo in the caps, in my opinion.  I really like the way Paper in Oil caps sound and I use them at almost any chance that I get.  Just really smooth and clean with the tiniest hint of warmth.  I'm sure some nice film caps would do well too.

You can play around with transistor selection, and bias voltages, if you want to get tweaky with it or explore the range of this circuit.  I just quickly arrived at a working version and left it there.

I did a version with a 220 uF filter capacitor too, because I hate noise and hiss and hum.  My pedal build with the big cap is very quiet even at very high gains, just a small bit of hiss is left.  The stock version from Emerson does not use this cap.  I will include both layouts below.