Mini EVH

From the vendor

Mini EVH

Apr 17, 2022 4 min read

Updated: Aug 22, 2022

Originally posted on the Freestompboxes forum

This is my version of the EVH overdrive by Bajaman. Just to be clear, I have nothing but respect for him, all the work he has freely shared and which has become popular in many ways such as between sold PCBs. I don't claim this version is better or worse than the original, I just liked the idea and wanted to get to the same goal through my own means, using the circuit as inspiration and with the goal of making it more compact and easier to build without compromising anything.

Let's go through it:

• Removed the JFET input buffer. JFETs are expensive or tiny these days, and the 074 can do the job just as well. Input impedance is 1MΩ, input cap plenty large, input RF filter and current limiting have been scaled down for lower thermal noise while still avoiding pickup interaction. 68k was a common value for valve grid stoppers.

• Input stage scaled for more E6, resulting in an error of maximum 0.2dB. Capacitors go to ground, resistors still go to Vref, because I like to save caps too :).

• Gain control network: impedances scaled down, removed the resistor in parallel to the gain pot, pot made log taper, removed input cap. The bias for the next stage was already coming partly through the gain pot, but JFET op amps make this acceptable. The other values have been tweaked to get a very very close response across all the pot travel.

• Here's my bold move: before hitting the LED, we have three stages. Of those, IC1b and IC2b have some frequency-dependent gain characteristics, while IC2a features a gain of 2, but it's after a 0.68x attenuator with flat frequency response (the low-pass before IC2a cuts in the hundreds of kHz). Attenuation is before amplification, so headroom isn't affected, only noise. I thought i could make those three stages into two, and so I did. At the same time I saved some components along the way (the 33k and 10n at the output of IC2a aren't doing anything that I know of) and re-scaled to my E6 taste.

• You thought that was a bold move? The thing I found most interesting about the baja EVH was the idea of separately clipping the two half waves in different stages, but I really wanted to make this a 4 op amp circuit if I could. So I went looking at what's in between the two clippers. There's an attenuating low pass filter with an attenuation of 11 and corner frequency of 17kHz or so. I thought I could ignore the latter part. Following it there's IC3a, with a gain of 13 and flat response (low-pass at 49k). Combining this with the attenuation before it, since the series resistance is the same 2.2k and the diodes never conduct at the same time (I think we can say that ), the transistor diode is getting a signal 1.18 times bigger than the output of IC2b. In the realm of dB and gains in the neighborhood of 70dB, that's not much, so here's my fudge and maybe something you can lay blame on if the clipping (not the frequency respons