Tremdriver preamp / harmonic tremolo

Hey, fellow guitar freaks! I thought I’d share some of my design philosophy with you and how I came up with the Tremdriver circuit.

I’ve noticed that a lot of the older effects units from the ‘60s and ‘70s - the ones that were typically large units that sat on top of the amp and plugged into the wall for power - not only had great effected sounds but they also added that something-something to your basic tone. Even without using the effect they made the guitar sound bigger and warmer and somehow more magical and complete. I’m talking about things like all the old classic echo units - Echoplex, Binson Echorec, Univox, Tel-Ray, Klemt, Space Echo, etc. And also units like the Univibe or some of those big Maestro phaser shifters. Stuff like that. They really added character to your basic guitar sound but they were big, bulky, and often needed constant maintenance.
Of course, the EP-3 Echoplex became a standout in this area with a lot of people taking the preamp section out and trying to make a standalone preamp pedal with it. What made the EP-3 preamp special? It somehow made the guitar sound fatter and “boosted” feeling even though it didn’t really boost. I remember a long time ago in the ‘70s as a teenager I asked a local guitar hero what that big black box was on top of his Marshall half-stack. He told me it was an Echoplex but that he only used the echo function on a couple songs. But, he emphasized that he always had it as part of his signal chain because it made his guitar sound better and easier to play, even with the echo off. Remember that these units were not “true bypass” - when the effect is off your guitar signal is still going through the preamp circuitry. His comment left an impression on me though - how could an echo unit make the guitar easier to play even when the echo was off?
In recent years, I took it upon myself to analyze those circuits and figure out what was creating all the mojo. The main thing I instantly noticed with all these plugged-into-wall units is that they ran on much higher voltages than your typically guitar pedal. While the majority of pedals use a 9 volt rail these big units would typically have 15, 18, 22, 24 or more volts on the preamp gain stages. Also, since these units had built-in power transformers they also had their own power supply filtering schemes. These are the larger can style electrolytic capacitors you’ll find under the hood. Power supply filtering is there ostensibly to provide a rectified DC power source with acceptable ripple, but the whole rectification/filtering circuit also has a major affect on the playing feel of the audio stages. It can determine how firm or how soft the playing feel is. It can determine how much bass response there is. And then when I peeked inside an EP-3 I found some nice, big coupling capacitors in there, unlike the tiny little ones you find on most modern pedals. Do they make a difference? Yes. I’ve done extensive breadboard comparisons over the years and the different types and makes of capacitors play a significant role in the sound and response of a circuit. The third thing I noticed was that the preamp circuits were quite simple, with the primary preamp circuit being comprised of a single JFET, biased in a particular way and fed from the EP-3’s particular power rail scheme. JFETs tend to have a warm, tube-like response and do just the right thing to your signal before it hits that first tube in your guitar amp. So, what you find in these old units are a) higher voltage rails and DC rectification/filtering circuits, b) high quality capacitors, c) relatively simple all-discrete audio paths. There’s other stuff too, like the passive mixer stages loading down the output, and more, but for now, you get the idea!

When I started working on my take on the harmonic tremolo sound I kept coming back to “how do I get that plugged-into-wall sound but without the hassle of a large unit and having to plug it into the wall?” Well, every effect unit needs audio path circuitry so I tried using preamp blocks based off of the classic EP-3 JFET preamp for the audio path. They worked so well I ended up using three of them to create the harmonic tremolo sound. That’s right, I used three EP-3 preamps! And since I wanted something that was convenient and would work well in a modern pedalboard setup, I chose to run it from a standard 9 volt DC supply but internally convert that to a highly filtered, zener-regulated 24 volt power rail to mimic the response of those old plug-into-wall units. Basically, I wanted to have my cake and eat it too! I wanted the sound and feel of the old classic plugged-into-wall devices without the inconvenience of them. And when I started working on the LFO circuit to achieve the harmonic tremolo modulation, I tried all kinds of LFO circuits, and wasn’t feeling any of them. They had limited range and didn’t have the magic. One day for shits and giggles I decided to rig up the LFO from the classic Univibe circuit. I didn’t think it would work since that circuit is designed to drive a light bulb and not as a way to modulate the gain on a JFET preamp. And guess what, it indeed didn’t work. But I decided to be undaunted and to keep messing with it. It was like Frankenstein’s monster what I was trying to do, stitching parts from one beast to another lol. Finally, after a few weeks of experimentation, I found a way to make it work. Eureka! OK cool, the monster is alive. Now that I had my basic circuit architecture I could relax and begin to do what I do - tune the fuck out of it. I spent over the course of a year tuning and refining the sound and response of it until it sounded and responded the way I wanted it to. Part of the recipe was to measure and select JFETs for each of five JFETs in this circuit. It’s a bit more work and cost to do it this way. I could have just used some generic op-amp gain stages to create this pedal but I’m not willing to do that compromise. If I’m going to design pedals for a living I want to design awesome pedals that bring the magic.

Thanks for reading this. May inspiration strike you and carry you forward on your musical dreams!

Howard