A Double Dose of Preamp Gain

Early in its history Electro~Harmonix produced a very popular guitar preamp/booster, the LPB1. One use for this pedal is for clean gain in front of a tube amp to drive the preamp tubes to clipping. You can also use it to boost the input into your effects pedals. This is particularly useful if you want to get the opamp stage of your distortion pedal to clip. Overall, this is a great little booster and if you put two in series you've created an interesting sounding distortion pedal with lots of tonal variety.

I built two of these as a mirror image on a Radio Shack protoboard. I used polyfilm capacitors, since they have better tonal qualities than ceramic. The build was based on a schematic found at Tonepad.

I used a DPDT toggle switch so I can bypass one of the preamps if I just want a clean boost. Both have their own volume pot. When I run the single preamp stage into my Crate V-Series tube amp it overdrives the first stage tube buffer circuit in the amp. Even at low amp volumes this produces the rich tube amp distortion that I've been trying to coax from my amp.

When I connect the preamps in series I can get a variety of distorted tones that depend on the setting of the first stage volume pot and the volume pot on my guitar. One unexpected and very cool characteristic of this double effect is that when I have the volume pot of the first preamp stage between about 2/3 and full volume I get some subtle phasing effects that gives the transistor clipped tone a rounded and mildly vowel like sound. When the volume pot on the first stage of the preamp volume pot is around 1/3 it distorts the second stage with a mild coloration of the tone that works well even with open chords.

Warning: If you wire two of these in series and have the volume pots on both set to full, you will get tons of gain. Take care not to blow out your amp or even worse your ears.

Welcome to Tonevelope

Today I finished my build of a modded Dr. Q envelope filter. A subtle envelope filter (auto-wah) that adds some nice filter sweeping modulation controlled by the volume of the incoming signal. The build was based on the schematic found at the AMZFX site and built around a board I purchased from General Guitar Gadgets.

After building it I ran across an article about envelope controlled filters by Mike Beigel and Elliott Randall. Yeah, the Elliot Randall of Steely Dan Guitar wizardry fame. He's not only an amazing guitarist, but also quite an accomplished sound engineer and techie. The most interesting point in the article was that an envelope filter works best when the original signal of the pick attack is presented to the effect. Turns out that when you use a distortion pedal, or most any other effect before an envelope filter , you've flattened the signal and lost much of the dynamics that you created with your picking. After a brief explanation of how an envelope filter works, I'll tell you how sound engineers get around that problem.

The first part of an envelope controlled filter is an envelope follower that generates a control voltage that follows the volume contour of the input. The control voltage is then used to control the filter stage of the envelope filter. Ideally the other effects in your chain, especially distortion pedals would be inserted between the envelop follower and the filter section of the envelope filter. That way you generate a control voltage that more closely follows the dynamics of your pick attack. Turns out that that's what professional studio quality envelope filters do. Unfortunately the envelop filters found in guitar pedals effects don't have an effect loop insert between the two stages of the envelope filter.

So, what's a DIY'er to do but further mod the modded circuit and see if he can place an effects loop insert between the envelope follower and the filter section of the envelope filter. In an upcomming post Ill let you know how things went.