Clearglass - some new glass for your bass

From the vendor

Clearglass - some new glass for your bass

May 2, 2022 10 min read

Updated: Mar 24, 2025

I'm a fan of the sound of the popular Darkglass pedals, and about a year ago I engaged into making the Fairglass, my first re-adaptation of the VMT and B3K circuits. The Fairglass is still a great sounding pedal and I can't really pick favorites between it and these ones, but the noise could be better even if it's acceptable, and I think I can be more accurate too, while preserving the original goal of making the circuit more compact without compromises. The real hold-back was the inverter chip, both for the unavoidable noise and because I can't accept using only one section of a 16 pin chip. There are many other things I wanted to do to the circuit too, reason why I had to find a way around the inverter first. I've already shown a prototype of the Clearglass before, which is to say an alternative circuit that tries to substitute the CMOS clipping with something else, and here it is brought to a point I'm finally satisfied with.

Let's start with the schematics for both so you can stack up on questions before I try to answer them one piece at a time:

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The mu-amp and the notch

My first challenge was to replace the mu-amp stage and following Hall notch, both to make it simpler and to save on JFETs, which have become either too small or too expensive. The main thing to notice here is that the notch is an heavy load on the amplifier output, and they behave completely differently than each taken in isolation. This gives some interesting results regarding gain, headroom and frequency response, but also made my job trickier because I had to consider them as one unit.

First, I've built this part as in the original and did some measurements with ARTA and Soundcard Scope. Here's the frequency response. These are a bit noisy because I've used a very small input to avoid clipping, although I then tried overdriving it and it led to the same exact results (clipped white noise is still white noise). Here's both the VMT and the B3K in all three modes:

• VMT • Bk3 three modes

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These measurements mostly agree with simulations. As you can see the notch is much more narrower and shallower than if driven by an ideal source. Probing in simulation shows that's because the such loaded mu amp output peaks right next to the notch. Just as interesting, the gain potential of the mu amp is completely gone, and the output is at unity at about 1kHz (red line shows bypass).

I've struggled for a long time to reach a similar response without the mu amp, with bridged T, hall notch, increasing the source impedance, the loading, adding frequency dependent gain, but I couldn't get a notch that was as narrow or as shallow as the original, although I got close, without being overly complex. Then I took a step back and thought “is a notch this narrow even audible or desirable?”. If you account for the encompassing high-pass, the Q of the notch ends up being about 4.5 or m