Post-Christmas Update!

Posted: January 2, 2013 in Build Your Own, FX, Scallywag, Square Earth Theory
Tags: , , , , , , , , , , ,

Over the Christmas break I’ve been pretty busy playing with 2 new pedals I have been working on. The first I have mentioned a few times before – the Square Earth Theory, which still isn’t working too well to be honest. Here’s the schematic I have so far, trying to filter out the popping is proving to be such an enormous pain in the ass you wouldn’t believe. This uses a FET to switch the signal, in the past I used a 4053 and 4066, with OK results. The 4053 worked the best of the 2 CMOS IC’s, set up so that the signal is shunted to the Vref rail, and is also broken, but the FET seems to be the quietest option when set up to allow a slower switch on time as in the schematic. If you have any luck getting this to work better than I have please let me know!

Square Earth Theory

I have also been playing with an op-amp clean boost circuit that uses a charge pump to internally double the +9v supply run the op-amp that does the boosting at +18v which allows a decent amount of headroom before the signal starts clipping when boosting. I’m pretty sure the Klon used a charge pump to get ~18v without at 18v supply, and a clean boost seems like a great application for using a charge pump.

When you talk about clean boosts everyone throws the term ‘transparent’ around as if it is the holy grail, and it kind of annoys me as what most people are really after when they think of a clean boost is an all around ‘tone fattener’, which is far from transparent! Op-amps get discarded as being slightly sterile when used as clean boosts, but I don’t necessarily agree. Mosfets do tend to be slightly ‘warmer’ than op-amps and do other things to the tone as they boost, but I actually really like the way the op-amps stay so clean (especially when you have a the increased headroom that is provided from a larger voltage than the standard 9v) even when the boost is many decibels.

Anyway, here’s the schematic.


The amount of boost the pedal can kick out is set by the trimmer, which sounds great around +24dB (trimmer set to 8k8). This worked well for the pick ups of my PRS Custom 22 as it allowed loads of boost without any clipping. I made adjustable with a trimmer so that it can be adjusted to suit your guitar, as some with really hot pickups, especially active, may clip earlier. The available range is between 9-35dB. This could have been a pot, but I would rather have it set internally and then just forget about it. With the setup I have here the signal is always boosted by x amount, and then attenuated with the volume control. I think it was Jack Orman (who incidentally designed the Mosfet Booster, which is the basis for many commercially available clean boost pedals) who said that this is like driving a car and always having your foot on the accelerator and jamming on the brakes to slow down, which is a great analogy of what is going on here. I opted to do it this way as in practice it felt more natural to me than adjusting the gain when using it. You could substitute the 100k trimmer (R7) with a 100k pot and do away with the volume control to control the gain directly, which, on paper at least, makes more sense (and actually my original plan). If you are thinking about doing this I would consider changing R9 to 510k, R3 to 10k and using a 500k pot, which will give you around +34dB of gain on tap, but not all of which will be clean.

I think I’m going to call this pedal the Scallywag, I’ll get it built up and boxed asap, and may make a small batch.


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