Oh! Dee Pre 2500

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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asbestosaurus
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Post by asbestosaurus »

A fun variation on the classic D+/250 style circuit that nobody asked for. I like the original but wanted a little more flexibility from it.
2500.PNG
IC1 is a precision Vref supply which I like to include when I can.

IC2 is a super useful variable capacitance multiplier, borrowed from the page on gyrator filters at https://sound-au.com/ you can read more on it there. Here's its used to get a Timmy-style pre-clipping bass control but by virtually changing the capacitor value in the feedback network, as opposed to gradually blending in a larger cap. This works in conjunction with IC3 to recreate the bulk of the original circuit. Is it worth the extra op amp stage? Up to you, I guess.

I've grouped IC1 and 2 together since they're not in directly the audio path and so can be halves of a fairly cheap dual opamp like an LM358, which works just fine on my breadboard proto.

IC3 is the 250 style input and gain stage, with a slight reworking of the drive pot action and values to decrease noise and get a slightly wider gain range than the original. I like the interaction of the high pass filter and the gain sweep so I've preserved that as much as possible with the added versatility of a range of capacitance values without the need for a big rotary switch. I've tried to select values so that a 'stock' sound is achieved with the pot centred.

The clipping section is mostly per the original(s), again just with more flexibility. D+ sounds available with more reliable BAT41s in place of Germaniums, since they behave basically interchangeably. I also included 880 nm Infrared LEDs which have a Vf around 1.2 V and that I like the sound of a lot, still the classic LED sound but at a headroom that's not such a drastic leap from a pair of bog standard 4148s.

IC4 is a bit of extra control and some housekeeping for my own peace of mind. In general I don't like large value volume pots directly on the output, especially with tone controls or other impedance-sensitive parts directly preceding them. As such I'm not a big fan of the originals 100k pot potentially driving a long cable run so I wanted a nice low output impedance. While I was at it I thought at least some control over high end would be nice.

What I came up with is effectively a passive Boss style tone control (think BD-2 or OD-3) wrapped around a negative feedback loop.
This stage has a flat boost of 4 dB, around which frequencies above ~1kHz can be boosted to about 10dB (the 1.5n cap means they peak at about 8kHz) or cut back to unity.
2500 tone-Grapher.png
I've optimised this stage for an essentially flat response with the pot centred, bar some extra rolloff above 10kHz or so. The 390k resistor here is necessary because the op-amp gets grouchy without DC feedback, it's also the lowest value (for noise reasons) I was happy to use before it started rolling off too much bass.

Any dual opamp can be used for IC3+4, try your favourites.
I'd love to hear if you find any of these bits useful for anything!

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