Alembic - SF-2 [schematic]

[Ferniff]

I forgot to provide the bench-test frequency response of the clean (unfiltered) signal in the pedal adaptation. It's quite flat. With a 200 mV p-p input, relative to 1 kHz, with the Preamp level knob fully counterclockwise (gain = 1), the -1 dB points at the output are 13 Hz and 41.7 kHz. With the Preamp level knob fully clockwise (gain = 10), the -1 dB points are 14 Hz and 26.5 kHz.

To be clear, is the Q factor is what Alembic calls what would normally be called resonance on synths?

[mauman]

Welcome to the forum! Q factor is the ratio of the center frequency to the bandwidth. In a filter, if you narrow the bandwidth, you increase the Q and create a sharper and more audible "peak" in a bandpass filter or "valley" in a notch filter. Higher Q = higher resonance = lower damping. So yes, Q and resonance are related, and are inversely related to damping. In the SF-2, Alembic called their Q control "reciprocal damping ratio" which is just 2 x Q.

[Jarno]

This was under my radar

Pretty cool, nice work!
I made a dual filter preamp for my bassguitar, but I still need to verify the PCB (I couldn't get the first iteration to work, so I did a board spin, and also went for mostly SMT, still need to verify it). It uses the "CAMPUS" filter tone control (Alembic work-alike) times two, each pickup going into a separate filter, with a mixing stage at the end (I think 10 opamp stages in total, not sure how long the batteries will last..... ). I did not include a noise cancelling system with a dummy coil like on a series model Alembic.

[mauman]

I got a PM question about how to decrease the upper limit on the frequency range, which is currently 6kHz. Here's the process if you have a similar need, using Filter A as the example.

To halve the maximum center frequency from 6kHz to 3kHz, double the values of R7 & R8 from 755 ohms to 1.51k ohms. Or if you want a 4kHz upper limit, use a pair of 1.13k resistors for R7/R8. In either case, leave C2/C3 as they are (35.1 nF.)

R7/R8 set the maximum center frequency, and the pot value (100k) sets the minimum. The minimum of 45 Hz won't change more than 1 Hz when you change R7/R8. The formula for the center frequency is [1 divided by (2 * pi * R * C)] where R = [(F7 or F8) + the pot value] which depends on where the knob is set, and C = (C2 or C3). For the upper limit, let the pot be 0 ohms, and for the lower limit, let the pot be 100k ohms.

[twotees]

Just wondering how you would go about voicing the Superfilter circuit from mauman above to suit better the Mid frequencies and higher Frequencies associated more with a 6 string electric guitar? Thanks

[mauman]

Hi twotees, I think it would work fine as-is with guitar, I tested it with guitars while I was building it. The center frequency is adjustable from 45 Hz to 6 kHz, which is wider than most guitar EQ's. Guitar frequencies run from ~ 82 Hz on the low E string (which the 45Hz will cover) to ~ 660 Hz high E string on 12th fret. Even if you triple the high end to allow for harmonics you're still at less than 2 kHz, so a 6 kHz range should be more than enough. If you want to reduce that top-end range to 3 or 4 kHz, the previous post has details on which resistors to modify.

[twotees]

Sounds good maumon, I figured those frequencies you put up were pretty right for 6 string, was just curious if you thought otherwise. Thanks for all the work you did putting this design together, It looks interesting and effective, and time permitting hoping, I might get a chance to put it together..
Cheers, Trevor