Alembic - SF-2  [schematic]

Tube or solid-state, this section goes to eleven!
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Ferniff
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Post by Ferniff »

mauman wrote: 16 Jun 2022, 05:04 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?

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mauman
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Post by 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.

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Jarno
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Post by Jarno »

This was under my radar :D

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.
"It crackles....., but that's ok"

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Post by 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.

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