I recently completed a pedal version of the Alembic SF-2, and I found this thread helpful in the design process, so I’m posting my results here rather than starting a new thread. There’s still no authoritative schematic of the SF-2 available, and I didn’t have an original unit to trace, but I’m pretty confident I’ve come up with a decent work-alike, or so says the bass player I made it for. I’m posting the schematic and a verified vero for the pedal adaptation I built. The original should be similar, with a few additions listed below.
In addition to this thread, I am indebted to the following sources:
Alembic SF-2 specs, description and block diagrams,
http://alembic.com/support/SF2_Manual.pdf
Rod Elliott’s state variable filter (4 op amp version),
https://sound-au.com/articles/state-variable.htm
Universal Filter by Uwe Beis,
https://www.beis.de/Elektronik/AudioMea ... ilter.html
State variable filter for guitar by Vigstrom,
http://blogg.vigstrom.se/2017/11/19/fil ... ic-guitar/
Alembic-like state variable filter schematic,
https://www.musikding.rocks/gallery/ind ... schematic/
The original SF-2 was a two-channel second order state variable filter with constant gain, voiced for bass, rack mounted. It had an instrument-level input with preamp, two line level inputs and two outputs. It had three filter types (low pass, high pass and band pass), adjustable center frequency from 45 Hz to 6 kHz, and adjustable Q from 0.625 to 5. (The Q knob was marked in “reciprocal damping ratio” units of 1.25 to 10, which are just 2 times Q.) A mode switch selected either stereo (A and B channels independent with separate inputs/outputs) or mono (one input, split to A and B channels, which were mixed again at the output.) The clean input signal (called Direct) and filtered signal were mixed at the output using a pot for each. Direct signal level could be adjusted from zero to unity, and filtered signal level adjusted from zero to +20 dB. Power was 10.5 to 19.5 VDC, presumably regulated and inverted for a dual rail supply.
In my pedal adaptation, I’ve kept the instrument level input, associated preamp, and one output, eliminated the A/B line level inputs and buffers and the second output, and retained everything else. The input preamp gain (up to 20 dB) is pot-adjustable rather than the 3-position switch in the original. The mono/stereo switch now selects either channel A, or both A and B mixed, there’s no independent use of B. This A/B switch is a 3PDT toggle, the three sections are widely separated in the schematic and marked “A only” or “A+B”. The odd routing of the clean signal in A+B mode is retained from the original SF-2. I also added a notch filter for a total of four filter types, selected by a 1P4T rotary switch. Power supply (not shown on schematic) is +18VDC, regulated and inverted to +/- 15VDC (the max recommended for the op amps) with a total current draw of 80 mA including a 1 mA LED. Bypass switching is true bypass.
- Schematic of pedal version, page 1 of 2
- Schematic of pedal version, page 2 of 2
The nonstandard R and C values in the filters are made up by hand-selecting two series resistors or two parallel caps for each. Those caps should be high-Q, COG or similar composition. The filters have the same frequency range (45 to 6k Hz) and Q (0.5 to 5) as the original. The Q boundaries are set by R4 (low) and the value of the Q pot (high). The impedances within the filter are workable for TL074 op amps throughout the adjustment range of the F and Q pots. I started with 50k Frequency pots, but that put R7 & R8 around 392R which seemed low. Using 100k pots for Frequency and scaling the caps gave me a minimum op amp load of 755R which seemed safer.
The tapers of the pots were selected for intuitive response and to avoid bunching-up at either end of the range, as much as possible. Frequency pots are dual, and I could find slightly better matches between the two decks with B taper but the spread is much better with C taper. The Frequency pots were hand-selected for the best match between decks. The labels on the Freq pots are a little off the actual settings, 45 and 6k are correct, but 440 is actually 1k. I forgot that the tracks on reverse-log pots are actually multi-segment approximations… but there’s a chart in the user doc that gives the correct settings.
- SuperFilter Guts 1 of 2
- SuperFilter Guts 2 of 2
- SuperFilter Front
I put this in a 1590XX enclosure to ensure plenty of working room, but it could easily be reduced to a 1590BB with a few changes. I think I’d swap the Q and Freq pot positions next time, since the Freq pots are taller and they prevented easy mounting of the main vero to the backs of the other pots. The Filter Type rotary switches are C&K A10405RNZQ 26mm, but any 1P4T will do. Knobs are 15mm and pots are 16mm Alpha. Since this was a one-off, I didn’t design a PCB, but you may be interested in doing that. The vero could also have a smaller footprint, you could integrate the preamp vero, etc. I like to keep charge pumps away from signal leads, so the power here is on a separate vero in the corner. I’ve been told my power protection and filtering practices are overkill, so you could simplify that portion. The heat sink of the MOSFET polarity protector is common with the drain which is +18V, so if you use one, insulate the tab with some heat shrink. I wasn’t sure if the final current draw would be over or under 100 mA so I used a 1A 15V regulator, but you could probably get away with 100 mA. I regulated the 18V down to 15V before the inverter. Q3/R2 on the power board prevent the op amp loads from pulling pin 5 of the LT1054 positive on startup. The circle around C1 is a grounded cylinder of aluminum tape to suppress square wave radiation from that cap, it’s very effective and eliminates any need for shielded signal wiring, the noise floor is very low. The LED +18V supply includes an RC circuit (Rpop, Cpop) to slow the LED turn-on and reduce popping.
- Main and preamp vero
- Main and preamp vero cuts
- Power vero
- Power vero cuts
- Switch wiring diagram
About The SuperFilter Dual Bass Filter pg1.pdf
