Cgs synth

Nice work, John! I printed out your email for my notebook. This is
really helpful, as I am about to build two Steiner VCF boards.

Thanks,

-Richard Brewster

John Loffink wrote:

>To recap, I built my Steiner VCF and noticed that the entire frequency range
>was covered by knob positions 0 through 5. I also wanted the direct control
>voltage input to track to approximately 1 V/octave. Here are the tweaks I
>did to bring this circuit up to snuff. Your circuit may vary, so proceed
>with caution if changing anything from the nominal values recommended by
>Ken.
>
>Output "level" jumper/resistor: I put 180K in the level position, in
>addition to the 47K already in the op amp feedback path, or just put 220K
>total in the 47K position. This gave the maximum resonant oscillation
>amplitude without clipping and brought the normal non-oscillating output to
>a better level.
>
>FREQ knob adjustment. As stated, my original range of 0 to 5 (out of 10)
>covered 20 Hz to 20 KHz. There are two resistors to adjust for this.
>First, decreasing the size of the input divider resistor on the CV input
>section, 1K in Ken's circuit, increases the range of the frequency control.
>I put 2.2K in parallel with the 1K resistors, or you can swap the 1K with a
>680 ohm resistor. Note that this also affects the Control voltage inputs,
>so they will need to be adjusted. I also added 220K of series resistance to
>the FREQ knob. This tends to center the frequencies on the FREQ knob so 200
>Hz is near the center. With these values the knob range 0 to 10 sweeps
>approximately 5 Hz to 25 KHz. This was measured by putting 20 Hz and 20KHz
>sine waves into the LPF input and measuring the amplitudes at min and max
>FREQ positions. I also tested audibly using white noise as an input.
>
>I wanted my direct CV input (no knob) to sweep approximately 1 V per octave.
>Just realize that the tracking on this filter is not that accurate. I
>measured this by putting the filter into self oscillation and reading the
>frequency with a frequency counter. Note that the filter oscillates from
>about 200 Hz to 6.5 KHz, so that's the only range that was measured. For
>this adjustment I added 148K in series to the existing 220K input resistor,
>or just exchange the 220K input resistor with a 360K. Note this setting is
>dependent upon the above mentioned input resistor divider modification.
>
>For the attenuated CV input, I wanted a 5 volt MOTM envelope to sweep the
>entire frequency range. In the original circuit the range was fine, but
>when I changed the CV input resistor divider the range became too small.
>This requires decreasing the size of the input series resistor from the pot
>wiper. In Ken's circuit it is 220K, I put 300K in parallel, or just swap
>the 220K resistor with a 120K resistor.
>
>Ken, if there is this much variation per circuit you may want to consider
>adding some trim pots to any future revision of the PCB. I double checked
>all transistors, resistors and diodes in the circuit and everything was
>originally built to your BOM.
>
>This is a great filter, by the way, with lots of character. It sounds very
>Moog ladderish with the low pass input, and mixing VCOs into the LP, HP and
>BP inputs gives some great effects.
>
>John Loffink
>The Microtonal Synthesis Web Site
>http://www.microtonal-synthesis.com
>The Wavemakers Synthesizer Web Site
>http://www.wavemakers-synth.com
>
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