Steiner VCF Tweaks DIY
2004-01-02 by John Loffink
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
[Non-text portions of this message have been removed]
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
[Non-text portions of this message have been removed]