I'm working now on the Quad VC Equalizer/Resonator. Already finished simulation works.
Your comments/critique are welcome.
I used as reference 'Digitally-Controllable Audio filters and Equalizers' paper of Gary K. Herbert and Fred Floru.
http://www.thatcorp.com/datashts/AES13- ... ilters.pdf
The device has four identical bands with BP filters. Also first band is switchable to HP mode and fourth band to LP to make low and high shelves. Also each band is switchable to boost or cut modes.
Every parameter has voltage control, also I plan master CV ins for all bands.
Controls are:
FREQUENCY is peak frequency of a band. Range is 10 octaves. V/oct responce and initial frequency are trimmable with trimpots. CV range is 0...10V.
BANDWIDTH range is from 2 octaves to 0.06 octave at 3dB point. CV range is -5....5V.
LEVEL range is 14dB. CV range is 0...5V.
LM4040 precise voltage source for all offset voltages.
Temperature compensation.
Filters are based on hi-end quality THAT2180 chips. VCA for resonance gain (level control) is discrete.
Chips used:
8 x THAT2180
2 x THAT300
3 x TL074 (for CV)
12 x dual opamps for audio. I will use OPA2604 but TL072 possible.
24HP Eurorack.
Here are simulation plots:
BANDWIDTH
from -5V to 5V CV in 1V steps. Level CV is 5V. Freq CV is 5V.
Simulation is made for 5V amplitude modular input signal level (14dB).
I didn't changed level for cut mode but for boost mode I decreased level from 14 to 0 dB to have headroom for boosting.
cut:
boost:
LEVEL
From 0 to 5V CV in 0.5V steps. Bandwidth CV is -5V. Freq CV is 5V.
cut:
boost:
FREQUENCY
From 0 to 10V CV. Bandwidth CV is -5V. Level CV is 5V.
I expect a bit of selfoscillation on high frequencies for thin bandwidth.
This is plots for 3 last volts of frequency and 3 last volts of bandwidth.
LOW AND HIGH SHELVES
They are not shelvig filters really but HP for low self and LP for high shelf.
So if we want to cut low freq, we need to swith in boost position.
We can very effectively cut (or boost) low (or high) freqencies. Bandwidth setting changes slope in this case.
On the plots bandwidth setted to min, middle, max (-5V, 0V, 5V). Freq setted to min for low shelf, to max for high shelf.
TEMPERATURE COMPENSATION.
I made temperature compensation for the filters, it affects to frequency and bandwidth.
But I didn't made tempco for level. It makes 1dB loss from 25 to 75 °C. I think it's not so important.
Plots for 25, 50, 75 °C.
Without tempco:
With tempco:
...but thats me 