[EDIT]
Think I ballsed up the Q resistors, hadn't noticed they were log-revlog pots...will post updated version asap....if anyone is interested LOL
/EDIT]
Had a play in the simulator but its behaving funny.
These are the circuit values I ended up with:
[EDIT]
Circuit removed - Q values were wrong
[/EDIT]
Getting the right frequency sweep, did without the slugging resistor (33k) on the Q pot as I won't use a pot, but a switch instead for wide/narrow. Using the simulator I ended up with 8k45 for wide Q of 0.8 and 453R in parallel (would be switched in) for a value of 429.9R and a narrow-ish Q of 3.
Would love to know the math for that. Something feedback related obviously. The changes in positive feedback caused by the Q resistors have quite a large affect on the overall cut/boost range (resonance I guess).
The Dove article claims a variation of +/-1dB over the range of the slugged Q control... so with switched resistors there should be little variation right?
Here is a graph I generated from the LMF values in the Dove article (25 to 500 Hz):
You'll notice a massive loss in overall cut/boost when the bandwidth is set wide. Is this correct for this topology? I thought the CAPS provided variable bandwidth independant of gain.
Buzz Audio use the CAPS topology in their MPE1.1 EQ... these are the graphs from it at two bandwidths, centred @ 1k.
Only a 1dB change in the gain... what's up with my simulator? LOL Maybe the Buzz isn't swinging inputs? How else can the CAPS be used?
These are the sweeps from my LMF values (100 to 3 kHz):
How much of an issue is the rise in H.F towards 1Meg, I ran some transient analysis on 10kHz square waves, wondered about stability, played with the 100pF cap around the swinging input amp, didn't result in much.
Also this thing gets screwy with more bands in.
I tried a 4-band with one CAPS section and three inductive ones.
The slopes looked all skewed, bandwidths all out of line - which I guess is due to interaction of the nearest bands.
I set centres at 50Hz, 500Hz, 3k2 and 10k... Dove mentions using centre-tapped pots for minimising this but there is a noise penalty.
For a four band EQ do you think it would be best to run 2 opamps and have a 2x2band cascaded stage to minimise overlapping bands?
Put LF and HMF on one and LMF and HF on the other?
Cheers Tom