MatthisD said:
Any thoughts? what are the flaws with this idea?
The good thing about this idea is that it is a cut only - no boost - which makes it an easier task - it is boost that is the tough one.
The flaws are twofold:
1. The poor man's EQ has its pot and cap values tweaked to work with a 10K:10K transformer rather than the 600:600 one used in the original Pultec so you would need to do the same again for the pot, cap and inductor values for a mid cut.
2. The hi and lo cut circuits actually start working at between 5 and 10 times the frequency setting on the switch. Even so, with the highest lo setting and the lowest hi setting together, there is little interaction between the hi and lo circuits. This would not be so with a mid cut and it would be quite possible to overlap the responses of the mid section with parts of either the hi or lo sections. This may or may not be a bad thing - it might produce some interesting curves or it might just sound awful. That said, it is quite common for three band EQ to overlap in this way so I may just be seeing a problem where there is in fact none.
Despite that you got me thinking and I tried a quick simulation of a mid cut addition to the poor man's pultec and it seems to work quite well. It is a simple series resonant circuit connected from the junction of the hi boost and hi cut pots to ground. I worked out some cap values for a Carnhill VTB9050 inductor which seem to give usable frequencies. One difficulty with mid cut circuits is knowing what value of Q to aim for. Manufacturers seem to go for a higher Q for cut than for boost with about 3 being typical for boost and twice that for cut. With that in mind I worked out two sets of values of caps and frequencies with the VTB9050 one for a Q of 7 and the other for a Q of 3.5.
In an attempt not to compromise the hi and lo circuits or to require additional gain boost, this circuit works in a slightly unusual way. The pot is simply in series with the LC which varies not only the attenuation but also the Q - the Q starts out low and reaches its maximum at maximum cut. I added a fixed resistor in series with the pot to limit the cut to about 14dB like the hi and lo controls. One small problem with this simple circuit is with the pot at maximum there is still a small cut of about 0.6dB. To eliminate this you could add a switch in series with the pot as a sort of mid cut on/off switch. I attach a screen capture of the circuit taken directly from LTSpice and annotated with the calculated values.
I have not looked at interaction with the lo and hi controls.
Let me know what you think.
Cheers
Ian