Digitally Controlled Analog EQ

[Bo Deadly]

I've been thinking about using a high voltage CMOS gate chip (ADG1414) to select inductor taps and capacitors in a resonant filter to make a EQ based on the following circuit:

This is a very simple circuit. It's just like the API553 but CMOS gates would select the tap of an inductor like VTB9050 or similar and gates would also select some combination of capacitors and thereby make possibly many frequencies depending on the number of useful capacitor combinations.

The ADG1414 has pretty good specs to +- 15V. Distortion is low as long as the load is relatively high which in this circuit I think it should be 5k so it is.

However, there is one possibly fatal flaw. The voltage at the intersection of the inductor and capacitor can be quite high at the resonant frequency (let's call it the "flyback" voltage). At max boost, at the resonant frequency and with a high Q, the voltage can reach many times the supply (100V+). This is a deal-breaker because the protection diodes on the gates will just clip the signal at +-15V.

If I limit the boost to 12dB, and use resistor values as shown and limit the Q to 4, the flyback voltage is not too high and in theory the circuit should work fine.

So the question is:

is a Q of 4 too low for an eq

or is there a similar circuit to this one that would yield the same result but limit the "flyback" voltage effect

or is there a popular EQ circuit that can compete with the noise performance of an LC circuit and which can be reasonably constructed using conventional panel controls like DP12T Grayhill switches)?

 

[abbey road d enfer]

However, there is one possibly fatal flaw. The voltage at the intersection of the inductor and capacitor can be quite high at the resonant frequency (let's call it the "flyback" voltage). At max boost, at the resonant frequency and with a high Q, the voltage can reach many times the supply (100V+). This is a deal-breaker because the protection diodes on the gates will just clip the signal at +-15V.
If I limit the boost to 12dB, and use resistor values as shown and limit the Q to 4, the flyback voltage is not too high and in theory the circuit should work fine.

The issue of elevated voltage at resonance is a concern when designing such an EQ with simulated inductors (so-called "gyrators"). Being opamp-based, these simulated inductors have the same voltage limitations as the clamping diodes you mention. It doesn't prevent thousands of graphic EQ's to work satisfactorily. One very basic reason is that one generally does not boost a frequency that's already "hot", rather the contrary.
The resonant tanks in 1/3 octave graphic EQ's have a Q of about 8. When used at 12dB boost, the voltage at the inductor (real or simulated) is about 15dB above the input signal. That means that distortion due to any clipping at this point will happen about 3dB before distortion due to basic signal clipping; so there is a risk. But it's not likely to happen if the user is not completely decerabrate (think DJ 8)). Now if you want to have narrower BW, like 1/6th or even 1/12th octave, you may find that boost has to be limited, although in most cases, these narrow settings will be used for notch rather than boost.
In conclusion, I would say the operational possibility of running into overload/clipping at the resonant tank is minimal.

  So the question is:
is a Q of 4 too low for an eq?

It all depends on what you want to do, but using switched elements instead of continuously variable definitely puts your design in the camp of "tone controls", not" surgical instruments", so a relatively moderate Q is called for. 

or is there a similar circuit to this one that would yield the same result but limit the "flyback" voltage effect 

No; this effect  is intrinsically tied to this topology.

 

[ruffrecords]

Classic passive EQs like the Pultec, Helios and  active EQs like Neve tended to have a maximum boost Q around 3.

Cheers

Ian