Could someone help me understand max cut/boost in this EQ

Hello,

What sets the max cut/boost in this circuit? Would've thought R5 would be important but what other resistor(s) is it working with.

Many thanks.

try R4

That looks like a simple parametric EQ.

The top 3 op amps form a state variable filter...

The 10k dual varies the R for the two integration stages of the SVF that set the two BP filter poles.
The VR2 10 varies the Q or bandwidth of the  EQ
VR1 varies between boost and cut by panning between the input and output of the inverting stage U1A

The BP output of the SVF is feeding the U1A + input. The BP output of the SVF is unity gain inverting ,

It is easiest to understand in full boost mode, An inverted version of the input signal is appearing at the + input. So
for +1V at input, + input is at -1V and op amp output will be at -3V, so this is roughly +10dB boost.

When set for cut the bandpass is fed from the output and reduces the gain a similar amount.

JR

So if I understand it right, R5 sets the gain for the BP out? Which in turn increases or decreases the max cut/boost?

FETlife said:

So if I understand it right, R5 sets the gain for the BP out? Which in turn increases or decreases the max cut/boost?

Click to expand...

The SVF BP gain is set by R4 and R5 similar to an inverting op amp stage. The overall Boost/cut is also influenced by R1 and R2.  Note: if you change the BP gain that will also shift the Q or bandwidth of the SVF (higher BP gain will track narrower bandwidth IIRC).

Back in the '70s I sold a parametric EQ kit and I incorporated my Q pot into the SVF BP gain, so that the amount of boost/cut varied with the Q pot. This was nice for equalizing full range program sources as there was a first order correction  for loudness change when you broaden or narrow boost/cut. Narrow bandwidth delivered more boost/cut, while broad bandwidth delivered less boost/cut. I did not include this Q/boost-cut interaction in the professional parametric EQs I did.  Since it is not desirable for equalizing single instruments.

JR

Thanks John, what still confuses me is that if R1, R2, R4 and R5 are all 22k-doesn't that imply unity gain?

Suppose what I'm getting at is, how to calculate the gain  given the above values? Or is it a bit tricky and easier to just build it and measure?

FETlife said:

Thanks John, what still confuses me is that if R1, R2, R4 and R5 are all 22k-doesn't that imply unity gain?

Click to expand...

As I tried to explain in my earlier post the boost cut is the combination of the SVF BP gain and and that BP output fed into the + input of U1A.

The gain of U1A for signal input to the + input is 2x not unity, For full boost you get that 2X plus the 1x normal input, for 3X.

Suppose what I'm getting at is, how to calculate the gain  given the above values? Or is it a bit tricky and easier to just build it and measure?

Click to expand...

Should I type more slowly...  ;D

You need to visualize the signals. When the input to U1A goes up 1V and the boost pot is full left the BP output goes down 1V. Now the input r R1 is dropping 2 V from +1V down to -1V where the - input is. Since R2 is the same value it will drop another 2V to cancel that current, making the output -3V.  Note: this simple DC analysis ASSumes an AC signal within the SVF BP frequency.

JR

Aaaaaaah I'm with you now, thanks ever so much.