I’m having troubles understanding this equalizer circuit. I understand there are there is some kind of amplification going on at the first op-amp. Then there are several parallel wires with filters (which I assume are band-pass) that are letting different frequencies through as you vary the resistors.
The way I’m guessing it works is that the variable resistors control how much of the voltage goes over the filters, and thereby controls how much the filter let through. Is that right?
Then the frequencies that are let through the filters are amplified, and then they go through a voltage-follower for some reason.
I’m wondering. Is my understand of the circuit correct? Also, I have no idea what the first capacitor (C1) or what the voltage-follower does.
I didn’t look at this in detail, so hopefully I’m not wrong.
The voltage follower (unity-gain buffer) usually serves to isolate the output and the intermediate stages from each other, which is what it’s doing here - this way whatever’s hooked to the output won’t affect the filter.
There’s also no voltage gain in the first section (the resistor ratio is 2:1 on both the input side and feedback side). I’d say it’s there to provide a high input impedance and filter out some low frequencies. This is what the cap is for. It blocks any DC offset, for one, and with the input resistors is a high-pass filter (probably to block line noise).
The blocks in parallel are all filter stages, yes, although I think they’re set up to be notch filters instead of bandpass. It also doesn’t look like there’s any voltage gain in that stage either (5 50k pots in parallel would be 10k).
qualifier - its been years since I have down any real electronincs, but I would agree the bottom op amps are acting as notch filters - but as they are then passing the signal to the inverting input of the top op amp, this turns the whole stage into a pass filter?
They are wired in paralell, so why notch? That way, if several of them were turned to the full position all frequencies would still be let through, as the frequency that’s blocked by one would just pass through the other filters instead.
First of all, there’s got to be something wrong with that schematic. Shouldn’t the common lead that connects to R23, R26, R24, R27, and R25 go somewhere? Like to ground? I should think so.
Each of the five filters is nothing more than a capacitor and inductor connected in series, thus making a resonate circuit. Don’t see the inductors? That’s because they’re simulated using gyrator circuits. (A “better” inductor can often be made using a gyrator circuit.) In the 100 Hz filter, for example, the inductor/gyrator is formed by U2B, R18, C7, and R23. This inductor is in series with C2, thus making a series LC resonate circuit.
For each of the five filters, the LC resonate circuit & potentiometer are in U1B’s feedback path. Varying the overall impedance will vary the gain. Because each filter is “tuned,” each will apply gain (or attenuation) only around its center frequency. This is achieved by adjusting the potentiometers.
