Single knob [unidirectional] band pass filter?

[Rob Strand]

FWIW,

What you sometimes find is someone bends things around a bit to simplify the circuit,  perhaps not working 100% like it but good enough.

[Andon]

FWIW,

What you sometimes find is someone bends things around a bit to simplify the circuit,  perhaps not working 100% like it but good enough.

I appreciate the idea, Rob! I'm trying my best to understand how to achieve this using an active circuit, but pragmatism and Occam's razor may get the better of me.

Going backward, you can maybe lose the coupling cap from the first stage - but the hi-pass filter cap breaks the DC bias into the second amp, so the 100k reference resistor still needs connecting to 4.5v.  If that filter cap wasn't there, yes you could bias from the output of the first amp and not need the reference 100k resistor either.

In short, you don't need a cap on Rin but Rin must go to 4.5v, never ground.
When you have a cap after that resistor, it's either because it made an easier board layout to use the ground instead of 4.5v, or you want a hi-pass filter effect on the gain the amp adds. Have a look at the Proco Rat circuit to see this being exploited in a more complex way. The cap if used can either go to 4.5v or ground, but if you need a cap and the value is so high it would be a polarized electrolytic type, the ground is the best choice as the negative plate of the capacitor will be on the most negative point so can't get a reverse polarity voltage that can damage or degrade it.

Thank you, Jim! So, revision number whatever:



Does that seem right? Can the HPF resistor go to Vref, or are you saying to add another 1M to Vref after R2 and leave R2 to ground? I also changed the Rin (R5) to Vref based on what you said about how it should never go to ground, unless that's incorrect here.

I also found this example that splits the HPF and LPF across the input and output of the opamp:



There are a few other examples given at the source: Active Band Pass Filter

[Rob Strand]

I appreciate the idea, Rob! I'm trying my best to understand how to achieve this using an active circuit, but pragmatism and Occam's razor may get the better of me.

No problem.   It's a good exercise getting the opamp version to work.    The problems you have seen will come-up again in the future.

My only comment is we don't know the current circuit sounds good enough.

It's quite possible the circuit Fancy Lime mentioned might work-out better.  This circuit won't be without practical issues though because you want the bass to drop when the treble is rising.   You need one gang of the dual lpot to increase while the other is decreasing.  That's going to screw-up an audio taper pot set-up.    That leaves a linear pot as the easy choice however a linear pot might not work well over the large range of frequency adjustment.   The control will get cramped (maybe my ckt will to  ).    Ideally the pot would be a ganged log/reverse-log but you won't find one of those in a hurry.

(I have a circuit which behaves like Fancy Lime's circuit except it only requires a single gang pot.   The idea behind the circuit is very different to what Fancy Lime outlined.  I'm not putting it up because it needs some care in the design which I haven't looked at closely.)

[anotherjim]

Yes, that's all right.
Let's have a look at the influence of those 4.5v connections.
R2 is setting the amp bias voltage level. Whatever voltage that is, the amp will always copy to its output. When signal happens along at the input, it adds and subtracts its voltage to the reference and that also appears at the output. If the bias connects to ground (that is also the amp negative supply in our case), then the output biases at ground and the signal cannot swing negative of that so you get severe distortion. Using 4.5v, in theory, means it can work with signal voltage swing anywhere between 0v and +9v. In practice, most op-amps can't work the whole way either at the inputs or outputs.  Biasing at half of the total supply voltage gives the best compromise but you can find examples where a different reference voltage is chosen for special reasons.

R5 is a lot different but only because the voltage at that -input pin will be inverted by the amp. Connecting to ground would send the output voltage higher than 4.5v depending on the ratio of R5 and R4 (which is 1) so it will rise by +4.5v and try to hit +9v. Again the amp is forced into severe clipping. So let's connect R5 to 4.5v. Now it's telling the output to be 4.5v. I think of the -input as the pivot of a seesaw and the Rf and Rin values are how far the ends are from the pivot.
This is key information for troubleshooting. All 3 op-amp pins should read close to the reference voltage (4.5v).

[duck_arse]

Yes, that's all right.

R7 and R5 both show in two places.

[Andon]

The pivot point analogy makes total sense - thank you!

My only comment is we don't know the current circuit sounds good enough.

I'll be breadboarding this later today hopefully and will report back - I just haven't had much time for actual application as of late.

Yes, that's all right.

R7 and R5 both show in two places.

Ah ha, "R7" is actually R? because they hadn't been annotated yet, and the second R5 (1M to ground after the blend pot) should be R6 now, just hadn't updated the annotations. Good catch!

[Rob Strand]

I'll be breadboarding this later today hopefully and will report back - I just haven't had much time for actual application as of late.

Don't be afraid to play with the cap values to tune it up by ear.

If you find you need to increase the high-cut cap and/or decrease the bass-cut cap by more than a factor of two of the starting values you might consider dropping the 10k  "R1" value to 4k7 and doubling the cap C1.   That will prevent too much gain drop at the center frequency.

[ElectricDruid]

Shouldn't there be a cap between the blend pot wiper and the 1M to ground?

There's 4.5V across that 1M resistor and without the output cap I don't really see what good it is doing there. Plus you'll get a god-awful DC thump whenever you plug anything into that output without the cap.

[Rob Strand]

Shouldn't there be a cap between the blend pot wiper and the 1M to ground?

There's 4.5V across that 1M resistor and without the output cap I don't really see what good it is doing there. Plus you'll get a god-awful DC thump whenever you plug anything into that output without the cap.

I guess the other way to spin it is the 1M output resistor should return to 4.5V.

The right thing to do depends on the circuit context the added circuit appears.