Op amp filter as output buffer?

[Dr Bliss]

Hey guys, nice to meet you, my first post in an electronics blog ever...
I have a fuzz pedal I'm working on for which I used an inverting input buffer because I liked the sound better... Therefor, I also installed an inverting output buffer to flip the phase back to it's original position.

I'm happy with the overall sonic result but want to add a high pass filter. So my question is: Is it possible to use the inverting side of the op amp to design a filter? Or do I need to build the filter section into the existing circuit with a third op amp?

Thanks,
Ariel

[thermionix]

With no schematic, I'm using my imagination.  It sounds like you're talking about IN buffer > Fuzz circuit > OUT buffer.  There's a very good chance you already have one or more high pass filters already in the circuit, in the coupling between stages.  If you want to shave off more bottom end, it may be as simple as a smaller cap somewhere.  But where in the chain do you want to reduce the low frequencies?

[antonis]

I used an inverting input buffer because I liked the sound better...

The above statement, by its own, precludes the absence of "some" kind of filter..
(meaning, the obvious reason for you to like the sound better is "tonal" matter of taste..)

If you are sure your buffer is inverting (no hard feelings, just to make sure we're using same terminology..  ) there probably is some coupling cap creating HPF with inverting (-) input resistor (R_G, which should be of equal value with R_F, the feedback resistor from output to inverting input..)

You may tweak that cap value to set your preference cut-off frequency (1/(6.28*C*R_F), either on Input or Output buffer..
(there should be different tonal results according to HPF placement in the sense of already filtered signal distortion vs already distorted signal filtering..)

But, as thermionix already said, post a schematic..

P.S.
Wellcome..!!

[MrStab]

hi Ariel,

just to add to what Antonis is saying, if your fuzz stage has a volume pot on the output, turning this pot will add to the inverting buffer's input resistance and affect the gain, which may affect the response of any filter in the buffer. it's worth being aware of.

also, i can't remember the equations just now, but if you place a series resistor and a capacitor in parallel with the feedback loop resistor, you can vary the amount of gain for specific frequencies while letting the rest through. you can also do this on the input and compensate on the output, but that would probably affect the response of the fuzz stage.

is it possible that it wasn't the phase inversion you like about the sound, but the large input resistance on the first inverting buffer? maybe it interacts with your guitar or previous pedals better? inverting amplifiers are more stable but maybe switching to a non-inverting input could simplify things a bit.

[Dr Bliss]

Thanks for your answers guys,
I realize I didn't give enough details...
At the moment the circuit consists of In buffer > Fuzz > Out Buffer.
The buffer I chose for both input and output is this one:


The actual Fuzz is more or less a fuzz face circuit.

I left the buffer values as they are in the schematic except for a 2.2uF rather then the 10uF leaving the Input buffer and 0.1uF after the output buffer going to a 1M VOL pot.

Also, maybe I should mention that I'm using this with a bass guitar.

I want this to be a variable filter which from my research seams a bit tricky.

My intention is to add the hp filter right after the fuzz circuit. And if it makes sense to combine the functions of the inverting output buffer I'm using at the moment with a Sallen-Key type filter to kill two birds with one op amp.

is it possible that it wasn't the phase inversion you like about the sound, but the large input resistance on the first inverting buffer? maybe it interacts with your guitar or previous pedals better? inverting amplifiers are more stable but maybe switching to a non-inverting input could simplify things a bit.

I A/B'd the two buffer options with my bass and an amp and the inverting one sounded like it preserved more of the high frequency range so I picked the inverting option. I'm not sure what the reason is, it just sounded better to me.

also, i can't remember the equations just now, but if you place a series resistor and a capacitor in parallel with the feedback loop resistor, you can vary the amount of gain for specific frequencies while letting the rest through. you can also do this on the input and compensate on the output, but that would probably affect the response of the fuzz stage.

Do you think that this would be applicable in the buffer I'm using by changing the 5nF cap on the feedback loop?
If so maybe I could do a rotary switch utilizing a few cap values to achieve a somewhat variable tone control?

Thank you all again for your time.

[antonis]

I left the buffer values as they are in the schematic except for a 2.2uF rather then the 10uF leaving the Input buffer and 0.1uF after the output buffer going to a 1M VOL pot.
Also, maybe I should mention that I'm using this with a bass guitar.
I want this to be a variable filter which from my research seams a bit tricky.
My intention is to add the hp filter right after the fuzz circuit. And if it makes sense to combine the functions of the inverting output buffer I'm using at the moment with a Sallen-Key type filter to kill two birds with one op amp.

In theory, you shouldn't have any problem..

BUT... As you can see, you'll have 4 additional HPFs (other than those of Fuzz Face circuit..)
These are (Buffer 1): Input Cap/Input resistor, Output cap/FF input impedance, (Buffer 2) :Input cap/Input resistor, Output cap/Input impedance of whatever comes next..
(IMHO, many unpredictable phase shifts..)

also, i can't remember the equations just now, but if you place a series resistor and a capacitor in parallel with the feedback loop resistor, you can vary the amount of gain for specific frequencies while letting the rest through. you can also do this on the input and compensate on the output, but that would probably affect the response of the fuzz stage.

Do you think that this would be applicable in the buffer I'm using by changing the 5nF cap on the feedback loop?
If so maybe I could do a rotary switch utilizing a few cap values to achieve a somewhat variable tone control?

Feedback loop cap value is 5pF..
(not 5nF - or you should have a LPF of cut-off point at 3.2kHz instead of 32kHz, which is the point set by specific buffer designer - greetings Jack..!!   )

That cap and feedback resistor R2 form a LOW-pass filter of 1/(6.28*R*C) corner frequency (-3db cut-off)..
(highs pass more easily through that cap so they are by-passed from R2 hence left out of feedback loop gain - they are amplified less than lows..)

The opposite stands for Input cap and R1 - they form HIGH-pass filter, of corner frequency calculated by the same formula as above,  in the mean of highs pass more easily through R1 hence they are amplified more than lows..)

The above two cut-off points form a BAND-pass filter, if LPF cut-off point is set lower than HPF cut-off one..
(or band-REJECT filter in reverse case..)

You can vary those cut-off points either by varying cap values or resistor values..

In your case, it would be advisable to vary cap values (input cap value for only HPF..) to keep buffer's ratio (R2/R1) close to unity..

[Dr Bliss]

Thanks Antonis!
I'll play around with the feedback and input capacitors on the output buffer and see what I come up with.

Surprisingly though, I'm not missing the high frequencies due to the 3.2kHz cutoff so possibly it's a good low pass cutoff point for my fuzz after all.

[antonis]

Surprisingly though, I'm not missing the high frequencies due to the 3.2kHz cutoff so possibly it's a good low pass cutoff point for my fuzz after all.

Obviously, nobody reads what I post  or everybody take my word for calculations accuracy..

-3db cut-off point of 1M/5nF Low-pass filter is 32 Hz (and not 3.2 kHz as above mistakenly mentioned by me..)

So, if you use 5nF with 1M feedback resistor, you loose almost all frequencies for bass guitar..
(anything higher than E at 7^th fret and A at 2^nd fret will exibit more than -6db attenuation..)

I can't say how audible will be at low frequencies, due to unity gain buffer, but you'll surely have significant attenuation at high frequencies..

In your case, I shouldn't use any cap of value higher than 47pF (cut-off lower than 3.4 kHz) for LPF - but, after all, your ear decides..