Another handy little tone circuit?

[samhay]

At little while ago, I had a project, which had a spare op-amp that I thought I might use for a tone control. I wanted a one-knob option, which had as much of a tonal variation as possible (I like somewhat arbitrary design challenges). This is one option I played with that I thought someone might find useful or amusing. It is non-inverting, so it works quite nicely as a standalone booster as well as a single-op-amp module that doesn't load down the preceding circuit. It can also be switched off (to give a flat response at unity gain) by lifting the ground point in the middle of the filter.

It is not, at all, new. It is simply a Twin-T network in an op-amp feedback loop that I 'de-tuned' (for lack of a better word). It uses a single-gang pot, yet the peak gain - it boosts signal rather than cutting it - is fairly constant across the audio range. With 100k resistance across the pot, you have a ~100 Hz bandpass filter and at the other extreme of the pot, you have a treble booster - the simulation below should make more sense than I can. I used reverse log pot so that bass was CCW.

Schem:

Sim (pot at 100k, 30k, 10k, 1k, 1R from left to right):

[garcho]

Nice! Will definitely bread board this within the hour     Thanks!

[Kipper4]

So It's not inverting?

[samhay]

garcho - hope you like(d) it.
Kipper - nope, it's not inverting, which makes it potentially a lot more useful. I did try and turn it into a notch filter by subtracted dry signal from it (or vice versa), but this is a little tricky as calculating the gain for it is not trivial (I think). 

[Kipper4]

I guess you mean the tone is not inverted because I was under the impression that when the signal goes into the negative input the signal will be inverted on the output.
Bare in mind I'm still a fresher and more than willing to learn.
Thanks Sam and I have to say I'd like to implement it in a build sooner or later.

[ashcat_lt]

It really looks as though it is a non-inverting scheme with the + and - inputs mislabeled.  Not sure how the sim actually works, seems to be running open loop.  I would expect the output to slam to one of the rails (maybe the negative?) and stay there.

[garcho]

I did everything in the schematic the way it's written, except coupled the signal to the non-inverting input instead of the inverting input. It works well, bravo. I'll mess around with the values a little just for fun and get back to y'all. The 'switched off' mode seems much louder to my ear than the filter. Strat into a Tiny Giant. Thanks Samhay!

[PRR]

>m I was under the impression that when the signal goes into the negative input the signal will be inverted on the output.

Usually.

In this case, the schematic is mis-marked. The input is to the + input, not - as marked.

[samhay]

Garcho - glad you like it.
As noted, I have drawn the op-amp inputs the wrong way round. Sorry, will update this later today.

Edit - updated.

[samhay]

The 'switched off' mode seems much louder to my ear than the filter. Strat into a Tiny Giant.

I didn't use the swich in the end, so will have to play with this on the breadboard some more. With the switch lifted, you should get a unity gain buffer with an input impedance of ~1M if you use a TL072.
If you didn't use a TL072 (or other FET input) op-amp in your Tiny Giant and/or you have a long cable run, then the buffer may clean up your sound a bit and make it sould louder.

[garcho]

...coupled the signal to the non-inverting input instead of the inverting input.

Just realized my post didn't make any sense.      It would have made sense had I mentioned that I redrew the schematic and swapped the '+' and '-'.

[samhay]

garcho - I figured if you got it working, then that must have been what you did (like the redrawn schematic?). I just put this back on the breadboard and I get unity gain if I lift the ground from the middle of the filter (the switch in the schem).

I had a think about what we could do with the other op-amp if one wanted to use this as a standalone circuit. The filter does some weird things to the phase, and I had to empirically work out what the max gain is, but I have something that might be interesting. I won't try and draw it in Eagle, as I will likely goof again, so here is a simulation screenshot. The second op-amp is working as a difference amp. We subtract our filtered signal from clean to get a notch filter. Because of the phase weirdness (I think), the notches are sharper than the original 'humps'. As you play with the gain of the second op-amp (it now has 2 knobs - a gain/mix pot) you go from clean signal to a notch, to the boost we started with. It works, but the gain/mix pot is a bit funky, Not sure if it's a keeper. If you still have it on the breadboard, it might be worth a try though.

[arma61]

Hi guys very nice little crk here! Have it on my breadboard atm just after a toneless guvnor workalike :-)  the only thing i ve noted is a high volume boost towards the treble part of the pot. is this.a normal behaving in this setup ? from centre to bass side is fine.

thx
Armando

[samhay]

Hi guys very nice little crk here! Have it on my breadboard atm just after a toneless guvnor workalike :-)  the only thing i ve noted is a high volume boost towards the treble part of the pot. is this.a normal behaving in this setup ? from centre to bass side is fine.

thx
Armando

Glad you like it. If you just have the single op-amp version (in the first post), then a volume boost is normal. Probably best to think of it as a booster with a variable frequency response - it doesn't attenuate any frequencies in the audio range. However, you could use a voltage divider to knock the overall level down if you like. The max gain is about 15 dB, so a 40k/10k divider should get you a maximum of ~ unity gain.

[arma61]

Thanks samhay


yes I'm using half opamp here, where do you think I should I stick that voltage devider    ??
Surely not at the output pin as it will knock down also the level when on the bass side....

btw tried also the last version posted, not bad and less or totally missing the volume increase, though I like the first version, seems bass are more "strong" and also because it has only 1 knob  !!

Thanks

[samhay]

I was suggesting you could try to put the voltage divider at the output, but ONLY if you want no more than unity gain. You do get a bass boost, but it will be harder to hear as it is a relatively narrow bandpass. In any case, it was a bit of a through-away comment, and I am not sure I would really recommend this option. However, you could put a volume pot after this thing if you want a but more flexibility, but then you have another pot...

The dual op-amp version was mostly a thought experiment. Glad you tried it, but it sounds like we came to the same conclusion.