What to do with an extra op-amp?

[soggybag]

Here's a Rat like circuit cribbed from the 1981 Inventions DRV. The original uses a charge pump and runs on 18V. I figure with the diode clipping there isn't any use for a charge pump so I made my own version without.

If I use a TL074 there's only 14 pins but I end up with an extra op-amp. What to do with this?

- VRef buffer?
- ?

[GGBB]

Use it to provide your bias supply. Something along the lines of the following (specific values might vary):

[antonis]

You can use it to make a more "elegant" tone control" (e.g. Tubescreamer style..)

>off topic<
In cace you don't realy need the C2/R15 subsonic HPF (about 3Hz), you can omit both items and bias IC1_B directly from IC1_A output..

[ElectricDruid]

+1 what Antonis said. If you've got an extra op-amp, you've got an opportunity to make a more interesting active tone control. How about sticking a simple two-control active cut/boost on it? (Frequency and cut/boost?) Ok, it won't be an authentic "Rat-like thing" any more, but it might be more interesting, and it'd certainly be yours.

[Fancy Lime]

I would redesign it so that you can ommit IC1_A by including a second ground leg on IC1_B. You know, like the Rat does it. The TL07x series of opamps is plenty powerful enough for that and you can then use a TL072 dual instead of the harder to get quad. The frequency-gain structure will be slightly different (more like the Rat, obviously) but I doubt that the difference will be dramatic. Seems like your goal isn't to do an exact copy of the 1982 anyway, right?

Andy

[mark2]

You got some recommendations for productive things to do with it...
But in the event you truly have no use, here's a safe way to terminate it:  +input to a reference voltage, -input to output

[antonis]

On a second glance, there are some peculiar items values/combinations, like R7 (480R) and R12/R13 (10k/2M2)..
(not to mention almost all filters cut-off frequencies..)

[Vivek]

Pre distortion mid control

Or

Blend for dry / rat mix

[iainpunk]

stick a wave folder stage in between the 2nd gain stage and the normal clipping.

it won't sound great, or even good, but it will be unique



cheers

[Elijah-Baley]

What does this stage do?

[anotherjim]

Clever.
When the signal is below the diode Vf it's unity-gain and non-inverting. If you feed both inputs of the opamp the same signal, the gain formula Av=(Rf/Rin)+1 doesn't work as usual. (Rf/Rin) cancels itself out leaving the +1.
When the signal is above Vf, its unity-gain inverting (the +input is "grounded" when a diode is on). So the output signal waveform gets folded upside down.

Of course, with a 9v single supply, the diode ground will be Vref 4.5v.

At least, that is how I think it works.

[antonis]

It "folds" signal waveform..

It can be more easily understood if you replace diode pair with a "switching" resistor..
(OFF for signal amplitude less than 650mV - ON for more than 650mV..)

When OFF, output is unity (1 + 10k/10k - 10k/10k) and when ON output is 650mV*(1 + 10k/10k) - signal amplitude*(10k/10k)..

P.S.
Iain's circuit is intended for bi-polar supply..

edit: Jim is by far faster..!!

[antonis]

I have to disagree with Jim's second case interpretation..

Non-inverting input isn't grouned.. It's set on Vf.. Output is the sum of 2XVf + (-Vsignal)
In case of Vsignal isn't greater than 2XVf, output isn't folded upside down..

[anotherjim]

I put "grounded" in quotes. Be it 0v as in the schematic, or virtual ground of 4.5v.

[soggybag]

Thanks for all of the help. Here is what I did with all of this. I'll get around to the wave shaper at some point.

Extra op-amp buffers the bias supply:



Extra op-amp adds a tilt tone control. Not sure if I grafted that on correctly.



Extra op-amp adds Baxendall tone stack. Not sure if this is correct? I think this needs a bias voltage at the + of ID1_D (pin 12)

[antonis]

Bias supply is OK..
Baxandall configuration is also OK if pin 10 is connected to VR AND pin 12 too (via a 2M2 resistor, like you use for R3 & R20..)
Tlt tone control puzzles me a bit, so I've no comment..

[antonis]

I put "grounded" in quotes. Be it 0v as in the schematic, or virtual ground of 4.5v.

OK but I insist for 2 X Vr "crossover handicap" beyond an ideal signal folding..
(regardless of non-inverting input bias point..)

[ElectricDruid]

Extra op-amp adds Baxendall tone stack. Not sure if this is correct? I think this needs a bias voltage at the + of ID1_D (pin 12)

Yes, it does some bias, but you could just remove C8 and let it pick up the bias level from the previous op-amp stage.

In general, if you do that multiple times or with high gain stages, you finish up amplifying all the offsets up to a serious level, but in this case I doubt that would be a problem and the following cap C10 blocks any offset from making it to anything downstream anyway. So just put a wire link where C8 is and you're good for that one too.

The others all look fine, btw. Nice work.

[antonis]

The others all look fine, btw.

Is output stage of 1.0045 gain included in all the others..??

[iainpunk]

Thanks for all of the help. Here is what I did with all of this. I'll get around to the wave shaper at some point.

looks good

general advice from a sober version of me: don't take the ideas i post (under the influence of one to many) in threads too seriously. i build pedals that are weird and whacky one trick pony's, not pedals that have a broad range of applications.

cheers