Low-gain opamp

[ghiekorg]

Hi everyone,
yesterday i built this pedal (black cat OD1 http://effectslayouts.blogspot.com/2016/12/black-cat-od-1.html)
i just changed a couple of things:
- i didn't have an OP275 nor a TL072 so i went for a 4558
- i didn't have a 430R so i went for a 470R
I still have to proper test it (i am not home now) but the sound is really distorted, even when the drive is really low.
Which opamp can i use to reduce the gain?
Thank you

[iainpunk]

you can increase the 22k input resistor, higher value = less gain

cheers, Iain

[OiMcCoy]

Is there a schematic for this? I have been wanting to breadboard an op amp drive.

[antonis]

[OiMcCoy]

Cheers!

[ghiekorg]

you can increase the 22k input resistor, higher value = less gain

cheers, Iain

Thank you Iain
Is there a formula i can use here? I mean, let's assume i want to have, on drive 100%, the amout i have now on 50%. Do i have to double the resistor to 44k or is it a bit more complicated (like often is )? I actually used a B pot not a Log for the drive
Thanks

[antonis]

Is there a formula i can use here?

Just the good old inverting op-amp Gain formula..
( - R_F/R_G, where R_F -> 470k & R_G -> 22k..)

[R.G.]

Changing the specific opamp will do pretty much nothing. All modern opamps will act very much alike in that circuit. They all have enough gain that the operation in the circuit will depend on the feedback and input resistors. So as Iain said, either increase the 22K in series on the opamp inputs, or decrease the feedback resistor from the output pin to the negative input.

From a look at the schematic, it appears that the distortion comes mostly from huge accumulated gain. The first opamp has an effective gain of 470K/22K = 21, and the second opamp has a gain 0f 1M/22k = 45.

As an aside on hacking circuits, adding a parallel resistor is easier than pulling out a resistor and putting in a new one. You could also decrease the two feedback resistors (the 470K on the first stage and the 1M on the second) with another resistor. Maybe try tack-soldering another resistor in parallel on the top of those resistors. Try another of equal value to start. That halves the gain down to 11.5 and 22.5.

[ElectricDruid]

Not only is that going to be very distorted (there's no control over the distortion in that first stage beyond the level of the signal you feed it) but it's also going to be very loud. On a 9V supply, you might well expect 7Vpp output. That's a big signal for a guitar pedal. With the volume turned up, you can probably overdrive the input of whatever you've got next in the chain too!

It's a simple circuit, but it's pretty brutal. Calling it an "overdrive" makes it sound soft and gritty, but I'd expect more of a harsh fuzz sound tbh.

[anotherjim]

Yeh, I think you are wanting to make this something that it is not. The Overdrive description is often misused.

You can get opamps which might sound a little nicer when clipping flat out. This might include the MC1458 which is a slower & noisier ancestor of the 4558. However, when there is a lot of gain, noise can be a problem.

Such a low input impedance of 22k! And this is a commercial design?

[antonis]

Such a low input impedance of 22k! And this is a commercial design?

Maybe for electric ukuleles of 1k2 or so output impedance.. 

[PRR]

...... the sound is really distorted, even when the drive is really low. ....

Have you checked the DC operating points? You expect most nodes in here to be 4V-5V. Power pins at extremes of course. If other pins are way off midpoint, it's a wiring error.

[iainpunk]

it just baffles me that they call it an overdrive, especially with the OP275, which is particularly harsh when clipping to rails, with some nasty overshoot oscillations.

i do like an overdriven opamp, especially the CA3130 if implemented right, but the gains available in this pedal are just to much to call it an overdrive.
i have fuzzes that have less gain than this!

cheers, Iain

[antonis]

it just baffles me that they call it an overdrive,

What buffles me is that they implement "overdrive", as a term, on semiconductor devices..
(but that's another story..)

[Elektrojänis]

Thank you Iain
Is there a formula i can use here? I mean, let's assume i want to have, on drive 100%, the amout i have now on 50%. Do i have to double the resistor to 44k or is it a bit more complicated (like often is )? I actually used a B pot not a Log for the drive
Thanks

Using a linear pot for that drive control (as it is essentially wired as a volume control) will probably cause the usable control range to pile up in the start of the pots range. If you change the gain pot to log, the max gain will be the same, but the center (50%) of the rotation will be same as about 10% on that linear pot.

[j_flanders]

[amptramp]

I would beware of the 220 pF capacitors to ground from the inverting inputs on both stages, especially the first where there is no corresponding capacitor across the feedback resistor.  This reduces the feedback at high frequencies and can cause oscillation where the rising gain given by this reduction of feedback meets the falling open-loop gain of the op amp itself.  This capacitor needs to be isolated by adding resistance between it and the inverting input or by eliminating it entirely.  I concur with those who have said there is too much gain in this circuit for it to behave without extensive shielding and this gain really does not do much musically.

The gain of the op amp is not all that important in this circuit in the frequency ranges where the op amp gain exceeds the closed-loop gain (that is, the gain defined by the input and feedback resistances).  I would prefer the first stage at least to be non-inverting to get a higher input impedance but the second stage can be either inverting or non-inverting.

Op amp distortion is not derived from a specified characteristic of the op amp.  Op amp clipping tends to be abrupt because the amplifier goes from following the input signal to running up against a hard limit in an internal stage and this is not well characterized either in the spec or in examination of the op amp internals.

[mdcmdcmdc]

The "OD" in "OD-1" might be misleading; black cat have always called this a fuzz pedal. The 'overdrive' might have been added by whoever drew that schematic.

[anotherjim]

The original has a 50nF input cap. This happens to be worth 30k ohm at 100Hz and 3k ohm at 1kHz. This input looks like a brute force pre-fuzz filter to me and intended to interact a lot with guitar pickups.
It would be an interesting mod to switch a back to back pair of "coring" diodes in series between the 2 stages. This can greatly reduce noise by gating the first stage off during silence.

[Gus]

Note that the first stage gain is the 470k/ (22k + the guitar LRC and the input caps as anotherjim posted)

Looks something like a muff fuzz

Something from the past an adjusted muff fuzz
https://www.diystompboxes.com/pedals/mfuzz.gif