Critique my design - guitar/bass overdrive

[fryingpan]

OK, so this is the guitar/bass overdrive I was working on:



A possible addition could be a switch on the feedback to ground path of the first opamp, as a "guitar/bass" switch, with a second path to ground with a smaller resistor + cap to provide some bass cut and higher gain.

The design is inverting as it is now, but I could mirror the de-emphasis stage (U4) from the pre-emphasis stage (U2) (make it into a non-inverting stage).

The pre-emphasis and de-emphasis pots would be respectively treble-boost and bass-boost only. The design provides some soft clipping with few higher harmonics, most harmonics are below 10th at transients (around 30% THD for a 2Vpp input, which is on the very hot side).

I expect the opamps to clip a bit on very hot transients, specially U2 (a rail-to-rail opamp would be nice, but most opamps are limited to Vrail - 1V or thereabouts, and that means the highest input voltage, assuming a sinewave, assuming maximum gain, would be around +/-400mV), but I'm not too bothered. U1 could have some treble cut, but that's just a small parallel capacitor in the feedback.

[antonis]

You could omit J2 buffer either by using a JFET input op-amp for U1 or a bipolar input one with bootstrapped bias resistor..

Also, are you sure for that "tilt" EQ configuration around U2..??
(it's non-inverting and it's beyond my knowledge..)

Also, R6 is superfluous..
(U1 output biases directly U2 non-inverting input..)

Also, a cap of generous value (10X C16 value, at least) is needed right after U3 output..
(I let you guess why..)

[fryingpan]

You could omit J2 buffer either by using a JFET input op-amp for U1 or a bipolar input one with bootstrapped bias resistor..

The buffer is there to provide some flexibility as regards opamp choice. With a TL074 you don't really need it, provided it works well enough at +9V, single supply (IIRC TI advises against that).

Also, are you sure for that "tilt" EQ configuration around U2..??
(it's non-inverting and it's beyond my knowledge..)

ElectricDruid pointed it to me, and it simulates well.

Also, R6 is superfluous..
(U1 output biases directly U2 non-inverting input..)

I was thinking the same, and in fact I was thinking of removing it.

Also, a cap of generous value (10X C16 value, at least) is needed right after U3 output..
(I let you guess why..)

Why? U3 outputs Vref.

[antonis]

I do know what Tom pointed..
https://www.diystompboxes.com/smfforum/index.php?topic=131108.0

Why? U3 outputs Vref.

Which Vref is totally useless there..
(U4 is Vref biased via R36..)
U3 output exhibits a DC Vref, which can't be considered as AC GND..
In the presence of a DC blocking cap, U3 output should serve as GND,,
(in the mean of almost zero resistance..)

P.S.
I'm a zealus exponent of buffers but only where they are really needed..

[Steben]

Well that is a triple soft clipping stacked... with an EQ amidst of clipping 2 and 3... interesting.
But remember in second stage the opamp might clip: amplitude goes to 1.4V + 1.5V + signal... this means in a gentle strat let's say 3.1V to 3.9V with a HB easily. The EQ cuts the signal a lot.
Would like to know the result.

[ElectricDruid]

You could omit J2 buffer either by using a JFET input op-amp for U1 or a bipolar input one with bootstrapped bias resistor..

The buffer is there to provide some flexibility as regards opamp choice. With a TL074 you don't really need it, provided it works well enough at +9V, single supply (IIRC TI advises against that).

I'm with Antonis on this one. I can't see much need for J2. Pretty much any op-amp would be able to manage a decently high input impedance. Change R5 to 1M, and then use C19 as the input. It won't needd to be be as big as 1u, so that makes life simpler. Add an extra resistor on the outisde to bleed charge off the cap if required or you feel the need. RG has the definitive article about all the potential "extras" you can add beyond the basic "get it working" stuff: http://www.geofex.com/circuits/what_are_all_those_parts_for.htm

Whatever TI say, the TL07x series has been used in single supply applications as long as I've been alive, which is starting to be long enough.

[fryingpan]

Well that is a triple soft clipping stacked... with an EQ amidst of clipping 2 and 3... interesting.

The opamp soft clipping is just there as "protection", to avoid abrupt hard clipping. With 2Vpp @ 100Hz, the first opamp provides 0.8%THD, the second opamp around 6%. The soft clipping before the JFET stops the transistor from hard clipping and should also provide some protection against blocking (the 1k resistor onto the gate also limits gate current during gate conduction).

But remember in second stage the opamp might clip: amplitude goes to 1.4V + 1.5V + signal... this means in a gentle strat let's say 3.1V to 3.9V with a HB easily. The EQ cuts the signal a lot.

What do you mean with 1.4 + 1.5 + signal? Are you talking about Voutput limits? To be honest, I was looking at other opamps instead of the TL074, which has some issues when used as a buffer (not very stable apparently if the input voltage goes more than 0.5 negative with respect to the signal ground, with 9V single supply that is). Also it is a little bit noisy for modern standards (but it's good enough, mind you). But it does have dynamic range limitations. Having 1V greater range with an opamp that can get a bit closer to the rail would be nice.

The EQ insertion loss should be around 10dB or so, but that actually helps with keeping levels sane.

[fryingpan]

You could omit J2 buffer either by using a JFET input op-amp for U1 or a bipolar input one with bootstrapped bias resistor..

The buffer is there to provide some flexibility as regards opamp choice. With a TL074 you don't really need it, provided it works well enough at +9V, single supply (IIRC TI advises against that).

I'm with Antonis on this one. I can't see much need for J2. Pretty much any op-amp would be able to manage a decently high input impedance.

If it's a bipolar input, impedance might be as low as 100kOhm. With a JFET buffer, wired that way, you get little distortion, little loss (it needs a trimpot but nothing too hard). Also because of the bootstrapping, that 1M resistor could be a small as 200kOhm and still look like over 1M.

Change R5 to 1M, and then use C19 as the input. It won't needd to be be as big as 1u, so that makes life simpler. Add an extra resistor on the outisde to bleed charge off the cap if required or you feel the need. RG has the definitive article about all the potential "extras" you can add beyond the basic "get it working" stuff: http://www.geofex.com/circuits/what_are_all_those_parts_for.htm

Whatever TI say, the TL07x series has been used in single supply applications as long as I've been alive, which is starting to be long enough.

Of course, it needs a pulldown resistor.

[antonis]

Change R5 to 1M

I think OP aims to even higher input impedance (judging from R38 bootstrapping - although JFETs are very inferior to BJTs in bootstrapping due to much lower transonductance..) which could also be obtained by connecting C3 GND to Vref, exchanging C3-R4 order and connect R5 (100k to 1M) to C3/R4 joint..
Also, in such a case C19 could be much smaller (a couple of nF should be able..)

[fryingpan]

About that GND reference for U3 output/U4 input. What if I add a 100k resistor (or smaller) to Vref in between instead of a cap?

[antonis]

WHY you insist to provide Vref to U4 inverting input..??
(it doesn't need it..)

Do you realize that U4 will exhibit a DC gain set by [(R30+R31) // (R37 + Diode static resistance)] / (R32 + R35) ,,??

P.S.
What U2 Vref respective connection uses is Vref's cap (C1), only..!!

[fryingpan]

WHY you insist to provide Vref to U4 inverting input..??
(it doesn't need it..)

Do you realize that U4 will exhibit a DC gain set by [(R30+R31) // (R37 + Diose statric resistance)] / (R32 + R35) ,,??

Right. So cap it is.

[antonis]

That *&^%$$ Vref connection in various circuits has "ruined" a lot of people..

[fryingpan]

So I don't need a Vref connection in U2's feedback circuit? I don't understand. Why would U2 be connected to Vref's cap only? The node is shared by the cap and the parallel of R1 and R2.

[antonis]

So I don't need a Vref connection in U2's feedback circuit?

No, you don't..

Vref is externally applied to non-inverting input, only..!!
Output tries to make inverting input look identical to non-inverting one..
In case of inverting input also biased at Vref, output is out of job..!!

I don't understand.

https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electronics/Operational_Amplifiers_and_Linear_Integrated_Circuits_-_Theory_and_Application_(Fiore)/04%3A_Basic_Op_Amp_Circuits/4.03%3A_Single_Supply_Biasing

https://www.ti.com/lit/an/sloa030a/sloa030a.pdf?ts=1693899633718&ref_url=https%253A%252F%252Fwww.google.com%252F

http://fab.cba.mit.edu/classes/863.16/section.Architecture/people/Ge/bias_opamp.pdf

https://www.ieee.li/pdf/essay/single_supply_op_amp_design.pdf

[antonis]

Does this makes thing clear..??

[fryingpan]

Yes, yes. I see what you mean. Vref is Vbias outside the feedback loop, and signal ground inside.

[Elektrojänis]

You could omit J2 buffer either by using a JFET input op-amp for U1 or a bipolar input one with bootstrapped bias resistor..

The buffer is there to provide some flexibility as regards opamp choice. With a TL074 you don't really need it, provided it works well enough at +9V, single supply (IIRC TI advises against that).

I'm with Antonis on this one. I can't see much need for J2. Pretty much any op-amp would be able to manage a decently high input impedance.

If it's a bipolar input, impedance might be as low as 100kOhm. With a JFET buffer, wired that way, you get little distortion, little loss (it needs a trimpot but nothing too hard). Also because of the bootstrapping, that 1M resistor could be a small as 200kOhm and still look like over 1M.

On an overdrive (or anything else that is meant to distort) it might not be too much of a problem if the input impedance is just 100kohm. It may cut some treble from passive pickups, but the difference to typical 1Mohm can be small enough that the harmonics created by the clipping will mask it. And even if it is noticeable, it could actually sound better. Which is totally subjective ofcource, but worth a try.

[fryingpan]

You could omit J2 buffer either by using a JFET input op-amp for U1 or a bipolar input one with bootstrapped bias resistor..

The buffer is there to provide some flexibility as regards opamp choice. With a TL074 you don't really need it, provided it works well enough at +9V, single supply (IIRC TI advises against that).

I'm with Antonis on this one. I can't see much need for J2. Pretty much any op-amp would be able to manage a decently high input impedance.

If it's a bipolar input, impedance might be as low as 100kOhm. With a JFET buffer, wired that way, you get little distortion, little loss (it needs a trimpot but nothing too hard). Also because of the bootstrapping, that 1M resistor could be a small as 200kOhm and still look like over 1M.

On an overdrive (or anything else that is meant to distort) it might not be too much of a problem if the input impedance is just 100kohm. It may cut some treble from passive pickups, but the difference to typical 1Mohm can be small enough that the harmonics created by the clipping will mask it. And even if it is noticeable, it could actually sound better. Which is totally subjective ofcource, but worth a try.

The idea is that the pedal should function as a sort of "transparent" preamp/overdrive of sorts, just providing some very gentle fattening/"bloom" at lower gains with little in the way of tone shaping (the three capacitors in the tone section are selected as to provide either around 6dB mid cut @ 800Hz-ish, flat response or a gentle 1kHz boost respectively, when centered). It would be a replacement of my EHX Hot Tubes pedal, which is nice but sounds a bit congested. With higher gains, and pre/de-emphasis, the pedal should be able to provide stronger overdrive without sounding congested by limiting boost at lower frequencies.

[antonis]

Vref is Vbias outside the feedback loop, and signal ground inside.

Not exactly 'cause "feedback loop" is considered the circuitry between output and inverting input (for a linearly working amp - comparators, vibrators, oscillators, etc excluded..)
It might be as simple as R2 in following schematics fig.1 or even simpler as a short in fig.2

https://medium.com/supplyframe-hardware/designing-with-op-amps-the-virtual-ground-1bf84b6ee13c