Critique my design - guitar/bass overdrive

[antonis]

As for input impedance and op-amp choice flexibility, proceed as follows and don't bother with input buffers anymore..



P.S.
Simulations show R5 apparent value as high as 250M so C19 could be as low as 47pF..
(for input HPF of 13.5Hz cut-off frequency..)

[Elektrojänis]

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.

Well for that I can see the point for keeping the input impedance high. You have a lot of clipping stages and complexity for subtle, but I quess that's how it goes if you try to maximize the softness of the clipping.

It might be worht experimenting how much the sound actually changes with the lower input impedance as it could have the benefit of lower noisefloor. The experiment would be as easy as changing the value of R38. After trying that you could decide if you want to experiment by removing the buffer altogether.

[fryingpan]

As for input impedance and op-amp choice flexibility, proceed as follows and don't bother with input buffers anymore..



P.S.
Simulations show R5 apparent value as high as 250M so C19 could be as low as 47pF..
(for input HPF of 13.5Hz cut-off frequency..)

What about the opamp's input noise? Bipolar opamps usually want a very low impedance feeding into it to maximise their noise performance.

[antonis]

I don't wish to skip from one point to another but why you think that the JFET input buffer noise reduction (refered to input source) is higher than the noise addition to the following stage due to itself and relevant circuitry..??

P.S.
I start to regret for pointing out the chance for items/space save..

[fryingpan]

I don't wish to skip from one point to another but why you think that the JFET input buffer noise reduction (refered to input source) is higher than the noise addition to the following stage due to itself and relevant circuitry..??

Well, JFETs tend to be low noise, and the buffer provides an easy path for the opamp. Does it matter much? I don't know. The noise calculation in LTSpice (which doesn't include the opamps) for the present circuit is a healthy -100dB relative to a 100mV signal. Let's say it's ten times higher, even on low intensity signals (10mV) you would still have 60dB SNR.

[antonis]

One last definition:

Are we dealing with a Hi-End preamp or an overdrive pedal..??

[fryingpan]

Tell you what. I'll veroboard the design, adding provisions for both (the JFET buffer and the resistor bootstrapping) and I'll check.

That said, what about the following circuit?



My only doubts now are the following:

- opamp choice might be the NJM4565/NJM4580; decently low cost, decent noise profile, good input common mode voltage range, should be able to get close enough to the rails; of course the opamp's power supply will be bypassed with a 100n cap to ground as close to the opamp as possible
- U3 should presumably see no more than 2Vpp, so I shouldn't have any phase reversal issues?
- how should I go about wiring the ground paths? The JFET(s) need a ground reference, the opamps need a Vref reference, and I presume signal ground and power ground should be well separate
- R43-R28 are now a 1M pot (the aforementioned opamps state a 1M typical input impedance, 0.3M minimum; the tonestack was calculated at 250k impedance for the following stage but there isn't much difference over 300k)

The Schottky diode for polarity protection will be at least a 0.5 amp device (with 50 ohms in series, I shouldn't see more than 180mA @ 9V, double that at 18V should I make any mistakes setting voltage on the power supply).

[antonis]

That said, what about the following circuit?

Let me see..
Except for some "weird" items values, it's almost identical with the initial one..

You do realize the disdain of the whole discussion, don't you..??

[ElectricDruid]

Ping me an email and I'll send you a pot symbol for LTspice. Makes your schematics easier to read and it's less work too.

[fryingpan]

That said, what about the following circuit?

Let me see..
Except for some "weird" items values, it's almost identical with the initial one..

You do realize the disdain of the whole discussion, don't you..??

Some of the weird item values are trimpots. The rest are taken from the E24 series (resistors).

If I removed the input buffer, would you like it a bit more...? U3 is unfortunately necessary because U4 needs to be fed from a very low impedance.

[antonis]

If I removed the input buffer, would you like it a bit more...?

I think YOU have to like your own design..

U3 is unfortunately necessary because U4 needs to be fed from a very low impedance.

Not if you wire U4 like you did for U2..

P.S.
I've noticed that, in spite of your particular "anxiety" about noise, you do add a not essential noise injector (R43 - 750k) of about 9μV (-100 dBv)..
Can't see the reason for further dominating (25% of what comes out of C13) the already existing Tonestack loss..
In case of you do wish for a low amplitude U4 input, set it by J1 gain (or even turn it into a Source follower) and get rid of R43..

[fryingpan]

So, I did a few changes to the design, namely a clipping indicator for the first two opamp stages (I didn't really need both, but since an LM358 comes as a dual opamp, why not), an LPF at the end, added some gain after the tonestack loss, changed a few cap values to cut more of the subsonic frequencies.

I would like to add in a variable HPF, such as the one described by Rod Elliott here: https://sound-au.com/project155.htm

He says that it isn't suitable for mic level signals (I'm assuming dynamic mics, since condensers can be quite hot!), and since guitar/bass signals should/could be in the same range, I'm assuming it needs some preamplification.

Where would you place it (just the second opamp, I'm assuming, since most gain stages should be fairly low impedance)? Of course before clipping. It's either after U1A or U1B.

Here is the updated design: https://docdro.id/5RrwDRm (don't know how to share PDFs here on the forum). There is one extra low value (dunno, 100-300ohm) resistor at the input of U1B that is missing in the schematic. (The idea is that C18 may impose a capacitive load on the input). Maybe one at the exit of J1?