Critique my guitar/bass preamp

[fryingpan]

During the lockdown, I started designing a completely solid-state guitar/bass preamp centered around opamps for clean gain and diode clipping + JFET colouration for overdrive and distortion. It features pre-emphasis and subsequent de-emphasis in order to allow higher gain (as of now, the pre-emphasis is a +10 or +16dB high shelf, with the two "shelves" being approx. below 80Hz and over 1 kHz, and the de-emphasis is a corresponding and reversed bass shelf; this would also allow a +6dB bass boost or +6dB added brightness). I might rework the pre-emphasis and de-emphasis to allow bypassing it (and have essentially a flat response within the passband) and tweaking the values a bit (for example +6dB and +12dB). Also, I might add a HPF for guitar use (as it is now, the HPF is at around 30Hz).

The diode from gate to source on the first JFET is to avoid heavy gate conduction, and is there only for pathological conditions (extreme peaks), but I could use an appropriately chosen diode + zener diode to ground I suppose (or just not bother). The second JFET employs essentially a semi-forced source voltage, fixed by an opamp, and it behaves well in simulations. I have quite a few OPA2134 at home, but I think that some of the opamps would work well with an NE5532 in their place. The tone control is essentially a Big Muff tone control with selectable values (I have a 6-way switch, why not put it to use?). The attached images lack the clipping indicators, power supply and regulation (which is dual +/-15V; as for regulation, I was thinking of just using a zener regulator for the opamps, as they have very high power supply noise rejection, and a regulator chip just for the JFETs, which could even run at a higher voltage if necessary), active EQ, vibrato and spring or digital reverb (I could use either).

[anotherjim]

Well, that's a big one.
I can't see anything that won't work.
You may need another lockdown to get it built and debugged.

[Keppy]

Looks like fun! A few minor notes, since you asked:

• It looks to me like the R26/C25 should be on the (-) input instead of the output, unless I've mistaken the intent.
• You might need coupling caps after opamp stages. It depends on whether there's enough gain for the opamps' DC voltage errors to compound. Checking the output pins for stray DC would tell you for sure. I would definitely put a coupling cap, series resistor, and pulldown resistor on the final output though.
• You can save a cap by removing C4 if you parallel R7/R8 and just switch one of them in/out. Or put them in series and use the switch to short one of them. Or whatever, you get the idea, just switch resistance and reuse the cap.

Like I said, minor stuff.

[Vivek]

A) Should R13 and R14 be 10 times larger ?

B )Are both tp3 and tr5 needed or one of them can be replaced with fixed resistor ?

C) There is a situation where u5a does not have a ground reference on + pin

D) What do R30 C28 do ? I think there could be an error here

E) What do R31 R32 C29 C30 do ? I think there could be an error here

F) TP2 should receive voltage that is completely ripple free. Better to add a ripple rejection cap and resistor here.

G) I'm not sure if pin 1 of U7a will benefit from a large resistor to ground, as a ground ref

H) I'm not sure that U6a is configured properly. I feel it needs one big resistor in between - pin of opamp and wiper of TP2.

[anotherjim]

Yeh, U5a has no 0v ref with the switch to C20. Also R26 - C25 is on the wrong end of the feedback.
SW4 filtering doesn't really do anything as the low output impedance of the opamp won't feel any effect from the filter caps.
SW3 filtering depends on the source impedance of Q2 and we don't know that until the JFET is trimmed.

[fryingpan]

A) Should R13 and R14 be 10 times larger ?

B )Are both tp3 and tr5 needed or one of them can be replaced with fixed resistor ?

C) There is a situation where u5a does not have a ground reference on + pin

D) What do R30 C28 do ? I think there could be an error here

E) What do R31 R32 C29 C30 do ? I think there could be an error here

F) TP2 should receive voltage that is completely ripple free. Better to add a ripple rejection cap and resistor here.

G) I'm not sure if pin 1 of U7a will benefit from a large resistor to ground, as a ground ref

H) I'm not sure that U6a is configured properly. I feel it needs one big resistor in between - pin of opamp and wiper of TP2.

A) if Q1 has low output impedance, and according to the model and simulation it ought to be < 1kohm, 10kohm is an adequate load for Q1. The purpose of that inverting opamp is to allow the boost to increase the asymmetry of the distortion, instead of essentially offsetting it. It could even be bypassable, with little consequence.

B) yeah, I suppose both, or at least TR5 could be set at an appropriate value.

C) noted.

D and E) U6B is the de-emphasis, essentially a low shelf. All those resistors are needed for the operation of the low shelf.

F) noted.

G) yes, I suppose it could be at most around 50Kohm.

H) why? It's just an inverting opamp.

[Vivek]

A) The output impedance of FET is not known till the trimers have been set
For production runs, I suppose it will help if the input impedance of the next stage is increased so that a wider range of FET and trimmer settings can be accommodated

D and E) Please relook. I do not believe they will perform as you expect.

Regarding R30 and C28

The gain of a Non-inverting Opamp depends upon the ratio of two resistors + 1




If one or both of those resistors are converted to a frequency dependent impedances, we get a filter

The gain at a particular frequency then depends upon the {ratio of (The impedance between output pin and inverting pin) to (the impedance between inverting pin and ground)} + 1

In your case, the (Impedance between inverting pin and ground) is infinity

Hence the opamp U6B will have gain of 1 for all frequencies. Ie it is not a filter.




Re R31, R32 C29, C30.

They could have been a filter if they were one leg of a voltage divider.

But the other leg of the voltage divider is absent.

hence BB_out will always be same a pin 7 of the Opamp U6b for all frequencies

Ie it is not a filter.


Please consider the parts in red color:

[antonis]

I like R6, R7, R8, R15, R24 & R27 values..

[Vivek]

> H) I'm not sure that U6a is configured properly. I feel it needs one big resistor in between - pin of opamp and wiper of TP2.

> H) why? It's just an inverting opamp.






Let's look at the situation where trimmer wiper is at the very top

Pin 1 of U5A is the voltage source. That means there is no resistance in series between the voltage source Vin and the - pin of the Opamp. Or maybe 0.000000001 ohms of trace resistance.

The gain of that opamp U6A then depends upon internal impedance of the Opamp and the feedback resistor. I expect extremely high gain (Close to open loop), but I am not sure.


Please consider the part in red (5 to 10 times larger than trimmer TP3 value) :

[Vivek]

Are R26 and C25 at the right place ?

Please consider that red connection

[fryingpan]

I like R6, R7, R8, R15, R24 & R27 values..

Values were taken from a table. That said, I've just checked and different websites show different tables. Anyway some tolerance is to be expected

[Vivek]

Normally the idea is to get a rough idea of component value from calculations or table, and then replace with closest standard value

[fryingpan]

> H) I'm not sure that U6a is configured properly. I feel it needs one big resistor in between - pin of opamp and wiper of TP2.

> H) why? It's just an inverting opamp.






Let's look at the situation where trimmer wiper is at the very top

Pin 1 of U5A is the voltage source. That means there is no resistance in series between the voltage source Vin and the - pin of the Opamp. Or maybe 0.000000001 ohms of trace resistance.

The gain of that opamp U6A then depends upon internal impedance of the Opamp and the feedback resistor. I expect extremely high gain (Close to open loop), but I am not sure.


Please consider the part in red (5 to 10 times larger than trimmer TP3 value) :

The idea is that the trimpot is never fully to ground, and is only there to allow for attenuation into the opamp. That said, I could very well set the trimpot to a fixed value (eg. 10kohm) and adjust gain using the trimmer only (a 22k trimmer for example), considering that headroom is very high being a dual +/-15V power supply.

[antonis]

The idea is that the trimpot is never fully to ground, and is only there to allow for attenuation into the opamp.

The above should be true and correct in case of trimpot's wiper was connected to non-inverting input..
As it is, it serves as gain trimpot with both its outer lugs grounded..
(max value 5k)

[Steben]

I'm guessing the 600 ohm / LED clipping stage is all about letting the diode's impedance do its thing?

[Rob Strand]

Having the cap C7 right on the output of the pot TP1 means the low-pass filter formed by R10 through C7 isn't well defined.   The correct implementation depends on what you are trying to achieve here.    The first solution would be to put a resistance (say 10k, at least 4k7) between TP1 pin 2, then re-select the value of C7.  Another would be to bundle all the low-pass at C6 and use a small C7 - it won't sound the same as C7 won't be filtering post diodes.