Critique my guitar-bass preamp!

[fryingpan]

So, I'm designing a preamp, to go with Rod Elliot's guitar power amp. It should ideally work with both guitar and bass (with, at most, few component changes).

This is the general idea:







Some notes:

- I haven't settled on opamps, I think I'd use an OPAx134 for U1, U4 and U5; U2 would end up driving a 220 ohm load when its output goes over about 1.8V, according to the specs an OPAx134 could work because it is rated for maximum 35mA current (and I would be well within that spec even with a very strong guitar signal, say, 1V peak - unlikely but possible), but I assume that opamps capable of driving low loads could be better suited (ideas?). What about gyrators, or U7?

- I suppose that I could do without U1 and let U2 do the slight full-range gain boost and use the volume control afterwards (the idea of U1 is that I can use a 10kohm pot, so less noise), but according to my simulations the soft clipping characteristic given by the 220 ohm resistor in series with the diodes wouldn't be as effective, so I'd rather leave it.

- I will probably need some level control pre-eq as the output is rather high.

- I could eventually switch the two JFET stages around, but I'll try them as they are in the schematic.

- U3 has to be an "inverting buffer" because I'd like J2 to accentuate the asymmetry, if I leave U3 out J2 would square out the wave making it more symmetrical. Symmetrical clipping is not necessarily a problem, but I want to play with this idea first.

- the preamp is lacking parametric mids, subsonic filter and reverb ATM.

- I'd like the DI to be post-power amp, since Rod Elliot's power amp is a high impedance one and that changes the frequency response; a pre-power amp DI would be all weird tone-wise.

Thanks!

[fryingpan]

Oh, power supply is +15V.

[fryingpan]

One last consideration: JFETs are not overly critical, and Rs/Rd values are just chosen in order to work with the SPICE models. In general, I would need (or would I?) a JFET with a Vp around 1.5-2V for the first stage and a JFET with a Vp around 1V for the second stage (the first stage should have more headroom, the second can and should distort more easily). Ideally the first stage should have lower impedance than the second one because I have an inverting opamp right after it.

[Bunkey]

I suppose the acid test would be how it sounds irl.

I don't know what your experience is and I can't comment much on the layout as I've only built discreet with BJT's but what I will say is; things like clipping diodes, different IC's, stage impedancies etc. will all have their own tonal character that's going to influence your sound/tone shaping; running simulations, whilst useful for checking functionality, won't tell you what actually sounds good.

If you've designed a lot of amps and already know how a lot of this will play out then fair enough you likely know more than I do; but it's certainly a lot more complex than I would begin with (not in terms of ability, but more like jumping right into the middle of the book instead of starting at page 1); if you haven't yet built the proposed amp on a breadboard and listened to how it responds irl, I would personally be starting simple and building off a core idea with one or two ICs and no tone shaping so that I can hear exactly what influence every part is having on the overall picture and add things as necessary (like the treble boosting in the feedback loop for example, and the experimenting with LPF values further on).

From my own experience I'd say the BMP tone stack is good for balancing the overall response of an amp and taking out some intrusive mid harmonics within a particular band - as determined by other factors like clipping diodes, power supply/decoupling caps and their emphasis on the overall EQ etc. - however, I've found this topology in a pre-amp is more of a set-and-forget thing to bring the amp into balance; if you want user-adjustable tone control you might be better following your 'built-in' tone shaping with a typical baxandall or something like that (edit: I see you have additional tone controls at the last stage). Again, I can only speak from my own experience here and YMMV but the BMP has not been a particularly useful 'adjustable' feature for me in a pre-amp and is more akin to pole-piece heights on a pickup than it is to your guitar's tone control.

Build the thing on a solderless breadboard, use your ears and take it from there - That is my critique!

Nice work though, you've certainly put a lot of thought into it.

[iainpunk]

i have some doubts about the Jfet's, generally they have a trimmer to set the correct bias in DIY pedals, so you can account for individual Jfet tolerances.
the opamps seem all-right as far i can see, but my eyes don't perceive sound that well. it puzzles me why you have 2 tone stacks tho.

its a lot of fun designing an intricate preamp from start to finish, but i recommend starting on a breadboard and building part by part
first build the active tone stack, get it to work on your bread board.
then add the pre and de emphasis.
then put the distortion channel in between those two, and tune the emphasis and distortion channel to eachother.
then, add the clean channel path on to your breadboard, and make it work with the existing emphasis (which is tuned to the distorted channel) until the clean channel sounds the way you like. clean channels can be very interesting to design, way harder than distorted channels!

i wish you all the luck,
cheers, Iain

edit:
an interesting way to build a clean channel is designing a good attenuator that lowers the volume before the distortion section, and has some tone shaping to make the cleans sound nicer, since overdrive channels generally have a bunch of mid push, you can have a slight mid scoop and a bunch of volume drop in front of the drive section to even out the response.
edit 2:
i found this video of a pedal that does the same:

[fryingpan]

I suppose the acid test would be how it sounds irl.

I don't know what your experience is and I can't comment much on the layout as I've only built discreet with BJT's but what I will say is; things like clipping diodes, different IC's, stage impedancies etc. will all have their own tonal character that's going to influence your sound/tone shaping; running simulations, whilst useful for checking functionality, won't tell you what actually sounds good.

If you've designed a lot of amps and already know how a lot of this will play out then fair enough you likely know more than I do; but it's certainly a lot more complex than I would begin with (not in terms of ability, but more like jumping right into the middle of the book instead of starting at page 1); if you haven't yet built the proposed amp on a breadboard and listened to how it responds irl, I would personally be starting simple and building off a core idea with one or two ICs and no tone shaping so that I can hear exactly what influence every part is having on the overall picture and add things as necessary (like the treble boosting in the feedback loop for example, and the experimenting with LPF values further on).

From my own experience I'd say the BMP tone stack is good for balancing the overall response of an amp and taking out some intrusive mid harmonics within a particular band - as determined by other factors like clipping diodes, power supply/decoupling caps and their emphasis on the overall EQ etc. - however, I've found this topology in a pre-amp is more of a set-and-forget thing to bring the amp into balance; if you want user-adjustable tone control you might be better following your 'built-in' tone shaping with a typical baxandall or something like that (edit: I see you have additional tone controls at the last stage). Again, I can only speak from my own experience here and YMMV but the BMP has not been a particularly useful 'adjustable' feature for me in a pre-amp and is more akin to pole-piece heights on a pickup than it is to your guitar's tone control.

Build the thing on a solderless breadboard, use your ears and take it from there - That is my critique!

Nice work though, you've certainly put a lot of thought into it.

My experience is in recording and mixing, I like doing lots of the tone shaping on my own and through the years I have learned what to expect from stuff. I also have some theoretical background (I'm an EE part-time student, sort of; I'm an English teacher in actual reality) although lots of the stuff I worked it out just by observation and not by actual calculation.

The BMP tone-stack has been tuned to give a sort-of Fender tonestack response (7dB mid cut centered around 500Hz), considering that you can also go deeper with the cut but in real life I find myself tuning Fender tonestacks to this sort of cut (not too scooped on the mids), but you can leave it flat (switching to the second capacitor option, the tone-stack becomes a simple tilting EQ) or boost the high mids (with the third option, which should come in useful with bass). In general it is intended to be a "character knob". The pre and de-emphasis is intended to sound crisp both with cleans and distortions, while the first stage is intended to be played clean to mild crunch (the second boost stage can be made to sound very distorted if you want to). The tone-stack before the boost is unconventional, but during my mixing experience I have often found myself shaping the tone before the drive. I could give the boost section a bit more tone-shaping capabilities but in fact that's the most tentative part of my design, together with the amount of pre/de-emphasis.

My request for comments was not aimed at getting pointers on how to voice the amp (I will refine it during breadboarding) but rather on the general design, whether I had made some gross design mistakes etc.

There are some things I have to work out (switching: relays, FETs? I'd like the boost to be footswitchable, and I'd like the reverb to be adjustable with an expression pedal) and of course the PCB design will be a challenge, but with Covid and my total lack of social life these days I have ample time on my hands anyway

Oh, anyway, Rs/Rd for the JFETs (and the opamp forcing the voltage on the source of the second JFET) will obviously be adjusted with trimmers, after selecting the right JFET anyway.

[Bunkey]

Ah I'm glad I went with the book analogy in that case 

Given your experience it makes a bit more sense to me now; I wondered why you'd complicated it with so many elements relating to tone but yes, that you know how to use them and this is how you approach your sound by all means it makes a good focal point for the design!

I employed a similar (although much simpler) switchable treble boost/bright control right before the clipping stage in my own pre-amp (inspired by the global feedback presence control in Fenders) and it does have some interesting effects on the mid/high emphasis and texture of the distortion, so I'm with you on that.

I'd like to be more help but afraid I'm not qualified on the finer points of the devices used; I don't see anything glaringly obvious.
I'm sure once you have it on the breadboard everything will fall into place.

Good luck!

[edvard]

...

edit:
an interesting way to build a clean channel is designing a good attenuator that lowers the volume before the distortion section, and has some tone shaping to make the cleans sound nicer, since overdrive channels generally have a bunch of mid push, you can have a slight mid scoop and a bunch of volume drop in front of the drive section to even out the response.
...

I have discovered that you can do exactly that if you put a standard Fender/Marshall/Vox passive tone stack in front of a low-medium gain overdrive stage.  The lossiness of the tone stack pulls back the gain and at the same time adds that classic FMV tone shaping that can transition into a nice, mushy, "vintage" overdrive if you crank it.  Early tube amplifiers had the same topology, which only changed after folks started demanding higher-gain preamps, where EQ *after* the gain stage made more sense.

[fryingpan]

...

edit:
an interesting way to build a clean channel is designing a good attenuator that lowers the volume before the distortion section, and has some tone shaping to make the cleans sound nicer, since overdrive channels generally have a bunch of mid push, you can have a slight mid scoop and a bunch of volume drop in front of the drive section to even out the response.
...

I have discovered that you can do exactly that if you put a standard Fender/Marshall/Vox passive tone stack in front of a low-medium gain overdrive stage.  The lossiness of the tone stack pulls back the gain and at the same time adds that classic FMV tone shaping that can transition into a nice, mushy, "vintage" overdrive if you crank it.  Early tube amplifiers had the same topology, which only changed after folks started demanding higher-gain preamps, where EQ *after* the gain stage made more sense.

That was my idea, I am after a "vintage" low-gain sound (or at least the choice to set it up like so). The boost can be kept low and add just a few harmonics, or you can set the tone stack flat or with a mid hump and overdrive that. I could make the tone stack switchable by the way (pre-post boost stage, it wouldn't be a big deal, it's basically a 2p2t switch). Anyway a tone stack before overdrive makes sense because you're shaping the overdrive itself, just like a post-EQ compressor. It makes sense sometimes.