2 channel guitar preamp

[marcelomd]

Hi,

I did a thing.

This is a 2 channel solid state preamp for guitar. My idea is to pair it with a cheap TPA3118 power amp module in a compact enclosure. I also wanted to try one of those chinese PCB manufacturers and this was the perfect excuse.

Any comments or suggestions are welcome.


There is nothing new here. One side is a Box of Rock with a Marshall Bluesbreaker tone stack. The other side is a SHO followed by a Fender Bassman tonestack. Plus a MOSFET buffer after the switch. I wanted to keep it as simple as possible but have some room to tweak values.


Schematic and PCB were designed in KiCAD. I wanted to make it as compact as possible.


I used JLC PCB. Boards (5 of them) came in less than 10 days for USD 10 including shipping. They are super high quality. It makes no sense to make boards at home anymore. Sorry for the out of focus edges. My phone is too smart for me.


First stage of assembly.


Testing. I didn't have any enclosures ready and was in a hurry to get this working. Also, cardboard backing for mojo.

This thing sounds really really good. Surprisingly not noisy. The clean side is... clean. It goes up to a very nice mild overdrive. The crunch side rocks. Dirty but very tight. Both channels are very touch sensitive.

I'm not a tone sommelier, and don't have golden ears, but this sounds as good as any brand name amps I've played with.

Things to do now:
- Maybe add some gain to the last stage, to make it crunch too;
- Add the high gain switch to the crunch side, like the Distortron;
- Convert it to SMD.

[BubbaFet]

Very cool  .
I've been having such great success with JFET guitar preamps feeding inexpensive Chinese class-D stereo-power-amp boards, that I have stopped my 30-year insanity of building guitar tube amps. I actually prefer the tone of the solid state JFET preamps.
I can build a 50W + 50W stereo guitar amp for under $100, in a day, and have it fit on my pedal board, and even run it on batteries (with some added weight) for ultimate portability and convenience. Goodbye BIG IRON.

[Elijah-Baley]

I like this kind of things!

[Ben N]

Nice! How does it do with pedals?

[marcelomd]

Nice! How does it do with pedals?

I only had a Timmy clone at hand and it sounds really good in any of the situations I tested: Clean side + pedal providing crunch, clean side at break up + mid boost, dirty side + pedal to tighten it. For my tastes, the pedal isn't really needed to get a good tone, but I like the option of having a "more" button.

What this pedal doesn't have is that saggy feel some tube amps have. This one feels like it has zero inertia. Personally I don't care, I like it snappy, but I'm told some people love it.

[amptramp]

I used to do electronic design reviews all the time and I could always find something to pick at, but this looks great as is!

I like the use of the 2N7000, it is  a real sleeper of a device with a tight gate threshold spec of 0.8 to 3 volts (nominal 2.1 volts) and a high transconductance of 100 to 320 millSiemens at 200 mA drain current.  Your drain current cannot exceed 2 mA, so your transconductance is not that high but it is still probably higher than most tubes.  Feedback from the drain to the gate stabilizes the MOSFET operating point, easier to accomplish with a MOSFET than a JFET.  Unlike clipper-type distortions, you distortion increases with gain.

Top marks for a great design!

[marcelomd]

I used to do electronic design reviews all the time and I could always find something to pick at, but this looks great as is!
...
Top marks for a great design!

Wow, thanks! Credit goes to mr Vex, who created the SHO and Box of Rock, and all of you for the lessons over my time here.

[printer2]

What voltage are you running at?

[marcelomd]

What voltage are you running at?

Regular 9V. A bit less since there's a diode and resistor in the power supply.