Author Topic: Boba FET 4 watt guitar mini amp  (Read 5415 times)

jonny.reckless

Boba FET 4 watt guitar mini amp
« on: September 16, 2017, 02:12:51 PM »
I've recently designed and built a small, 4 watt guitar amplifier suitable for practice and home studio recording. It has a JFET preamp topology, and a very simple power amplifier. It runs from an 18V center negative pedal supply. All of the electronics are on a small double sided through hole PCB.

There is a lot of gain available on this amp. It feels and sounds quite tube-like in its response, and has a wide variety of tones. The overall tonal characteristic is reminiscent of the "brown" sound. The preamplifier circuit might be adaptable to a stomp box overdrive pedal.

You can listen to a demo of the various tones here:


If you would like to build one of these, I have a few spare blank PCBs available which you can have for $12 each.

https://drive.google.com/drive/folders/0BzLBW4IzZybIR19RT1QtMVhPdWM?usp=sharing
« Last Edit: September 16, 2017, 02:31:54 PM by jonny.reckless »

Gus

Re: Boba FET 4 watt guitar mini amp
« Reply #1 on: September 16, 2017, 03:34:52 PM »
Circuit looks nice.
I like the old school way you are driving the output transistors by the current draw of the 5534.
I like the use of cap multipliers in the power supply 
« Last Edit: September 16, 2017, 03:59:52 PM by Gus »

mth5044

Re: Boba FET 4 watt guitar mini amp
« Reply #2 on: September 16, 2017, 05:44:14 PM »
Cool EQ!

Would it be possible to up to voltage in the power amp to get more power, but keep the rest of it regulated?

jonny.reckless

Re: Boba FET 4 watt guitar mini amp
« Reply #3 on: September 16, 2017, 05:55:37 PM »
Thanks! I've always liked the Marshall style mid and treble controls, but the bass control might as well not be there, so I designed a hybrid. I wanted a bass control that actually works!

In theory you could go up to about 36V on the supply which is the maximum the NE5534 would take, but you would also need to replace the 35V rated caps with a higher voltage, and the output transistors with bigger devices, probably Darlingtons for that power, and mount them on a heatsink. You'd probably get somewhere around 20 watts into 8 ohms that way. You would need to adjust the preamp biasing resistors based on the higher VDD, but it's possible.

If I was planning a more powerful amp I would probably use a different power amp topology though. This was designed to be for low volume practice and home studio recording. It works really well at "bedroom level".
« Last Edit: September 16, 2017, 06:20:06 PM by jonny.reckless »

PRR

Re: Boba FET 4 watt guitar mini amp
« Reply #4 on: September 16, 2017, 09:50:38 PM »
> get more power

For how long?

That scheme is good for a couple Watts. OK, 4W in DIY-world where occasional repairs are not a big problem.

It has no protection, and is a little shy on current gain. It will struggle or die at higher powers, unless you do major up-design. These days it is a ton more practical to just buy a Power Amplifier Chip.
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jonny.reckless

Re: Boba FET 4 watt guitar mini amp
« Reply #5 on: September 17, 2017, 02:53:20 AM »
Yeah, I tend to agree. It was designed and built as a small bedroom blaster home studio recording amp, and for that purpose it excels. It's not a giggable amp!
« Last Edit: September 17, 2017, 02:57:36 AM by jonny.reckless »

Steben

Re: Boba FET 4 watt guitar mini amp
« Reply #6 on: November 25, 2017, 06:17:35 AM »
Unless you put PA on it with mic's....... ;) A friend of mine uses Blackstar 1W amps on stage

Great job. I am so convinced you can make a great tube emulation amp with jfets, including all sag and bias shifting by just adding some signal tracking and controllable power voltage / bias.
Rules apply only for those who are not allowed to break them

Mark Hammer

Re: Boba FET 4 watt guitar mini amp
« Reply #7 on: November 25, 2017, 10:27:25 AM »
Since so much of what we hear can come from it, what are you using for a speaker and cab?

You have the bypass caps for the gain stages built around TR3 and TR5 ganged to the same switch.  Any value added by splitting those into two separate switches, one per stage?

Other than that, nice job.

jonny.reckless

Re: Boba FET 4 watt guitar mini amp
« Reply #8 on: November 28, 2017, 12:38:29 AM »
It's a Seismic Luke 1 x 12 cabinet with an Eminence CV-75. Mic was SE2200A. I did experiment with separate switches but the effect was a bit too subtle. More details on the YouTube link itself.
« Last Edit: November 28, 2017, 01:34:17 AM by jonny.reckless »

jonny.reckless

Re: Boba FET 4 watt guitar mini amp
« Reply #9 on: June 14, 2020, 06:47:51 PM »
I finally got around to adding the PCB Gerbers and NC drill files to the Google drive shared folder:
https://drive.google.com/drive/folders/0BzLBW4IzZybIR19RT1QtMVhPdWM?usp=sharing

I ran out of my own PCBs but you are welcome to make your own boards using these Gerbers.
Please respect the J Reckless copyright on there if you're using my design. Thanks.