Firstwatt F1J

Yeah! It is working:). With some temporary wiring in place I have powered up one channel and it is working nicely. The voltages shown on the schematic and in the write up are matching well, except the gate of Q1 and Q2 is running at 7.96 VDC relative to ground versus the listed 7 volts. The speaker outputs are at 13.8 VDC and the DC offset between the outputs is ~6mV. I used an F4 transistor kit from the diyAudio store to get the matched transistors, and based on the low DC offset produced I am quite impressed with the matching of these transistors. A sound check with a beater speaker and an old iPod sounds pretty good. I am satisfied that all is well with the boards, and I am comfortable selling the remaining boards. In the next couple days I will PM those who have expressed interest to get address information to calculate shipping. The boards will be $20 + shipping for qty(1) power supply board and qty(2) amp boards.


  • IMG_5592.jpg
    1,003.9 KB · Views: 574
music soothes the savage beast
Joined 2004
Paid Member
Hi Chas, thanks for the boards again. I had a little time yesterday, so I put it all together. It works from the first moment. I am using variac, test speakers with cap, and fans to cool it down in the first testing stage. Its just test layout on the carpet at this moment. Case work will have to wait. I adjusted drain voltage no problem, it draws current as expected. I have small dc (0.60 and 0.45 V) on the output which I will address once it is settled after some run time. I am using cap before test speakers right now. It sounds nice so far.
Thanks Nelson for sharing the circuit and the outputs!
music soothes the savage beast
Joined 2004
Paid Member
I addressed the dc offset. Added 150k and 180k resistors to R105. That took dc from half the volt positive to 50mV negative. Instead of fiddling with those resistors again, I added 2M2 resistors on opposite side, R106. Offset is -11mV at start, as it heats up, it goes across zero to positive and stays ~20 to 30mV, depends how much I blow the fan on it.
If I knew, I would order another pair of ideal bridge rectifiers, to avoid some heat. This amp heats a lot. I may run it at slightly lower voltage. I do not like it hot. Some do :)
Otherwise it sounds great.
I agree this amp makes a lot of heat, I have not compared the actual power dissipation, but it feels about as hot has my BA3.

You obtained good results using the trimming resistors. I will look into adding a potentiometer in series with R105/R106 to make this easier on future boards.

BTW all of the boards that I had are now sold. I plan to make a few small improvements (feedback/suggestions are welcome) and make another small order for people who have expressed interest, or possibly try a group buy.
UMS Board Schematic

I have had a request for the schematic for the UMS boards, so I am posting them here. As expected they are pretty much the same as the FirstWatt schematic, with the addition of the trimming resistors, dedicated pads for various input capacitor configurations and the power indicating LED circuit has been moved to the power supply board using a single fixed resistor rather than the potentiometer of the original.

PS: Sorry about the smudges on the power supply schematic, I'm not sure what the kids got on the scanner.


  • F1 Power Supply Schematic.pdf
    345.2 KB · Views: 130
  • F1 UMS V3,5 Schematic.pdf
    58.7 KB · Views: 205
Last edited:
Build so far

Here are a few photos of my boards and the case i shall be using still waiting on some parts from mouser.Hopefully next week can continue the build of this f1j amplifier Again thanks to csample for releasing his version of the board .As well as thanks to Mr Pass for his designs .


  • 20201024_130925.jpg
    765 KB · Views: 282
  • 20201024_132413.jpg
    582.1 KB · Views: 275
  • 20201024_130922.jpg
    736.7 KB · Views: 268