I built this ACA “Amp Camp Amp” designed by Nelson Pass, just wish to try out the characteristic of this amplifier design.
By looking at the schematic, the first stage using 2SK170 as source follower, much like a buffer stage , and directly feed into IRFP240, operate as a class A single ended amplifier with another IRFP240 as constant current source. The detail of whole design had been explained in FirstWatt page.
I found a guy who sell ready made PCB with built-in rectifier/ capacitors, so I bought the blank PCB from that guy.
I also bought a small casing with two heatsink on its side and seems just enough to put everything in it. The heat measured 10mm thick which is more than enough to dissipate heats from the Mosfet. I found that silicone heat compound are needed to dissipate heat to heatsink fast.
My power supply measured around 22V DC with load. 10 pcs of 3000uF capacitors for each channel power supply, with a small resistor of 0.22R 3W in between capacitors to form CRC filter. A 120 VA toroidal transformer is more than enough for both channel. I can’t hear any hum from my speaker. The current measured are around 1.2+A but I will experiment more after this.
Most of the time, I will try to avoid any pre-amp if the power amp can provide enough gain. I used APLS blue pot volume control. I adjust a little bit of feedback resistor to increase gain slightly. The gain can’t be too high though , The damping factor will drop rapidly if I set it too high. currently using 56K/ 10K and my DAC can drive it to clip.
I’m not using speaker binding post here, because speaker cable already come with banana jack, I used banana socket instead, to match this small-ish casing.
The whole amplifier measured around 23cm X 26cm X 10cm, small enough for me to pass around among my friend to try out. Sound test proved that simple Mosfet in single ended class A like this sounds really close to vacuum tube amplifier, warm and a lot of detail.
Video uploaded to youtube: