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@Jonny69
So, onto testing at home. This is my first time with a class A transistor amp. All my previous transistor amp builds have been class AB, which sort-of run in class A at low outputs and switch to class B as the output increases above the quiescent value. I made some notes while messaging a friend who's also into electronics.
First thing is it definitely needs more cooling. I thought the heatsinks I used would be adequate but longer-term it's not going to do the transistors much good running that hot.
It has big, deep bottom end. The transient response is sudden, like unexpectedly so, and can take you by surprise for something that’s only putting out a couple of watts. I think it’s because the power supply doesn’t drop any volts when the music suddenly gets louder. This is a thing that class A amps shine at - on a class AB or B amplifier, even a big one with a big power supply, the power supply voltage drops as you make bigger demands on it. This means the amplification drops as well, so you don't get completely linear amplification between zero output and full output. On a class A amplifier you do. This one draws a constant 1.2A on each channel no matter what it’s doing, plus I’ve regulated the voltage so it just sits bang on 27V and doesn’t move.
So some tracks like Tasmin Archer - Sleeping Satellite and Moody Blues - Nights in White Satin have a lot more depth than usual. It’s less noticeable with electronic music which doesn’t have so much dynamic range. But for example I had Death in Vegas - Dirge playing and, although I was used to it building, it just got louder and louder and louder as the track went on. Sash! - Encore Une Fois sounded massive and I only had it turned up to a couple of watts.
Another thing is bass guitar sounds very detailed and you can hear the string detail on it as well as the bassline. My pal speculated this is the lows not swamping the higher frequency stuff, where class AB probably sags the supply voltage and takes out the details. As the supply voltage on this does not change, it does not do that.
My main amp at home is a pair of 150W Maplin MOSFET amps, another home-build that I originally built in the mid-90s and rebuilt in 2021. That's an AB design and it's always sounded great on my big speakers. I’m very tempted to make a regulated supply for the that amp. I bet it would have some attack!