A 1970 Power Amplifier - how good can it be !

RE: A 1970 Power Amplifier - how good can it be !

shabaz — Sun, 10 Apr 2022 13:02:26 GMT

Hi Michael,

Very nice looking amplifier! This is a coincidence, I too was looking at ancient amplifiers the other day. I picked this one from Ferranti. Completely different ballpark though, this is just 130mW output.

I just realized the other day how few power BJTs I have : ( I couldn't assemble any other higher-power discrete amplifier as a result.

I built it, but with some slight mods to suit what I had, and got distortion down to 0.2%, but performs poorly with higher distortion as soon as the input level is changed much (I left off the TR1 portion of the circuit, only constructed the remainder. This amplifier sounds really bad as a result (just irritating to listen to, that must be the Class B distortion I guess), requires a lot of fiddling to sound semi-reasonable. Also, least distortion is at 32 ohms, I'd need to change component values further for 8 ohm load.. so I used a 32 ohm resistor, with high-impedance headphones in parallel.

Incidentally, I have a solution for audio measurement, that could be smaller than the HP device and might perform close to it. I still need to write it up, but basically it uses a ready-made DSP board from Aliexpress.

RE: A 1970 Power Amplifier - how good can it be !

Andrew J — Sun, 10 Apr 2022 09:02:51 GMT

Really interesting blog Michael. Can I ask about your approach to mounting on the heat sink: when it comes to enclosing it, assuming you do or for the purposes of the question you do, how would you go about it? As it stands at the moment, the fins are ‘down’ but if you reversed it, there’s a lot of leverage on the transistor mounts. I ask because I’m currently working out how best to mount 5 TO-220 devices onto a heat sink and encase it all and the approach you’ve taken is likely to give a lot more options in terms of heat sink and enclosure choices.

RE: A 1970 Power Amplifier - how good can it be !

Jan Cumps — Sat, 09 Apr 2022 16:22:40 GMT

Nice!
Nitpick alert on the schematics: You draw the output transistors as a connector. For a reviewer, it's not immediately clear what kind of transistors they are. My OCD kicks in then ;)