Home built power amp

[Joe bags]

Hi guys

I have this home built power amp.
I'm trying to figure out
1. How to measure output wattage?
2. Whats the best speaker configuration?

So if I input a 1khz sine wave from a signal generator into a behringer mixer.
Output from mixer measured with oscilloscope to just before clipping.
Input this to power amp connected to 8ohm 400 watt dummy load.

Measure speaker out VAC with Fluke 25 DMM.

I'm trying to be careful here as I do not want o blow anything up obviously!!
as I think the dummy load is a bit under rated for this test.

With input volume knob on amp is turned up half way i'm getting 45Vac.

Am I right in calculating 45v x 45v =2025v/8ohm = 253 watts?



Some extra info.

There's 2 inputs at the front which are linked in parallel.
So I presume it's a mono amp.

There are 4 speakon outs which are linked in parallel too.
So each time I add a speaker to the outputs the resistance pretty much halves.



Caps are 6 x 15000uf @ 100v
Output transistors
2sc5200 x 4
2sa1943 x 4

[pdf64]

Yes, I agree with your power measurement method.
Though it may put out more power at 4 ohms.
Are the speakon outputs all wired in parallel too?
That may indicate that it's intended to be good down to 2 ohms.
Is there any label on the PT? Indication what the line fuse should be?
As there's a serial number sticker, it may have been built in a shop, eg as a sideline, when repair work was quiet.

[Mick Bailey]

I scope the output and measure the voltage at the point of clipping. You know that your input isn't clipping, but you need to know if the output is. Just measuring the output doesn't give you a full picture. Most Fluke DMMs will show increasing RMS output as you transition into clipping and beyond. Scoping the output also lets you see any instability creeping in as you increase the power.

A precaution is to briefly take up the amp to the clipping point. Running a power amp for very long with a sine-wave at maximum output isn't a good idea.

[eschertron]

Curious. Am I looking at it right, each speaker out is individually fused? and what's the LED above each hooked up to? Power indicator or blown fuse indicator?

If the fuse values for each of the 'branch' fuses and the value of the mains fuse were discovered, you might be able to get an idea of what the design limits were expected (or intended) to be. Pretty clean amp, BTW.

[J M Fahey]

Very well made, better than quite a few amps out there.

Scope the output to check actual clipping as suggested above, you don't need to make the preamp output "almost clipping" , setting it to about 2V RMS should be enough to drive that amp to clipping.

What you measured sounds about right anyway for what you have, it will probably be around 200W@8 ohms, some 350W@4 ohms and if they can drive them (protection might trigger at such low impedance) , some 450W into 2 ohms.
A beast of an amp.

I think rails must be around +/-70V but please confirm.

Individual output fuses are fine so if 1 of 4 speaker cables or cabinets shorts (or just some idiot trumps on it) the rest keep working.