TOA P-924A Mono Power Amp Output Connections

[TomCarlos]

MEF Team….

A friend dropped off a TOA P-924A mono power amp. I am guessing this is 1970’s. He states the amp works but then looses volume. I will get into the amp soon, start with a test, clean the volume control, etc.

If you would like to see the full schematic, you can find that on Page 10 of the manual. But I think the images below should help cover what I am asking here.

My question is on the output connections, and wondering how best to use this amp. This amp does have an output transformer (used for the 8 ohm, 25v line, and 70v line) and then there is a 4 ohm output. I see the output for the 4 ohm connection. The image on the right shows that the connecting bar should be removed when using those lugs. My friend was using the 4 ohm output with the bar connected – so maybe that was not a good idea. But is there any harm in connecting an 8 ohm load to the 4 ohm output point (assuming the connection bar is removed)?

Manual - P924A_Manual.pdf

Thanks in advance…

[Jon Snell]

Connect your speaker between Common and 8Ohms.
It is like a valve amplifier, all that will suffer is the bass response to your ears if you get it wrong.

[J M Fahey]

1) that is a beast of an amp

2) the "real" amp output is some 240W RMS into 4 ohm, no transformer involved so if unused disconnect it, why add a useless inductive load to the amp?

Now IF used, speaker load at some tap will reflect back as a regular 4 ohm speaker, but again,if unused just pull that strap.

That said, the amp is designed to drive a transformer, so that alone canīt cause any malfunction.

3) amp direct output (4 ohm) will be happy driving an 8 ohm speaker, like on any SS amp, only it will put out 120-150W RMS into it ... which is fine.

4) your amp might drop output if overheated, check heatsinks are not clogged with dust/fleece.
Does it have a fan?
Same thing.
Air vents?

5) or you might have dirty/corroded pots - connectors - etc.

Feed it 1 kHz to almost clipping into a direct 4 ohm load and check whether you can reproduce the fault.

[TomCarlos]

Thank you Juan and Jon...

The amp does have vents at the top but no fan. So I will open it up later tonight and give it a good blast with air. I'll also do a visual inspection where I can. Then, onto a sound test and let it run for a bit to see what it does.

I'll report back.

Much appreciated.

Tom

[TomCarlos]

An update on the TOA....

I checked with my friend once again who reported that the amp would "distort" after a short period, being played at a moderate level. He was using a bass guitar, into a preamp, and the preamp into the TOA. I did notice that the sensitivity switch was at the 100mv, 20dBv position. And the Bar was connected at the output.

For an old amp, it was surprisingly clean on the inside. A quick air blast with my blower and that was it. I cleaned the volume pot but decided to NOT remove connectors (as I learned a lesson to not touch something if I do not have to). Using a 4 ohm dummy load, I inserted a 1Kz tone and measured a clean 225 watts before clipping (at spec). I let it sit there for a bit and no distortion, no oscillation detected.

So I will go back to the iPod test and use an 8 ohm cab at a moderate level to see if I detect anything after the amp works for a bit and warms up. Maybe it was the sensitivity setting or the way he connected to the output terminals.

[g1]

Did the owner determine that there was no issue with his cab(s) or speaker cables?

[TomCarlos]

Good question G1... All I can do is some testing here to see if I can replicate the issue or not. I may ask him to bring his other equipment here so I can test those items as well.