BillFitzmaurice.info

Something I've been doing recently while gearing up for summer shows is checking out some amps and making sure they're functioning correctly, cleaning them up/out, etc..

I also purchased a Peavey DPC750 basically for nostalgia because these were some of the first amps I've DJed with and always liked them for some reason.

It's rated to do 350w @4ohm /channel. I decided to plug the DMM into the binding posts and feed it 100hz and give it gain till the the clip lights came on, right before clip I was getting 63vac. Now- I know some amps are overrated and some amps are underrated, but 63vac @4ohm puts this thing at just over 900w.

Could I really expect this thing to achieve that type of power? It's only rated to consume 850W from the outlet! Or would a load on the amp drag the voltage down (as expected).

All my QSC amps are exhibiting the same behavior. Our MX1500a's were able to do 71vac before the clip lights came on. @4ohm that would push it to around 1200w a channel. The amp is only rated to do 500w (4ohm per channel).

That being said, the procedure around these parts (and also how I set my limiters) is to measure voltage with no load, and introduce the limiter so that it will not allow you to go over said output voltage.

If what I'm seeing is known to happen, would that not mean our limiter settings are incorrect and we should be using some type of a load to set the limiter?

Measuring 63v with no load doesn't mean you'll get 63v at 4 ohms. The output transistors may be capable of it, but the power supply isn't.
SS amps will produce the same voltage irrespective of the load impedance, but only at small signal levels, ie., at least 3dB below the maximum capacity of the power supply. If you do set the limiter at a voltage that the amp should easily produce, in this case 37 volts, it should have no trouble delivering that into 4 ohms.

The safe method is to set the limiter with no speaker in place, with the worst that can happen being you have a higher safety margin. If you want to check it again with the speaker you can, but you need a place to do so without getting arrested for disturbing the peace.

My Jands920 (Aussie made iron feet breakers) amps give separate output readings on the case for one channel and two channels driven.
eg: both channels driven 280W/8 ohms or 450W/4 ohms
single channel driven: 550W/4 ohms
It's not something you see on a lot of amp cases, it makes me wonder if many amps are the same.

It would be interesting to see if this is the case with your amps too
eg feed 100HZ to one side and measure, then both sides and measure and see if there is any change...

itsnew2me wrote:My Jands920 (Aussie made iron feet breakers) amps give separate output readings on the case for one channel and two channels driven.
eg: both channels driven 280W/8 ohms or 450W/4 ohms
single channel driven: 550W/4 ohms
It's not something you see on a lot of amp cases, it makes me wonder if many amps are the same.

It would be interesting to see if this is the case with your amps too
eg feed 100HZ to one side and measure, then both sides and measure and see if there is any change...

Most manufactures list those ratings on the spec pages, but not on the amps themselves.

itsnew2me wrote:both channels driven 280W/8 ohms or 450W/4 ohms
single channel driven: 550W/4 ohms
It's not something you see on a lot of amp cases, it makes me wonder if many amps are the same.

This also points out that what limits the amps capacity isn't the output devices, it's the power supply. The highest quality amps made aren't even called stereo, they're called dual mono-blocks, indicating two amps sharing the same case but not the same power supply, so that nothing that happens with one amp will have any effect on the other.