Measuring the voltage rails of an amplifier.

You can do this, though your meter probably wont measure 1KHz AC. A lower frequency like 100Hz ought to work.

It will give you an idea of RMS voltage output, but it will not be truly accurate as it is likely the max output will sag slightly under load.

If the amp is clips you won't get an accurate measure. Either look at it with a scope or take off the case and measure the rails with reference to GND.
 
The significance may not pertain to you if you are not in pro audio and/or drive your amplifier for 10 – 15 hours non-stop at high SPL. Basically, my research evaluates how robust the power supply each amplifier offers using a simple ratio. The reference is the amount of output voltage the amplifier distributes with no load. I compare the amount of output voltage with no load to the amount of voltage drop that taking place using an 8-ohm in addition 4-ohm load.

The amplifiers that offer the highest output voltage drop using the above method are configured to drive lighter loads. If you still don’t understand, don’t worry about it.
 
Sorry Omnifex but I guess you are missing the point.

The peak voltage you measure in the speaker output is not the rail voltage by an important margin.

And it's not an accurate representation of what the PSU rails sag down to.

You are forgetting voltage drop across semiconductor junctions, loss in ballast resistors, loss in internal wiring, but more important: beta (Hfe) loss at higher currents and even more: what the short protection or current limiting is set to.

You must open the chassis and measure at the actual PSU, there's no other way.

You have 7 amps in a rack?
That's only half of the picture.

Are they all the same brand/model?: open just one and you're done.

Are all 7 different?

Well, do it 7 times, no big deal.
Opening a cover, measuring and closing it back can't take more than 10 minutes each.
 

turk 182

Member
2012-10-26 3:03 pm
omnifex
yes i do understand now that you've outlined the criteria for what your trying to determine.
if you care to take a moment to look at my profile it does state my primary interest and occupation.
with respect to your test i would say it would give a reasonable indication as long as we're talking about an amp that employs a simple transformer/ rectifier / filter supply but if the design incorporates regulation it would simply be determining the effectiveness/stability of the regulator leaving long term stability of the output stage in question.
 
Sorry Omnifex but I guess you are missing the point.

The peak voltage you measure in the speaker output is not the rail voltage by an important margin.

And it's not an accurate representation of what the PSU rails sag down to.

You are forgetting voltage drop across semiconductor junctions, loss in ballast resistors, loss in internal wiring, but more important: beta (Hfe) loss at higher currents and even more: what the short protection or current limiting is set to.

You must open the chassis and measure at the actual PSU, there's no other way.

You have 7 amps in a rack?
That's only half of the picture.

Are they all the same brand/model?: open just one and you're done.

Are all 7 different?

Well, do it 7 times, no big deal.
Opening a cover, measuring and closing it back can't take more than 10 minutes each.

Sorry JMFahey but I does not appear you understand what I am doing.

If you think pulling an amplifier out of a rack which each rack consists 7 amplifiers is only going to take 10 minutes each, then you are not in professional audio.
 
omnifex
yes i do understand now that you've outlined the criteria for what your trying to determine.
if you care to take a moment to look at my profile it does state my primary interest and occupation.
with respect to your test i would say it would give a reasonable indication as long as we're talking about an amp that employs a simple transformer/ rectifier / filter supply but if the design incorporates regulation it would simply be determining the effectiveness/stability of the regulator leaving long term stability of the output stage in question.

I am happy to hear you understand what I am doing.

They are basic amplifiers very straight forward (my avatar will give you a hint). I'm checking your profile right now...
 

turk 182

Member
2012-10-26 3:03 pm
amcron crown macrotech series if memory serves.good amps used racks of them on many tours.they aren't happy with low impedance loads that have reactive components as in Martin bass bins with the high pass caps still wired in. other than that i vaguely remember something about them going into protect prematurely but can't quite recall the specifics on that issue at the moment.
and by the way if pulling an amp from a rack to perform a test can't be done in under 10 minutes you obviously never had to work with the tour/road managers i've had to work with!
 
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