bench testing amp question

[Bad4health 10+ year member]

oh ok thank you....you guys are the s h iz.

 

[amprepairman 10+ year member]

yea..dont try to read current with the meter in the pic...its a dc current meter....

 

[dB-r 10+ year member]

Just remember to put the clamp around only ONE of the speaker wires, if you put it around both, which lots of people do that mistakingly, you get no reading...

Power in watts = voltage x current

Impedance = voltage/current

e.g.

625 watts = 25VAC x 25AAC

1 ohm = 25/25

If you got 25VAC and 10AAC you would be 250 watts @ 2.5 ohms

 

[dB-r 10+ year member]

Also note, in a lot of newb clamp tests (and pros) I see people claiming ridiculous numbers from some amps, and lots of times it shows as if the amp were more than 100% efficient.

i.e. Say someone says a D9 puts out 9800 watts @ 1 ohm...

Let's say they read 99VAC and 99AAC so 9801 watts @ 1 ohm....

BUT THEN they show the DC clamp and VDC on the power wires for the test as say:

625ADC @ 12VDC... Well, that's only 7500 watts.... So the amp is only using 7500 watts so WTF did the clamped output say 9800 watts? How is that possible....

When you run an amp into clipping, it doesn't draw anymore current, the output signal just gets overdriven and the clamp meters will show the distortion as clamped power..... simple. The meter picks up the clipping as extra amps or voltage...

So to do an absolutely accurate test you need to monitor DC input voltage, DC input Amperes, AC Speaker output Volts and AC Amps, and use an oscilloscope on the output of the amp to watch the output signal. When the tops or bottoms start to look flat or the signal starts to look square going to your speakers, you are clipping the signal and the meters will keep going up, the sub may get a little louder, but the amp is not putting out clean power, which would be a SINE WAVE, instead it is putting out a SQUARE WAVE which sounds like a chainsaw (or farting sound in the coil of the sub).....

 

[dB-r 10+ year member]

You figure at about 80% efficiency, maybe 85%, for the above class D amp, so you take the 7500 watts - 80-85% = 6,000 to 6,375 watts more realistically...

 

[dB-r 10+ year member]

So clamp test = 9800 watts... but DC clamp test says 7500 watts... so use the DC clamp test results since they SHOULD be more accurate and subtract 80-85% for efficiency losses.... Then you get you realistic power output which sounds about right for a 4KW amp board:

 

[dB-r 10+ year member]

NOTE... AC clamping is necessary for finding your impedance, so it is important... Good thing is most DC Clamp meters also do AC... So you don't have to buy two seperate meters, you can use the AC side to get your impedance, and use the DC side to get you more realistic power output numbers... Class D amps are all like 80-85%, I don't care what fluff they feed you... Class AB's are 50-60% at low volume, but can be as high as 80% or so at full volume... weird, I know, nature of the beast...

 

[thch 10+ year member]

I've had several notes about this and related subject. most are:

Comments on Gains, Measuring Power, DMM, Oscope, and Clamp - Realm of Excursion

on a side note,

i think 80% is a bit above the maximum efficiency of a classB output stage unless you drive it into clipping. the class B amp, fully clipped, can attain efficiency above that of the class D amplifier (if the output stage allows).