How to find watt output?

[ShetiPhian]

I have an old amp, that still works very well.
I want to add some Aura Base Shakers, the Power handling is 25 watts each.
I dont know what the output/per channel on my amp is, but I'm thinking its 65watts.

So I've got two questions:
How would I test the output of my amp to get the watts?
How would I prevent the extra wattage from damaging my shakers?

[wintermute]

If you have an 8 ohm dummy load and a multimeter it is pretty easy to test the power output (however without a scope it will be difficult to see if the amp is clipping). If the amp clips you will get higher readings usually.

hook up your dummy load to the speaker output and measure the AC voltage across it.

Ohms law states that V = I * R we know V and R so we can work out I. I just did it on my gc at 100Hz and 10Khz.

at 100Hz I got 13.2V

13.2 = I * 8 therefore I = 13.2/8 = 1.65A

Power = V * I = 13.2 * 1.65 = 21.78 W

Interestingly I got 20.5 V at 10Khz which equates to about 52W.

Unless you have a true RMS multimeter these calcs are not to be regarded as super accurate.

With regards to having an amp with more power than the speakers this is not at all a bad thing, provided you don't turn it up too loud. your ears should tell you when you need to cut it back a bit It is actually clipping (amp running out of power) that is more likely to damage your speakers

Tony.

[ShetiPhian]

good O' Ohms law, I never considered using it to get the amps.
I also never thought of connecting a dummy load.

Thanks for pointing me in the right direction.

[wintermute]

happy to help

please ignore that calc with the 10Khz sine wave, it seems my multimeter gets very inaccurate with high freq ac. double checking in TrueRTA revealed 14.4 V at 10Khz... the difference between that and the 20Hz reading being most likely due to the amp having started to roll off at 20Hz.

Tony.

[Nordic]

On an amp that is known to be working, can one use a speaker instead of a dummy load...?

[maxro]

Originally posted by Nordic
On an amp that is known to be working, can one use a speaker instead of a dummy load...?

I think the idea is to find out maximum amplifier power output without overloading your speakers (or eardrums). Also, a speaker's impedance changes with frequency, so you would need to know what the resistance is at the test tone frequency.

Max

[Nordic]

Thanks

[sagarverma]

roll off??

is it end of response of amp to the i/p??

[wintermute]

The amp starts to roll off (freq response no longer flat) at a certain point, though I suspect that my sound card preamp is doing the most rolling off (and the test I did shows it is only down about 0.6db at 20Hz).

The value of the feedback capacitor (if used) in conjuction with the feedback resistor which goes to earth through the cap, has an effect on the low freq roll off point for the amp. In my case I think the corner freq should be 5Hz.

This is an area I'm not too familiar with, so If someone else wants to elaborate.....

Coupling caps also have an effect though in my case I'm not using one. for a comparison of different caps on input causing different rollof have a look at this rmaa test I did before and after increasing the value of the coupling cap for my SC preamp http://home.swiftdsl.com.au/~tonywww...mpare_MKII.htm

Tony.