Re: Amplifier, impedance and power?

From: Jerry G. (jerryg50_at_hotmail.com)
Date: 03/19/05 

Date: 18 Mar 2005 20:40:47 -0800

What you want is a complex answer. Many of these consumer amplifiers
are not properly rated at their output.

The spec you mentioned may be for the both channels, or is it for one
channel?

Is it for one channel at a time, or both driven together?

Is this spec the power input of the output stage, or the actual
sinewave output to the speaker terminals, in consideration of one of
the first two questions?

Is the rating for the full bandwidth of the audio output response, or
is it at any one frequency?

I can see by your question, the spec you have is very abartuary, and
does not really mean very much.

To have 60 Watts at 8 ohms from one channel, the voltage output would
be a bit over 20 Volts of signal RMS. To have 1/2 the power at 8 ohms,
the voltage would be about 15 Volts.

Obviously there has to be some type of inverter inside the amplifier,
because the drive voltage to the output stage would have to be of the
peak-to-peak value that is much greater than the RMS value. Then there
is the deficiency loss.

Your best bet is to put a 10 amp fuse on the amp for testing, play it
at a fairly high volume (not distorted), and have a DC amp meter in
series with the fuse at the same time. Observe the amp reading, and add
about 10% to the average that you get. The value you come up with
should be very close to the fuse you want to use. If the amp is good,
and the 10 amp fuse blows, then put a bigger one to test. If the fuse
blows at a low volume setting, or with the volume turned down, then the
amp is defective.

If you want to know the true output of your amp, you will need the
proper setup with a dummy load, scope, true RMS meter, distortion
analyser, and an audio generator. With these tools, and someone who is
experienced at it, the true specifications of your amplifier can be
detrmined. To do this on the bench, a very big power supply will be
required. These draw a lot of current when working.

Jerry G.
======

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