Can I find out the output impedance of my amp?

You can put DMM only when 1v/1000Hz signal is given to the amp.

Hi Mark,

I strongly suspect that the output impedance of the amplifier is NOT 8 ohms, and that the "Output Impedance: 4 and 8 ohm" specifications given at the Art Audio site are actually (and confusingly) referring to the speaker impedances that the output taps are designed to work into. The output impedance of the amplifier will usually be considerably different than the load impedance it is optimized to work into.

And if, as I suspect, what you are trying to accomplish is to verify that the output tap has not been customized to work into a load that differs from 8 ohms, a measurement of the amplifier's actual output impedance won't help, at least without being analyzed in the context of a schematic and other detailed design information.

You might try contacting Art Audio UK, and asking them.

FWIW, though, the one method that occurs to me of accurately measuring the amplifier's actual output impedance is to play a test tone into, for example, an 8 ohm high power resistor, then playing the same tone at the same volume control setting into say a 4 ohm high power resistor, and measuring the voltage that appears across the resistor in each case. The output impedance of the amplifier could be calculated from the amount by which the voltage decreases as the load impedance decreases. While the measurement would have to be done on only one channel, to avoid possible damage to the amplifier BOTH channels should be loaded, either with a resistor or with a speaker.

Best regards,
-- Al

Al,

"And if, as I suspect, what you are trying to accomplish is to verify that the output tap has not been customized to work into a load that differs from 8 ohms,..."

Yes, you are correct. Not even sure how it left the distributor when new.
Thanks for your suggestion. I actually think I can handle that. I have an email into Art Audio UK, I'll first see what they say.

I don't know of any output taps that are exactly as stated. VTL/Manley set their output impedance to 6 ohms so their amplifiers would be compatible with either 4 or 8 ohm speakers.

I actually think I can handle that. I have an email into Art Audio UK, I'll first see what they say.

Hi Mark,

If they can't provide the information for your specific amplifier, ask them what the damping factor is for the standard configuration that is optimized for an 8 ohm speaker.

The output impedance of an 8 ohm tap equals 8 ohms divided by that damping factor number. You could compare that result with the output impedance of your specific amplifier, as determined via the measurements I described.

I worked through some equations, and the output impedance of your amplifier can be calculated from those measurements as follows:

Let V8 represent the test tone voltage measured into an 8 ohm resistor (connected to the 8 ohm tap, if there are multiple taps).

Let V4 represent the test tone voltage measured into a 4 ohm resistor (connected to the same 8 ohm tap, not the 4 ohm tap, if there are multiple taps). The volume control setting should of course be the same as for the previous measurement.

Let R represent the amplifier's output impedance.

R = (8(V8 - V4))/(2V4 - V8)

I suspect that the answer will be around 1 or 2 ohms for the PX-25.

Also, for several reasons the test should be performed at a low volume level, as monitored with a speaker connected to the channel you are not measuring. Given the 101 db efficiency of your speakers, at low volumes you'll probably be measuring voltages that are in the rough ballpark of 20 or 30 millivolts or so.

Best regards,
-- Al