Useful And Simple Amplifier Test

A test that I commonly use on my servicing bench to verify performance of a repaired amplifier is as follows.

Take a Y-Lead and feed both channels with a music source.
Connect a single speaker between both output active terminals.
At medium volume setting, adjust the balance control to achieve nil output from the speaker.
Now adjust the volume up and down to gauge channel balance over the full volume range.
Turn the volume up to clipping to gauge behavior in overload condition.
Swing the tone controls to gauge tone control channel balance.

By adjusting the balance control left or right, speaker output level and absoloute polarity can be chosen.
By adjusting the volume control, the amplifier operating point can be chosen.

I find this quick and simple test to be very informative of the channel matching charcteristics of stereo amplifiers.

Polarity & Level Settable Amplifier

Yes, this test will quickly and easily determine any non-matching of the two channels, and without test equipment and with real world music signals.
This test at clipping level is also particularly revealing.
Many amplifiers actually sound friendlier imo with an active load, and the ability to invert absoloute polarity easily is very handy.

With a four channel amplifier box (4 * LM3876 for example), and a dual gang linear pot, output level and polarity can be easily adjusted without any switching or other change in the replay system - see attached diagram.



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    4 ch amp.jpg
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Maybe this test would be more informitive if, for example, the output of each amplifer channel were loaded with an 8 ohm, wire wound, high wattage resistor. (OK give me just a little inductive loading here.)

This is a simple test I have used for years and does what you say it does but I have found it more revealing if I load the amplifer outputs and then use a high Z speaker or headphones across the outputs. (When using headphones remember to keep the volume low or add lots of series resistance, a pot would be good, to keep from blowing up your headphones or you ears.)

Have fun
Improved Testing

Hi Bruce, I agree both channels should have a proper speaker load.
I agree that if using enclosed headphones (to exlude speaker room sounds) you can have a more correct listen to the difference to the channels loaded with real world loudspeakers.
Resistive load on each of the two channels is not really a proper dynamic test.

roddyama said:
Hi Jan,

Not OK. When the two channels are balanced, the voltage differential between the channels is equal to zero. Zero Volts = zero current, regardless of load.;)

Time for bed?:confused:
Rodd Yamas***a

Not OK not OK. Not wanting to split too many hairs, but for me this is important. "Regardless of load", no the point is that the load for the amp is infinite. Maybe you wanted to say "regardless of impedance between the amp outputs", that I agree.

Goodnight Rodd,

Jan Didden