On Mar 5, 7:18*am, "john reves" wrote:
I have a very small sony amplifier (the portable type such as you would
travel with and attach to an Ipod). I dont have the 9 Volt DC transformer
that came with it, so decided to use one of those 'variable voltage'
transformers that I bought from Lidl some time ago.

It's not clear from the transformer which of the polaritys the polarity
switch is indicating. So to check the polarity i used a voltmeter.

The choice of output voltages on this transformer is 3, 4.5, 6, 7.5, 9, and
12 volts. *The output I require for this particular amplifier is 9 volts.
But when taking a reading from the voltmeter it only reads 7.5 volts on the
9 volt setting.

Checking the others, 3volts is really 2.5volts, 4.5 is really 4, 6 is 5, 7.5
is 6.5 and 12 is 10v.

I read somewhere that a voltage reading from a voltmeter is different from
the reading taken when something is 'under- load'.

Would you use the 9 volt setting (which reads 7.5v) or use the 12v voltage
setting which reads 10volt, for use with this 9volt amplifier. *Thanks for
advice.

I like the switch mode power supply drooping under no-load idea. Can
you put a bit of load on it and test again? 1 k ohm might be enough.

George H.