Welcome back to the North Balwyn Repair Cafe online channel. Today we have just a
short video for you showing you how you can check you amplifier that you just picked up
to make sure it doesn't blow your speakers. It's very easy for this to happen and there are lots
of faults in amplifiers that can make this occur, but using a multimeter you can do some quick check
just before you connect up those speakers. anyway, I hope it's useful for you.
So it's a pretty common situation. You find yourself with a second-hand amplifier that
you bought cheaply off eBay or Gumtree or something like that or one of your
neighbours and you think you'll take it home and connect it to your speakers at home.
What I'd like to do is to give you a couple of things for you to check before you connect
the speakers to save them from potentially being damaged. All you need is a multimeter.
Now you don't need anything fancy at all. I'm showing you what I use in
my office here at the moment, but you can use any multimeter you like as long as you can measure
DC voltage, which they all do, and you need to use it in it's lowest possible setting.
I'll just rearrange this amplifier so that you can see the bits that we need to test.
So you can see now that I've turned the amplifier around to show you the back face and also to
just make sure and reinforce the point that there is nothing connected to the inputs, and there
are no speakers currently connected - you don't want to do that yet until you've done the test.
So the amplifier is turned on, and what we're going to measure, and the important bit about
this video, is we are going to measure what's called the "DC offset". The DC offset is basically
the DC voltage measurement on the terminals of the speaker, so in this case
I'm measuring the left channel and the DC offset is measuring 0.027 - 27 millivolts.
And so I'll go and do the same measurement on the right channel, and that's about 20 millivolts.
So it's as simple as that - measure the voltage across the terminals for each of the channels,
with the amplifier turned on and with no inputs connected.
Now in fact what you really should do is let the amplifier warm up for five or ten minutes before
you take these measurements - the measurements are likely to stabilise a bit better after five or
ten minutes and so you measurements will be more representative of the DC offset of the amplifier.
So why do we care about DC offset of amplifier outputs?
Well to explain it in a very, very simple way, your speaker drivers are driven by oscillations or
variations in the DC voltage across the terminals and if that is stuck at a higher level then what
it does is it drives the speaker cone to it's limit and it can actually cause excessive current
to run through the wires of the speaker and burn them out and so it's to be avoided.
So what's a safe level of DC offset? Well you can probably find lot's of debate about that online,
but the rules of thumb that I generally use is that anything less than about thirty-five
to forty millivolts is probably OK. If it's less than 10 mV then it is pretty darn good.
If it's more than 100 then I'd be starting to look at whether I need to do
some adjustments or potentially do some repairs of the amplifier and
certainly if it's several volts and up to 30 or 40 volts, which is common if there's a
particular failures in the amplifiers, then it's time to actually get in and do some repairs.
So what you are looking for when you measure it, and you saw the the numbers for this amplifier
a bit earlier - around 20 - 25 millivolts - I'm happy that this is a safe amplifier
to connect to my speakers and I'll continue on with further testing and enjoying the music.
So anyway, I hope this is useful and if you have any questions please send them through
and if you like the video, if it's useful for you, please give us the thumbs-up
and we'll look forward to giving you a few more of these sort of advice videos.
Thank you again for coming to our North Balwyn Repair Cafe YouTube channel.