Help with JVC JA-S11 amp please
 

Hello everyone

I need some help with a JVC JA-S11 amp I got from my father if anyone knows more about them. He had it in storage for a while but gave it to me as a housewarming present.

It mostly works fine but there's one problem:

Usually on this amp after you turn it on you'll hear a click after a few seconds and then the amp starts working. When I turn it on the light comes on but no click. I opened it up to have a look and it started working. Problem is the light no longer comes on but you can hear the click from the relay (I have no idea what it is, someone told me it was the relay) and the amp works.

After I moved the amp a bit the light came on again and the amp stopped working and there's no more click when I power on.

Can anyone help me here? Thanks

Help with JVC JA-S11 amp please
 

"cave_johnson" wrote in
message ...

Hello everyone

I need some help with a JVC JA-S11 amp I got from my father if
anyone
knows more about them. He had it in storage for a while but
gave it to
me as a housewarming present.

It mostly works fine but there's one problem:

Usually on this amp after you turn it on you'll hear a click
after a few
seconds and then the amp starts working. When I turn it on the
light
comes on but no click. I opened it up to have a look and it
started
working. Problem is the light no longer comes on but you can
hear the
click from the relay (I have no idea what it is, someone told
me it was
the relay) and the amp works.

After I moved the amp a bit the light came on again and the amp
stopped
working and there's no more click when I power on.

Can anyone help me here? Thanks



Good amps have a relay - an electrically operated switch - in
series with the loudspeaker outputs to protect the amp and to
protect the speakers.

If the amp has a single positive power supply then the amp output
will settle at half the supply voltage, e.g. if the amp runs off
+50V then the amp output will sit at 25V. There will be a
coupling capacitor in series with the amp output to isolate this
voltage from the speakers and which will cause a thump in the
speakers at switch-on as it charges up. If the speakers are not
connected at that instant and the capacitor can charge up through
some other route provided then when the speakers are connected by
the relay activating there will be no thump. The capacitor can
also affect the sound quality of the amp.

It is more common these days for the amp to have positive and
negative supplies. As with the single rail amp described above
the total supply voltage may still be 50V but made up of +25V
and -25V which, by the description above, means that the amp
output will sit half way between these supplies, or 0V. The other
side of the speakers is connected to 0V, so is thus no need for
the isolating capacitor and sound quality can improve as a
result.

However, to make sure that there is no fault at switch on, the
speakers are not connected to the amp output until such time as
protection circuits are happy that the output is within about +/-
100mV of 0V. This voltage error is known as the offset. Anything
more than about 100mV offset can cause distortion and will
eventually damage the speakers. As most electronic components
fail at switch on if one of the output transistors went short
circuit the loudspeaker could have +25V or -25V across it
static - which would fry it in no time along with a burning smell
and smoke if indeed not a fire.

The click you hear is the relay switching to connect the speakers
to the amp output. If there is no click then the sensing
circuitry thinks the amp is faulty and is protecting the
speakers.

There could be a number of faults:-
One rail of the power supply could have failed or be
intermittent;
There could be an unacceptable difference between the supply
rails causing a static offset on the amp output;
There could be a fault in the output stages of the amp, indeed
modern amps are direct coupled throughout so almost any component
could upset the voltages (commonly called biasing) which could
again give a significant output offset and trigger the protection
circuitry.

If you think the amp is worth it get someone to have a look at
it - Richer Sounds and Comet both used to offer a repair service
but at a cost.


--
Woody

harrogate three at ntlworld dot com