Problem with muting function on TDA2794

[moonglum71]

Hello All.
first post here (so please be kind :-))
I'm just starting out in the playing with electronics to make my own amp game, and to start out I bought one of these kits:

http://www.quasarelectronics.com/kit...t-kit/1218.pdf

Good fun, and sounds great but the muting function doesn;t seem to work. The kit applies the power supply (+39V) to the muting pin which charges a capacitor. When +3.5V hits the mute pin, the amp comes out of mute. Great - unfortunately it makes a massive thump (though my gash speakers fortunately.) I seem to have found a solution, but I'm wondering if its a good idea.

I've wired a 1M pot between +39 and the earth star. When I use this as a potential divider, I can wind the voltage appled to the mute pin down to 10V, and at this point there is no thump. Is there likely to be any unwanted effects of using this potential divider across the power supply? The current should obviously be down in the milliamp range with only milliwatts dissipated, so my purely physics brain says this is absolutely fine. Any ideas?

Sorry for the complex post - there will prolly be lots more as I tinker!

Cheers

Nick

[jazzman06]

may be TDA7294

[mjf]

hello.
have a look at
r8 =47k
and c9 built in reverted?(mistake in the schematic).
you can compare this with the original datasheet of the tda 7294........
greetings..........

[Eva]

The first chip-amps that I built 10+ years ago were based on TDA7294. I still use them (4 channel).

It's perfectly possible to use the positive supply rail to drive the mute/standby circuit without any thumps. So your problem is probably some mistake in component values or with overall connection.

R6=22K
R7=100K
R8=47K
C8,C9=10uF 50V

Note that C9 is erroneusly wired backwards in the schematic, although it's shown with the right polarity in the PCB.

Never omit C1.

[moonglum71]

Thanks for the replies guys - will closely check everything.
One oddity that seems unlikely to caus a problem, but who knows...
In the construction details, it says to use 0.25mm wire to connect to the mute channel. Is this spec likely to be important?

[Eva]

The gauge of this wire is not relevant, they recommend a thin wire because it's much easier to handle.