HELP!- Hum noise on o/p of LM3875 AMP

[craigS]

Hi,

I've just finished building my first LM3875 amp with reg power supply.

The amp is very good except for the fact that you can hear a slight hissing noise from the speeker (tweeter) if you hold your head up against the speeker.

Is this normal? - I was expecting the amp to be 100% silent when there was no signal present?

Please help!!!!!!!

Craig

[lineup]

No, this is not normal.
There may be a little detail you have to fix.

How does it work otherwise?
When you play music?

[Nordic]

It probably also gets a bit hot, and may be oscilating. Connect a 10R cap and 0.7uH inductor in parallel, then connect that to the output... this should help, alternatively you can try another Zobel arrangement.

[craigS]

In reply to lineup

"There may be a little detail you have to fix"

What detail were you thinking of?????

I've noticed when listening that the bass / lower mid range decays??!!!

[lineup]

Originally posted by craigS
What detail were you thinking of?????

I've noticed when listening that the bass / lower mid range decays??!!!

It is impossible to say, without knowing some more facts.
You have any link to a website/schematic that shows your setup?

If you study the application instructions in LM3875 datasheet
you will for example see there are some issues how to wire Ground.

When designing a layout, it is important to return the load
ground, the output compensation ground, and the low level
(feedback and input) grounds to the circuit board common
ground point through separate paths. Otherwise, large currents
flowing along a ground conductor will generate voltages
on the conductor which can effectively act as signals at
the input, resulting in high frequency oscillation or excessive
distortion. It is advisable to keep the output compensation
components and the 0.1 μF supply decoupling capacitors as
close as possible to the LM3875 to reduce the effects of PCB
trace resistance and inductance. For the same reason, the
ground return paths should be as short as possible.
In general, with fast, high-current circuitry, all sorts of problems
can arise from improper grounding which again can be
avoided by returning all grounds separately to a common
point. Without isolating the ground signals and returning the
grounds to a common point, ground loops may occur.

Many think you can not go wrong.
Not with a simple chip.
Unfortunately things are not quite that simple.
There are many details to think of
and a number of things that can make LM3875 not working properly.