Entertainment Center: TVs, Stereos, VCRs and DVDs - Advent AV570 Powered Speakers, One with HISS

I've got a pair of AV570's a friend gave me. These are amplified two way speakers I believe originally designed for multimedia. The units can each take a 110 or 14v power source, have a standard RCA in, and utilize a 35 watt amplifier. Controls are a powerswitch, volume, bass, and treble controls.


One of the units has a hiss that stays on constantly the minute you power it up. Sometimes it will crackle and pop but the hiss is constant. Changing tone or volume does not effect anything. Having an input plugged in or not does not matter. I've disconnected the woofer and tweeter individually and the sound comes through both.

I was hoping it would be a bad ground or something loose in the inside. I think it pops and crackles more when I shake things but I'm not certain. At any rate nothing so far has reduced the amount of hiss.

Any opinions on what direction I should go to tackle this?

A bad ground will show up as something like a humming noise (the mains frequency gets amplified through a ground loop) or you may get sqealing noises (due to instability caused by positive feedback somwhere in the circuit).

Hiss is the classic noise caused by the so called thermal or "white" noise. This is just the random movement of the electrons inside the semiconductor devices. My best guess would be that the gain of one of the amplifier stages after the volume control has changed. Another possibility is a faulty component like a transistor. If the volume of the one with the hiss is louder than the one without for the same setting of the volume control while playing music this could be the cause.

White noise is exactly what it sounds like. Thanks for the advice.

The volume is slightly louder on the one with the hiss, but not much louder.

If the volume is not hugely different another guess would be that one of the resistors in one of the bias chains has gone high in value thereby injecting unwanted noise into the circuit. In a resistor white noise is proportional to the resistance (I think total noise power is 4kTBR - is was a long time ago since I did this calculation). So if it has gone up in value it could be injecting noise. If this has happened the bias point could also change and you may notice the sound is either distorted or gets distorted quicker than the one without the hiss as you turn the volume control up.

Unfortunately the only way is to troubleshoot the circuit. Normally one would use a scope and a circuit diagram. However, since you have two identical units what you can do is to buy a cheap audio signal tracer and use it to listen to the signal thorugh the various stages and compare both units. You then may be able to figure out where the hiss is coming from and do a fix. You will still need to probably look at the circuit and figure out how it works so you may need to get hold of a circuit. You also need to have some understanding of how the different parts of the circuit operate.

IMHO the simplest way to deal with this problem is to bypass the amps and use them as regular speakers.