[milkweg]

My old Audiolab 8000A was loaned out to someone for years and now that I have it back it has hiss when turning the volume up to full. I don't remember having that issue before so something must need replacing, but what? I took it apart and cleaned the inside with compressed air and tuner cleaner but that didn't help. Needs a new PSU maybe?

 

[billybob_jcv]

Are you listening to it with the same upstream and downstream components as before? It could be lots of things - from an electronic component (capacitor probably) that is breaking down to a gain mismatch with either your source or your 'phones.

An amp *amplifies* so any noise in the signal (plus what may be added by the amp itself) is magnified by the gain of the amp.

 

[Gollie]

A bad capacitor or improper grounding of crucial components (input or output) can cause a hiss. This is a huge pet peeve of mine also. I can't stand floor noise.

 

[deltaydeltax]

Background noise is, in its simplest form, input resistance multiplied by your input bias current. R*I = V. Amps with a lot of gain in one stage of the amp usually tend to exhibit more background noise than necessary. This is common where someone uses some ridiculous value like 1M ohm for a feedback resistor.

Pull all of the source cables, leave the speakers connected and see if it still makes that hiss you don't like.

 

[milkweg]

OK, just tried that and there is hiss and a small amount of hum too. When I switched it to phono the hiss was a lot louder too. I'm not using the same source as I used to years ago (used to use Rotel cd player and am now using a PC via digital connection to EMU 0404 DAC) but neither my EMU 0404 USB amp or Onkyo SR505 receiver has any hiss at all so it is not the source causing it.

 

[billybob_jcv]

Just because the source has no audible his on its own doesn't mean that it would not cause hiss when combined with the other components. As an example of that, I point to perfectly good DAPs combined with excellent high gain amps and efficient IEMs - the result is often hiss. I'm not saying that's the issue in this case - I'm just trying to point out what might be a flaw in your assumptions.

The others have given some good info - I'm out of ideas...

 

[earwicker7]

Quote:

Originally Posted by billybob_jcv /img/forum/go_quote.gif Just because the source has no audible his on its own doesn't mean that it would not cause hiss when combined with the other components. As an example of that, I point to perfectly good DAPs combined with excellent high gain amps and efficient IEMs - the result is often hiss. I'm not saying that's the issue in this case - I'm just trying to point out what might be a flaw in your assumptions. The others have given some good info - I'm out of ideas...

x2!

I once had a problem with hiss while using my CD player; however, it only effected one of the three amps I had. Everyone's assumption was that the amp was faulty, but it turned out being an interaction between components and the RFI/EMI/dirty power at my old place. I was actually able to solve it with an interference rejecting power cable to the CD player.

 

[milkweg]

Quote:

Originally Posted by billybob_jcv /img/forum/go_quote.gif Just because the source has no audible his on its own doesn't mean that it would not cause hiss when combined with the other components.

Yea, but I listened to the amp over headphones connected to no source as suggested and the hiss and slight hum is still there when turning the amp up to full volume so it can not be caused by the source.

 

[deltaydeltax]

Sounds like you have one noisy amp on your hands. Does the power plug have three prongs?

 

[milkweg]

Yes. It never used to be noisy so something needs replacing but I don't know what so will just have to live with it. Like I said though, you can only hear it at loud volume level.