[Zero]

I got my brand new Odyssey Khartago amplifier in not too long ago (thanks Klaus, she is finally opening up and is going WELL beyond my expectations!!!!)

I also got a vintage Adcom GFP 565...  which is one of the few Adcom pieces I have actually enjoyed. Well, there is only one problem........  this renouned quiet pre amplifier is anything but!!

There is a huge audible hiss coming from my speakers (which are 86-87 db efficient). You know the sound - like a noisey turn table. I've went through and checked each input and output of the Adcom and everything measures up. All I can figure is that there is an impedence mismatch.

I read somewhere that the Odyssey Khartago inputs are at 10k where as the Adcom outputs at 100. The red flag is going off but I have NO idea how to cure this issue. Any suggestions would be GREATLY appreciated....!!!  

This combination sounds fantastic and there is nothing in it I am wanting to change if at all possible!

[warnerwh]

There should be no impedance mismatch. Usually the minimum should be that your amps input impedance is at least ten times what your preamps output impedance is.  At 100 times like your situation that is not the issue.  As for what could cause a loud hiss I'm not sure. Try a different amp in your system then a different preamp and see if it goes away. There may be something wrong with one or the other.

[yo2tup]

sounds like a ground loop

[rustydoglim]

If you leave the amp on without input connected, is there any hissing noise?
Try this: use a wire to connect RCA ground to chassis ground on the amp.
I think some trial and error with re-routing the ground wiring could cut of the ground loop (or make it worst ).

[_scotty_]

If he leaves his amp on with nothing in the input he runs a good risk of the amp going into oscillation and blowing up. What you are probably thinking of is putting shorting plugs into the RCA jacks on the amps input and seeing if any noise remains.  When an amps input is shorted to ground the amp will exhibit the best signal to noise ratio it can have because there is no noise
from any source going into the amp. The only noise present is that which the amplifier itself produces.
Scotty

[Zero]

Gentlemen,

Thank you for taking to the time for suggestions!

Warnerwh,

That is something I did not know - thanks for sharing that bit of information with me! That puts to rest that theory/concern. Thanks!

Yo2tup,

I do need to resolve a ground issue within the house wiring at some point in the near future. I have been putting it off and now it looks like I can no longer. I believe I have narrowed down the hiss to the pre amplifier.

Jason / Scotty,

I completely failed to mention that in effort to determine the source, I ran the amplifier straight from two sources (a cheapo dvd player, and an E-MU1212M sound-card). It was completely silent.

Now, the Adcom pre amplifier is the only piece not grounded - but I'd imagine this would not be an issue since for the rig intended, both the source and the amplifier are. I got the case open and plan on doing some investigating later this evening. I am also going to take it over to a friends house over the weekend (if possible) who runs all Adcom gear. Theres no reason for this pre amp (that again is acclaimed for its silence) to cause any sort of noise on that equipment unless something is wrong inside.

[Fife12]

I powercord with no ground will probably help.

[craig223]

Make sure the ground loop isn't coming from a source component.  My Direct TV does the very same thing to my system.  If it is unplugged, the hiss disappears.

Good Luck!

[warnerwh]

You can also try a cheater plug and see if this is it. When you say hiss do you mean a humm? As a ground loop will be low frequency, hiss is high frequency.  Also get yourself a polarity checker from the hardware store. These are only 5 bucks and easy to use.M ake sure your outlets are wired correctly as this is a common problem. If you change outlets be sure to switch off the power to that outlet. You can use the polarity checker to make certain there's no power there. Last thing: if you change outlets buy the commercial or better ones for a couple of bucks, don't use the 59 cent ones.

[avahifi]

Step one:  is the hiss gone with the amp connected to the speakers and no input cables connected to the amp?  I know of no rational amplifier that would be unstable in this configuration.

Step two:  Connect preamp to amp with no sources connected to preamp, and preamp volume control turned all the way down.  If there is hiss, it is a fault with the line section of the preamp.  If no hiss, then connect sources to the preamp one by one to find the culpert.

Bad house wiring or ground loops may cause hum, but will not cause hiss.

Another possible source could be large amounts of RFI interference, which would be worse with some exotic interconnect cables that are poorly shielded.  Try replacing cables with another brand and see if that helps.

Frank Van Alstine

[Zero]

Craig,

I do actually have a ground loop issue going on, but this is running through the whole line. I gotta figure out how to go about solving grounding issues in the home. Thankfully, Google is going to be right there to help lead the way.

Frank,

Thanks for your steo by step instruction! Your time is much appreciated! I've come to find that it indeed was the pre amp causing the hiss - at first I feared the line-outs..  but I tried it into a HK 3470 stereo receiver with no issues at all. It looks like a mis-match going on here - gain maybe? Who knows. Kind of a bummer...  

Again, thanks for your instruction as I found the process of elimination very useful. It is also a good piece of mind when the information comes directly from a guru.