How much noise is considered normal?

[Leigh]

Hi folks, first post here.

Background: Have been running the Foreplay III for 2 years until a couple weeks ago when I did the upgrade to the extended. The III was a huge sonic upgrade from my old NAD preamp and I have very much enjoyed it, and still do.

My power amp is a heavily modified/upgraded Dynaco Stereo 70. I have the Triode Electronics driver board, the SDS Labs power supply upgrade, and I've replaced all the wiring with teflon wiring and have replaced all the tube sockets. All that remains original are the transformers and chassis, I think.

My question is, how much background noise (when the Foreplay is just idling) is "normal"? I ask because while the amp was on the bench for the upgrade, I plugged my source (Squeezebox Touch + Cambridge Audio DacMagic DAC) straight into the ST-70 (I split the output from the DAC into the ST-70 and the powered subwoofer) and the background noise with just the ST-70 when I put my ear an inch from the speakers (ProAc Super Tablette monitors) was almost nil, just a tad of 60/120 Hz, no white noise whatsoever.

With the Foreplay III I have a fair amount of whitish/pinkish noise plus some hum that is more of the light buzzing sort, suggesting perhaps a grounding issue. The Extended background noise is, as far as my memory goes, identical to the original FP III.

The noise occurs with the volume attenuators all the way off and gets louder when they are turned up. The noise is slightly louder when I select a source with no input. Behavior is identical in each speaker. I have the volume padded such that the highest volume pot setting is as much as I'll ever need, but does not overload my speakers.

When I sit in my listening chair, I can barely... barely detect a whiff of the hum/buzz when there is no source. So it's not very loud. But, it's louder than I'd like.

Any suggestions? Or is this normal?

[Grainger49]

The FP III has gain, and a lot of it.  With that gain there is also an associated noise.  The ST-70 is pretty sensitive too.  The problem has cropped up before.  There are a couple of ways to go about shedding some of the gain.  

PJ suggests a voltage divider at the input of the ST-70.  I believe you will find the post somewhere in the FP III area.

Found it:

http://www.bottlehead.com/smf/index.php/topic,626.0.html

[Leigh]

Thank you for the reply. I had read that thread before and wondered if maybe something like that might help.

I moved some things around (cables etc) and changed inputs around nand somehow, magically, the buzz is now almost completely gone, just the pink noise remains - and I have no problem with that. So maybe it's a shielding issue. I will have to keep experimenting. This makes me fairly confident I don't have to reheat solder joints etc... but I may not be out of the woods.

[Doc B.]

If the buzz goes away when you move stuff around it's a 90% certainty that it is an issue with the cables. We are constantly making and trying out new cables around here. With all of the plugging and unplugging we do they can get roughed up a bit, particularly where the shield attaches to the RCA plug and at the interface between the plug and the jack. Wiggle each cable with the amps on to see if you can recreate it and you may be able to get things even better. A contact cleaner like Caig works wonders on jacks and plugs.

[Leigh]

Thanks for the advice, Doc. I have a small vial of Cardas contact cleaner I got a few years ago and almost forgot about. I'll give it a shot.