Pre-amp dependent amp hum - is this normal?

[Neuronal]

OK, beginners question I think, but I'm trying to learn and couldn't figure this out online. When I turn my amps on (Paramounts) with my pre-amp (EFPIII) off, I get a ton of noise in my system that sounds like 60 Hz hum; this nearly entirely goes away when I turn the pre-amp on. If I short my amp inputs and measure hum, it is not perfect but acceptable (2.5-3 mV). Music sounds great (although background noise is slightly higher than I'd like). If I have hum caused by a ground loop, why would it go away when I power on the pre-amp (isn't the ground plane the same with the pre-amp on and off)? Is this normal behavior or something I should dog down? I've rewarmed every ground connection I can see, with no change.

 

[Paul Birkeland]

I would not turn your amplifiers on before turning your preamp on.  What you're experiencing isn't necessarily abnormal. 

[Paul Joppa]

The preamp, when operating. provides a low impedance to ground at the grid of the power amp driver - i.e., it acts as a shorting plug. With the preamp off, the impedance to ground is much higher and more susceptable to electric fields.

[Neuronal]

Thanks to both for those replies - much clearer to me now (and I won't be turning on the amps without the pre-amp anymore!).

[Paul Birkeland]

Along that line of thought, you could change the output to ground resistors at the output of the Foreplay to a lower value.  50K might be enough to knock that out.

[Neuronal]

I'll try it and let you know!

[Neuronal]

OK, I'm having a different issue that seems conceptually related to my first question, so I thought I would post it here.

I just got stereo powered subs (Omega DeepHemp 8s) to go with my Omega CAMs (which are terrific with my Bottlehead gear). I'm running speaker wire from the posts of the CAMs into the DeepHemps. Here is the issue - with my main system off, and with the sub "on" but not receiving signal (it has one of those signal detection circuits, so there is a little red light that goes on under these circumstances, which turns to green when it sees a signal from the amp) I hear a hum in my CAMs (not in the sub). It is loud enough that you can hear it if you are sitting in the chair that is next to the speaker. If I actually turn off the sub this goes away, and it is not a ground loop (because it is not resolved by an isolation transformer), and it is not RF noise because if I switch to shielded speaker wire it is still present. Here is the kicker - it depends on my main amps also being hooked up to the speakers - if I unhook the speaker wires coming from my Paramounts (which, again, are off when this happens), the hum goes away as well.

Any thoughts as to what might be going on here? Anything worth trying (short of switching to RCA inputs from my pre, which is impractical given my room configuration)? I could of course turn off the subs whenever I'm not using the system, but that is a bit of a pain, and I wonder whether this noise is adding to my tube noise when the main system is actually on. And of course, I'm not hurting the Paramounts or their tubes as a consequence of any of this, am I?....

[Jamier]

Is this connection scheme recommended by the manufacturer?

Jamie

[Neuronal]

Yup, this is what Omega recommends for stereo subs; I asked Louis, and he is at a bit of a loss as well....

[Paul Birkeland]

In the Paramounts there are wires from speaker - to the chassis.  You could try removing that connection in each amp to see if it helps.  Traditionally with series feed amplifiers with high voltage on one side of the output transformer and your speaker on the other, earthing the transformer secondary provides some extra protection just in case there's a catastrophic failure.  On the Paramount, the tube side of the transformer only sits at about 70V, so the risks are not so substantial.

Another way to determine if this might help is to pull the power cords out of the Paramounts while they are powered down to see if that changes the noise.  If it doesn't, also try pulling the RCA cables out of the Paramounts.

[Neuronal]

PB - thanks for your advice! Taking out either the power cord or the RCA cable does nothing, but both together kills it entirely!

Is the wire you are recommending I cut the one from OT terminal 2 to the black binding post? Is there some equivalent to the "diode trick" that might preserve some of the protection here (I'm probably thinking about this all wrong, but can't hurt to ask)?

thanks for your help on this - super cool you were able to do a remote diagnosis like this! What do you think is going on to cause this?

[Paul Birkeland]

Is the wire you are recommending I cut the one from OT terminal 2 to the black binding post?

Is that the wire from the negative speaker terminal to the chassis? 

If you remove one of the wires from the output transformer to the speaker posts, you'll get no sound out of the amp.  There's an incredibly obvious piece of buss wire soldered to the speaker - binging post that goes back to the solder lug attached to the chassis.  This is the connection between the speaker - post and the chassis.  This is the connection that should be lifted, not the black wire from the output transformer.

[Neuronal]

gotcha, I'll try it and let you know....

[Neuronal]

OK, I did it (don't worry, I popped the correct bit of buss wire but, alas, no change in the buzz. Still goes away if I unplug the power cord + RCA, but not if I do either alone. Anything else worth trying? If this helps, there is slightly more buzz from one speaker than the other (where it is also power cord + RCA dependent).

[Paul Birkeland]

Removing the bit of buss wire from the chassis to the output transformer provides the same type of isolation that you get when unplugging the power cords and RCA cables.

Are the power cords on the sub amps 2 wire or 3 wire?  Are there line level inputs on the powered subwoofers?