New noises

[cpaul]

I've shelved Quickie at different times while I've had it, sometimes to mod or experiment with it, sometimes to use other gear.  I just resurrected it over the last few weeks and I'm getting a bunch of noise I've never had before.  First, with Q sitting on a shelf above my Marsh A200 amp (125wpc) I now get a substantial hum.  It's maybe 3-4 inches above the top plate of the Marsh, and just to the side.  It's the same location and a pretty much the same setup to earlier when I had no hum.  The Marsh is 56k input impedance and 1Vrms input sensitivity.

Second, after moving Q to another new location as a preamp to a single-ended 6BM8 amp which has it's own volume control (10k PEC pot with a 27k inline resistor before it, so 37k input impedance I believe, no idea of input sensitivity - I'm planning to bypass the 6BM8 volume control assuming I like what I hear) the system now has a hum and a buzz with the 6BM8, which has always been DEAD QUIET.  I've re-checked by taking Q out of the picture and it goes back to dead quiet running the CDP directly to the 6BM8.

With Q in the system, if I run the 6BM8 at full volume and use Q to adjust the volume of the system I get the hum and buzz.  The buzz is adjustable by the Q pot but the hum is not.  The hum and buzz, on the other hand, are both adjustable using the 6BM8 pot.  If I run the 6BM8 pot at just under 1/2 turn and us the Q pot to adjust volume, both the hum and buzz largely disappear.  Somehow, the 6BM8 seems to pick up some hum with Q in the system.  Ground loop?  Not sure what the buzz is.  I'm hoping to try out some new tubes for Q, as mine are pretty microphonic.

Finally if I keep the 6BM8 pot at about 1/2 and control the sound level with Q's pot, I get pretty noise free operation but have to turn Q to near full to get higher sound levels (the 6BM8 is only about 1.5wpc and I'm using 88-89dB speakers).  It sounds clean and undistorted to my ears with Q's pot near or even at full, but I wonder if doing so does increase distortion?  In other words, will the signal distort as it passes through Q with a typical CDP output or a mm phono run through 40dB of gain in a phono stage (the CDP says it's 2V max output)?

Thanks a ton for any comments.

Q Details:
PJCCS installed
Startup and shut-down caps installed per earlier thread

[Doc B.]

Hum that is like a pure 60Hz sine wave without any buzzy content is not from the Quickie, as it doesn't have any AC power. So it has to be picking it up from a nearby power transformer or other source of EM radiation. The 120Hz buzzy stuff is a ground problem. Resolder your connections looking closely at the RCA jacks in particular, check your interconnect cables, etc. - the usual things one does to make sure you have good ground connections everywhere.

[cpaul]

Thanks for the quick(ie) reply, Doc.  I will do a thorough check of grounding.  I know the hum can't exactly originate with Q.  In fact, since it doesn't even adjust with Qs pot, I figured it must be a new ground loop.  But, since it doesn't exist without Q in the signal path, there's a connection.

I also found this while perusing posts about tube sources:

The Rotel is a 70 watt amp with fairly high sensitivity, and the Tekton speakers are pretty efficient - you have much more gain in the system than you need. Try this:

Crank the Quickie volume to maximum, then turn down the Rotel until music is about the loudest you want to listen at. This way, the Rotel will not amplify the microphonics nearly as much. The Quickie will amplify the input signal more, so the total net signal gain will remain the same, it just gets redistributed.

Then listen to be sure the Quickie is not being overloaded on signal peaks. You may have to back off a little to get to the sweet spot, but there should be plenty of improvement available this way. Once you have taken the edge off, you can shop for different tubes more carefully and with less stress.  :^)

Maybe some of it - particularly the buzz - is microphonics being passed on to the 6BM8?

Hmmm.  And now after perusing the forum, listening a bit and writing this, the hum and buzz have substantially diminished.  All I've done is re-wired interconnects trying different combinations to see what's going on, then back to the setup that was a problem for me in the first place.  Aaarrrgh.

[Paul Joppa]

Aaarrrgh indeed!  I feel your pain, believe me - hum and buzz is an evil that is everywhere.

What is called "ground loop" is usually circulating AC currents in the ground lines, with a voltage dropped due to the not-quite-zero resistance of the ground line. For example, if there is current between the CDP and the power amp, then adding a quickie will increase the resistance between the CDP ground and the power amp ground, increasing the noise. Cleaning up the solder joints and interconnect tightness within the quickie would possibly reduce that resistance and hence the noise.

Just a thought.