Noisy

[alanh]

Hello,

I'm new to the forum but not new to Bottlehead. I built a first-generation Foreplay many years ago. In the meantime I've built a few PTP designs of my own and a few I found by other people. My Foreplay's power transformer died and my choke-loaded 5687 preamp's regulated heater supply died so I was listening to a solid-state NAD pre. That prompted a Quickie (how can you resist?). The problem: it's got some kind of oscillation or at least a constant thumping noise at a consistent frequency in both channels. At first I thought it was something wrong with the PJCCS so I replaced with Hammond 156Cs. No change. I checked and rewired the ground scheme. No change. I have the grid stoppers installed. Any thoughts? Larger grid stoppers? It's not horrible when the music's on, but I'm currently using it to drive my headphone amp. It's not tolerable directly into my ears.

Alan

[Paul Joppa]

About the only thing I can think of that is within the Quickie itself is the cathode bypass cap, which might be installed backwards, or otherwise faulty. You can disconnect one lead temporarily to completely remove it from the circuit as a test - it will reduce gain and increase output impedance but should otherwise perform normally.

If that is not the problem, then if it were mine I'd go through a bunch of tests to examine other components and possible interactions with them and the cables between them.

[alanh]

Thanks. I'll check the caps. Don't think I'll have time to get it on the bench until Saturday or Sunday.

Further explanation of the symptoms: The noise remains a constant volume regardless of where I set the control. And it never changes when I change input (phono pre or CD) or when I swap power amps.

Alan

[alanh]

I probably should be doing something else around the house right now since I was out of town for a couple of days, but I decided to work on the Quickie. The PJCCS is back in (not too many hours of comparison, but I think I like it better than the choke load) and the bypass caps are out. The noise is still there, though it's quieter. Not sure what the next step should be.

Alan

[Grainger49]

As PJ mentioned removing the cathode bypass cap will lower gain.  If you mean it is quieter at the same volume, once you crank it up to make up the difference, then that is odd.  It should be much the same.  You might have jostled the offending component when you removed the bypass caps.

Have you narrowed the noise to the Quickie, that is eliminated the interconnects and all other components?  

The Quickie is battery powered.  So try swapping batteries.  Probably not the problem but it is quick.  If that changes nothing put the old batteries back in and go from there.  

Short the inputs of the Quickie with jumpers, remove the input interconnects and see if it thumps.  If not, it is the interconnects or the source.  If it persists then short the inputs of your amp and see what it does.  Note: this does not absolve the interconnects between the Quickie and the amp.  If it still thumps it is the amp.  If it doesn't then you have to back up and try shorting the RCA plugs that were in the Quickie's outputs, plug the interconnects back into your amp and remove the amp shorting jumpers.  See if the interconnects are the cause.

After all this you can say for sure that it is the Quickie if it is so indicated.  

[alanh]

Problem solved. Thanks for the replies, PJ & Grainger. After ruling out cables, batteries, sources, and power amps I had resigned myself to living with it while I built something else. Then tonight I had it on and didn't hear any problems. After dinner I came back into the living room and it was ticking away. Then my wife said that it started when she turned on the computer. She was right: it was the wireless modem. I changed the channel and the Quickie got quieter. I turned off the wireless and it quit making noise altogether. Now I finally hear the dead quiet everyone else talks about.

[Paul Joppa]

See if you can fins a couple small capacitors, maybe 100-200pF and preferably silver-mica or polystyrene or NPO ceramic (NOT other ceramics!). Place them from pin 3 of the tube socket to the nearest ground - probably T3? This should drain high frequency energy to ground and away from the tube grid, without noticeably reducing the treble.