HUM: Never Give up!

[DRBILL]

There is almost always a posting about hum somewhere on the Forum. Hum is the bane of every tube owner's existence. There is always a little. Sometimes you have to put your ear up to the speaker, but it's there.

Most equipment would pass when used with typical modren low-efficiency speakers. But hook them up to heratige Klipsch speakers and there it is in all of its cloying glory.

I had a very faint hum in the right channel of a pristine Dynaco PAS-2. It was not the common 60Hz hum, but the series of harmonics above the missing fundamental. This preamp was feeding Mark IIIs feeding Klipschorns.

I did the usual search including ground loops, power cord polarity, proper dressing of internal leads, shorting plugs on unused inputs, etc. Nothing. I replaced the 12X4. Nothing. Going for bigger bucks, I replaced the quad filter cap. Nope. I added additional chassis ground to the PCs. Still humming, right channel, only.

Drastic measures were needed. I decided to lift the lead from eyelet 9 of PC-5 (volume control to grid) to see if the hum was originating from the board or before the board.

I grasped the wire with a long nose and before I touched the iron to the eyelet the wire lifted cleanly out! A 40+ year old cold solder joint! This joint would have passed the most descriminating QC eye.

Maybe twelve hours work, $50+ in parts, and a one second fix. My guess is that the cold joint was acting as a detector and was picking up the diode whine from the heater supply. With an antenna, I could have probably gotten the news!

This is a long story. But it goes to show how elusive hum can be. Never give up. Something causes it and that something can be found and fixed.

Any other hum stories out there?

DRBILL

[BillH2121]

My hum involves my turntable - a Scout. When I turn the volume on my preamp up all the way on all inputs except the phono input, dead and absolute silence. However, when I turn the volume up to approx 2:30 or 3:00 o'clock position with the phono input selected, a hum is heard - not loud, but nonetheless heard. The hum intensifies when I touch the interconnects from the tonearm to the TT. Also, the tone arm becomes microphonic when I tap on it with the volume at the 3:00 o'clock and above level. I made a new TT ground wire last week which helped some. I've checked all the interconnects and had the preamp checked by a McIntosh service tech. The hum is not audible at normal listening levels, but it bugs me anyway.

[DRBILL]

With the phono leads unhooked from the preamp, do you get hum? If, so, it is a preamp problem. But if it only hums when the phono cables are attached to the preamp, it would have to be the TT, cartridge, or cables.

I am a superannuated citizen (old F**T) who doesn't keep very well up to date on new equipment, so I doun't know about your Scout TT. How does the cartridge shell connect to the arm? It there a quick disconnect so that you can change the cartridge/shell? If so, there are probably spring loaded contacts. These are terrible offenders. You can clean them with alcohol after you have brightened them with a common pencil eraser. That may take care of everything.

Back in the heyday of audio, we always used shorting plugs on any unused preamp input. It is a practice that should be put back into general use. It will make a quiet preamp even quieter. If you can't find any, you can make some out of common RCA plugs. Just solder a link between the pin and the ground shell.

You are attending to the ground problems which is good. Be careful not to allow audio cables near power cords. Never run them parallel.

I've probably told you a lot of stuff you already know. Good luck.

Let me know if you are successful.

Regards,

DRBILL

[homemade]

I would assume that a preamp turned up that high (3:00 o'clock) would run the amp into clipping and the Khorn would be extremely loud at that point! I'm sure I would hear turntable hum also at that volume level on my cheapo Yamaha reciever if I still had the turntable hooked up!

[DRBILL]

Well, yes. Assuming that he was actually playing a record. We are talking about quiescent state. You might well expect some thermonic "whoosh", but it is not unreasonable, in quality equipment, to expect it to be reasonably hum free.

I don't think his expectations are unreasonable. He is conducting a perfectly valid test of his equipment. I wish there were more people like him. Perfection is impossible, but we can at least nudge it a bit. It is the Klipsch "way".

Thanks for your thoughts.

DRBILL

[BillH2121]

DRBILL - thanks for your response and I apologize for my late reply (I spent the weekend painting our living room). Yes, when the inputs are disconeected from the preamp the hum disappears although there is a sound resembling white noise at full volume. I'm pretty sure my problem is in the interconnects from the TT to my preamp. Complicating matters is the fact that all my home theater equipment and TV is sitting a foot or so from my turntable and all equipment is plugged into one power bar. Plus, because my McIntosh preamp is over 30 years old, there is no 3 prong plug - there's not even a ground side (both prongs are same size. I notice from my reading of this Board that a ground loop is blamed for many problems like this - I wish I understood more about ground loops and how to diagnose and solve the problem.

[NOSValves]

There is almost always a posting about hum somewhere on the Forum. Hum is the bane of every tube owner's existence. There is always a little. Sometimes you have to put your ear up to the speaker, but it's there.

DRBILL

It is? Are we talking general noise or true hum? to me hum comes only from the woofer. Both my tube systems have ZERO hum with your head stuck in the lascala base bins. Now both my systems have a very faint hiss out of the tweeter/mid horn with you ear up to the horns. To me this is completely exceptable.

Craig

[DRBILL]

Well, I guess this would be "exceptable".[]

DB

[Daddy Dee]

Dr. Bill,

Thanks for the encouraging account of finding the problem causing the

hum. To a non technical person like me, hum is the kiss of death in a

piece of vintage gear. One reason, it's pretty unlikely that the source

would be identifiable by me. The second reason, no one can "afford" to

pay someone to look for it. The tech guys who repair and rebuild amps

for a living actually have to be compensated for their time.

One more thought, someone might ask a friend to take on the gear as a

labor of love if the friend is a technical type. However, I know when

people ask... "pray for me," they usually have no idea what they are

asking.[]

[DRBILL]

"One more thought, someone might ask a friend to take on the gear as a labor of love if the friend is a technical type. However, I know when people ask... "pray for me," they usually have no idea what they are asking."

Send 'er on over, or better yet bring it! It will soon be cool enough to return to the bench.

DRBILL

[Daddy Dee]

DRBILL,

Thank you. You are most kind. Actually my gear is in pretty good order

for the present. I do hope it won't be too long before I can make a

trip to FW.

Sounds like my idea of a good time to hang out at your bench and watch.

[richinlr]

I have an article somewhere by Sumiko that talks about equipment polarity and discusses the fact that a three prong polarized plug is no guarantee of correct polarity. The article goes on to tell you how to determine the correct polarity of your comments by measuring the potential on the chassis with the neutral leg on the power cable (if I remember right???). I once did this for every component in my system and about half of the plugs were wrong way around. On the three prong and polarized plugs, I had to unsolder them and swap them around. Once I did every component the difference was pretty incredible - hum vanished.

Maybe you don't want polarized plugs - you can just flip 'em around. If I can ever find the article I will scan it (yes it is paper and it's old) and post it for everybody.

[russ69]

DRBILL, you said; "Hum is the bane of every tube owner's existence. There is always a little. Sometimes you have to put your ear up to the speaker, but it's there."

I agree that it is somewhat common but I have 2 complete tube systems that are dead quiet. I don't know why some tube systems have more hiss and hum than most solid state systems, perhaps due to power supplies and noisy tubes mostly or too much gain. I always ask before I buy a component if it is "dead quiet" and so far I've been lucky. You did a good job of finding your problem, I'm not sure most of us would be that clever.

Thanx, Russ

[WMcD]

Good to see you again DRBILL.

I had an odd problem with one of the original Sony Walkman FM radio. A very good performer to this day.

The headphone jack went intermittent. Wiggling the plug made it seem like a problem with the plug but the 'phones were good when used on other units.

Upon taking it apart I saw there was a hariline crack in the solder connecting the jack to the printed circuit board. The joint was not gritty or lacking solder.

A touch of an iron and a little solder solved the problem.

I was surprised that the soft solder could crack at all. None the less, this might be further to the notion that solder joints can fail because of mechanical fatigue.

Who knows about expansion and contraction from heating in other situations. This is apparently an issue in microelectonics I wonder if older tube type units have joints which look good but have hairline cracks.

Best,

Wm McD

[jamesV]

Check the thread date, 2005

James

[philipbarrett]

I have an article somewhere by Sumiko that talks about equipment polarity and discusses the fact that a three prong polarized plug is no guarantee of correct polarity. The article goes on to tell you how to determine the correct polarity of your comments by measuring the potential on the chassis with the neutral leg on the power cable (if I remember right???). I once did this for every component in my system and about half of the plugs were wrong way around. On the three prong and polarized plugs, I had to unsolder them and swap them around. Once I did every component the difference was pretty incredible - hum vanished.

Maybe you don't want polarized plugs - you can just flip 'em around. If I can ever find the article I will scan it (yes it is paper and it's old) and post it for everybody.

I'm building a balanced PSU which negates the whole polarity thing as each leg caries 60V, I'll keep you posted on the results. One of the problems of owning ultra-high efficiency speakers systems!