Need experts for vexing hum problem

So, you've passed 'Old fart', and 'Geezer'. What's the next level? ;-)

Curmudgeon ?:-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

I am fairly certain I haven't switched providers.

I guess that will work for the next decade or two. :)

You're still on Comcast, and in California.

So this isn't your problem?

I doubt there will be any joy for news server users working on it third-hand. It's bad enough when the OP is posting directly.

RL

But the amplifier hums when the preamp is its signal source, and doesn't hum with any other signal source. And it hums with the preamp as it source whether or not the preamp is powered, unpowered, or unplugged.

As I said, it's 'vexing'.

Whoever solves this one will be given genius/hero credit, at diyaudio.com .

- T

Ah, I've seen this problem, your input MIC/SIGNAL ground maybe opened. I used to trace stereo and I've found a broken wire inside a MIC cable.

beware of missing words on my posts. My typing is bad, it can't keep up with my thinking speed.

Hi RL,

Well, it's not actually MY preamp. But a few of us have made it our problem. There's a collaborative effort underway to help a previously- despairing preamp builder whose hum problem has persisted for far too long, and has defied many valiant efforts to defeat it.

I copied your earlier long post about it to the thread at diyaudio.com, where it helped spark a flurry of new activity and discussion, and also helped to attract new participants. It has helpd to build both momentum and morale.

I don't know how gratifying all of this might be, for anyone here who contributes ideas and suggestions. But it HAS given new hope to Gary, the preamp owner/builder, and to those who have been doggedly pursuing The Answer, with him.

There have been many bright people involved in this effort, and still 'no joy'. And I must admit, that even when participating in the original thread, trying to help diagnose and solve this type of problem remotely, via a discussion thread, is often a frustrating, slow process. That is one reason I finally decided to post a request for help, here, in SED. We needed some fresh perspectives and ideas. And I figured that '200 heads would be better than 20'. And I am quite familiar with SED, and have the greatest respect for the minds, here.

The link to the original and ongoing diyaudio.com thread is in post #1, of this thread, by the way.

Doh! (Slaps forehead.) I hope that everyone here will consider joining diyaudio.com, not only to directly participate in this effort, but, mainly, because there is a whole lot of cutting-edge electronic and scientific work being discussed there. (I kid you not in the slightest. There are discussions covering in-depth component and device physics, new and old digital stuff, PCB effects, extremely- advanced transistor and MOSFET modeling topics, ALL kinds of circuits, details of component selection, RF and electromagnetic field theory, magnetics, waveguides, phased arrays, SMPS, every known type of voltage and current regulation, materials science, optics, woodworking, metalworking, and on and on and on (oh, and amplifier and speaker designs of every type, too); plus... music!) Not only that, but there are a lot of world-renowned designers and scientists who are regulars, there, as well as a large number of other very-technically- sophisticated people, from all over the world. And many of the people there are just simply great to communicate with. (It is also very well-moderated. So there are virtually no out-of-control jerks allowed, which, after seeing what has happened to SED, is very, very nice; quite civilized, by comparison.) As a bonus, there is also a vitually unlimited supply of highly-motivated newbies who would be forever grateful for even the smallest bit of mentoring (And as they say, "You can't take it with you.".).

..

ly

e quoted text -

Thanks for the further suggestion, MooseFET!

I will pass it along. (Actually, I already posted this thread's URL in the diyaudio.com thread. So your suggestion will be seen whether I mention it there or not.)

- Tom

For those people who are using actual newsgroup-traffic feeds, who might not be able to easily view previous posts in this thread, the entire thread can be viewed and navigated at

- Tom

Does "not powered" mean "turned off but not unplugged"? If so, the hum comes from its transformer - capacitance from primary to secondary...

Yes, it really sounds like a lousy patch cord with an open ground on one end. - the extreme case of 'ground loop'.

RL

I copied the thread, but it still sounded like there were computers and such still attached. Keep it simple.

RL

Looking at the schematic, it seems that the output impedance of the preamplifier is extremely high.

I mean 330K? Get real. Although the reciever may have an input that's equally high, the idea is always to get the source impedance lower than the receiver, if noise is a concern.

I think these tube could drive 33K and still develop a 3V signal with only a 2% change in cathode current. (coupling cap 0.12 for 40Hz)

Hell, why isn't it 3K3? ......1uF /240V coupling still gives -3dB at

RL

Thanks for taking the time to respond, Robert!

Either or both, unfortunately. Unplugging it from the AC Mains does not change the unpowered hum behavior at all.

Regards,

Tom Gootee

Since this has been mentioned at least twice, I will definitely re- emphasize checking it, in the diyaudio.com thread. Thanks!

I'm afraid you've completely lost me, there!

On Mar 1, 12:56=A0pm, legg wrote:

Hi RL,

This is a very interesting point, that had not yet been raised. Thank you VERY much for the insight.

The fix-finding effort had mostly been concentrated toward star- grounding problems, loop-area/coupling issues, and possible faulty components, with (some but) relatively little thought given to whether or not the actual circuit toplogy and component values were reasonable. Since this was a commercial 'kit' (i.e. build-it- yourself), which had been purchased by the builder, everyone tended to assume, especially at first, that the fault could not be because of such basic original design considerations. However, after seeing the abysmally-inept grounding scheme, we should have become more suspicious of the rest of the design, too, and probably sooner, but mostly hadn't yet. Even with the grounding scheme issues, when I first mentioned modifications that it appeared to need, I was reminded that since it was a commercial design, it 'must be' OK, and any actual design changes should not be needed. Whether or not my approach was correct, in that case, that illustrates the mind-set that we all have had to try to evolve beyond, apparently too gradually, as all of the usual suspected problems have been found to not be the cause, or at least as far as we can tell from remote locations. (And I don't know whether or not it reinforces or negates the possibility that the original design is at fault, but, the unit was also already sent back to the manufacturer of the kit, who apparently could not fix the hum problem.)

Your first point, I think it was, in your first post in this thread, still both haunts and encourages me: Probably especially because all of us except the unit's owner are trying to do all of this 'remotely', I still often find myself assuming that there must be some simpler problem that has been missed, or a proposed solution that would have worked but didn't appear that it would, when tried, because of some trivial mistake in interpretation or implementation, which couldn't have been noticed without being there. Those types of lingering doubts are probably not so good for remote contributors' morale. Naturally, there are also problems with keeping the unit's owner on track well-enough, so that he does provide needed answers, measurements, etc, and try suggested modifications, which is understandably difficult for him when there are too many suggestions hitting him at once, for example. But it must be almost like hell, sometimes, for HIM. :-) He has to try to discern what the concensus is, and what the exact details are for suggested tests etc, after he has tried to discern weightings for which contributors he should be listening to, etc etc etc. However, the good news is that it appears that the group is getting better at coordinating itself, and at working together, better, to try to keep the unit's owner on one path at a time, and following logical progressions, etc. But sometimes I wish we could collect the entire 'best' progression of tests and modifications, etc, and have him do it all over again.

Sorry to go on about all of that. It's still slow and unsure, doing it remotely. And the peamp's owner has limited time to work on it, usually. I'm still hoping it's something fairly trivial that simply hasn't been mentioned, yet, although that appears to be getting less and less likely.

Thanks again, very much, for the output impedance insight! I will definitely pass it along to the group.

Highest regards,

Tom Gootee