I'm Hum-swoggled

Bill Epstein · « **on:** 15 Jan 2009, 11:48 am »

I just pulled the RCAs and measured the holes at 5/16". (I guess I counted steps wrong) Vampire CM Hex want 3/8" so I drilled them out.

Re-inserted an RCA with it's teflon shoulder washer carefully centered in the hole, and......the dreaded continuity BEEP

Where's that teflon tape?

How come it didn't hum with the Cinemags installed on the inputs with the same RCAs?

All my stuff uses CM Hex and nothing else hums. That's 9 other pairs, 18 RCAs and they're all isolated properly but not the Cornet? And not just in the one hole, all 4 holes

tubesforever · « **Reply #1 on:** 15 Jan 2009, 09:56 pm »

I have been double checking rca ground to chassis continuity for some time now. Wrapping with teflon is a great idea.

Bill, on my original C2 build I had just one rca touching and the hum was frightful. Luckily on one of my centering washers, I had not checked to make sure the shoulder was inside the chassis hole. This was a fast fix. Found it quick too with the ol dmm.

As for other aspects of ground loops, electronics are amazing. Within any circuit or even straight wire I understand you will find eddy currents. What should be a straight forward ground path can become mirky and mired.

Heck, it took me months to track down one hum. It was a cracked cartridge clip wire that finally broke off. After I resoldered it, the hum was gone. Luckily it was extremely low level.

Damn nasty critters those gremlins....they love to twist us slowly over the fire pit.

BobM · « **Reply #2 on:** 15 Jan 2009, 10:15 pm »

Seems there's been a lot of Coronet related HUMMM issues posted lately. I also had a hum problem for a while before I was able to track it down and fix it.

Hey Jim - this issue seems to be a bit prevalent. Perhaps a FAQ would be in order, listing the most common ways to identify, then reduce or eliminate hum in the Coronet if it is a problem.

Just a thought. It would save the builders and yourself a lot of back and forth on the issue, at least.

Bill Epstein · « **Reply #3 on:** 16 Jan 2009, 01:59 am »

Most of this afternoon was spent wrapping the RCAs in teflon tape (a PITA), re-inserting and continuity testing each, some 4-5 times until I had both pairs isolated.

Back in the system: HUMMMMMMMM

So now I'm looking at a foto of the original build and see that I had, besides the Cinemag ground wires to chassis ground, just a wire from EARTH to IEC Ground. UL Approved? No... But deathly quiet.

flocchini · « **Reply #4 on:** 16 Jan 2009, 06:31 pm »

Bill

When I built my Cornet 2 I also had hum problems. I used a 6SN7 in place of the 12AU7 so I had a weird adaptor thing going on and additionally I chose to mount my RCAs on the side of the cabinet rather than at the back. Moving the input wires around the perimeter of the board so that they did not cross over the board cured my problem. Not sure that applies but I had also done all the continuity checking things. Another trick I have used is to mount RCAs in wood instead of metal.

Good luck

Bob

Bill Epstein · « **Reply #5 on:** 16 Jan 2009, 07:40 pm »

Thanks, Bob, but the huge irony in all this is that my Cornet is dead, balls-on stock, (except for off-board RCAs) and the stock grounding scheme yields hum

Earlier I tried connecting AC ground to the chassis and left Earth unconnected to the chassis: that got rid of the hum with the Cornet on it's own but inserting the Cinemags 'tween arm and Cornet brought back the hum. Godfrey Daniel!!!

tubesforever · « **Reply #6 on:** 16 Jan 2009, 08:58 pm »

Bill,

I think if lifting the ground between the earth ground and the board earth quiets the hum, that is because the board is finding ground from a different source. It is happier without the second pathway.

To check for this, just pick up a board ground and see if it tones without that earth ground connection at the chassis ground point. If it tones, you don't need to worry that shorts will kill someone. The unit will be safe.

I remember seeing pics of the burned out piece that exploded on your board. Did it fry any of the electrolytics? They can look fine on the outside but still leak voltage. You might want to check each one for leakage just to eliminate that from the realm of possibility.

Usually hums are silly stupid things that make us say....how'd I miss that! Yours seems a little more perplexing.

I had to use double insulated IC's for my CineMag unit. They would pick up hum from the Hammond tranny. It also sounds best with a short run from the SUT to the Cornet 2. It did not sound as good with a long IC. Double shielding was a requirement for the CineMag in my rig.

You deserve a hum free Cornet. Keep at it and let me know if the board is grounding without the pcb ground wire connected to the star point.

hagtech · « **Reply #7 on:** 17 Jan 2009, 05:36 am »

Yeah, been meaning to write up a hum article for about 5 years now...

jh

hotrod · « **Reply #8 on:** 6 Feb 2009, 01:58 am »

I'm in the same boat as far as hum goes(maybe buzz?).I was reading over these hum threads and this one has me stumped.If I'm dumb let me know, but if the RCA ground wires are soldered to the board ground and the earth connection on the board is connected to the grounding lug won't there always be continuity between the RCA's and the chassis?
There is no doubt the C2 is the culprit,with the inputs disconnected from the C2 there is still a hum so its not SUT or TT related.With just my preamp,amps,and speakers the system is dead quiet.
Not sure where to look next.

Bill Epstein · « **Reply #9 on:** 6 Feb 2009, 03:16 pm »

Quote

If I'm dumb let me know, but if the RCA ground wires are soldered to the board ground and the earth connection on the board is connected to the grounding lug won't there always be continuity between the RCA's and the chassis?

No dumber than me, who had to have it explained not that long ago. We don't want continuity between the RCAs and chassis without the ground wires attached to anything else. Otherwise, it's a loop.

I now have the cure, thanks to Tubes ofhand remark, better than teflon tape or just being careful with the isolation washers:

Forthcoming Clarinet chassis in glue-up.

It's wood, baby! No stinkin' continuity here

analog97 · « **Reply #10 on:** 6 Feb 2009, 03:39 pm »

Bill,

That is a beautiful case!! Congratulations on what looks like a truly sublime build.

tubesforever · « **Reply #11 on:** 7 Feb 2009, 07:59 am »

Bill,

Great work on the wooden casework. I just love wood. If I decide against using a professional rack in the future I have some plans for a cherry figured set of Hagerman kits!

BTW, I need to work out a deal to buy some of that Copper foil from you. My Michaels did not have any. I want to use this on my CineMag and Piccolo projects. HELP!

Bill Epstein · « **Reply #12 on:** 7 Feb 2009, 12:15 pm »

Yikes! The cherry case got the last of my stash I bought prolly 2 years ago in Ohio.

I just searched Michaels site and gornischt!

Fortunately, Google has a lovely first page with many choices for http://www.google.com/search?q=copper+foil&rls=com.microsoft:en-us&ie=UTF-8&oe=UTF-8&startIndex=&startPage=1'copper foil'.

Bobzilla · « **Reply #13 on:** 7 Feb 2009, 10:03 pm »

Bill,

Nice work on the wood cabinet!

I've given some thought to doing the same. I have few metal working tools and feel much more comfortable working with wood. It is far more forgiving with the unskilled! However, I have one question ...

How important is the shielding? If it is important, is thin copper sheeting adequate? It certainly is easy to work with!

I see some audio equipment that places all components, controls, and connectors on the top metal plate and apparently no shielding beneath. I'm not a big fan of the esthetics of this approach, but if this is acceptable, from a shielding/noise point of view, how important is the copper sheeting you are forming in the interior? Are you doing it because IF it is necessary, this is the point it must be done, or are you doing it because it IS necessary? It seems that this shield itself would have to be star grounded thus leading you back to a similar hum inducing situation that you were experiencing before.

What 'cha think?

Bill Epstein · « **Reply #14 on:** 8 Feb 2009, 01:16 am »

From what I've read the copper serves only as an RF shield, not grounded to any of the components or the top plate. The top plate, which has everything connected to it, does get grounded to earth and AC ground. That worked well in a previous wood chassis Cornet.

I only do this for pre-amps, the stronger signal and hum pot of the amp I built doesn't seem to require it, or I've been lucky. The Simple 45 is absolutely stone quiet with the 97 dB Pi 4s and no foil.

Hum is so danged elusive; RFI, EMI, placement of components, phases of the moon. The copper is kind of a chicken soup thing for me. "Couldn't hurt", right?

Like you say, wood is so much more satisfying than metal; to work and to handle

jameshuls · « **Reply #15 on:** 9 Feb 2009, 12:45 am »

Tubes, THE place to get copper foil (or brass, or teflon, stainless steel. carbo fiber etc.) This place is a great resource! It's where I got all the copper and brass for my Cornet chassis.
http://www.onlinemetals.com/.

If the shipping is too excessive let me know - I have a roll of foil 10' by 4'. I could just fold some up and ship it in an envelope for next to nothing if you wanted some and didn't mind creases, au gratin

!

analog97 · « **Reply #16 on:** 9 Feb 2009, 01:56 am »

What thickness do you folks recommend for chassis shielding?

hagtech · « **Reply #17 on:** 9 Feb 2009, 04:22 am »

For E-field shielding you can go very thin. Aluminum or copper foils will do the job no problem.

jh

hotrod · « **Reply #18 on:** 12 Feb 2009, 12:36 am »

After a few weeks of tracing down a hum issues with the Coronet2 it turned out to be a simple fix.After reading JH's post on IC's and their proximity to the tranny this is what I found.With my SUT on the same horizontal plane as the C2 with IC's running off to the side I had a large amount of hum,but when I placed my SUT 2 feet below the C2 on a lower shelf with the input IC's coming out from the C2 and running straight down to the SUT the hum was completely gone.With the volume way past what would be normal listening range all I hear is some slight tube hiss.
Now on to the Clarinet build. aa

analog97 · « **Reply #19 on:** 12 Feb 2009, 01:09 am »

Quote

For E-field shielding you can go very thin. Aluminum or copper foils will do the job no problem.

jh

So, Reynolds Wrap will work as well as copper? Sounds good to me!!