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Old 14th January 2007, 07:08 PM   #1
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Default Aikido hum has me stumped !!!!

I spent a pleasant afternoon with Bruce Anderson yesterday, well it was frustrating. We siimply couldn't nix the hum. Cap-coupling PCB ground, etc, etc. did nothing to improve things, although I did learn that high quality tubes are far less microphonic than my Russian cheapies!

We even tried a few things such as disconnecting the B+ to see if hum was comming from the AC. With heaters on and no HT, it was still there. "Ah" we though it's AC heater hum. However, our hopes were dashed. When we disconected the heater transformer and ran the tubes on B+ only, the hum persisted.

So, we are both totally stumped on this one, although it seems to be a ground loop. Now this morning, I tried to test ground connections from all kinds of places to the chassis ground. Something that I did notice was this:

Input and Output jacks are isolated from the chassis. There seems to be no chassis connection between signal ground and chassis via the pot.

So, I lift all connections to chassis/star-earth, and I get no connection between signal GND and chassis/star-earth bolt. I replace the PSU ground and I get 0 ohms from signal ground to chassis/star-earth - so there's a connection.

Now, I swap PSU ground for PCB ground and do the same measurements and I also get 0 ohms from signal ground to chassis/star-earth.

Finally, I reconnected everything, but removed the signal-gnd from input to PCB. Of course I get no sound from preamp/amp. But...no sound also means no hum!

So, could I have a ground loop in my signal wiring???

If so, how can I get rid of it????

Charlie
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Old 14th January 2007, 07:26 PM   #2
SY is offline SY  United States
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Remove all your grounds from chassis (safety) ground. The only thing that should be connected to the chassis for this go-round is the 3rd wire from the power line. Connect the now-isolated audio/power ground point containing all the other grounds to the chassis via a 20 ohm resistor. See if the hum goes away. If it does, put the lid back on and enjoy the music.
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Old 15th January 2007, 05:26 AM   #3
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Sy,

Maybe I have my PSU/audio GND wired incorrectly. I have attached a sketch of my Aikido layout.

Should the PSU GND connect to the Aikido PCB GND pad? Which would then connect to star-earth via a single wire and maybe 20 ohm resistor???
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Old 15th January 2007, 11:57 AM   #4
SY is offline SY  United States
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Sketch doesn't show here.

Without the boards in front of me, I can't remember exactly how the grounds are set up, but basically, ground the green wire from the power line to the chassis. Run all other grounds to the usual star point, but isolated from the chassis. Then connect the star point to the chassis via the resistor.
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Old 15th January 2007, 03:44 PM   #5
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Sy,

OK, any particular wattage resistor???

Also, I did have a brief period of quiet last night. It's a little like Awakenings. I redid all my solder joints, anyway I still get hum this morning. I could also try to couple the PCB to the chassis via the PCB mounting bolt and a cap or resistor. Broskie does say that it can be done this way. It would move the chassis/PCB/PSU GND away from the IEC GND, and maybe that would be a good idea.

I tried to post my circuit layout, but I don't think it got put with my postings.


Charlie
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Old 15th January 2007, 04:31 PM   #6
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For test purposes, no particular wattage, just whatever you've got on hand that's 1/4w or better. Resistance is also uncritical, just make it in the low double digits (39 or less). If this works, we can discuss options for changing it out.
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Old 15th January 2007, 07:23 PM   #7
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Charlie,

seems as if hum is being really pesky to you...

Not that I have alot of experience with hum but having built almost TWO ( ) headphone amplifiers I know my side of the story.

After trying almost everything in the end I found a major ground loop (I grounded a shielded input cable on both sides...duh). Another thing to check when you are using a metal top plate...Are the rca's really isolated from your top plate? Sometimes the punched holes are too small and make contact with the inner metal part of the rca...

And last: carefully wiggle all cables or even bigger parts (with an isolated screwdriver) and check whether hum changes somehow...

good luck,
Oliver
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Old 16th January 2007, 12:12 AM   #8
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I'll stop at my local electronic store and pick up a 20 ohm 1/4W resistors. I thought I had a pretty good collection, but nothing low.

As for my jacks, my chassis has wood sides, front and back with a metal top plate which carries all components except for the IEC, switch and jacks which are located on a back plate. It is couple to the top chassis via the same bolt as my earth bolt, which is actually one of the transformer mounting bolts. Maybe I need to move this bolt away from the transformers???

My jacks show open-circuit to ground, so they are likely not the problem.

Thanks for the replies.

Charlie
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Old 16th January 2007, 03:53 AM   #9
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Charlie,

What are you using for your input wiring? I used 4340 mic cable and floated one end of the shield and took the other end to ground.

Joe
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Old 16th January 2007, 04:08 AM   #10
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Thanks to Sy, who gave me the clue to hum removal from the Aikido. I had been connecting my PSU ground directly to the chassis ground. There is another ground wire from the Aikido PCB to chassis ground. The bolt for the chassis ground was the same used for the 6.3VAC transformer.

So, Sy's suggestion was to lift my PSU/PCB ground and connect them to the chassis ground through a 20 ohm resistor. Well, I don't have the resistor in place yet, but it got me thinking.

Now the Aikido PCB has a pad marked GND. It also has a pad marked chassis which connects to the chassis via one of the PCB mounting bolts. There's also a jumper to connect the GND pad with chassis pad. So, I connected PSU ground to the PCB ground pad, and jumpered to chassis directly.

The hum is discernable, but only just. Maybe swapping the jumper with a 20 ohm resistor or 0.1uF cap will be even better. Now I have a Wima 0.1uF cap, which Bruce and I used on Saturday in our search for hum reduction. Of course, one leg finally gave up the ghost and snapped clean off. I will buy a cheap polyester cap tomorrow when I buy my 20 ohm resistor.

Sy, when you said that some folk use back-to-back diodes for the earth connection, which kind would you suggest, and do they go cathode-to-cathode or anode-to-anode.

Right now, my Aikido is humming (in the sense of great sound, no hum) I have already tried a number of music CD's - Michael Hedges, Chanticleer, and it all sound bl**dy amazing.

Of course, I'll wake up in a couple of minutes to find that I've been dreaming......

Anyway, in light of tonight's Golden Globes, I would like to thank the following people without whom, this production would not have been possible: Bruce, Sy, BurnedFingers, JoshK, GingerTube. This is a great forum and in the next couple of weeks, I'll be asking for someone to check my resistor values for wiring a blue led to my B+ voltage.


Thanks,
Charlie
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