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Old 2nd June 2013, 12:30 AM   #331
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I cant really see anything unusual. Ill have to look at ground plane on boards, but it doesnt seem likely. You said there was no issue on the F4 using same configuration in chassis?
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Old 2nd June 2013, 02:40 AM   #332
6L6 is offline 6L6  United States
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Quote:
Originally Posted by buzzforb View Post
You said there was no issue on the F4 using same configuration in chassis?
No problem at all. That's what's stumping me...

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Old 2nd June 2013, 03:44 AM   #333
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i think its unfixable. i will be happy to "get rid of it" for you.
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Old 2nd June 2013, 04:31 AM   #334
6L6 is offline 6L6  United States
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Nah, everybody knows that the place for proper disposal is Serbia...

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Old 2nd June 2013, 09:31 AM   #335
biju55 is offline biju55  India
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how is its sound , Same like F5 ?
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Old 2nd June 2013, 10:43 AM   #336
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My first gues is small connector wire tying (-) input to ground. A major difference in the topology of the F4 and AJ is the FE. F4 is SE and AJ is balanced capable. If you know the F4 is dead silent in same case and same configuration, then I would be looking at the innput first. That little wire seems easy to pick on. I would have suggested transformer and PSU size for an amp that draws almost 10A current total, but i am less inclined to think so and the fac that it is quite with input grounded suggest it is a non issue. THe PCB has signal and output ground coming fromsame point, so i dont think it is a layout issue. Try replacingthe wire(mine is done same exact way, but heavier wire) and see what happens.

Did you watch the output when you hooked up RCA to make sure no oscillations were triggered?
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Old 2nd June 2013, 10:54 AM   #337
henryve is offline henryve  South Africa
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Those multiple GND connections on the PCB; is that functioning as the audio (star) ground? I would not have done it that way. Also, the cable for the power ground to the PCB seems very long. Don't know if that can cause problems, but would have tried to keep that power ground as short as possible.

Edit: On second thought I don't think it is the problem though. The fact that I would not do it does not negate the fact that your scheme seems technically correct. If the hum is gone with the input shorted then by process of elimination the hum must be caused by a problem on the input circuit grounding. Just as an experiment; what happens if you disconnect the signal ground on only one if the input connections?

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Old 2nd June 2013, 10:59 AM   #338
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Sorta of a tree star. It doesn't have to be a single star point to work. Only way to better it is to have all ground connections going to PSU PCB. Starts to get hairy with wiring that way unless whole amp is designed for it. Ideal grounding scheme can be seen in XA 30.5 review at six moons. Nelson has it all on one sub board. this is how i will be doing it in my clone.
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Old 2nd June 2013, 03:51 PM   #339
Mr. dB is offline Mr. dB  United States
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Quote:
Originally Posted by buzzforb View Post
My first gues is small connector wire tying (-) input to ground.
Well if you look at Nelson's amps with balanced inputs, he uses a balanced audio line from the rear panel to the circuit board, and then for unbalanced operation just sticks a jumper in the XLR connector. I imagine the resistance of that arrangement would be more than the short length of small-gauge jumper wire that 6L6 has used?
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Old 2nd June 2013, 03:58 PM   #340
6L6 is offline 6L6  United States
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Originally Posted by henryve View Post
Just as an experiment; what happens if you disconnect the signal ground on only one if the input connections?
It's actually louder... the loop problem from the inputs is being carried on the signal + more than the ground. As the RCA plugs are being inserted they are really loud when the grounds are not touching, and get a little quieter once the grounds contact.

One input by itself is hummy, two sounds like an angry hornet, and inputs shorted is quiet.

Also, I tried a bigger piece of wire connecting -in to gnd, with no difference.
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