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Old 19th August 2010, 06:12 PM   #8691
sangram is offline sangram  India
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Can you still have a hum loop with the safety earth removed?
Yes

Hum can be transferred over the air and moving transformers is the way to solve that issue.

It is also possible that the return currents for the signal input and the speaker output are mixed up somewhere. Among some possible solutions are improper star ground, RCA input jacks not isolated from chassis, high currents from speaker ground return flowing through the input ground loop, and safety earth connected to two different pieces of equipment which share a second ground connection, such as through interconnect cabling It would seem you have already eliminated this last possibility.

The way I connect my F5, the star ground is a point just after the main power capacitor. To this point are connected the speaker return wire, the main power ground from the power capacitors, and a thin wire that connects to the signal ground point on the PCB. I use Peter's PCBs, if that helps, but you can also connect the ground from the RCAs to the star.
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Old 19th August 2010, 06:27 PM   #8692
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Originally Posted by Tripmaster View Post
I appreciate these questions are a little tedious, but please bare with me.

I've been re-checking all of my connections and I cant see where the hum problem may be. I know this isn't safe, but I decided to quickly remove the safety earth lead from the IEC mains inlet (now reconnected) to see if it made any difference, and it didn't. Can you still have a hum loop with the safety earth removed?
I think you can. I think that the earth is still connected via your RCA's, no?

What he said ^
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Old 19th August 2010, 06:52 PM   #8693
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Most helpful. I will draw a quick sketch of my amp as you may spot it straight away.

The hum is very slight. I have to place my ear close to the speaker to be able to hear it, so its really not a major issue. I've heard so many say their amps are silent I'd like one too
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Old 19th August 2010, 07:07 PM   #8694
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Richard

May I ask you:

are you sure, it is hum and not some oscillation?

Do you have hum, when you shorten the input connectors to ground (not on the rca connectors, but on the pcb?

Franz
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Old 19th August 2010, 07:15 PM   #8695
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Originally Posted by Franz Gysi View Post
Richard

May I ask you:

are you sure, it is hum and not some oscillation?

Do you have hum, when you shorten the input connectors to ground (not on the rca connectors, but on the pcb?

Franz
Hi Franz

You may...

It's silent when the input is shorted.

I'm currently amending Cvillers suggested layout and will post it when completed. If my error is easily spotted I will get the soldering iron out, but I would rather not wield the iron willy-nilly
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Old 19th August 2010, 07:21 PM   #8696
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Oh, it is silent when the inputs are shorted!

My two cents: just try input caps on the signal and also on the ground wires of the input rca's to completely galvanic isolate the input loop circuit.

When you still have hum: I think then it is not a ground problem, rather oscillation.

Franz
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Old 19th August 2010, 07:25 PM   #8697
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Originally Posted by Franz Gysi View Post
Oh, it is silent when the inputs are shorted!

My two cents: just try input caps on the signal and also on the ground wires of the input rca's to completely galvanic isolate the input loop circuit.

When you still have hum: I think then it is not a ground problem, rather oscillation.

Franz
What value would recommend?

I plan to make a DCB1 at some point. I remember Salas suggesting a 1uf cap at the input.
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Old 19th August 2010, 07:28 PM   #8698
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Quote:
What value would recommend?
Just for a test!

Everything more than 0.1uF is O.K with 100k input impedance.

It is building a highpass filter, this caps / input Z.

But the point for this test is to galvanic isolate both: the signal AND the input grounds.

Franz

Last edited by Franz Gysi; 19th August 2010 at 07:31 PM.
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Old 19th August 2010, 09:15 PM   #8699
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If its silent with the inputs shorted, wouldnt that indicate something upstream,i.e. whatever is serving as pre? (or source?)

russellc
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Old 19th August 2010, 09:19 PM   #8700
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Originally Posted by EUVL View Post
This article explains why you might want to degenerate the 2SJ74 to make it truly complementary with the 2SK170.


Patrick

PS You can open and read the pdf, but not extract and not print.
... I make my living on memory chips, so I don't support paper copies.


.
Is this the case with all 2SJ74 and 2SK170, or just the Toshibas? I understand your instructions until the very end, then the part about getting a new fet or something? I am getting ready to build with my new fets, you guessed it, Toshibas, so I'm trying to understand exactly what I need to do here.

Thank you,

Russellc
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