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Old 17th May 2006, 04:32 PM   #11
AndrewT is offline AndrewT  Scotland
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Hi,
try hard wiring the dirty returns together using the shortest insulated wire links possible.
Then run the shortest route to the central ground.

The short wires are to minimise inductance in the connections.
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Old 17th May 2006, 05:08 PM   #12
Bazukaz is offline Bazukaz  Lithuania
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Hi,
I think that resistors must be removed after regulators , as Andrewt suggested.
Also , zeners will open at something like 0,5mA or so (i am not sure).
You may try putting resistors to ground after zeners , so that they draw constantly a few mA of current and ensure that zeners are always open.
If the zeners would start opening/closing there mayl be some noise present...
Not sure about this , just some ideas ...

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Lukas.
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Old 17th May 2006, 06:52 PM   #13
vzs is offline vzs  Europe
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I missunderstood what Andrew wrote in
Quote:
Now that you have the supply working you may want to consider running a resistor Zener//cap AFTER the regulator
I drawed the resistor in the red sqare just to ask if that is what he suggested, but no, I do not have any resistor there.

In the quote he suggested to add an extra stabilizing stage using a dropping resistor (270r) and a Zenner (15V 500mW), but first if all I will try to hard wire the dirty returns toghether and run them to the central ground.

Regards,
Zsolt
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Old 22nd May 2006, 06:43 AM   #14
vzs is offline vzs  Europe
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Thank you guys,

I was able to resolve the hum/noise problem .

The first noise problem was that I haven't connected the input sockets to the chassis directly at the point they were mounted.

And, indeed, the faint ssss noise in the speakers was due to bad grounding of the regulators and decoupling caps used by the input operational. Hard wiring them to the central ground removed the faint hiss too.

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Zsolt
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Old 22nd May 2006, 07:15 AM   #15
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A friend of mine was trying to chase down a mechanical hum from an early 405. He tried mains cleaners, DC blockers on the AC-line, the lot. He figured out eventually it was the laminations in the mains transformer itself.
Listening closely, with the lid removed, he managed to strap his EAR across the 240vAC terminals on the top of the transformer!
I learnt new words that day...
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Old 7th June 2006, 08:42 AM   #16
vzs is offline vzs  Europe
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Hello all again!

I'm still upset with something and I couldn't find a solution for it:

Despite that there is no noise (hearable only if you stick your ears to the speakers) when the input is plugged to the CD player or grounded (volume to min), there is still a hearable noise (both 50-100Hz rail hum and noise) when the input is in the air. Also when I plug the audio cable to the input RCA (and the cable is not connected to the cd player) this hum+noise increases.

Although I know this is not a big problem it bugs me, so if you have some suggestions...

Regards,
Zsolt
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Old 7th June 2006, 09:26 AM   #17
AndrewT is offline AndrewT  Scotland
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Hi,
it's good if there is no noise when the cables are connected and no signal is present.

Do you ever listen music on your system with the cables disconnected?

Have a look at the schematic. Is there a DC blocking cap on the input? Is there an input impedance setting resistor to ground from the amp side of that same cap to signal ground? Is there a DC referencing resistor on the other side of the cap (RCA end) to ground? If not then adding a high value 1M0 to 2M2 resistor from RCA input pin to RCA ground might help. But equally it might not, worth trying.
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