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Old 31st January 2011, 08:07 AM   #921
h_a is offline h_a  Europe
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Hi OllBoll,

likely that for the same bias not so much is happening.

Nelson wrote a few times, only total bias matters.

Of course, with half the mosfets, the remaining cook gently at twice the original dissipation (with same total bias current), which should be avoided due to reliabilty.

Lowering bias will certainly raise distortion, as will drain source voltages lower than 15 V or so.

Hannes
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Old 31st January 2011, 09:17 AM   #922
OllBoll is offline OllBoll  Sweden
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Hi OllBoll,

likely that for the same bias not so much is happening.

Nelson wrote a few times, only total bias matters.

Of course, with half the mosfets, the remaining cook gently at twice the original dissipation (with same total bias current), which should be avoided due to reliabilty.

Lowering bias will certainly raise distortion, as will drain source voltages lower than 15 V or so.

Hannes
If one instead of halving bias to halv lowered it to ~ 62.5 % thus increasing the bias of the pair left by 25 %, would this bias increase compensate for higher usage / mosfet or is it then a better idea to use other mosfets, for example use parallel Toshiba 2SK2013 with a lower rail voltage?
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Old 31st January 2011, 02:21 PM   #923
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With 20V rails you can increase the bias considerably.

Just build it, you will like it.
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Old 31st January 2011, 05:36 PM   #924
Manu is offline Manu  Europe
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I would like also to thank Zen Mod


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he's just acting nicely in public ..... sometimes

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Old 6th February 2011, 01:54 AM   #925
anilva is offline anilva  India
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Default Hum on my Aleph J

My Aleph J started having hum coming out of the speakers, after fixing a perfectly working table top assembly into a chassis. Can't figure out why. I am using a standard papa's zen type power supply schematic with 4 x 15000uF on each rail and toroid at 2 x 18v, 600VA. When i disconnect the input wires from the pcb, there is absolutely no hum. But when i connect the inputs to the RCA socket and ground the -ve input for SE operation, there is hum, even without any input being connected to RCA sockets.

The hum is mild but audible. Amp works very well as far as sound output is concerned.

What could be the problem area. There are no ground loops. RCA sockets and speaker outputs are isolated from the chassis.

Any pointer to where I should look or add any small cap somewhere to suppress the hum?

TIA.
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Old 6th February 2011, 02:53 AM   #926
anilva is offline anilva  India
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My Aleph J started having hum coming out of the speakers, after fixing a perfectly working table top assembly into a chassis. Can't figure out why. I am using a standard papa's zen type power supply schematic with 4 x 15000uF on each rail and toroid at 2 x 18v, 600VA. When i disconnect the input wires from the pcb, there is absolutely no hum. But when i connect the inputs to the RCA socket and ground the -ve input for SE operation, there is hum, even without any input being connected to RCA sockets.

The hum is mild but audible. Amp works very well as far as sound output is concerned.

What could be the problem area. There are no ground loops. RCA sockets and speaker outputs are isolated from the chassis.

Any pointer to where I should look or add any small cap somewhere to suppress the hum?

TIA.
To add, when I short the input at the RCA socket, the hum disappears completely. Leads me to believe that the amp in itself is pretty quite. Seems to me that the shielded input wire that I am using is picking up hum. I have also tried two thin wires twisted around as input wires. Still picking hum.

Is there a small cap that I should add at the input socket to stop the stray pickup?
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Old 6th February 2011, 03:17 AM   #927
Zen Mod is offline Zen Mod  Serbia
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have NTC from PSU gnd to chassis ?
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Old 6th February 2011, 03:23 AM   #928
anilva is offline anilva  India
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have NTC from PSU gnd to chassis ?
Yes, ZM. The power earth is connected to the chassis and the chassis is connected to '0v' of the PS through 10ohms NTC as per Papa's schematic.
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Old 6th February 2011, 06:12 AM   #929
cviller is offline cviller  Denmark
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Do you also hear hum if you connect it to a preamp? The amps I have built of this typology were a bit noisy with inputs not connected.
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Old 6th February 2011, 06:35 AM   #930
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Anilva
As the only thing that changed is connecting to the Pre ..
Are both screens (L & R ) on the cables to the preamp connected both ends?

Only the preamp end need to be as maybe you got ground connection on the mains cable as preamp chassis is grounded.

Maybe just snip snip on one of them and then can alwayse solder back
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