Aleph 5 clone issue

[dhaman]

Hello all,
I build Aleph 5 clone. I have little issue with both channels. 100Hz hum.
Current bias via all Fets is set around 550mA.
AC current is set to 50%.

Do you have idea what could be wrong ?

measurement:

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scheme:

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dps:

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photos:

 

[albertNL]

I don't pretend to actually know anything but a few things come to mind. First of all a 100Hz hum should be a psu related problem (with it's source located after the bridge, or it would have been 50Hz).

1. Out of curiosity, when going to the trouble of dual mono psu's with dual secondary toroids, why did you not use dual rectifier bridges in your psu (like in the standard FirstWatt psu's and KK's old Aleph psu's schematics)?

The way I understand it you now have V+ at the -> side of the bridge rectifier and V- at the <- side of the bridge, which leaves your PSU GND floating (or is it fluctuating?) somewhere .....
(Note that I have no idea at what potential (fluctuaating or not) your PSU GND will be and leave that to someone who understands this better to explain).

2. As far as I can see your audio GND is connected directly to the chassis (via the mounting screw below fuse F1). In most Pass amps I have seen the audio ground is connected to chassis ground through to CL-10 to avoid noise.

3. I can't see on the photo's where safety ground from AC input is connected to the chassis. Is it?

Hope this is of some help.

Good luck!

 

[Zen Mod]

try connecting all grounds (wires from xformer and signal trace to amp electronic) ditto between C13 and C3

and yes , one CL60 from each audio gnd to case.safety gnd

on case just one screw for all grounds

 

[dhaman]

Thank you for your time. As i understand:

1. Disconnect GND from case below fuse F1 (brake path on pcb)
2. install one case.GND screw on case (in the middle of case ?)
3. Connect both amp.GND (fast on connector between C13 and C3) to case.GND via CL60
4. Conect ac.GND via CL60 to to case.GND

I'll try and let you know.

 

[albertNL]

4. AC ground DIRECTLY to case.

 

[Zen Mod]

try connecting all grounds (wires from xformer and signal trace to amp electronic) ditto between C13 and C3.......

audio gnd to case directly or through CL60 , choose what is (more) silent , I prefer through CL60 , that way monoblocks are more isolated

but - what I quoted above is more important

 

[propitious]

Do what others have said, but there is one more thought. I cannot tell from the pics if you have a separate bridge rectifier for each secondary, but as I understand it, in an Aleph amp, the positive rail passes more current than the negative rail. So each secondary from each toroid transformer needs its own bridge. Therefore, you need two bridges for the left channel and another two bridges for the right channel. Several PS schematics show this kind of setup.

 

[Nelson Pass]

Nice looking build. I'm sure that with a little experimentation you can
find the problem, which is probably power supply and grounding.

BTW, the Alephs will draw the same current for both + and - rails.

 

[dhaman]

Thank you All.
I did experimentation with grounding but looks like it is PSU issue.
I measure on scope small AC on filter capacitors.
300mV / 100Hz it could be the cause.

An externally hosted image should be here but it was not working when we last tested it.

So maybe 40mF is not enough

 

[ElectroNick]

I had the same problem.
A CRC did the trick.

After that no more audioble hum.

Regards,
Nick

 

[radule]

Thank you All.
I did experimentation with grounding but looks like it is PSU issue.
I measure on scope small AC on filter capacitors.
300mV / 100Hz it could be the cause.

An externally hosted image should be here but it was not working when we last tested it.

So maybe 40mF is not enough

With 0.3Vpp of PS voltage ripple there should not be hum on the output.

If all previously suggested do not fix problem, consider rerouting GND side of the R13 resistor.

There is a small ripple current flowing trough R13 resistor and doing contamination of signal GND.

 

[propitious]

Nice looking build. I'm sure that with a little experimentation you can
find the problem, which is probably power supply and grounding.

BTW, the Alephs will draw the same current for both + and - rails.

Thank you for correcting my misstatement. I thought I read that somewhere...but then again, maybe I was dreaming I read it...

 

[Nelson Pass]

Can you show a scope shot of the output noise? This would be with the
input shorted to ground.

 

[ElectroNick]

So maybe 40mF is not enough

I used 3x47000uF for a stereo psu, had same irritating humm.
Between 2 caps, i placed a 0,2 ohm resistor and now amp is dead silent.
(=CRC filter)

So invest a few cents in an extra resistor....

 

[dhaman]

@Nelson here is output noise with input shotred.

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@ElectroNick yea i'm going to break path on PCB (behind C2/C12) and create CRC = 20mF + 0.33ohm + 20mF.

 

[dhaman]

CRC solve the issue. It's acceptable now.
Supply ripple is 28mV now:

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now

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before

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[Nelson Pass]

Most Excellent.