mini Aleph hum

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Hi,

built a mini A for a friend using 94dB sen. speakers. He said there is a lot more hum coming from speakers vs. other amps in his system. He is using a tube pre.

if adding 100ohm between signal input and ground, the hum remains, then what to check next?

Does increasing to higher bias possibly cause more hum?

I am pretty sure I used the X2 .1uf cap in parallel to 1K resistor between circuit ground and the chassis.

Solving hum issue is so tricky so asking for help here.

Thx
 
pictures and complete info about PSU

Hi ZM,

PSU is c r c, 15,000uf to 0.1ohm to 15,000uf per rail
each rail has its own full wave bridge rectifier. smaller resistors in photo
are bleeder resistors. Also Solen 0.1uf on the outputs of the psu. Transformer is Antek, shielded wire to chassis ground with steel cover on it.

I have tried 100k shorting input to ground. Adding 120ohm parallel to x2 cap between AC ground and chassis. The star ground at the CRC's has a CL60
between its star ground and chassis. the minus speaker terminal goes from star ground directly to speaker post isolated from chassis.

I swapped twisted pair wire on one channel with shielded wire and that did not
help.

Is it possible for this aleph build to have zero hum when you put your ear next to the speaker? that is my goal unless it is not possible with brute psu.

I also biased higher, not sure if this is contributing to the hum.

Since I put 100k between signal input and ground, hum still present,
what else could I try? its low constant hum and present in both channels.
sound wise is excellent but on very sensitive speakers, its not doable.

Thanks
 

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I'm curious if the hum changed at all when the inputs are shorted to ground?

I had a mini-A many years ago and it was pretty quiet, at least no worse than
my other amps at the time. My speakers are only ~90dB or so. However,
I had about 100000uF per rail in CRC and was only using the basic 1A bias.
If the issue is ripple then the high bias would exacerbate the situation.
 
I read an older thread about aleph hum issues, he solved the issue by soldering jumper wire from minus sig. input to ground. I have already done this and using shielded wire on one channel. both show same amount of hum.

I have a 1uf solen cap at the psu outputs not 0.1uf but that would not help with the hum
nor the cause.
 
it is pretty much this CRC supply with dual FWB mounted to chassis
resistors also mounted to chassis

Antek 12V 400VA with shield cover--->33,000uf 50V--->0.1ohm 25W--->33,000uf 50V--->1uf solen mkp cap

there is a 2k bleeder resistor off the last 2 caps
nothing fancy here

2 CL60's on transformer mains and 1 CL60 from psu star ground to chassis
AC Ground and chassis has X2 cap in parallel with 120ohm resistor

I was hoping to get some other ideas before I move the transformer, gut it and rewire one channel at a time, still might not resolve issue.
 

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anyone???

try moving the toroid?
try another CRC filter?

how can you isolate AC hum from mechanical transformer hum or pin point it?
other previous post on aleph hum mentioned rotating the toroid,
would the steel shield Im using reduce this emf?

I dont have a scope :-(

Thanks
 
Use only one channel-cinch for ground preamp - Aleph.
Connect cinch-ground to ground star.
Use not the copperplate, use solid core, max! 1,5 diameter.
Remove the crimps, connect per solding.
Do not use channel-separated psus.
Separate the trafo, the rectifier, first caps in a separate case.
...
my first suggestion.
 
Thanks but cannot rebuild.

I took the toroid out of the center of the chassis and move up and out to one side.
There was no change in the hum so its not emf.

On these Aleph boards, I cut a small strip and mounted all 3 irpf9610's together,
with isolation of course or it never would have worked. Can this pick up hum?

Only thing left I see is lowering the bias back to default values and
some rewiring of the power supply wires.

Shield wire frok antek toroid disconnected or connected to chassis made no difference either.

I have built few other aleph and F5, some tube pre's. This one seems to be more troublesome.

I only have this one antek toroid so also wonder if its the toroid itself that is the cause.

Thx
 
Can you unhook one channel from the psu? This would effectively mean halving the current drawn from it, thus lowering residual ripple voltage. If the remaining channel's hum improves this could indicate insufficient psu filtering.

great idea. just tried it. no difference.

my guts telling me its the toroid or cap bank. anyone ever experience issues with pc grade power caps? these are surplus stock, though if they were faulty, it would not be a consistent hum but all sorts of other issues?

can you help with this question, is 12V per rail enough where the mini aleph circuit will conduct correctly or needs minimum of 15V? I have 2 12v 5A power supplies, I could connect this directly to the aleph board just to see if I hear any hum or other noises.

Last thing is to lower bias thought I dont think that will remove the hum
possibly reduce it.

Thx
 
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