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Old 9th January 2016, 05:43 PM   #1
Bengali is offline Bengali  United States
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Default mini Aleph hum

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
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Old 9th January 2016, 05:52 PM   #2
Zen Mod is offline Zen Mod  Serbia
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pictures and complete info about PSU
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Old 9th January 2016, 09:07 PM   #3
Bengali is offline Bengali  United States
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Thx ZM

I asked him to remove signal wires first from pcb and see if hum goes away.
I had used twisted pair wires and not shielded. hoping thats the issues.

its the basic brute C-L-C power supply
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Old 18th March 2016, 03:20 AM   #4
Bengali is offline Bengali  United States
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Quote:
Originally Posted by Zen Mod View Post
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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Old 18th March 2016, 03:33 AM   #5
Bengali is offline Bengali  United States
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sorry, they are actually 33,000uf 50volt caps, 2 per rail
nippon chemicon brand bought from bg micro.

can these caps go bad and cause hum or unlikely?
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Old 18th March 2016, 03:39 AM   #6
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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.
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Old 18th March 2016, 04:09 AM   #7
Bengali is offline Bengali  United States
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is 100k between input to ground similiar to just shorting input to ground?

Thx
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Old 18th March 2016, 05:18 AM   #8
Bengali is offline Bengali  United States
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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.
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Old 18th March 2016, 05:50 AM   #9
plasnu is offline plasnu  United States
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Are you connecting bridge output (noisy) to the star ground? If so, I have read that it is not a good idea. I'm not an expert, so I'm not really sure, though.
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Old 18th March 2016, 06:57 AM   #10
Bengali is offline Bengali  United States
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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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