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Old 26th May 2002, 07:45 PM   #1
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Default aleph hum

hey all,
i know that grounding amps has been beaten to death, but i searched and couldn't find a relative answer to my current delima. maybe the answers were there, i just couldn't recognize them. sorry if that's the case.

here goes:
when i fire up my amp, i measure 27 mV and 49 mV on the outputs of my channels. when i hook up speakers, all's quiet.
when i introduce my preamp, it hums like crazy and i reak 340 mV on the output.

current ground scheme:

earth point on chassis to ground on outlet and my input neg leads

star for each channel with pcb ground, including output ground, center taps and that channel's filter cap grounds.

same for other channel.

i think it is something obvious, but i'm not comfortable enough with this to start hooking wires up... or can i? is there any danger in using a wire to hunt down the cause of this loop?

and is it a ground loop? is that what's giving me the increase dc at the outputs?
thanks for any input,
scott
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Old 26th May 2002, 08:18 PM   #2
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Could it be that your preamp is DC coupled and showing some offset at its output?

If so, put a capacitor at the input of your aleph, say a 2.2 uF MKP or something like that. Check if the offset/hum problem at your aleph's output is solved. Please use a test speaker before hooking your aleph to your main speakers.

Later you could take a look at your preamp and see if you can do something about the offset.

Regards
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Old 26th May 2002, 09:12 PM   #3
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no,
i measured the preamp and no dc at the outputs. oh, and don't worry about the spkrs, i learned the hard way a while ago. i have a throw away spkr that i use for testing. once bitten....

what's weird is that it used to work with this pre and others really well. very quiet. i redid the amp boards and now tha i have it up, sort of, i tried to improve the grounding. however, i went back to the old way, that was quiet and now i get this bothersome hum/buzz.

thanks,
scott
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Old 26th May 2002, 10:11 PM   #4
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Star grounding should help you prevent ground loops. But connecting it to the ""outlet ground"" could cause trouble again.
I suggest diconnecting this temporarly and see if there is any change.
I had some ugly experiences in the past with ""dirty grounds on an outlet"".
Please try my previous suggestion as well (input cap), since it only takes a very little DC to cause all sorts of mishap.

Regards.
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Old 26th May 2002, 11:47 PM   #5
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my star ground is floating, for each channel. so, in other words, each star has:
pcb ground
output ground
centertaps
and that channel's ps filter cap ground

the only components hooked to earth are
input gound
chassis

the cap, how exactly do i install it, in line with the input? please advise and i'll give it a try.

thanks alot!
scott
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Old 27th May 2002, 11:33 AM   #6
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Hello Lowfi,

seems to me you´ve got a nice earth loop.
To remove this you can do the following:

Connect your central earth point(s) via a resistor(10 ohms) or ntc to the chassis ground.

Connect the input ground only to the pcb or to the central earth point, NOT to the chassis. (you will have an earth loop cause you preams output is probably also at chassis ground level)

I´ve build all my amps this way (after experiencing the same problems you have) and never have any "humm" problems.

william
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Old 27th May 2002, 11:47 AM   #7
Helix is offline Helix  United Kingdom
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At the moment you have a star ground for each channel, try combining these in to one
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Old 27th May 2002, 04:25 PM   #8
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william and all,
i connected the two star grounds for the channels and the dc at the outputs dropped right down to 20-30 mV. but that hum is there, it's everywhere! i removed the input ground from the earth and started touching it to various parts on the amp to see if i could get the hum down. no go, it's jus this loud, clean hum. it does get louder when i touch it to the pcb ground, but not by much.

any thoughts?
thanks,
scott
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Old 27th May 2002, 05:42 PM   #9
BDP is offline BDP  United States
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Location: SE Washington State
lowfi,

If you are using BrianGTs PCB boards and not isolating the ground trace from chassis you could be getting your ground loop from this. If you are using metal standoff and they are hooked to chassis, guess what, ground loop. You could use plastic standoffs or cut the traces around the mounting holes.

BDP
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Old 27th May 2002, 06:16 PM   #10
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i filed a break in the ground traces to isolate the boards from the chassis.
the pcb grounds are isolated, at the moment, they are floating along with the ps cap grounds and xformer center taps.

i have taken the input ground wires and "touched" around trying to eliminate the hum with no luck. they, too are just floating now.
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