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Old 16th June 2002, 02:41 PM   #1
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Default static noise and hum

Hello,

I have build an Aleph 2, which is working better and better. However I have a difference between the channels in noise and hum.

In channel 1 I have a little bit of hum in the woofer but no noise in the tweeter at all.

In channel 2 I have a little bit of static 50hz 'ticking' noise in the tweeter but NO hum in the woofer at all.

So channel 1 of the opposit of channel 2... Very funny. The levels are very low (so a it is not an earth loop I think) you really need to put your ear in front of the speakers. The hum in the woofer of channel 1 is a bit higher of level than the noise in the tweeter.

When I checked the output with a scope; channel 1 shows a kind of 50hz saw form (the hum). Channel 2 does not show this saw but has a thicker line (the noise?!).

I have about 0.02V DC offset in both channels.

My questions;

1) What is the single most important part for hum cancelation in the circuit, as channel 2 does this better than channel 1.

2) What is the single most important part for noise pickup. As channel 2 is more prone to pick up...

I drive them single ended, but I am going to pickup a balanced cable tomorrow to test this.

Both channels have there own 1000VA power supply and currently I am running them as mono blocks on a piece of MDF wood. No connection of grounds whatsoever between them. They only share the 220V inlet. The ripple of the powersupply is about 200mV top-top.

Thanks,

Edwin


Click the image to open in full size.

P.S. As I type this, I thought of swapping the power supplies between the boards... I am going to try this now.
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Old 16th June 2002, 03:02 PM   #2
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Default Re: static noise and hum

I just swapped the power supply between the channels. This does not help. The static noise and hum, stay at exactly the same channel ... So it is not the ripple of the power supplies.

What can it be?

Edwin
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Old 16th June 2002, 05:11 PM   #3
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Have you checked the inputs? I have not built one of these and don't even have a schematic but assume that the red and black leads coming from the bottom of the photo are the inputs. They ought to be twisted together or shielded, don't you think? The right side of the photo shows and extra black lead. What is that?
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Old 16th June 2002, 05:15 PM   #4
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Quote:
Originally posted by Bill Fitzpatrick
Have you checked the inputs? I have not built one of these and don't even have a schematic but assume that the red and black leads coming from the bottom of the photo are the inputs. They ought to be twisted together or shielded, don't you think? The right side of the photo shows and extra black lead. What is that?
Thanks,

the black and red are the speaker cables. The 'extra' black cable on the right is a shielded input cable.

Edwin
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Old 16th June 2002, 06:07 PM   #5
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If I were trouble shooting this I'd look at the output with a scope and with a pair of wooden tongs a move some of the wires around a bit to see the effects, if any - particularly around the input cable which might also be suspect of marginal shielding.

It's hard to tell but it looks like the wiring from the supplies is going to the main board rather than the output boards and that the output devices get their power from the main board. I would have done it the other way around.

As an aside, I'm wondering if the wiring from the supplies isn't a little on the skimpy side.

Before I started looking for problems on the main boards, I'd eliminate all other possibilities.
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Old 16th June 2002, 07:49 PM   #6
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Quote:
Originally posted by Bill Fitzpatrick
If I were trouble shooting this I'd look at the output with a scope and with a pair of wooden tongs a move some of the wires around a bit to see the effects, if any - particularly around the input cable which might also be suspect of marginal shielding.

It's hard to tell but it looks like the wiring from the supplies is going to the main board rather than the output boards and that the output devices get their power from the main board. I would have done it the other way around.

As an aside, I'm wondering if the wiring from the supplies isn't a little on the skimpy side.

Before I started looking for problems on the main boards, I'd eliminate all other possibilities.
The power supply wires are 2.5mm^2 for the blue and brown ones, and 1.5mm^2 for the temporary white ones. The white ones will be removed when the amp gets in the alu case.

Edwin
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Old 17th June 2002, 03:21 AM   #7
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Perhaps the decoupling caps on the amp board are busted? I had a hum issue with my JLH; turned out a 1000uf decoupling cap wasn't working. Switching it for a new 100uf one made the problem dissapear.
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Old 19th June 2002, 03:00 PM   #8
nar is offline nar
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Hey Edwin!!!!!

Your power supply rails are way,way to long....!!!!!!.....
Suppose the pick up lot of noise,for 50 or 100 Hz don't run a cable near capacitors,cross everything at right angle and shorten to MAXIMUM all voltage rails.
Perhaps a pi filter would give you less noise,and use a balanced signal to drive the amp

Regards
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"... Audio needs the thinnest wire ..." Rowan McCombe ... I hear it !!!
"Just 'cause they can't hear or sense it themselves doesn't mean you can't !" Allen Wright Allen was right ...
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Old 19th June 2002, 04:25 PM   #9
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Default decoupling caps

How do I test or tell the decoupling caps are working or not ?

Thanks,
Chris
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Old 19th June 2002, 06:51 PM   #10
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The noise goes away.
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