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Old 20th June 2010, 10:58 PM   #1
zoubour is offline zoubour  Switzerland
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Default 50 Hz in home made power amp

Hi,
I got a very good ALBS power amplifier from Ebay.
It works very nicely except for a strong 50Hz hum that gets loud when the RCA input is connected to a source not playing.
Actually, it's stronger when using a macbook as a source than an Iphone.

I replaced the big capacitors but no improvement.
Any ideas would help.
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Old 21st June 2010, 12:17 AM   #2
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Well the computer has a power supply which is grounded so you may have a ground loop. But the iphone isn't grounded and you said you still get hum with it, so there's another possibility that should be investigated.

Short the amp input to ground. Does the hum go away? If it doesn't, the problem is in the input stage. Make sure any signal cables inside the amp are shielded, and that they don't come close to the power rails or to the transformer powering the whole thing. You say the amp is homemade, there could also be a design error on the PCB.
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Old 21st June 2010, 01:56 AM   #3
gootee is offline gootee  United States
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Also, make sure that each input's signal and ground conductor stay very close to each other (preferably twisted tightly together, preferably inside a shielded cable with shield grounded to chassis on the end near the input), as much as possible, ALL the way from the (non-grounded, I hope) input jack to the input of wherever they go next (probably the top and bottom/gnd of an input resistor?). That's because any little bit of loop area that is formed by the conductors will be able to have a current induced in it, by any changing magnetic field (which your transformer and AC wiring, among other things, so-thoughtfully supply). Needless to say, if the input signal/ground pairs aren't even twisted together, then that is likely to be your problem, since plugging in a source completes their loop, and the induced current would then be able to induce voltages across the source's impedance, the first-encountered input impedance, and even across the conductors themselves to some extent.

The same "minimize the loop area" idea should be applied to ALL other conductor pairs inside the amp, such as DC power/ground pairs, AC power pairs, all signal/ground pairs, output signal/ground, etc. The conductor pairs should be twisted tightly together. The low-level ones should preferably be inside shielded cables, with each shield grounded to chassis at one end only. You should try to keep everything away from the AC stuff, and keep the low-level signals away from the AC and everything else, too. Things that must cross should do so at a right angle to each other.

In case that's not the problem, the following article is very good:

Audio Component Grounding and Interconnection

Cheers,

Tom Gootee

Last edited by gootee; 21st June 2010 at 02:20 AM.
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Old 21st June 2010, 10:54 AM   #4
zoubour is offline zoubour  Switzerland
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Default Shorting signal and ground RCA

Thanks for these advices.
The amp card connection to the RCA is very close to the RCA input. It is in a shielded cable.
The rest of the connections are made with heavy duty speaker cable.

Unfortunately, connecting the RCA shield to the chassis or the ground point does not take the buzz away.

I know it is not a ground loop because the computer is not grounded nor some other sources I have been using.


Connecting the shield and the signal part of the RCA input removes the hum but attenuates the signal drastically.
Is there any component I should use between these instead of a naked wire that would leave the volume intact?
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Old 21st June 2010, 11:08 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
get ready to send it back to the seller and ask ebay and/or the seller for a full refund including the return postage.
Once this is all agreed, send it recorded delivery to the seller.

Or.

arrange a big discount on the price to cover the cost of repair if you are confident you can repair it.
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Old 24th June 2010, 03:31 AM   #6
gootee is offline gootee  United States
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Quote:
Originally Posted by zoubour View Post
Thanks for these advices.
The amp card connection to the RCA is very close to the RCA input. It is in a shielded cable.
The rest of the connections are made with heavy duty speaker cable.

Unfortunately, connecting the RCA shield to the chassis or the ground point does not take the buzz away.

I know it is not a ground loop because the computer is not grounded nor some other sources I have been using.


Connecting the shield and the signal part of the RCA input removes the hum but attenuates the signal drastically.
Is there any component I should use between these instead of a naked wire that would leave the volume intact?
Since this is something you purchased, you will probably just want to try to return it, instead of modifying it.

But I want to point out, anyway, just for the sake of clarification, that by "shielded cable" I meant TWO conductors inside the cable, with an additional outer shield. I did not mean that the outer shield carries the signal ground. And the outer shield would not be connected to the RCA jack's outer "shield"/ground, since that should not be connected to the chassis, which is where the outer shield of a two-conductor+shield cable should go. To re-iterate: A single-conductor+shield "RCA cable" is NOT what I meant by a shielded cable. Also, you would not want to connect the RCA jack's "outer shield" to the chassis, anyway. That would probably create the potential for a ground loop.

Note, too, that I was not suggesting that it was a ground loop. I was suggesting what could possibly be called a Faraday loop, which basically acts like an antenna and can pick up the mains hum very easily and well, and induce a hum voltage right across your amp's internal amplifier input. You might want to try testing it with a small-ish resistor (maybe 50 to 500 ohms, but it's probably not too important), between the outer and inner contacts of one or both of the RCA input jacks, and see what that does to the hum. I am thinking that that might make the hum either stay the same or get worse.

Regarding connecting the signal and shield of the RCA input: Yeah, that would tend to "attenuate the signal drastically", since you would then have a zero-volt input signal. You should not do that when the source is connected and playing, since that would short its output. If you short both inputs, is the amp's output completely silent?

Seeing some good pictures of the inside of the amp might help a lot. And I am curious to see it.

By the way, there is a good article on this (diyaudio) site:

Audio Component Grounding and Interconnection

Cheers,

Tom
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Old 1st January 2011, 11:26 PM   #7
daminos is offline daminos  France
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Hi,

Pretty new to this forum, I will use a [1 conductor + 1 shield braid] cable for internal wiring.

When wiring a RCA input with a pair of these wires, say [1 conductor on RCA signal] + [1 conductor on RCA ground],

should I twist them together ? and connect each of their shields together to the chassis ?

Thank you.
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Old 3rd January 2011, 09:59 AM   #8
AndrewT is offline AndrewT  Scotland
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no.

use coax for both flow and return in a circuit.
or
use screened two core, the cores carry the flow and return and the screen is grounded at it's input end.
or
use twisted two core as flow and return.
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Old 3rd January 2011, 10:54 AM   #9
h_a is offline h_a  Europe
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Lots of nice info there, but I'm not sure all of it is going to have success. It depends too much on details.

For example, in my big block power amp, the only effective measure was to use a spectrum analyzer (ARTA+Sound card) to find the proper measures to be able to achieve 100dB dynamic range.

Braided wires (koax if you wish) were not effective at all; much less than twisting wires and positioning them carefully (fix them with clips or wires to stay in place).

I can only recommend to stay away from modding commercial equipment, even less when it's brand new. Who knows what else might be messed up.

Have fun, Hannes
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Old 3rd January 2011, 10:59 AM   #10
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It could be as simple as your 0V cables are too thin.

All 0V connections should go to a common SINGLE point as close as possible to the main reservoir capacitors. All 0V connections need to be heavy gauge cables, I use 30A auto cable to give you an idea of how heavy.
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