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Old 20th June 2007, 03:25 PM   #1
ThomasS is offline ThomasS  Denmark
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Unhappy Power transformer snerring.

I just got a 2A3+300B Music Angel from ebay - nice sounding right out of the box :-) I will post a review once I get more familiar with the amp.

However - the power transformer makes a low snerring sound (you can also feel the vibration when touching the cover of the transformer) - it does somehow seems hum related - a low level hum is also comming from the speakers. The transformer gets fairly hot (40-45 degree C). I'll try to get the schematic from the supplier...

Is there a simple "cure" for the power transformer (apart from buying a decent sized one)?

Thanks,
Thomas
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Old 20th June 2007, 09:09 PM   #2
Klimon is offline Klimon  Belgium
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Apart from mechanically decoupling the trafo from the chassis (to prevent the chassis and other components from resonating along) there's not much you can do IMO. I wouldn't worry about it too much. A larger trafo is a costly affair and will only yield mechanical silence, you'll still have humm on the outputs.

Simon
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Old 20th June 2007, 10:58 PM   #3
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Yes, hum on the outputs has not necessarily to do with transformer mechanical hum. The obvious: You did make sure that the clamping bolts are tight? Otherwise if it is troublesome, take off and open up, and apply mains - one could often use a finger to feel where the trouble is. (Also just to comply with forum rules: You would know to be careful NOT to feel where the voltage is!!) If you can determine the spot, applying some lacquer there or just generally often helps - in the past I was fortunate to have had that work. A little more problematic if the noise comes from the centre piece for E-I laminations; a lot more problematic if you have a C-core transformer!

Regarding electronic hum, if you have a drop-through transformer and steel chassis, you also have my sympathy. But as it is an existing amplifier one would presume that it was originally quiet. Then the "usual suspects" apply: Poor filter capacitors, corroded earth connections et al.

Good luck!
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Old 20th June 2007, 11:49 PM   #4
jnb is offline jnb  Australia
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Quote:
Originally posted by Johan Potgieter
if you have a drop-through transformer and steel chassis, you also have my sympathy.
Would you say that a drop through transformer is generally a bad thing, or only with a steel chassis?
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Old 21st June 2007, 12:20 AM   #5
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Especially on a steel chassis, I would not use it. Yet again, there are commercial equipment using this quite successfully. So if you have one, do not throw it away; try first. But starting from new, there are enough on-chassis models available not to choose a drop through, and it is simply less of a risk (like for me, all the gremlins usually come rushing up, so I chicken out and avoid rather than cure!).

Regards.
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Old 21st June 2007, 06:23 AM   #6
ThomasS is offline ThomasS  Denmark
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I'll make a seperate (solid) enclosure for the transformer and psu - it will leave room for improvements, solid fastening of transformer and remove heat and vibration from the amp.

This will furthermore "protect" the amp in case the transformer decide to selfdestruct.....
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Old 21st June 2007, 06:28 AM   #7
ThomasS is offline ThomasS  Denmark
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I guess that the hum would be really annoying on "full range" speakers - the amp is driving my OB, which is ~9dB down @60Hz

Let's see what the psu improvements will bring....
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Old 21st June 2007, 08:52 AM   #8
ThomasS is offline ThomasS  Denmark
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I just got the schematic - it's going to be interesting to check how it matches the actual amp

Any recommendations for improvements?
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Old 21st June 2007, 10:32 AM   #9
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Separate filament supplies might end up reducing the hum problem on the main transformer. All of those SS rectifiers dumping into big capacitors is tough on the transformer. Better filament supplies might sound better anyway. You have a lot of options: choke input, CCS, ...

To be honest, I don't know what the 3A5 heater supply is all about, but I don't like halfwave rectifiers anywhere. They are especially hard on the transformer actually putting DC through the winding. That is never good as far as I'm concerned.

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Old 21st June 2007, 11:45 AM   #10
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I had a commercial ss amp long ago where the transformer hummed and ran very hot. I don't think it had a shorted winding or anything, it was just a cheap transformer. The amp performed fine electrically, but you could hear it in a quiet room. It seems to be common that a transformer design will start out at a high quality level, using a decent grade of steel, and then be transferred to a cheaper source. First to go is the good grade of transformer steel, followed by wire gage and construction practice. Losses go up, then you have hum and high temperature even under quiescent conditions. Grommets and careful mounting will help, but the real cure is a well made transformer. I prefer non-drop through transformers, but haven't had a bit of trouble with either type.
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