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Old 10th November 2004, 10:38 PM   #1
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Default Linear Power Supply With AC On The Outputs

I have build a simple linear power supply, as per the circuit diagram with a +/-25v, 25VA transformer, and 15,000uf capacitors.

This power supply will power my gainclone amplifier. I realise it is underpowered, but my gainclone never draws more than 200ma from the supply, and over 150ma of that is for the cooling fans, so I feel it will be adequate.

The DC is fine, ~+/-35v output, but there is also ~0.5v AC on each output. So my amplifier now has a nice 60Hz tone along with the music. How can I get rid of this voltage?

Sorry about the messy diagram. Everything is connected correctly, even if I have drawn it incorrectly. Both 0v outputs are connected together, to the mains earth, and to the case.
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Old 10th November 2004, 11:00 PM   #2
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Default 60Hz tone

Faulty cap?
Try replacing, and maybe a 10 ohm 5W+ resistor to gnd(E)

Arne K
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Old 10th November 2004, 11:27 PM   #3
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Nope, they are both fine.

Where should the resistor go? between the ground connections and mains earth?
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Old 10th November 2004, 11:55 PM   #4
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Aside from the diodes drawn backwards..... you can save a FWB by connecting the windings in series. Which reminds me, what is it with these SS guys and their whacked up rectifiers anyway? Not to mention the capacitance, a 25VA transformer is what, 1A, maybe 0.5A DC? At 15mF that's more like 300mA...or less! Certainly no more than 200mA with +/-35V output.

And 200mA for a what, 20W amp? Sounds to me like your chip is toast...(have you tested it at full power?) It should have hum immunity built in anyway.

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Old 11th November 2004, 09:24 PM   #5
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Matttcattt, you will blow the trafo if you really think in using it with a GC.
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Old 12th November 2004, 12:08 AM   #6
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Quote:
Originally posted by Sch3mat1c
Aside from the diodes drawn backwards..... you can save a FWB by connecting the windings in series. Which reminds me, what is it with these SS guys and their whacked up rectifiers anyway? Not to mention the capacitance, a 25VA transformer is what, 1A, maybe 0.5A DC? At 15mF that's more like 300mA...or less! Certainly no more than 200mA with +/-35V output.

And 200mA for a what, 20W amp? Sounds to me like your chip is toast...(have you tested it at full power?) It should have hum immunity built in anyway.

Tim
Im not bothered, the current power suppy is a bench supply rated at 1A max and it has never drawn more than 200mA.

I dont know the output of my amp. My sub reached xmax before full power so I havent tested the amp exept into a dummy load. It still hums.

Quote:
Originally posted by carlosfm
Matttcattt, you will blow the trafo if you really think in using it with a GC.
How?
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Old 12th November 2004, 12:35 AM   #7
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Originally posted by Matttcattt
How?
How?
As soon as you feel like listening at a little louder volume the primaries will go pffffffffffzzzzzzzzzzzzzzt.
Even at idle that trafo will run very hot.
If you want to use a small trafo at least use a low voltage, like a 2x12V trafo at 2~3A.
With a low PSU voltage the chip will demand much less current.
I have a small amp with a PSU like this and no problem, works very well.
You will have 10~15W RMS.
Feeding the chips at +/- 35V with such a weak trafo won't do.
Sorry.
Use that trafo for something else, like a preamp, not a power amp.
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Old 12th November 2004, 01:09 AM   #8
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Originally posted by carlosfm
Matttcattt, you will blow the trafo if you really think in using it with a GC.

I dobut it.

I'm probably the only one cheap (and dumb) enough to think of using 2 16V 10VA doorbell transformers I got for about a dollar apice at a garage sale.

And guess what I powered with it??

2 x LM3886 driving small speakers, and 2 x OPA549 bridged driving an 8ohm sub.

And it worked fine. Although the transformers got quite warm (could feel it was very warm even through the 1/2" MDF case.)

Now I have upgraded to 2A transformers I got for cheap at science and surplus, and it sounds much better, though I get a low frequency humm from the sub. Thats because the transformers humm

Are you sure the humm you are hearing isnt transformer humm?
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Old 12th November 2004, 02:46 AM   #9
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Heh. Just seems funny to see a sig that states one's amps need more magic smoke, following a post suggesting a reason why they have so little......

Tim
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Old 12th November 2004, 05:59 PM   #10
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Quote:
Originally posted by soundNERD
Are you sure the humm you are hearing isnt transformer humm?
Are you sure it's humm, or the small trafo vibrating like a mad cow because it's working on it's limits?
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