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Old 8th March 2005, 02:32 PM   #1
HarryK is offline HarryK  United Kingdom
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Default LM3886 - What Transformer ?

Hi All

Sorry for the really stupid question but......

I have just bought one of Brians LM3886 kits and I now need to get a transformer.

I live in the UK and I bought a standard LM3886 kit (2 LM's and one PSU).

What type of transformer will I need and where is the best (not really the cheapest) place to get one.

And if its not too much trouble, what wires go where ?.

Thanks in advance.

H
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Old 8th March 2005, 03:25 PM   #2
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Hi,

Maplins do some very good torroidal pretty cheap.
I have just received 2 * 120va 22 volt one for my dual mono setup.
If you want they have larger but the prices are far cheaper than anything esle in the u.k. that i have found.

For example mine cost 30 for two delivered. Maybe you want the higher rated one but in the confines of the case i bought they were a no go. I think that 22v ones are the best and they have 120va and 160va which are ideal for the dual mono setup.
For the stereo kit maybe the 330VA 2 x 24V, but this might be a little high voltage wise.

Hope that helps

Phil
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Old 8th March 2005, 04:05 PM   #3
ljk11 is offline ljk11  Trinidad and Tobago
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I am interested in starting a gainclone also and would like to know what is a good transformer for 8 ohms use, available in the US
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Old 8th March 2005, 04:14 PM   #4
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Quote:
For the stereo kit maybe the 330VA 2 x 24V, but this might be a little high voltage wise.
I am wondering if a person can't step down the voltage a little in this case? BrianGT had said he was going to use the above mentioned tranny, and i myself have got a 2*30V (300VA) toroid. I hope this'll be okay for this application.
thanks,
Mantisory
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Old 8th March 2005, 11:23 PM   #5
colinc is offline colinc  United Kingdom
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Perhaps I got lucky but if you're not in a rush check out what's on Ebay. I bought four ILC 300VA torroidal transformers with dual 0-21v secondaries for 2.50 each a couple of weeks back. I have a set of Brian's boards/parts to go with them.

Unfortunately bathroom DIY has overtaken audio DIY but I'll get there soon!
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Old 9th March 2005, 12:52 PM   #6
steenoe is offline steenoe  Denmark
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Quote:
2*30V (300VA) toroid. I hope this'll be okay for this application.
That is definately too much. You will have to use the regulated supply for that trafo.
It is not advisable to use a trafo close to the limit of these chips.
I am using 2x18 volts and there is plenty of power. Furthermore, the amp can safely be connected to speakers rated at 4ohms, without getting sweat'y I would say 2x22 volts is about max.

Steen.

BTW You could also unwind the trafo to get the right voltage.
It is actually easier than it sounds.
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Old 9th March 2005, 01:13 PM   #7
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Quote:
You will have to use the regulated supply for that trafo
Isn't Brian's offering regulated? I thought so....if not, what are the MUR860's in the power supply? Or do you mean i'd need to further regulate it.

Quote:
It is not advisable to use a trafo close to the limit of these chips.
On the national 3886 spec sheet here , it says that the max power supply voltage is 84V, which is 42 per side?
I am not sure where the problem is...
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Old 9th March 2005, 01:41 PM   #8
boholm is offline boholm  Denmark
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Quote:
Originally posted by mantisory
. . . it says that the max power supply voltage is 84V, which is 42 per side?
I am not sure where the problem is...
30V AC * 1,4 = 42V DC - so you are practically on the edge.

Furthermore when a transformer is unloaded it gives out more voltage than indicated, so in practice you may get out more than 42Volts DC of your PSU - even when your LM3886 is connected, because it doesn't use much idle current.

If you have a voltmeter (multimeter), some loose resistors, a calculator *and* know Ohms law, you can test it for yourself - *always* be careful messing with transformers .
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Bo
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Old 9th March 2005, 01:50 PM   #9
steenoe is offline steenoe  Denmark
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Quote:
Isn't Brian's offering regulated?
No. The Mur's are rectifierdiodes! Check the datasheet again regarding railvoltage/outputpower.
I guess if you want to run a 84 volt supply, you would need a speaker with an impedance of 30ohm or something like that. Impossible.

Steen.
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Old 9th March 2005, 02:49 PM   #10
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so...I bought this transformer thinking it would be the right one, but apparently i was wrong. Now what? Should i try a regulator circuit (based on the lm375 or something) or should i try to trade in my transformer?
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