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Old 17th March 2006, 08:46 AM   #1
Garnett is offline Garnett  United Kingdom
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Default What is what on an EI Transformer

As some of you will have gathered, I'm very new to the world of electrics. I'm about to start putting together a LM3875 kit from Peter at Audiosector (very nice looking kit), and I want to se if I can use an old EI transformer from an old faulty Marantz PM66SE amp.

Trouble is it's been seperated from the Amp and I don't have a clue what I'm looking at. Can anyone tell me which connections are what.

Click the image to open in full size.

Thanks again for any help you can offer a newbie.

I promise to put any info on my website so other newbies in my position can hopefully be referred there...
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Old 17th March 2006, 11:54 AM   #2
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The thinnest wire and coil with highest resistance is the primary. Looks like the terminals on the right. There may be two primaries which get connected in series/parallel depending upon the primary voltage (220/110).

If you want to be extra careful feed the assumed primary with very low voltage (12v?) and see if the secondaries get the expected lower voltages.
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Old 19th March 2006, 02:52 PM   #3
Garnett is offline Garnett  United Kingdom
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Thanks for the help Analogue SA. I'm going to do what you said.

In the meantime I've done a search of this website and found a reference to the pm66se transformer which says the secondaries are 28.7 v AC which gives 38.4 V after rectification.

IS this accptible? It seems higher than what is usually recommended on here. Is there anything I can do to make it work?
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Old 19th March 2006, 03:06 PM   #4
Nordic is offline Nordic  South Africa
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Little heavy on the headroom, but with a good heatsink it can work.
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Old 20th March 2006, 09:09 AM   #5
Garnett is offline Garnett  United Kingdom
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Thanks Nordic. I was a bit concerned because the consensus seems to be "transformers with 18-22V secondaries are well within reason for many common commercial and DIY speakers. A transformer with 25V transformer secondaries can also be successfully with less of a safety factor."

Is overheating of the chips the only thing I need to worry about?

I'll take out the two large heatsinks from the same amplifier and use those.

Will this affect sound quality and/or reliability? I read somewhere (can't remember where now) that a lot of people like to run Gainclones with low voltage for better sound (albeit quieter).

In the long term I have started to consider bridging or running two or more chips in parallel (saw this post on www.gainclone.com and thought I'd wait and see what the more experienced contributors thought. Bridging appears to produce a less defined sound, but I haven't found any reviews of parallel circuits.

Could I power both from the same transformer and would this mitigate the high voltage problem?
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Old 20th March 2006, 09:27 AM   #6
Nordic is offline Nordic  South Africa
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You also don't want to drive 4 ohms with that, so watch the impedance.

If it heats up enough, Spike will kick in, and yes its audiable...

I'm not qualitfied enough to be able to judge the validity of the following statement but I do believe it to be true.
As you drop the supply voltage, you increase crossover distortion.

It is all about which compromises you are willing to take.
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Old 20th March 2006, 09:34 AM   #7
Garnett is offline Garnett  United Kingdom
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I'm in luck there, because my speakers are rated 8 ohms.
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Old 20th March 2006, 09:42 AM   #8
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Just playing around with ohm's law will show that (2 x 38.4) / 4 will demand jsut short of 20A. now if you check the datasheets for the chips you'll see the LM3875 will handle 4 to 6A (varying from chip to chip), the LM3886, about twice that... either way, you'll be looking at seriously fried chips.
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Old 20th March 2006, 10:15 AM   #9
Garnett is offline Garnett  United Kingdom
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Thanks Nordic.

Jees. I can't see the reference in the pdf from Nat Semi. When you say "the LM3875 will handle 4 to 6A (varying from chip to chip)" is that the maximum?

As I've said before I'm new to all this.

If I use one rectifier bridge:

38.4 Volts output from the transformer/ 8ohms rating of my speakers = ~5 Amps current. This obviously falls right in the middle of the range. So I guess it's important I find out whether that rating range is a variable maximum (which varies from chip to chip) in which case I could be in trouble or whether that's a prefered range, within which the chip operates fine...
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Old 20th March 2006, 10:20 AM   #10
Nordic is offline Nordic  South Africa
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Look, its never going to draw that amount of current playing music. At least not before you turn the volume down.

If you want to get realy anal I suppose you could take off the 1.6 to 2.7V output dropout voltage from the supplies at the very least. Which draws numbers further in your favour.

P.S. on that datasheet look for Io (Output Current Limit)
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