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PostPosted: 18 May 2010, 10:41 
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Joined: 18 May 2010, 09:33
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Hi all,

I'm new here and gathering parts for my first Audio project. My background is in low voltage electronics, building test fixtures based on Microcontrollers and such, so forgive me if my question are a bit lame. My goal is to use as much "stuff" from my junk bins as possible but still maintain a quality amp. I have 2 LM3875's on order and the first part I am scrounging is the Power Transformer, I found an old broken Yamaha CR-820 in the basement and have pulled the transformer, speaker connects, some switches and other "things". The power transformer has 2 secondaries (Red,Red Bla) and (Org,Org, Blu). I found a schematic on line but it is a very poor copy, it looks like one of the secondaries is 51VAC before rectification and the other secondary is hard to make out but looks like 30VAC, is this normal? The 51V leg goes to a bridge that is snubberized, the other goes to a bridge that is not. My main question at this point is it acceptable practice to use the same 51V leg for both sides of the amp, left and right I mean. The original Yamaha CR-820 was rated at 50W/CH and had 4 output transistors.

Thanks!
Alan


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PostPosted: 18 May 2010, 21:41 
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Joined: 28 May 2008, 21:53
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Location: Winnipeg, CANADA
Hi Alan, welcome to the forum.
That transformer may work for the "single supply" circuit shown on the datasheet. For a "gainclone" you need a symmetrical supply in the +/- 22 to 35VDC range (depending on speaker). To do this you need either a CT or dual secondary transformer with about 18 to 28V on each side.
Cheers

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[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


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PostPosted: 18 May 2010, 22:07 
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Joined: 18 May 2010, 09:33
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Thanks for the reply,

So just to make it more clear for me, to use two LM3875's in a left and right channel configuration, I will need two separate bridge circuits (8 diodes) and four 10,000 Uf caps (two for each bridge) ?

And would it be possible to point me to schematic showing two LM3875's in this configuration?

I really appreciate the advise,

Thanks,

Alan


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PostPosted: 18 May 2010, 23:12 
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Joined: 28 May 2008, 21:53
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Location: Winnipeg, CANADA
Hi Alan, yes you will need two bridges. Here are a couple of LM3875 examples for you to follow:

Breadboard / point to point LM3875 gainclones
http://diyaudioprojects.com/Chip/Nanoo/
http://diyaudioprojects.com/Chip/Synerg ... Gainclone/
http://diyaudioprojects.com/Chip/P2PLM3875/

LM3875 Kit based gainclone
http://diyaudioprojects.com/Chip/Beast/

Cheers

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[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


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PostPosted: 18 May 2010, 23:58 
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Joined: 18 May 2010, 09:33
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Well, I followed the links you provided and in two of them it looks like only one bridge is being used. I have attempted to attach a capture of this, can you explain this?
Attachment:
5-19-2010 12-52-48 AM.jpg

Thanks


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PostPosted: 19 May 2010, 12:17 
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Location: Winnipeg, CANADA
Yes, my mistake.

With a dual secondary transformer you need two bridges.

With a center tapped transformer (as shown in the schematic above) one bridge will suffice.

Both configurations will work just fine.

Post a schematic of where that transformer came from. The 51V may work if it is 25.5V-0V-25.5V and can take the current demands (160VA minimum for stereo).

Cheers

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[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


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PostPosted: 19 May 2010, 13:43 
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Joined: 18 May 2010, 09:33
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Great, Thanks again,
My plan was to "cap off" the top center taped winding, and use the bottom center taped , that has 51VAC on each side of the tap.
I was considering using a solid state regulator to lower the DC voltage after rectification if need be. I will be driving 8 Ohm speakers , my klipsch La Scala's in the long run, but before I let the amp any where near them, a pair of old 8 Ohm speakers that are just sitting around in the cellar.
Here is the (hard to read) drawing of the transformer. This came out of a very well respected Yamaha Amp, that had met its demise for other reasons!



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PostPosted: 20 May 2010, 15:21 
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Joined: 08 May 2009, 08:20
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Location: Duesseldorf, Germany
Hi there.

Well, there is a chance to use the 51VAC outputs if you're living somewhere around the US (110 / 120VAC Mains).
If i saw it right, the transformer has a switchable primary input from 110VAC to 250VAC?...

If your mains outlet is 110VAC / 120 VAC you can connect the transformer to the 250VAC primary
(or whatever printed on the schematic) and you'll get half the voltage on the 51VAC rails..
That should fit your needed voltage for a LM chip...

For me it's hard to read out the values on the schematic.. Maybe someone can help here?
I guess it's Blue, Red, Grey, Yellow, Orange, Brown and somewhat elese...
Maybe you can follow the connections to the positions from the voltage selector...

Or keep the selector "as is" and use it as "buck switch"..

Have fun ;-).

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Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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