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Old 3rd March 2004, 08:23 AM   #1
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Default Help: LM3875 GC PS/Traf Help needed

Hellow everyone,

For some time now i've been working on my first DIY amp. Its gonna be a 4 channel LM 3875 GC(from the schematic in the LM3875 datasheet, which appears to be the simplest one to make, hence the attraction).

I've been reading decibel dungeon a lot but there are some questions which have come up which i cant really find the answers to.

First thing is about the bridge rectiier. Will this work?

http://www.partsexpress.com/pe/showd..._ID=1167&DID=7

I've already ordered it actually, i assume that being rated at 100V, 25A means that its more than really needed for a GC but means it'll still work fine right? God i hope i'm right.

Secondly, the thing that scares me more, is that this is the transformer i bought:

http://www.rsaustralia.com/cgi-bin/b...toid=-25770865

Its the one with the 0-25V twin secondaries.

At one point, i was sure this is the one that will work fine. But after receiving it, i'm not so sure anymore! Because i dont see an earth wire anywhere on the secondaries! Sholdnt there be one? Is there a special way to wire it up given this odd situation? Does this transformer look suitable? If i got this wrong, i would have wasted 100 bucks.

I was originally planning on running 2 channels per rectifier bridge, and both bridges powered off the same 300VA trafo.

Wiring the secondaries to the rectifier seems easy enough, but where does the earth go? Do i just join the signal ground and power ground together and join them to the earth socket in the wall power point?(if you know what i mean)

Thanks for the help guys, and sorry for the long post on something so simple!

P.S Just a last quick note to any aussies from sydney, if any see this. I'm new to this whole electronics thing, is there some place which sells every concievable part i could ever want in building any hifi equipment? Like a place which sells all the exotic brands of caps/chokes/trafos/resistors/pots/casings/chips/tubes etc. The only places i've been to thus far is dick smith and jaycar, and these places are WOEFULLY inadequate.
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Old 3rd March 2004, 11:33 PM   #2
Albo is offline Albo  England
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Try this link. It clearly shows how to link the two bridge rectifiers, in order to get the necessary 0 volt connection that you should find on your circuit diagram.

http://www.tnt-audio.com/jpg/psu3.jpg

This is the method I use and it works fine.

You will need to know which of the secondaries are 25v and which are 0v. So that you can pair them up.

I had a similar confusion with mine at first, because sometimes things are not obvious for beginners.

Hope this helps.

Al
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Old 4th March 2004, 02:56 AM   #3
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can't someone help? Please?
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Old 4th March 2004, 03:03 AM   #4
BrianGT is offline BrianGT  United States
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Default Re: Help: LM3875 GC PS/Traf Help needed

Quote:
Originally posted by lovely slut
Hellow everyone,

For some time now i've been working on my first DIY amp. Its gonna be a 4 channel LM 3875 GC(from the schematic in the LM3875 datasheet, which appears to be the simplest one to make, hence the attraction).

I've been reading decibel dungeon a lot but there are some questions which have come up which i cant really find the answers to.

First thing is about the bridge rectiier. Will this work?

http://www.partsexpress.com/pe/showd..._ID=1167&DID=7

I've already ordered it actually, i assume that being rated at 100V, 25A means that its more than really needed for a GC but means it'll still work fine right? God i hope i'm right.

Secondly, the thing that scares me more, is that this is the transformer i bought:

http://www.rsaustralia.com/cgi-bin/b...toid=-25770865

Its the one with the 0-25V twin secondaries.

At one point, i was sure this is the one that will work fine. But after receiving it, i'm not so sure anymore! Because i dont see an earth wire anywhere on the secondaries! Sholdnt there be one? Is there a special way to wire it up given this odd situation? Does this transformer look suitable? If i got this wrong, i would have wasted 100 bucks.

I was originally planning on running 2 channels per rectifier bridge, and both bridges powered off the same 300VA trafo.

Wiring the secondaries to the rectifier seems easy enough, but where does the earth go? Do i just join the signal ground and power ground together and join them to the earth socket in the wall power point?(if you know what i mean)

Thanks for the help guys, and sorry for the long post on something so simple!

P.S Just a last quick note to any aussies from sydney, if any see this. I'm new to this whole electronics thing, is there some place which sells every concievable part i could ever want in building any hifi equipment? Like a place which sells all the exotic brands of caps/chokes/trafos/resistors/pots/casings/chips/tubes etc. The only places i've been to thus far is dick smith and jaycar, and these places are WOEFULLY inadequate.
I will try to help.

That bridge rectifier will work fine. I prefer to stick to discrete diodes, MUR860 from On semiconductor.

2x25vac should be fine for the amp, as I am using 2x24vac and it works fine for my 4 ohm speakers (320va for 2 channels). Try using it, and if the voltage doesn't drop too much, or the transformer doesn't get too hot at high volumes, it should be fine.

I would stick to one bridge now, as it seems to be rated more than powerful enough.

Good luck for your first project. I would recommend just getting whatever you have working, then tweak it with different parts. I started with the bare minimum, then worked up from there.

--
Brian
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Old 4th March 2004, 03:18 AM   #5
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Thanks brian! I may be wrong here but are you the one who made the decibel dungeon GC website? If so, its great, thanks so much, it really answered many many many of the questions i had about my first project ever! I'm really looking forward to starting on it.

Either that or i got you mixed up with another guy who built the SEAS Thor kit i think? I cant remember lol.

Thanks a lot for the help. Decibel dungeon only mentions using the MUR860 diodes so i was concerned about using something else(although intuitively, it looked fine to me, good thing it is)

I've ordered all the remaining parts for the amp from partsexpress and it should reach me in a couple of weeks hopefully. All i'm, sitting on right now is the transformer and the chips and the heatsink and pots. They're all cheap parts, and i hope to get it working before going for the exotic stuff heh

Just a few small questions for anyone who can spare the time(not as important as the earlier ones, but thought i'd ask anyway)

Are the snubber capacitors for the bridge rectifier bipolar or polar? Is there a diagram someone which shows me how to wire them up? The decibel dungeon mentions 120ufd as being enough, but does this figure increase with 4 channels? At this point i wont be using snubber capacitors(because i dont know which to use!)

Since my transformer has no earth, is it enough to simply connect the circuit signal ground and power ground together and not join them to the earth pin on the power plug(i.e. there will be no earth output on the power socket of the GC)?

Lastly, is a log pot suitable for the circuit? Thats all i could find with the correct value.

Cheerio!
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Old 4th March 2004, 08:52 AM   #6
Nuuk is offline Nuuk  United Kingdom
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G'day Lovely Slut. First bit of advice - slow down and read a bit more carefully Take each section at a time and be sure you understand it, then write down notes on a shematic so that you have all the correct information when you build.

If you go to the Gainclone FAQ page you will see a wiring diagram for the transformer.

You will also see this:

Should I use snubber capacitors on the rectifier bridge?

I prefer to as they are inexpensive and almost certainly don't make the sound any worse, and probably improve it. Use small types, ceramic or better and values of 0.022ufd are what I recommend. You connect them across each diode (or in the case of a bridge, across the terminals). Snubber capacitors are supposed to reduce the noise that the rectifier diodes make as they turn on and off.
----------------------------------------------------------------------------------

Now, if you go to the web site of one of the electronic component suppliers, eg Farnell, PE, you will find that 0.22ufd caps are small non-polarised types.

Connecting "across the diodes" means that the two pins of the cap are soldered to the two pins of the rectifier diode, or in the case of a ready-made bridge, to each pair of legs.

Just remember to get caps with the same pitch (distance between lead outs) as the rectifier diodes or bridge lead outs.
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Old 4th March 2004, 11:18 AM   #7
BrianGT is offline BrianGT  United States
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Quote:
Originally posted by lovely slut
Thanks brian! I may be wrong here but are you the one who made the decibel dungeon GC website? If so, its great, thanks so much, it really answered many many many of the questions i had about my first project ever! I'm really looking forward to starting on it.

Either that or i got you mixed up with another guy who built the SEAS Thor kit i think? I cant remember lol.

Thanks a lot for the help. Decibel dungeon only mentions using the MUR860 diodes so i was concerned about using something else(although intuitively, it looked fine to me, good thing it is)

I've ordered all the remaining parts for the amp from partsexpress and it should reach me in a couple of weeks hopefully. All i'm, sitting on right now is the transformer and the chips and the heatsink and pots. They're all cheap parts, and i hope to get it working before going for the exotic stuff heh

Just a few small questions for anyone who can spare the time(not as important as the earlier ones, but thought i'd ask anyway)

Are the snubber capacitors for the bridge rectifier bipolar or polar? Is there a diagram someone which shows me how to wire them up? The decibel dungeon mentions 120ufd as being enough, but does this figure increase with 4 channels? At this point i wont be using snubber capacitors(because i dont know which to use!)

Since my transformer has no earth, is it enough to simply connect the circuit signal ground and power ground together and not join them to the earth pin on the power plug(i.e. there will be no earth output on the power socket of the GC)?

Lastly, is a log pot suitable for the circuit? Thats all i could find with the correct value.

Cheerio!
I didn't do the DD website, but I did build the Seas Thor TL, and I love them greatly. I am also organizing a group order of gainclone pcbs and kits if you are interested:
http://www.diyaudio.com/forums/showt...885#post331885

You will find the schematics that I am using within that thread. Here is a summary. From the transformer, each of the secondaries goes into a bridge rectifier. The outputs from both of them go into the the amplifier pcb, where positive output of one and the negative output of the other bridge is referenced to ground to give you around +/- 34v for my transformer There is a chassis ground that goes into the board as well. The chassis ground is tied to the earth ground from the AC recepticle as well.

As to the snubber capacitor, I just have a 4.7uF bipolar electrolytic over the dc output on each bridge, and no snubber caps on the diodes. It seems to work fine.

For my first gainclone, I used a single bridge rectifier, with a central ground, grounding directly from my bridge rectifier, and it worked fine.

Feel free to drop me an e-mail if want to ask me any questions.

--
Brian
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