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Old 15th July 2003, 10:05 PM   #1
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Default My First GainClone Project is under way!

Hi all!

Last year my Technics SA-GX505 receiver's output stage fizzled. Since then I have been without the luxury of hi fidelity. I've been reduced to listening to computer speakers! Oh the humanity!

Necessity breeds invention...or in this case, construction. What I plan to do is rip the large proprietary amp chip out of my receiver, throw it away, and wire RCA jacks from the original chip's inputs effectively making it a pre-amp with a huge power transformer, and build a gainclone to act as the power amp.

Unfortunately, the large transformer in the receiver has too high a voltage rating to use with the LM3875. Too bad. I ordered a Plitron 25V-0-25V, 300VA toroidal tranny a while ago and it arrived last week. Now I'm scrounging up the rest of the parts.

Since this is my first amp project, I will resist the temptation to buy the most expensive parts. For the power cord, I will use a standard computer power cord with Molex connector. To interface it to my enclosure I will raid a Molex jack from a computer power supply. I have a spare panel-mount fuse holder in my parts bin, and a number of power switches to choose from.

I'm going to order my bridge rectifiers from Digi-Key, along with the Panasonic FC caps for the power supply, and mounting pads to attach the hot parts to the heatsink(s). I'll order some metal film resistors and reasonable RCA jacks and speaker terminals.

I will probably use the wire from the computer power supply for the internal power wiring. That is handy because there are three colours which will correspond to +V, -V and Ground. For internal signal wiring, I can probably raid some shielded signal wires from an old VCR. (I can see some people shaking their heads right now!)

As for heatsink(s), I've been looking on eBay. There seem to be a lot of CPU coolers on the eBay market. I would be interrested in hearing other peoples' experience in using CPU heatsinks without fans. Would a basic Pentium II sink be large enough without a fan?

Actually in the computer power supply I'm gutting, there are two identical aluminum sinks 2.25" x 2.25" with 7/16" fins on the upper portion. There are a couple of TO-220 devices currently mounted to each one. I wonder if they would be sufficient using one per LM3875 near the high end of its supply voltage? Possibly not without a fan...Perhaps I'll try them and see.

I have some VCR chassis to raid for enclosures. That should do for now until I make something more styley. Here's an idea -- motorized door on front to open the controls! Maybe not.

Anyway, I'll post my progress...and will also be documenting my progress on my website:

http://www.colp.ca/projects_gainclone.html
http://www.colp.ca/projects_technicspreamp.html

Hey, I understand there are a couple of you guys from Winnipeg...Elizard and Spind. Let me know how your projects worked out!

Cheers folks!
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Old 16th July 2003, 05:34 PM   #2
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Default Heat

From what I understand, you will need some hefty heatsinks (capable of dissipating 60 watts per chip?) because the LM3875 runs quite hot when you feed it plus and minus 35V (25x1.414 when you go from AC secondaries to DC).

Depends how hard you drive it though...best off experimenting.
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Old 18th July 2003, 08:59 PM   #3
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Yes, I'm aware that I'm supplying it near the top of its supply voltage range, and that it may make some heat. I'll leave my heatsink selection up to experimentation. I saw some sinks suitable for slot-1 Pentium II's on eBay for US$2 bucks a piece. One per chip might work. If not, then I might look for a bigger piece.

Years ago I had a large heatsink from a piece of printing equipment. I can't seem to find it in my junkbox though. I'm kicking myself. It had four TO-3 devices on it...would have been perfect! They are probably buried deep under boxes of antiques in my mother's garage, not to be seen again.

My living-room speakers are quite sensitive Polk Audio 3-ways, and not overly huge, so they shouldn't put much strain on the amp. If I find my power supply is too much then I may get a smaller tranny in the future and make another amp out of discretes once I've cut my teeth on this project.

My parts list is shaping up... I am opting for BlackGate input coupling caps, and Riken carbon-film input resistors.

Parts ConneXion has some Vishay-Draloric 0.1% tolerance precision metal-film resistors for $.20 a piece. At that price I can afford to try a few values on for size. Depending on what signal level my Technics receiver's pre-amp stage puts out I may be able to lower the gain of my gainclone.

Some people have reported that bypassing the PS caps with 1uF Auricaps has improved the sound. I wonder if a significantly less-expensive yet still high quality 1uF Blackgate cap would give a similar improvement?

Oh, did I say "Molex" in my original post? Sorry, I meant the one you connect the mains cord to your computer with... Molex makes disk-drive power connectors. I'll probably end up just chopping the tail off a computer power supply cord and wiring it directly to the tranny and fuse.

Hey, if it sounds at least as good as the Technics receiver originally was, then it will be a success!
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Old 24th July 2003, 04:47 PM   #4
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Default My parts arrived!

I'm like a kid at Christmas!

I received a package from Digi-Key on Monday, and more parts from Parts ConneXion today! They're beeeautiful!

All I need now are heatsinks and mounting hardware for the toroid. Oh, and a chassis too. I have some CD player chassis around the house... Not as pretty as 1/4" aluminum, but what the hey.

I have some left-over 1/4" plexiglass from my computer case project...perhaps I'll use some of that in there. I also have tons of CD-ROMs that I can cut up...perhaps I'll make cap mounts out of them. Hmmmm...
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Old 24th July 2003, 08:50 PM   #5
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Quote:
Originally posted by TechnoGeek
Yes, I'm aware that I'm supplying it near the top of its supply voltage range, and that it may make some heat. I'll leave my heatsink selection up to experimentation. I saw some sinks suitable for slot-1 Pentium II's on eBay for US$2 bucks a piece. One per chip might work. If not, then I might look for a bigger piece.

I think you should look for the AMD one, cuz they are hot!!!


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Old 24th July 2003, 09:01 PM   #6
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Default Re: My First GainClone Project is under way!

Quote:
Originally posted by TechnoGeek
I would be interrested in hearing other peoples' experience in using CPU heatsinks without fans. Would a basic Pentium II sink be large enough without a fan?

I am using Socket A coolers for my gainclone: one for each chip. No problem whatsoever. The chips run hot but remain touchable - I am actually running 37-38v rails on them.

I have looked into some P2 coolers - they are much bigger than socket a coolers but I haven't seen one in person - BTW, Compusa has them for like $5 each.

Quote:
Originally posted by TechnoGeek
Actually in the computer power supply I'm gutting, there are two identical aluminum sinks 2.25" x 2.25" with 7/16" fins on the upper portion. There are a couple of TO-220 devices currently mounted to each one. I wonder if they would be sufficient using one per LM3875 near the high end of its supply voltage? Possibly not without a fan...Perhaps I'll try them and see.

I had that idea as well. Unfortunately, my transformer is too big to fit into an ATX PS case with the fan. so mine now is fanless. I would suggest that you consider using the PS case, connectors and parts if you can: I am using the bridge, some decoupling caps from my PS.

on a general note: why cannot you just put the gainclones on the heatsink and drop in a new transformer so you can use the same chassis / everything else?
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Old 24th July 2003, 10:25 PM   #7
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Here's nice GC made out with computer's parts, if somebody needs a little inspiration
Sun Microsystems Gainclone?
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Old 25th July 2003, 01:40 PM   #8
Konrad is offline Konrad  Norway
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Bridgeclone:
Four lm3886 "controlled" by high performance opamps.

here is my idea
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Old 25th July 2003, 03:58 PM   #9
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http://crazy.muarock.net/~kllau/****_cpu.html

See how carzy the AMD CPU heat sink is.
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Old 28th July 2003, 10:47 PM   #10
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So I went garage saleing this weekend. Made out like a bandit!

I picked up a JVC VCR for 5 bucks. Switching power supply is shot, but it's the same product line as my other VCR, yet this one is a higher-end model with S-VHS, and a jog/shuttle wheel. If I can fix the PS, that will be a nice thing.

But back to the subject at hand... I found a car booster amp as well.

It is a Clarion 2x35W with a 5-band EQ, of late 1980's vintage. The whole back of it is a heatsink, about 6" wide by 2" tall, and 1" fins. I think it will be perfect for mounting a pair of LM3875's.

In fact, I may just use that enclosure for my gainclone and put the PS in another enclosure!

I should really take some pictures...
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