My First GainClone Project is under way!

[TechnoGeek]

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!

 

[julester]

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.

 

[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.

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!

 

[TechnoGeek]

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...

 

[S.C]

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!!!

 

[millwood]

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.

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?

 

[Peter Daniel]

Here's nice GC made out with computer's parts, if somebody needs a little inspiration
http://www.diyaudio.com/forums/showthread.php?s=&threadid=15240

 

[Konrad]

Bridgeclone:
Four lm3886 "controlled" by high performance opamps.

here is my idea

 

[S.C]

http://crazy.muarock.net/~kllau/****_cpu.html

See how carzy the AMD CPU heat sink is.

 

[TechnoGeek]

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...

 

[jleaman]

i used to fix swmp in vcr's man they can be hard. some simple resister will fail and cause it to buzz loud Pain the BUTt ..

 

[TechnoGeek]

Heatsinks...and an idea!

I went downstairs to look for a power cord this evening...and found a little treasure.

It's the enclosure from a 40W car amp, and it has two separate heatsinks that are larger than the one on the booster I picked up last weekend. They measure 5.5" x 2.25" and 3/8" thick with thirteen 1" tall fins! They form the sides of the enclosure. I am thinking of using that enclosure to house the amp, replacing the flat top cover with 1/4" plexiglass left over from my computer case project. I'll find another enclosure for the PS.

I'll also make capacitor holders out of plexiglass as well, inspired by Peter Daniel's phenolic blocks. I was thinking of drilling CD's for that purpose, but CD's are too flimsy. Two pieces of 1/4" plexi attached to make a 1/2" plate should do nicely.

Hey check this! I came up with an idea this evening as I was reading some threads about transformer selection. My 25-0-25 tranny has dual primaries. There were some concerns about excessive heat. So it hit me -- I should install a switch to either run the primaries in series or in parallel, depending on my mood. Running them in series will cut the supply voltage in half, effectively making it a 12.5-0-12.5. I will call this "peaceful audiophile mode", and the other setting "loud furnace mode"!

I'll post some photos of my amp parts tomorrow.

 

[TechnoGeek]

Some pictures!

I took some pictures of my receiver and my gainclone parts. They are posted on my website:

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

I'll take more during the build. Enjoy!

 

[TechnoGeek]

It's built!!!

Hi everyone...

Well I finally did it...I completed my gainclone! Sorry it took so long for an update -- been too busy to work on it the last several months. I finally decided to sit down and finish it last night.

The case isn't as pretty as I had hoped -- the power supply is in a PC power supply enclosure, and the amp is in a car amp enclosure. They don't match...but it sounds pretty good!

I was disappointed with my first test, which was listening to an Internet radio station. The music that sounded pretty decent through my PC speakers sounded like crap on my Polks.

But then I hooked up my turntable, and laid down some vinyl, and it sounded every bit as good as the blown Technics receiver that it's replacing.

There was some concern about heat due to my use of a 25-0-25 tranny, so I put in a DPDT switch to switch the dual primaries between series and parallel. This allows me to cut the power supply voltage in half if necessary.

It sounds great in either position at low levels, but it clips if I want to push it while on the low setting. I need to run it in the full voltage mode to get louder volume out of it.

On the full voltage setting the heatsinks get warm, but not hot under normal listening conditions. If I crank it to high levels, then they get hot. This won't be a problem though, as I never listen at those levels anyway.

So far so good! I'll post pictures once I figure out how to make my CompactFlash reader work under Linux.

 

[elizard]

Congrats, and welcome to the GC family. What do you have planned next?

I finished my GC last year, and i'm fairly satisfied. Made a few mistakes that are correctable, but I'm short for time/money.
Next up (VERY soon .. month or so and i start sourcing the parts) is a valve buffered igc ..

The only parts I have so far are the chips, but that'll change soon enough. Then I'll get a nice case built for it (the front panel will probably be made by some company that a forum member posted a couple days ago .. nifty tool they have!), and then we'll see how it compares!

Btw, where in south-eastern MB are you? Brandon? Portage?
I'm in winnipeg myself, good to see local interest

 

[TechnoGeek]

I remember you elizard -- you were tossing around the idea of of building a super-compact water-cooled GC last year, right?

I live a few miles out of Landmark, Manitoba. If you're looking at a map, it is about mid-point between Winnipeg and Steinbach. I grew up in Winnipeg, and still work there.

I was going to convert my blown Technics receiver into a pre-amp to go with my GC, but I'm not sure how soon I'll be able to get around to that.

My next project will probably be fixing the switching power supply in a JVC vcr I picked up at a garage sale for 5 bucks. Hopefully I can track down the problem with my oscilloscope. This vcr is much nicer than my current one if it weren't for the fact that it doesn't turn on!

 

[elizard]

Eh south-EASTERN (not western, i always get east and west mixed up for some reason) .. yeah, I know where landmark is ..

It wasn't me who thought of the water cooled GC .. at least i don't think, now that i think about it i don't THINK it was me, but it could have been me .. hmm .. oh well

But that's awesome, you rarely see people from winnipeg on here (i think there are 1 or 2 active ones), so its good to see local interest as i said.

Btw, I finished gathering up almost all the parts on digikey (making an order) for the VBIGC, except for the 3 caps. Now I gotta wait a little to get money (about a month) and then the orders are gonna start coming in, and it'll start taking shape

 

[zbuickman]

I was wondering if the Technics project has come to fruitation yet?
I had the same thing happen to me (right side as well) and was looking into doing the same project and ran into yours.are these as acurate as the original amp in there. and am I missing any thing or are we looking at 30 watts @ 24v and 40watts at 36v?. also how about the preamp wires into the SVI 3206 D. have you figured out that pinout. so many questions from the noob. thanks for any help.

 

[CarlosT]

WOW! 3 years later and you decide to resurrect this thread, Dr. Frankenstein?

 

[zbuickman]

Well yeah because the part where techno is using it from the 505 is just too perfect not too.