Gainclone

Through my work I was able to order 5 each of the National Semi LM3875 and LM3886 audio amplifier IC samples, as well as two Linear Tech LT1083 voltage regulators. I also bought some MUR860 diodes in bulk on eBay.

My inspiration? This article. I plan to use his circuit with some modifications to the power supply because of my supply iron.

The transformer I have available to me has only one 35VAC 15A secondary rather than two (for the + and - rails), but the current capacity of the tap is more than enough to run two separate rectifier bridges for the rails.

My cousin picked me up a couple industrial grade light dimmers from a TV studio he tore apart with his company; this one's a 7.4kW beast and I plan on reusing the chassis and heat sinks.


Maybe I'll parallel up the outputs for more power...


Thanks for stopping by!

Way cool!

That dimmer chassis should make an excellent gainclone housing. I'm a little fuzzy about your supply transformer -- is that a single 35V secondary with CT or untapped? If untapped, I can see your need for regulation. But you'd be dropping about 15V or more. LT1083s might need larger heat sinks than the amp chips.

Hope you plan to update this post with your progress. Parts are ordered for my own gainclone project. I'm also contemplating supply voltage regulation.

And taking notes ... :yes:

That is one of the coolest chassis I've seen.

It will re-purpose very nicely for your project.

Stuart Pedaso said:

Way cool!

That dimmer chassis should make an excellent gainclone housing. I'm a little fuzzy about your supply transformer -- is that a single 35V secondary with CT or untapped? If untapped, I can see your need for regulation. But you'd be dropping about 15V or more. LT1083s might need larger heat sinks than the amp chips.

Hope you plan to update this post with your progress. Parts are ordered for my own gainclone project. I'm also contemplating supply voltage regulation.

And taking notes ... :yes:

Click to expand...

My transformer is a single 35V 15A (no CT) secondary with a 105/115/125 primary-- I've been leaning more towards the LM3886 chip because it can use 10 more volts (|V+| + |V-| = 20V to 94V) than the LM3875 (|V+| + |V-| = 20V to 84V) on the rails, to maybe make it easier on the regulators... in the article he drops 5V with the regulators with some small heat sinks (I've got a set of those, they're not very substantial at all) so maybe dropping 10V wouldn't be too back, and I've got a local source for cheap heat sinks

If I used some sort of voltage divider circuit, I'd have +/- 25V after rectification, which might not be enough. To drop 50V to 47V (absolute limit for the 3886) might not be so hard on the regulators, as opposed to dropping 50V to 42V (absolute limit for the 3875). :scratch2:

It looks like the power dissipation as outlined in the application notes for the LT1083 regulator is dictated by the equation Pdis = (Vin - Vout) * (Iout), so if I put 50V in and drop it to 40V at 4A (out of my ass) I'll have to dissipate 40W of heat at full tilt.

The toroid in the photos is a choke that won't be used in the final build.

Naturally, I'll upgrade the thread as I build :thmbsp: I just can't decide whether or not to paint the face or leave it looking like an industrial-grade dimmer...

soundmotor said:

That is one of the coolest chassis I've seen.

It will re-purpose very nicely for your project.

Click to expand...

Thank you! That's the idea; I'm trying to get more of 'em-- apparently there's a bunch siting in storage because they don't want to dump them but they also don't need them... :scratch2:

Check out www.chipamp.com they have board kits for the LM3886TF / LM1875T chips. I have used them they sound good and are high quality.

Quick update-- I fried two 8A parts (regulators for my + and - rails) on account of some error in the reproduction of a schematic. Whoops! Won't go that again!!

I've patched up the old power supply in the meantime with a more robust design in the works-- one channel is now singing (I think I might just enjoy the mono too much, now!) and it's beautiful.

All that excitement was last week.

This week I've been working on laying out, reworking, and painting the chassis to accommodate everything. I'll try to get some photos up soon!

I reworked the face the last couple days; I had to open some holes in the aluminum to fit the screws I wanted to use, made up a backing plate out of a scrap piece of plexy I had laying around to house the power and speaker protect switches, and painted it all.

Tonight the paint's dry about to handle and I thought a test fit was in order-- great success!


A quick before shot;


And the after pictures;


The front is supposed to be hammertone, but I forgot to prep the orange surface with matte black so there were orange splotches thru the first few coats, then I threw down a really heavy coat and that's what makes it look all bubbly and weird (but I like it like a mother loves her ugly child).

thefragger said:

And the after pictures;

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The front is supposed to be hammertone, but I forgot to prep the orange surface with matte black so there were orange splotches thru the first few coats, then I threw down a really heavy coat and that's what makes it look all bubbly and weird (but I like it like a mother loves her ugly child).

Click to expand...

I am glad someone loves it. . . .. Just kidding. That should make a great housing. Nothing better than recycling!!! I am sure Al Gore would be proud!!!

thefragger said:

The front is supposed to be hammertone, but I forgot to prep the orange surface with matte black so there were orange splotches thru the first few coats, then I threw down a really heavy coat and that's what makes it look all bubbly and weird (but I like it like a mother loves her ugly child).

Click to expand...

Looks like classic "fish eye" (plus general issues w/ hammertone paint). There may have been some oil or other contaminant on the surface. The paint won't etch into the surface because of the contaminant and the original paint shows through like pin pricks. Rather than a primer coat, scuff it up with Scotch-Brite pads and then wash it down with dish soap and thoroughly rinse it, dry, then paint. Unless of course the orange paint wasn't paint but a powder coat. Then you'd need to use something like Krylon Fusion to get paint to stick.

I used to use Rustoleum hammertone paint but had sporadic success with it. I have exactly (1) item out of many that I kept that came out looking like it should. It worked best for me in the winter when humidity is low. It also worked best when I put down (1) thick, wet coat vs. several thin ones and you have to do it fast. The paint skins very quickly which is how the hammertone effect works.

I had the least success when the surface to paint was no more than ~4" in any direction. Just too much area to get the paint down evenly & keep it wet. By the time I would get to the edge the middle was drying out. Then the effect would come out very uneven & lumpy. Putting down more paint just made it worse.

Today was a fun day! I still have some scrap plexy kicking around so I thought I'd whip up a support bracket for the PSU board!



I first had to do some math;

Did a test fit of the bracket;

Added the supports for the heatsinks;

And I threw it all together for a test fit! There will be two of those big heatsinks once it's all together;