Junk Parts Gainclone Build Thread (BPA Chipamp)

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I've been "collecting" parts over the years and decided to use some up to build a "junk parts" gainclone. I've got LM4780 boards from .... I forget! They are either from Peter Daniel or Brian GT? Anyway, I've also got some really neat monoblock chassis cases, a few transformers, XLR connectors, speaker binding posts, sufficient parts for 'parallel' LM4780s, AC inlets and mostly everything you need for a gainclone. This will be 'junk parts' wherever possible, and will be "bridge-parallel" gainclone with XLR inputs. So lets see what kind of Junk i'm using (its really not that junky when you clean the stuff up-- I say junk cause most of this has been sitting around for years.)

Here is the chassis, or at least one of them.



The chassis for these are a pair of neat little heat sink boxes of some kind that I found at the surplus store years ago. They were all grungy but some sanding cleaned them right up. You see I have already started mounting stuff in there, mainly and testing screw hole locations. A hole in the faceplate for a Red overtemperature LED is also drilled.

Overtemperature? This will be bridge-parallel and will probably run HOT. These gainclones run hotter than people think, this will be a little brick heater, hopefully it won't get painfully hot, but the case is only 10 inches long by 4 inches wide by 3 inches tall, very small for what might be a 200W amp. Yes, am worried about heat.

The holes will be covered by a metallic grill so screws and holes will not be visible. I also drilled some extra holes in the cover so that this does not turn into a little oven for the components inside.

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This will house two LM4780s, each in "parallel" mode, and each running out of phase with the other, to be a "bridge parallel" amplifier. People here call this a "BPA 200" I think for 'bridge parallel 200 WPC'?? Not going to try to make this 200 WPC because I've got small cases but certainly this will be more powerful than your standard chipamp.

So I have these two XLR inputs that have been sitting around for years, this amp will accept an XLR input, DC input, and have a speaker output. The amp is not intended to work signals that are not differential.

The + signal from the XLR goes to one LM4780, the - signal to another, and the output is taken across them to the speaker terminals.

The DC signal will be carried by a set of IEC 319/320 connectors and cables. Looks like IEC319/320 carries 220 V at over 15 amps, so should be more than sufficient. But the connectors are big. I just have enough room for them. I did buy these new as I had no parts in the junk box for making DC connections. These were far cheaper than any Neutrik part and cables and connectors are very plentiful.

So on we go-- how do you make large holes in a metal panel? With a hand drill and a file.



After drilling a 1/4" hole, pull out the file and go to town. If you are careful (outline the desired opening), it takes about 15 minutes to rout/ file a huge hole. No fancy punches needed. Looks crappy but just wash, sand, wash.

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In go the boards and chips.

Here is the Schematic I used- Bridge Parallel XLR Input Schematic.

EDIT- as you see this must be driven from a fully balanced source, and is a noninverting setup. Also, I have made absolutely no provision to reduce DC offset or incorporate any protection of any kind. Wish me luck (hey, I'm not that smart and this is a junk parts project).

Here is one of the mono blocks with some of the components installed.



I had some issues- the 2,200 uF capacitors I had were snap in and did not have the right pitch for the boards, so I had to drill out some holes. The current sharing resistors I had were .1 ohm, but it is better and safer to use .2 ohm. I used .1 ohm anyway.

I did not have enough .1 uF fancy Wima caps to bypass the electrolytics so I used the caps I had, mounted on the underside of the electrolytics. I think they are stacked film?

The chips are mounted to the roof of the case which is pretty solid. I used two standoffs to connect one edge of the PCB to the side of the case for extra stability. I am not bothering with two standoffs. One required enough precision to make me nervous that I could pull it off. Yes, I had some very long screws around to use with the standoffs; filled up a bag of various screws at the surplus store many years ago and have a lot left.

A Stancor bimetal switch is also mounted to the roof of the heat sink box, this will close at (I think) about 63 degrees C and connect the + supply rail to a red LED through a current limiting resistor, which indicates that the case is really hot. Had a bunch of these temperature switches left over from previous projects, I tend to use them a lot. I am going to have power LEDs in the separate power supply so these little boxes will run with no illumination other than the overtemp LED.

[AndrewT]

Hi,
BPA using a pair of 4780 sounds nice.
Most build a 4780 as a parallel. This draws double the current through the supply pins.
I think that building the 4780 as bridged might be better since the currents run into and out of the supply pins is not doubled. (double the power into double the impedance is equal to saying @ the same current as the single).

Then a pair of these 4780 to give your parallel combination.
I don't know if this would be measurable nor even audible but worth trying.

Your XLR feed. Is that a balanced hot and cold signal pair?
Are you going to use the chips as inverting or all as non-inverting?

Are you planning on using DC servos to balance the output voltages?

[lgreen]

Originally Posted by AndrewT

Hi,
I think that building the 4780 as bridged might be better since the currents run into and out of the supply pins is not doubled. (double the power into double the impedance is equal to saying @ the same current as the single).

Then a pair of these 4780 to give your parallel combination.
I don't know if this would be measurable nor even audible but worth trying.

Andrew, yes I thought of this but dismissed it thinking it wouldn't work. Had you provided this suggestion before I populated the boards I would have had the incentive to try it out. (Of course you did not know that I was about to build this so could not suggest anything). Another good idea from AndrewT, but its too late now.

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I bought a bunch of teflon insulated silver coated wire a few years ago. Its not the fattest wire out there but it is very dense and sounds fantastic. Just kidding, I have no idea how it 'sounds', but its pretty nice wire, so I'm using it to wire the AC and DC connections in the monoblocks.

Here are the monoblocks wired up.







There is a CL-60 thermister from DC ground (PS ground) to the case in order to ground but decouple the case.

For the XLR input, there is only one ground lead but I need two, one to go to each board. So I just jumpered the signal ground from one board to the other (yellow striped wire).

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Somewhere along the way I lost some of the short stubby screws that hold in the front and back panels. These were standard countersunk flat head screws.



I don't think they look all that good anyway, so I cut up some phillips head screws using my crimp tool.



These tools are supposed to cut the screw threads cleanly so it goes right into the threaded hole. Of course it really doesn't and is unreliable. You still may have to file or rework the end of the screw where the cut was made. The screws here need to be very short so I was forced to make these cuts or to drill and retap. Got it working after a while.



Gotta say that small things like getting the right look with the screws and fasteners mean a lot.

I think this looks better than the slot head version.

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I've got a small, and I mean small, box for the power supply. Its 9" wide by 7" deep by 3" tall. Just barely tall enough for a power transformer or power capacitors. I have to be really careful to get everything to fit.

I have a lot of those 15,000 uF 63V capacitors from Apex Jr., so in they go. I cannot use a PCB because any stand-off will exceed the 3 inches of height that I have. The capacitors have to be hot-glued to the case itself.



The case has grooves for fitting in the front and back panels, they slide into the base and are held in the grooves. The cover also has mating grooves around its periphery to hold the front and back panels.

Its kind of neat to have a case with absolutely no screws holding the front and back panels in. The bottom and top are connected by long screws and the grooves hold the panels in.

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I drilled and filed the back panel to fit the connectors. Again, no special tools needed. Then I used some 2-ton epoxy to epoxy small sheet metal cut outs behind the holes that I was not using. Finally I spraypainted the entire thing white. I had 2 cans of white and 2 cans of black, so used some black for the top of the case.



If I had to to it again, I'd spraypaint the metal pieces and back panel separetly BEFORE epoxing everything together. Duh!

Here is how it turned out.



Its ok, but could have been much better.

As you can see the fuseholder in the pics is different- I used a surplus fuse holder at first but it was so fragile (old) that the metal tabs simply broke off. I put some allen head screws in the back panel just to fill up the screw holes; had a lot of allen head screws left over from other projects.

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I know you are supposed to keep the DC away from the AC, which is hard to implement in such a small case. Here I have wired the DC connectors to the PS caps along the back panel. They will be perpendicular to the AC inlet wires which cross above them. Not a perfect situation since they should be remote from the AC wires, but since they are perpendicular they should not pick up too much of the 60 Hz.



Point to point soldering and hand wiring.

No room to fit in connectors for the bridge and transformer hookups.

The inside of this PS is not going to win any beauty contests.