Need help wiring transformer

[joecool85]

I'm working on a test board for an lm3886 from brian and I can't get the psu wired up right.

I have a 18-0-18 VCT tranny, the board has two holes labeled AC1 and two holes labeled AC2.  It also has a hole labeled GND.  I have 3 wires coming off my tranny, two hots and one neutral.  I split the neutral so that it went to one of the AC1 holes and one of the AC2 holes and then sent one hot wire to each of the other AC1 and AC2 holes - I didn't hook anything to GND.  All I get is a blown 1amp fuse.  So I replace the fuse with a 2amp, because thats what you normally use with an LM3886 anyway, and the tranny gets hot, the diodes get hot and the tranny hums.  And its not rectifying to DC either.

What am I doing wrong here?  Do I need to jumper some other wires and send the neutral to GND on the board?

[R.G.]

... the board has two holes labeled AC1 and two holes labeled AC2.  It also has a hole labeled GND.  I have 3 wires coming off my tranny, two hots and one neutral.  I split the neutral so that it went to one of the AC1 holes and one of the AC2 holes and then sent one hot wire to each of the other AC1 and AC2 holes - I didn't hook anything to GND.  All I get is a blown 1amp fuse.  So I replace the fuse with a 2amp, because thats what you normally use with an LM3886 anyway, and the tranny gets hot, the diodes get hot and the tranny hums.  And its not rectifying to DC either.

What am I doing wrong here?  Do I need to jumper some other wires and send the neutral to GND on the board?

You need to get out your ohmmeter and/or the schematic/layout of the PCB.

Mother Nature is telling you that the PCB is not what you think it is. Not knowing that PCB, I would guess that two terminals of the four in AC1 and AC2 connect to each other, and two others go from the pad on the board directly to opposite AC input sides of a bridge rectifier.

If you have a transformer with a CT, then hook the CT to either one of the two holes from AC1 and AC2 that the PCB has shorted together. Hook the non-CT wires to the other non-shorted holes in AC1 and AC2.

To save on environmentally sensitive fuses, make yourself a light bulb limiter. I *know* I've mentioned that to you before... 8-)

[joecool85]

... the board has two holes labeled AC1 and two holes labeled AC2.  It also has a hole labeled GND.  I have 3 wires coming off my tranny, two hots and one neutral.  I split the neutral so that it went to one of the AC1 holes and one of the AC2 holes and then sent one hot wire to each of the other AC1 and AC2 holes - I didn't hook anything to GND.  All I get is a blown 1amp fuse.  So I replace the fuse with a 2amp, because thats what you normally use with an LM3886 anyway, and the tranny gets hot, the diodes get hot and the tranny hums.  And its not rectifying to DC either.

What am I doing wrong here?  Do I need to jumper some other wires and send the neutral to GND on the board?

You need to get out your ohmmeter and/or the schematic/layout of the PCB.

Mother Nature is telling you that the PCB is not what you think it is. Not knowing that PCB, I would guess that two terminals of the four in AC1 and AC2 connect to each other, and two others go from the pad on the board directly to opposite AC input sides of a bridge rectifier.

If you have a transformer with a CT, then hook the CT to either one of the two holes from AC1 and AC2 that the PCB has shorted together. Hook the non-CT wires to the other non-shorted holes in AC1 and AC2.

To save on environmentally sensitive fuses, make yourself a light bulb limiter. I *know* I've mentioned that to you before... 8-)

Yeah...I may need to make myself one of those limiters.

[J M Fahey]

Hi Joecool
My crystal ball is somewhat foggy today, so I can't see your PCB clearly.
Please scan and post both sides of it.
A couple of sharp pictures will do.
Anything else is a *wild* guess, and that´s an understatement.
Your CT wire goes to ground, but having *two* AC1 and *two* AC2 holes puzzles me.

All I get is a blown 1amp fuse.  So I replace the fuse with a 2amp

Aaaaaaaaaaaaaarrrghhhhhhhhhh Joe: that´s what many do on stage on Saturday nights, then can't understand why the ironclad written 5-year guarantee doesn't cover the repair cost anymore.
(Also see: "a fuse I pulled from the car/motorcycle/heavy electrical appliance" and "fuse covered in aluminum cigarette paper") 8|
Good luck.

[joecool85]

Hi Joecool
My crystal ball is somewhat foggy today, so I can't see your PCB clearly.
Please scan and post both sides of it.
A couple of sharp pictures will do.
Anything else is a *wild* guess, and that´s an understatement.
Your CT wire goes to ground, but having *two* AC1 and *two* AC2 holes puzzles me.

All I get is a blown 1amp fuse.  So I replace the fuse with a 2amp

Aaaaaaaaaaaaaarrrghhhhhhhhhh Joe: that´s what many do on stage on Saturday nights, then can't understand why the ironclad written 5-year guarantee doesn't cover the repair cost anymore.
(Also see: "a fuse I pulled from the car/motorcycle/heavy electrical appliance" and "fuse covered in aluminum cigarette paper") 8|
Good luck.

Ok, the other thing with this board, it has 8, count em, EIGHT diodes on it -- what the??  I'll post a couple pictures of the board empty and filled hopefully later tonight.

**edit**
Also, I only replaced the fuse with a larger one because normally Brian recommends a 2amp slow-blow fuse, so I figured on powerup maybe the 1amp couldn't cut it.  I had only used the 1amp because it was already wired up with the transformer when I pulled it out of the subwoofer box.  The transformer can definetely pull more than 1 amp, I think the sub just had that small fuse in there because the chips couldn't draw more than that.

[J M Fahey]

the other thing with this board, it has 8, count em, EIGHT diodes on it -- what the??

[what the??] [2]
Now I *really* want to see your board.

[joecool85]

Ok, so now you so why I'm confused lol.  Alright, this is a prototype board Brian has done up - it has the poweramp and psu on one board, and as you can see in the pics it is pretty small considering.

[joecool85]

Pic 2

[joecool85]

Pic 3 (this is the board I've been working with, and it came stuffed by Brian himself, so I'm assuming it was done correctly)

[J M Fahey]

... Scratching my chin and puffing my pipe ..... "Well well well, I see our friend *loves* to have his power supplies generously filtered and rectified."
I´ll try to "connect the dots" (literally) and re-construct what that board is, although having the original kit schematic would help *a lot* (hint hint).
Specially considering that the board designer was very daring with his pad-to-track clearance, and in a couple of points I´m not sure whether a pad touches a track or just misses it by the tiniest of spaces.

[R.G.]

Pic 3 (this is the board I've been working with, and it came stuffed by Brian himself, so I'm assuming it was done correctly)

"Correctly" is not clear. That depends on intent. 

I've done some photo-retouching on the pics. The result is here:

This is a top-side image with the bottom traces imposed in transparent green. Eight diodes means two bridge rectifiers. The board is set up to take either two identical transformers, a two-secondary single transformer, or a single transformer with a CT.

I think. 

First how to hook up your power supply. Solder the CT in that Ground hole. Solder one of your secondary wires in AC1 and the other in AC2. Either AC1 will do, as will either AC2.

In fact, there are twice as many (expensive, by the look of them) diodes on that board as are needed. You can remove four of them and just use the pads for the places where you left the diodes. For a bipolar supply, you never need to do two entire bridge rectifiers, as you can always connect up the transformers in a Full Wave Center Tap connection.

Some observations on the board:
- twice as many diodes as needed
- vastly bigger diodes than are needed, even if they're supposed to be fast/soft recovery diodes
- very, almost overly generous board layout

If this not intended to power other LM3886-only board with this power supply, it's both bigger and more expensive than it needs to be. I'm guessing that whomever laid this out is not a pro. Not that that's necessarily a problem. It'll probably work fine when put together and hooked up correctly.

Which brings to mind that the instruction sheets for this build need some updating. 

[joecool85]

I don't have a schematic or build sheet for this one as it's a brand new prototype Brian is having me try out for him.  I'm going to email him and see if I can get him to post some insight on this thread.

[R.G.]

I don't have a schematic or build sheet for this one as it's a brand new prototype Brian is having me try out for him.  I'm going to email him and see if I can get him to post some insight on this thread.

I have some suggestions.
- Leave off half the diodes. You don't need them.
- Unless this is a try at super-mojo audio quality amplifiers for the Very Discerning, replace the TO-220 diodes with an integrated single-package bride. These can be had very cheaply and work very well. A 6A bridge will fit in the space of two of the TO220 diodes and probably be indistinguishable in sound quality.
- Think carefully about where the ground wire goes. Simply having copper bedsheets for ground is not always the best way to ground.
- Don't put R1 between the rows of pins on the LM3886. Sure, it could be conceptually nice to have that feedback resistor running between pins 9 and 3 there, but it's bad assembly practice, even if it fits.
- Move the i/o pads out to the edge of the PCB; especially the output pad and the input pins.
- Make a special ground pad goind to the center of the filter caps for the speaker return. Don't expect that tying the  return ground ot the "gnd" pin will keep speaker feedback off the ground line, especially if you're going to ground the input to that same hole. It looks like all the ground wires are intended to go to the "ground" terminal, possibly in anticipation of "star grounding". Unfortunately, what's set up there will mix the ground-side pulses of the power supply and the speaker return currents with the input ground if I'm reading the board right. The PCB will be exposed to hum and oscillation problems. It's possible it won't have hum or noise, because most of these things are somewhat forgiving, but there are better ways to do it.

What are the filter cap values? I'm about done with a layout of a similar circuit, but incorporating some of the things I just told you. Don't tell me what the rest of the values are.

[R.G.]

Quick note. I saw your post with the link to the LM3886 amp at chipamp.com. I think this board must be an attempt to put the power supply and one channel of LM3886 on the same PCB. If so, it explains how the double-bridge power supply got there, and it validates my comments on the power supply and ground.

I'm thinking that the MUR860 double-bridge must have come from some high-end hifi theorizing; it's certainly not needed on a guitar amp. The other comments on layout apply, I think.

[armstrom]

Here's the schematic for the power supply:
http://www.chipamp.com/images/ps.gif

I don't doubt that it could be done, but could you describe in more detail (maybe a schematic?) how to wire the rectifier to produce bipolar output from the dual secondaries (as shown in the power supply in the link)? I already have one of these power supplies (with the 8 diodes) built and was considering building a second one for an additional LM3886 chip I have. If I can get away with a cheaper power supply configuration then I'll do that

-Matt