Optimizing TDA7294 Output

[bcmbob]

Hi Guys,

A bunch of good ideas. Daniel you sound just like Andrew and madisonears - they both hate those little green things. Don't have enough fastons so I"ll do something with euros. What I had was "usable" with the short piece of perf-board. I'll probably rework that part later when new board is available. I also noticed the PS could easily be done modular with three umbilical wires. Maybe next time.

I considered the space between the buses yesterday. I thought the only way I could increase it was to attach the pins to the edge of the copper, but after looking at it I see I had almost a 1/16" off center to play with and still have a full circle to solder.

There is a half sheet of the copper plate left so the next one will use all the suggested changes. Should be finished in a few hours and I post more pics then.

[danielwritesbac]

The surge of current delivered at startup and hopefully for bass beat too, is far beyond the safe capacity of most convenience connectors. We need something sturdy that doesn't degrade, spark or loosen over time.

There is a type suitable for power board use:
A barrier terminal block.

The metal plates under the screws are springy, maintaining a secure connection that doesn't loosen over time.

Mouser search:
Barrier Terminal Blocks | Mouser
These are sturdy enough to withstand the charging surges and peak current of a power supply board.

This looks like a good time to mention mains fuses--a fuse series to the transformer primary.

Originally Posted by bcmbob

Daniel you sound just like Andrew and madisonears - they both hate those little green things.

Oh no! The lethal green blocks! The highest current area is an inappropriate place for the worst quality connection. They'll cause static, dump some bass power, defeat efforts for quality, burn up your amp, burn up your speakers, catch your house on fire, and kill pets and people. Worst quality ever!

Seeing those dreadful little green blocks makes me annoyed, anxious, afraid and sad. Please remove them.

Originally Posted by bcmbob

I considered the space between the buses yesterday. I thought the only way I could increase it was to attach the pins to the edge of the copper, but after looking at it I see I had almost a 1/16" off center to play with and still have a full circle to solder. There is a half sheet of the copper plate left so the next one will use all the suggested changes. Should be finished in a few hours and I post more pics then.

It could be good to have several millimeters more spacing between the rails. That center track could be reduced slightly in width, and a little adjustment is probably enough to avoid an erratic sparky noise or weird static. Remember that your signal does travel through the power supply, so that area is actually important to audio and it needs controlled/refined a bit so that we don't end up spending a lot of time chasing errata. After you do what you can to adjust the spacing, then you might consider spraying it with several coats of clear lacquer.

[danielwritesbac]

Due to surge currents involved, connections to and from a bridge rectifier need to be approximately twenty times stronger than a typical speaker connection. If you wouldn't use the dodgy little green blocks for speaker connectors, the matter is only twenty times worse at the power board. Quality control: Mail all of the green blocks back to China.

[danielwritesbac]

I just thought of it. We forgot to snub the secondaries for audio quality. Instead of those little green blocks, you can install Kean's 50R trimmer + 2u polyester cap (variable RC to snub the secondaries) at that location.

And Thanks!! I was wondering how to get the trimmer at the bridge rectifier without overheating the trimmer, but the jumper that you showed in your photos will drop the heat so it doesn't melt the trimmer.

[bcmbob]

Well, It's close but no cookie. In short - no explosions or stinky green smoke. The PS works but the values are not correct. I'll just run through the photos.

Copper pieces cleaned and scuffed.

Strips fitted on caps. Could have drilled closer to the edge.

Tinning process.

DC end fastened with epoxy.

Solid Cardas jumpers installed.

Insulated with Liquid Tape. Probably on all copper when finished.

Identify AC#1 and AC#2.

Hook-up.

So the first problem was the bulb tester didn't go all the way down. My temp gun showed no component above ambient after three minutes of operation. The first DC output check showed a range between 9 - 11 with constant fluctuation.

I went back to the AC end and found both rails read 12 VAC. When disconnected they are 24 on both.

Now here is where it's good I don't get embarrassed easily. When soldering the caps I reversed the orientation (should have stood on my head) compared to the diagram. To avoid de-soldering, I reversed the diodes - meaning the marking ring on the negative rail pointed back toward the AC end, and the positive installed toward the DC end. Although the PS is working (kinda) the polarities are reversed. The relationship between the diodes and the rectifiers may be more complicated that I assumed.

All in all I'm pleased with the progress so far but,

Help Daniel !!! Get me out of this

[bcmbob]

The rest of the pictures......

See ya'll in the morning - going to bed after a long day.

[danielwritesbac]

Oh, my. Well, you're going to want to de-solder all of those caps anyway because you need an insulator (like some perfboard or fiberglass board) between the cap and the metal rails. The cap bodies are aluminum and their label is very thin, and although I'd like to offer a quick fix, there isn't one.

After you turn the caps around, you need to remove and check the health of the 0.25R to see if they're not open circuit. And you need to remove the 6a standard diodes.
Leave the 0.25R and 6a off until you check out the bridge rectifier and first caps (this step is checking the power supply with just the first two caps active).
Next connect the 6a diodes and check the rest.
Lastly, reinstall the 0.25R.

Kudos for using the bulb tester!

[SoIL4x4]

Bummer Bob. Since you have some re-working to do I was thinking it may simplify things for you if you were able to lay all of the components out on one board. If you google the phrase fr4 sheet you will find many vendors of large blank fr4 sheets. It is fairly reasonably priced too. Doing a quick search I found green, natural and black, im sure some digging might produce some fancier colors.

Keep at it Bob.

[bcmbob]

Oh, my indeed.

As soon as my head hit the pillow I realized I had not done a full inversion. The rectifier flow pics you posted popped into my head.

I almost stopped construction yesterday to wait for more perfboard for a related but slightly different reason. That center plate is so large that even the heavy duty iron and the tinning weren't enough to overcome the heat dissipation. I gave it an assist with the torch on low, which made me think I might be burning that thin paint on the caps. Should have had the sense to push the "hold" button there - but you know how how we "kids" are.

After the dis-assembly and forensics I might remove some of the metal at the center of the large plate. Your report of shuffling the caps to remove the zig-zag placement prompted me to think of an elongated Romex doughnut as a solution. I might apply the idea with this flat stock.

I'll be placing an order at Mouser later this morning and might buy both fiberglass and paperboard. I'll add a couple one piece rectifiers also. I already have duplicates of everything but the caps as I doubled the component count on the original PS parts order.

The credit for the bulb tester has to go to Andrew T. He's got me convinced I need one even if i'm just plugging in my electric toothbrush.

I'll continue Saturday when the parts arrive.


Mr. 4X4, I will search for that and hopefully Mouser will carry the product.

I forgot to answer your question about the curved edges on the plates. That was just to gain a little clearance, but I did try to soften the corners (rather inconsistently) where I could to lessen the possibility of a spark jump.

Thanks for your encouragement.

[bcmbob]

Forgot to add - I had the heavy duty screw pad on hand but didn't have the room to install them on the perfboard scraps. Won't be a problem on the rebuild.