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Old 7th February 2013, 01:12 PM   #301
bcmbob is online now bcmbob  United States
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Looks like the caps survived the high heat. The plastic shell warped at a few spots but the paint on the aluminum shell is intact. I'm going to try simply putting them in the correct rotation (with a couple coats of the liquid tape between the caps and the copper) before I disassemble the rest of the build.

Edit: And turning the diodes around to the original orientation.
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Last edited by bcmbob; 7th February 2013 at 01:29 PM.
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Old 7th February 2013, 02:11 PM   #302
SoIL4x4 is offline SoIL4x4  United States
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Bob, I had an idea for your 0v bus. This would be a little labor intensive but it might help you localize your heat while soldering. As you can see from the attached "professional" rendering that you could first drill out larges holes to create voids in the sheet and then cut the rest with a bandsaw or scrollsaw etc. You could also drill your through holes closer to the edge which I know you mentioned. This should give your heat only one direction to escape and keep it where you need it just long enough.

The only problem I see with removing material is then the awg equivalent starts to drop, but I guess without some math we really don't know what it is now.
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Old 7th February 2013, 02:19 PM   #303
bcmbob is online now bcmbob  United States
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I've been working on a liquid cooler for the discrete amp I'm testing and thought about the leftover 1/32" (.75 mm) automotive paper gasket material I used. Will glue some on the copper for extra insulation.

Fear not - - I will do a build with all the suggested improvements after all these bugs are worked out.
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Old 7th February 2013, 02:40 PM   #304
bcmbob is online now bcmbob  United States
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Quote:
Originally Posted by SoIL4x4 View Post
Bob, I had an idea for your 0v bus. This would be a little labor intensive but it might help you localize your heat while soldering. As you can see from the attached "professional" rendering that you could first drill out larges holes to create voids in the sheet and then cut the rest with a bandsaw or scrollsaw etc. You could also drill your through holes closer to the edge which I know you mentioned. This should give your heat only one direction to escape and keep it where you need it just long enough.

The only problem I see with removing material is then the awg equivalent starts to drop, but I guess without some math we really don't know what it is now.
Thanks, I might try that on the next one. As you were posting I was about to off to the hobby store to replenish my Dremel bits and cutters. For now, I'm going to give what's in the picture a try on this one. I'll keep the remaining copper as least as wide as the outside rails.

In any event, the support of a full length perf or fiberglass board will be added.
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Old 7th February 2013, 05:20 PM   #305
bcmbob is online now bcmbob  United States
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So the moral of the story is not only "R T F M", but also "Build it as designed".

The rebuild produces + 33.1 and -33.0 VDC...

Thanks for hanging in there with me Daniel, and SoIL4X4 for your support.

Been a fun trip !! Now I are a expert.


BTW: A pic of the gasket material applied and the cut-out. Soldering was a breeze this time.
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Last edited by bcmbob; 7th February 2013 at 05:32 PM.
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Old 7th February 2013, 11:06 PM   #306
bcmbob is online now bcmbob  United States
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CAN YOU HEAR THAT ????

I almost forgot what all the power supply stuff was about. The amps are playing, the bulb tester is out and everything is at room temps. I am again really impressed at the quality of this chip (even on these tiny speakers) and am anxious to get on with the optimized design.

Thanks to all again.
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Old 7th February 2013, 11:46 PM   #307
SoIL4x4 is offline SoIL4x4  United States
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Sounds great Bob!

Great work!
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Old 8th February 2013, 12:10 AM   #308
bcmbob is online now bcmbob  United States
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Thanks Will, it was well worth the effort. The power supplies are great - providing very stable and responsive energy for the amps. I plugged in a regular power cord and the amps are really singing. I'll hook them up to the big speakers in the morning.
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Old 8th February 2013, 08:13 AM   #309
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Quote:
Originally Posted by bcmbob
The power supplies are great - providing very stable and responsive energy for the amps. I plugged in a regular power cord and the amps are really singing.
Oh, that's good to hear. KUDOS!!!

You just made a successful hi-charge (high dynamics) CRC (low noise) power supply. YAY!! And you did the audiophile features by layout, by design and by component value so that it didn't require audiophile parts. YAY!!!
Quote:
Originally Posted by bcmbob
I'll hook them up to the big speakers in the morning.
Heatsink:
Ah, before that event with the big speakers, you might want to add some u-channels or some free CPU heatsinks onto those heat spreaders. TDA7294's softie limiter (almost transparent except for muddy) is both thermally limited and thermally irritated.

I'm actually somewhat glad you started with small heat spreaders because that lets you find out if the chip is running reasonably (would be piping hot if oscillating), but I guess you found out already, so you might want to boost the heatsink before that event with the big speakers.


Bottleneck:
Basically, there's constipation at the 22u and 47u over on the small signal (left) side of the board that will cause 2nd order harmonic distortion (until those caps are replaced with larger).

The bootstrap and nfb cap (feedback-shunt cap), are a bit too small to show off well with big speakers.

Generally, you could transplant the 47u to the bootstrap, so now the bootstrap is twice as big.

And since 100u caps are really easy to find in decent quality and since the NFB cap (feedback-shunt cap) is near the edge of the board. . lolz, gotta nifty kludge here. . . 100u topside and another trackside, just sticking out the side. Makes 200u. That's slightly undersize, but 4 times larger than the 47u on the schematic at post 1. Even with 200u, it will still have a warm sound.

Clearing up the 2nd order harmonic distortion might make the midrange seem louder. The fix for that is larger amplifier board power caps and possibly a rail to rail cap. These components can be located at the dc power input are of the amplifier board.

So, after you get those areas balanced out, it should sound clearer.


Feedback resistor:
The kit has an insanely circuitous path for the feedback resistor. An inductor (long pcb traces) series with the feedback resistor is bad for higher gain at HF and RF as well as generally imprecise feedback. That resistor needs moved trackside between pins 2 and 14, right on the pins. Have a look at the datasheet's pinout chart. You'll see that putting the feedback resistor right on the chip pins is easy to do.
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Last edited by danielwritesbac; 8th February 2013 at 08:18 AM.
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Old 8th February 2013, 11:39 AM   #310
bcmbob is online now bcmbob  United States
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Quote:
Originally Posted by danielwritesbac View Post
........... 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. ..............
Hey Daniel,

The power supply is excellent. The dynamics, punch, bass and stage all sound better. I'll add some fins to the heat sink later today.

I couldn't find any information on the "Kean's" item. Will the Bourns trimmer work? Where do I put it and how do I use the variability aspect?

You have presented several versions/designs - some with stacked resistors, cap count from 10 - 16 and different cap values. I used 3300 uF. Do you consider that the best choice? In keeping with the tread theme, which parts combination would you choose as "optimal" for this particular chip/kit? I have delayed my Mouser order to accommodate that info as well as the mods you listed in the last post. I will keep my "learner" PS build and start form scratch on a final.

Also, though some claim no audible difference, would you recommend discrete PS units and/or transformers for the TDA7294? (BTW, the PS dissipates quickly within 1-2 seconds.)

Now that the power supply works, I'll take back half the bad things I said about you!
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