Why are there only disastrous PCB Versions for TDA7293/TDA7294/LM3886 etc. available? - Page 3 - diyAudio
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Old 11th November 2012, 06:32 PM   #21
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Quote:
Originally Posted by danielwritesbac View Post
Perhaps the middle ground between point to point versus PCB?
For kit boards, the easy three step solution is to:
1). Put the feedback resistor trackside, onto the chip pins.
2). Relocate the nanofared power bypass cap to the chip pins.
3). Stand up a 2.2R (or more) small signal groundlift resistor in any one of the bus ground spots and then hook all the small signal ground shunts and the input cable ground to the top of that resistor, thereby lifting most of the small signal up off the kit board.
good advices. Additional important from my view it is to divert a twisted line for the supply rails upward or downwards from the PCB but in no case conductor traces therefore on the PCB itself (except very short ones to the srew holes for electrolytic caps).
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Old 11th November 2012, 09:43 PM   #22
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Amplifier board power caps.
It is also possible to install an identical sample of the "main" cap, such as 330u or 220u electrolytic as your NFB cap, proceed to bypass it attractively (the goal is to avoid peaks), and then use copies of that setup for the amplifier board power caps. Sure, that's not perfect by ear; however, avoiding resonant peaks is a considerable improvement.
I'll bet you didn't want 100n for bypass!
I wouldn't want to blame the PCB layout for the schematic error of "arbitrary bypass" with guaranteed wrong values causing ringing.

Probably a 1u or smaller size electrolytic will do a nice job in bypassing a 330u or 220u electrolytic. However, 1u electrolytic on both rails at the audio amp might be a bit disturbing, since the centerpoint between the two caps is also the reference for inverting input. I really don't want to input all that. Therefore I used one of lossy polyester cap of 2u from rail to rail. This lossy cap is internally R-C-R (possibly a real RCR works?). Successful application makes for less congested and clearer midrange, and possibly a slightly cooler heatsink. Rail to rail cap doesn't attach to 0v and therefore it doesn't contact small signal input.

In either case above, the task of bypassing the 330u or 220u amplifier board power caps is mostly done. Now I can select range from 0.47n to 10n bypass caps to finish the job. These are so small that they don't taint the audio like peaky 100n arbitrary bypass error.

Probably my explanation is terrible, but what I hope to convey is this:
If a given bypass doesn't work for small signal then it doesn't work for power circuit either. Integrating a big cap with a smaller cap requires the same careful precision as integrating a woofer with a tweeter (and careless non-specific use makes a similar peaky mess). Successfully good power circuits have obvious audio benefits, such as the clear and level sounding response of decreased distortion. Arbitrary bypass is a schematic error, not a PCB error. I suggest to mail all of the 100n to the North Pole and then try for less peaky bypass methods. I believe that the PCB's will work better with non-ringing power circuits.

Gosh, I hope that helps. Probably the PCB layout designers weren't expecting the ringing caused by typical schematics. Those criss-cross mixup boards are expecting, totally clean, flat DC. And, I believe that expectation is invalidated by most schematics.
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Last edited by danielwritesbac; 11th November 2012 at 09:49 PM.
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Old 11th November 2012, 11:04 PM   #23
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If you feel LM3886 pcbs are poor why not design your own?

I fell foul of pcb design with a mixer. The first revision hummed like mad.
I looked up good pcb design and it suggested keeping the power supply ground separate from the audio ground. My second pcb was hum-less.
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Old 12th November 2012, 01:18 AM   #24
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I wish all PCB designers started out with small signal groundlift resistor for creating signal star ground. That makes it almost impossible to get mixed up.
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Old 16th April 2013, 09:16 PM   #25
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are there additional new knowledge and experience in the meantime??

In this case also this thread could be of interest:
Optimizing TDA7294 Output

Last edited by tiefbassuebertr; 16th April 2013 at 09:23 PM.
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Old 16th April 2013, 09:38 PM   #26
Art M is offline Art M  United States
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Kind of silly considering we have no control over the package pins/ bond wires/ routing and layout of the die. Like the $10000 line cord not accounting for the wires inside the walls.
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Old 16th April 2013, 11:15 PM   #27
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Default Mad pinout problem solved by Compact double-layer board

No problem! Drum roll please. . . Introducing, non-disaster TDA7294 and non-disaster TDA7293:

TDA7294
TDA7294 65W Mono Amplifier Board Kit 28 | eBay

TDA7293
1pc x TDA7293 Mono Amplifier Board Official Standard 85 w Original B Board | eBay

Parallel TDA7293
1 PCX Assembled Finished 170W TDA7293 2 Dual Parallel Mono Power Amplifier Board | eBay
(for 4 ohm speakers)

P.S. I'm not endorsing the component values or components on these boards; however, I do like the double-layer PCB. As the datasheet recommends, the mute circuit is overly complex. And, these boards don't have small signal groundlift feature. But those are minor issues that are easily corrected. The boards do still offer very short trace length and I think that's a useful feature for supporting these particular chips.

P.P.S. If you want a non-disaster LM3886, then I think you're looking for the MyRef-FE.
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Last edited by danielwritesbac; 16th April 2013 at 11:23 PM.
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Old 17th April 2013, 08:13 AM   #28
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Quote:
Originally Posted by Art M View Post
Kind of silly considering we have no control over the package pins/ bond wires/ routing and layout of the die. Like the $10000 line cord not accounting for the wires inside the walls.
But you can assume, that companies like SGS-Thomson or NS (now TI) goes completely different ways for reach perfect results than small audio companies for getting good results at their PCB routing layout.
Thus your statement is nonsense (except if the chip amp devices are fakes from China).
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Old 18th April 2013, 02:07 PM   #29
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Art M View Post
Kind of silly considering we have no control over the package pins/ bond wires/ routing and layout of the die. Like the $10000 line cord not accounting for the wires inside the walls.
That's the point.

Pin layout of TDA7293/4 is stupid crazy, the designers bring internal connections whick *must* be joined, to non contiguous legs. Stupid.
Pin7>13 and Pin8>15

Why didn't they spend 5 minutes more rerouting the *internal* layout , or even better joining said points *inside*.

Worst case, they could do it the hard way, adding the necessary couple wires at the die itself, before encapsulating.

This complicates PCB design, of course, and , as Art M correctly says, nothing the PCB designer does will change that.

On the contrary, LM3886 pin assignation is very good
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Old 18th April 2013, 02:18 PM   #30
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Whoa!?!?
It's the reason the TDA could be better than the LM - it has SEPARATE input and output stage supply pins, which is great.

I wish the LM had these.
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