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Old 26th December 2012, 01:47 PM   #1
bcmbob is offline bcmbob  United States
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Default Optimizing TDA7294 Output

The theme of this thread will be adjustments and modifications necessary to take advantage of and optimize the performance of the LM7294 device. The base or starting point will be the following schematic:


Base TDA.JPG

Due to a wide range of technical understanding, very basic definitions and descriptions of both primary and supporting components should be posted. Hopefully, this "Paint by the numbers" approach will attract and inform both novices like myself as well as those who wish to share their advanced knowledge and experience.
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Old 26th December 2012, 01:53 PM   #2
bcmbob is offline bcmbob  United States
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I'll start by taking advantage of a post by Daniel W. in response to a basic TDA7294 kit build. We can extract several items for definition and example with the goal of getting everyone on the same page.


Quote:
Originally Posted by danielwritesbac
Well, that's actually a really good result for a basic kit in stock condition. Both the gain and the feedback current may be a bit high and the NFB cap and bootstrap may be a bit small. That combination causes it to miss the highest highs and lowest lows, but that is what the datasheet example schematic does.

Try the bootstrap cap directly at 47u for slightly more low bass.
(some newer schematics show 47u)

And, try the NFB cap at least as large as 220u (or bigger)--Also try some tiny value polyester caps and/or electrolytic of 0.47uf or less, as bypass caps parallel with your NFB cap.
Then the lowest lows and highest highs should be working.
(the schematic shows 22u, which is a bass blocker)

Also for low bass this amplifier appreciates a large power supply reservoir capacitance, not located on same board as amplifier.

If it still does some midfi behavior, the gain might be set too high, since it doesn't really have enough gain capacity for a 70w amplifier. The quality drops if the gain is too high. If you want hi-fi, I think you want low gain power amp pushed by a preamp.

The feedback current might be too high due to feedback-shunt and feedback resistor values too small, which also causes fragility and current noise of overloading the small signal section beyond linearity.

To make a reasonable compromise, you could set the feedback-shunt resistor to 2k2 and the feedback resistor to 56k (or even 68k if going without a preamp). Nearby values make really similar results. This ballpark is a good spot to start exploring. That example is off a bit, but not nearly as far as the datasheet schematic.

Before using the amplifier in bridge mode, I'd like to see it running quite happily clear and definitely running cool temps. The operating voltage for bridge mode is with a 20+20vac transformer (max). So, if you're going higher than that, do consider output caps or speaker protector kit. I did test those baby latfets to 127 watts before I ran out of transformer, but I was using speaker protection, just in case.


So lets see if these can be explained and hopefully detailed on the schematic. I am contacting a vendor of a PCB of this type for permission to use a photo of one of his products on the thread.

1. NFB cap

2. Bootstrap Cap

3. Bypass caps

4. Large power supply reservoir

5. Feedback-shunt

6. Feedback resistor

7. Bridge mode

8. Output caps

9. Speaker protector kit

If links to other posted material on the DiyAudio forum and/or other sources is advised, that should be perfectly acceptable. Each of these items is complex and can occupy volumes to fully understand. Hopefully the information can be distilled down to just what is necessary for this project walk-through.
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Old 26th December 2012, 02:05 PM   #3
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Interesting Bob, I read Daniel's post two days ago when I as searching on google 'LM3886 vs TDA7293/4'. I was wondering if it really could rival LM3886 rev_c, apparently you were there on diyaudio too ;-0
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Old 26th December 2012, 02:09 PM   #4
KSTR is offline KSTR  Germany
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Let's not forget layout and wiring, Bob. Visually "small" design glitches can have big impact on the amp's perfomance additional to (if not even dominating) parts selection.
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Old 26th December 2012, 02:22 PM   #5
bcmbob is offline bcmbob  United States
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Hey metal,

No, I don't see the TDA chip to be in direct competition with a properly built MyRef. What I was impressed with was the quality of the sound from a build with so few components. All the latest MyRefs are or can be brought to a performance level that is simply unmatched IMHO. However, as is stated in the original thread topic "Which chip can play sweet/warm.....", that characteristic truly applies to what I heard. Had to build one and listen for myself to get any credibility base for my comments.

I have no idea if this chip could be the base of a sophisticated design similar to the MyRef. But for a simple and economical build with great sound - it looks very attractive. Again, we're talking about an exceptional two channel solution for ~$30.
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Last edited by bcmbob; 26th December 2012 at 02:42 PM.
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Old 26th December 2012, 03:06 PM   #6
wahab is offline wahab  Algeria
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Quote:
Originally Posted by metal View Post
II was wondering if it really could rival LM3886 rev_c, apparently you were there on diyaudio too ;-0
It is surely better than a LMxxx thanks to its mosfet output stage....
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Old 26th December 2012, 03:19 PM   #7
bcmbob is offline bcmbob  United States
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Quote:
Originally Posted by wahab View Post
It is surely better than a LMxxx thanks to its mosfet output stage....
I sincerely hope that this thread doesn't decay into a comparative and subjective soapbox.

The intent is to do a fairly simple walk-through of a process to maximize the circuits for one particular chip. The LMXXX have both their promoters and detractors, as does almost any other approach. This "hand-holding" (as Andrew T. might describe it) , could be a useful learning tool for those of us with limited or no design experience.

Hope we can stay on track !
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Old 26th December 2012, 03:21 PM   #8
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by bcmbob View Post
I have no idea if this chip could be the base of a sophisticated design similar to the MyRef. But for a simple and economical build with great sound - it looks very attractive. Again, we're talking about an exceptional two channel solution for ~$30.
I'm pessimistic by looking at the output stage. Phase inversion using opamp for class-B amplification. For small sgnal class-A this is okay. For high voltage swing I don't think so. And it seems efficiency is the main objective of this chip design. While good amps are rarely efficient.

This chip is capable of 10A. Minimum 6A is usually stated for stereo. And it does affect the strength point of this amp. But transformer is the most expensive part of an amplifier. With such a big transformer, I prefer going discrete. Or, higher (than 10V/us of the TDA7294) slew rate opamp driving mosfet output buffer, or CFB symmetrical amp.
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Old 26th December 2012, 03:30 PM   #9
wahab is offline wahab  Algeria
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Quote:
Originally Posted by bcmbob View Post
I sincerely hope that this thread doesn't decay into a comparative and subjective soapbox.

The intent is to do a fairly simple walk-through of a process to maximize the circuits for one particular chip. The LMXXX have both their promoters and detractors, as does almost any other approach. This "hand-holding" (as Andrew T. might describe it) , could be a useful learning tool for those of us with limited or no design experience.

Hope we can stay on track !
Was just pointing that it is a valuable chip as well since i used a few
as guitar or bass amps for home use and didnt find anything negative
about it.
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Old 26th December 2012, 03:33 PM   #10
bcmbob is offline bcmbob  United States
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Jay, I have been using an Antek 200VA transformer only with this build. I'll hook up a 125 later today and report any differences heard, The power supply boards were purchased to push two BPA 150 amps. I shot off a message to P Daniels for information on whether one of his Universal PS boards could be configured to match the TDA7294 requirements.
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Last edited by bcmbob; 26th December 2012 at 03:37 PM.
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