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TDA 7293 ---- done right ?
TDA 7293 ---- done right ?
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Old 4th September 2021, 04:59 PM   #91
bebo65 is offline bebo65  Germany
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Default symmetrical input, 2W @ 4Ohm load

for the measuments shown i changed the input of the circuit to a symmetrical connection by using a standard cable for telephone installation (type: shielded 2x2x0,6mm in starquad configuration) and changed the load to 4Ohm. The rest of the connections are left unchanged.

Attachment 1:
symmetrical input, no signal applied. clear to see that the power supply artefakte are lower than before, but not the "distortion" at 50Hz - i have actually no idea where this comes from, as no 50Hz "source" is nearby and the first rectifier artefakte is to be expected at 100Hz...

Attachment 2:
symmetrical input, testtone 1kHz

Attachment 3:
symmetrical input, dual testtone 19kHz and 20kHz (1:1)

Attachment 4:
symmetrical input, multitone testsignal


conclusion so far:
symmetrical connection is the way to go, it is clear to see that the signal arriving the circuit is much cleaner. looks like the single ended cable works as an antenna, even when the input pins are shorted.
Attached Images
File Type: png composite NFB symm no signal.png (96.5 KB, 226 views)
File Type: png composite NFB sine symm 4Ohm.png (98.6 KB, 224 views)
File Type: png composite NFB dual sine symm 4Ohm.png (100.8 KB, 226 views)
File Type: png composite NFB multitone symm 4Ohm.png (134.1 KB, 219 views)
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Old 5th September 2021, 07:03 AM   #92
Dibya is offline Dibya
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It seems error correction is working
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Old 5th September 2021, 04:25 PM   #93
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by bebo65 View Post
Attachment 1:
symmetrical input, no signal applied. clear to see that the power supply artefakte are lower than before, but not the "distortion" at 50Hz - i have actually no idea where this comes from, as no 50Hz "source" is nearby and the first rectifier artefakte is to be expected at 100Hz...
With 50 Hz mains, you're surrounded by 50 Hz sources that will couple inductively into your circuit if you let them. That's likely also why you see the other mains-related artifacts. I'd look for ground loops and minimize their loop areas.
You can also try a faraday cage. Ideally a sealed and grounded metal box. I've had pretty decent luck with a "circuit burrito" (i.e., wrapping the circuit in antistatic bubble wrap and then in tinfoil connected to ground with a clip lead). Just be careful you don't short stuff out or allow stuff to overheat in the burrito.

-100 dBc THD is not bad. That may be the limit of your instrumentation. It certainly indicates that the error correction isn't fundamentally broken.
What's going on at 20 kHz in the latest plots?

Tom
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Last edited by tomchr; 5th September 2021 at 04:27 PM.
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Old 5th September 2021, 10:30 PM   #94
MalleMike is offline MalleMike  Spain
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I do not know if it is possible to get rid of the TDA´s fingerprint . Probably it is due to the internal biasing circuitry of the chip . It is not a complementary circuit after all . But there is one thing one can try to check this out , by configuring the device as a slave and introducing a clean already amplified signal into pin 11 . If when doing that the same k2 and k3 distortion appears in the output it is introduced in the output-stage with its biasing and protection circuitry and so probably impossible to get rid of. if not then there are other possibilities as then it comes from either the inputs , the vas or power-supply somehow ???


I would do it myself if I had the equipment to measure very low distortion , which unfortunately I do not.

Last edited by MalleMike; 5th September 2021 at 10:40 PM.
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Old 6th September 2021, 08:32 PM   #95
bebo65 is offline bebo65  Germany
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A faradays cage is a real good idea, because i had an eye on building NO groundloops - the "circuit burrito" described looks as an easy way to get a real cheap faradays cage, thank you Tom.

I did some work to find out what is going on at 20kHz as i have seen this "distortion" too. I am almost sure i found the origin of it - on the oscilloscope i can see a real small periodic signal as soon as the connection to the computer starts to work, its frequency is an integer multiple (6x) of 20kHz...so it has to be some kind of aliasing (not sure if this is the right word, but i hope it is clear what i meant to say). This periodic signal shows up on both soundcards i have acces to (one firewire connected, the other one USB connected, both cards terratec)

I dont know the exact limit of my instrumentation, but looking at the soundcards selftest (shown a few posts ago) you could be right that it is close to it, Tom.

The TDAs "fingerprint" is so far down in level that i dont want to make any effort to get rid of it, Mike, thanks for your ideas anyway.
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Old 22nd September 2021, 07:08 AM   #96
bebo65 is offline bebo65  Germany
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while waiting for the components to arrive i did a quick test of my variation of the my_ref-circuit. The step-response test tells me it is working stable and the bandwidth (refering to my NFB circuit) is almost doubled (1W@8 Ohm), but shows a 15% peak in amplitude in the 400kHz region....
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Old 24th September 2021, 02:38 PM   #97
WLA is offline WLA  United States
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hello guys,
Has anyone seen that website - Low Distortion Composite Amplifiers LM1875 TDA7293 TDA7294
There is a lot of information about building super low distortion amplifiers based on TDA7293.
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Old 24th September 2021, 11:53 PM   #98
MalleMike is offline MalleMike  Spain
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WLA , yes , I for one have read and studied it and the circuit diagram since you mentioned it in another thread too . I think that what is being done here , especially by bebo65 (and Tom´s considerable input as well) is superior to whats on that page . If you have not done it yet I would suggest you read the whole thread. But of course all info is wellcome so thanks .


Hello Bepo , any news concerning the myref angle ? I am digging for more info and ideas constantly and have just received 10 TDA7293 so will start experimenting with them now that I can do it the things are kind of hard to come by these days I am even considering buying a few Ebay-boards then mount a TDA on them and start modifying , so just use them as an experimental starting point
And forgive me for repeating this but I really want to figure out if the slew-rate of the TDA´s output stage is a lot better than that of the rest of the chip - since it is Mosfet that might well be the case , so we could quite possibly get a much better upper frequency-limit /less phase-shift using a discreete VAS . Greetings Michael

Last edited by MalleMike; 25th September 2021 at 12:17 AM.
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Old 25th September 2021, 07:38 AM   #99
WLA is offline WLA  United States
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Hi Mike, good to hear from you.
I'm about to build an amplifier with TDA7293 and trying to find something really descent. According to the THD plots in the website mentioned above the design there looks really good. The type of the op amp used there is what I'd like to know. Any suggestions?
BTW what's the THD figure of the design discussed here?
Thanks
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Old 25th September 2021, 10:07 AM   #100
MalleMike is offline MalleMike  Spain
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Hi WLA - its hard to recommend an op-amp as opinions about them there are many and very divided. The best you can do is note down the denominations of the various op-amps mentioned in this and other threads , then type them into the search in the whole forum and see what you come up with . Or simply use the one recommended by bepo as he is , to the best of my knowledge , the main "motor" in this thread
You will find that some people prefer using an OPA , others an LM , and so on as apart from technical specs they seem to have a different "musicality" which to my understanding is due to different internal structure and specifications , so up to a certain point it is a matter of taste.
Personally I´d go with the LM4562 or if you are willing to spend a little more the LM49720 but be advised that both seem to be identical chip inside them. The LM4562 is an 8pin DIL package so can be put into a socket to be interchangeable without soldering which is one reason I prefer it.
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