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Class D Switching Power Amplifiers and Power D/A conversion

Big bro, the TDA8950
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Old 8th March 2016, 07:50 AM   #11
danielwritesbac is offline danielwritesbac  United States
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Quote:
Originally Posted by Kjeldsen View Post
...skip the onboard PSU...
That is an additional problem. However, it is entirely different and un-related to the fuzzbox problem. The cheap power supply will cause the amplifier to be unpleasant when it is cranked up loud, so that is impractical to use it at its maximum potential because of blare, which is a practicality-biased and slightly costly problem. Fixing that will not help the fuzzbox input locale problem. They're quite different. One problem is at small signal, the input, and the other problem is at large signal (supply=speaker level=large signal). The two problems that the amplifier has are not related. In my opinion, and because there is gain, I think we have to fix the small signal problem first. The presence of a voltage gain stage forces the order of repairs to follow smallest to largest. So, we really must accomplish the input repair first.
I do still think this amplifier is promising, but a determination is still pending.
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Old 8th March 2016, 08:43 AM   #12
Kjeldsen is offline Kjeldsen  Denmark
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I will dig up these modules again, and try out the solutions mentioned here.

They are indeed very promising and not particularly Class D like. For pure bass amplification they are unbeatable for the price.
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Old 14th March 2016, 08:13 PM   #13
danielwritesbac is offline danielwritesbac  United States
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Originally Posted by Kjeldsen View Post
I will dig up these modules again, and try out the solutions mentioned here. They are indeed very promising and not particularly Class D like. For pure bass amplification they are unbeatable for the price.
I suppose that the power supply costs won't be much different than for a high-end Class aB amplifier. Sure, the transformer and heatsink can be smaller sized for the Class D amplifier, but that's not much.

If you wanted to make thousands, then those economies might be important; but, if you wanted to make one for home use, the efficiency advantage, which wasn't much in practice, has evaporated.

Although some brags have not worked in practice, the remainder does seem to be capable of high fidelity audio... if you add the equipment for that.

It probably will be easy, but it won't be as cheap as claimed for Class D.
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Old 17th March 2016, 04:38 PM   #14
danielwritesbac is offline danielwritesbac  United States
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Originally Posted by abraxalito View Post
I managed to get the fuzz-box distortion with my TDA8932. I figure it was crosstalk from the output wires back to the input. I fixed it by fitting an input attenuator (i.e. series resistors) right on the pins of the IC with 0805 resistors. I think it was about -10dB. I also added a small cap but I think the fuzz-box effect went away even without the cap.
What was the value of the resistors?
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Old 18th March 2016, 03:02 AM   #15
abraxalito is offline abraxalito  United Kingdom
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From memory I think I used 56k either side. The overall gain reduction was about 3X, running in BTL mode.
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Old 18th March 2016, 03:16 AM   #16
danielwritesbac is offline danielwritesbac  United States
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Originally Posted by abraxalito View Post
From memory I think I used 56k either side. The overall gain reduction was about 3X, running in BTL mode.
Thanks man! That totally makes sense.
Also, the system gain won't go down unless the series elements also have a load. Was it the volume pot?
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Old 18th March 2016, 03:28 AM   #17
abraxalito is offline abraxalito  United Kingdom
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No, I put the volume pot prior to those resistors, on my input board.

The load is the built-in resistance of the chip. If you look at the DS, p12 shows what the input circuitry looks like. Remember in BTL mode you have two of these input circuits in parallel, so the impedance of each input to GND is about 25k.
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Old 21st March 2016, 09:16 PM   #18
danielwritesbac is offline danielwritesbac  United States
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I wonder if your resistors series to all four inputs could also remove/reduce the much quieter treble fuzz from a BTL TDA8932? Seems plausible?
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Old 22nd March 2016, 02:08 AM   #19
abraxalito is offline abraxalito  United Kingdom
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Yeah - I haven't tried 4 resistors because on the board I'm using the two inputs are already wired in parallel. So I went with just two resistors. If you wanted to use 4 they'd be larger valued because then each is looking into a higher load impedance (twice as high).
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Old 22nd March 2016, 02:47 AM   #20
danielwritesbac is offline danielwritesbac  United States
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Originally Posted by abraxalito View Post
Yeah - I haven't tried 4 resistors because on the board I'm using the two inputs are already wired in parallel. So I went with just two resistors. If you wanted to use 4 they'd be larger valued because then each is looking into a higher load impedance (twice as high).
Production of that has varied in the quality of decoupling caps provided. I sure do need to photograph that. Anyway, I do have that older one that was not suitable to use BTL but was delightful SE. I don't have a problem with prying (some heat involved too) all four inputs off the board to check this. Are you going to check it too?

Thanks for the speedy reply.

P.S.
As far as the board relevant to this thread, I bought it for two reasons; that it would suit a transformer that I already own (undersize for Class aB amp but not terrible for Class D amp's slightly lower current needs) and that it looks (and is) easy to wipe out that ugly/economy onboard power circuit, scrape off all the traces from that area and re-purpose that board space for something easily done that works slightly better, along with the annoyance of building an off-board supply (a straightforward task costs a bit). Can I build an unreg supply that has TDA8950 sound as pretty as little brother TDA8932 on a cheap linear reg (or outdated "gets hot" SMPS)? I don't know, yet. But, there is that particular "paywall" before they can be compared fairly--a big capacity amp compared to a little one. The matter might be linear in comparison to the output power.
Thanks again for the confirmation on the locale of the input trouble (too much current with those crossed up inputs); and now, for this thread, we can proceed to the power supply.
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