Point 2 Point (no PCB) for TDA7293, TDA7294, TDA7295, TDA7296. - Page 2 - diyAudio
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Old 20th January 2013, 10:07 AM   #11
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Plz have a look at this one.It follows your advice....Plz tell me any change in the values for Bass and mid +tweeter sound.
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Old 20th January 2013, 05:26 PM   #12
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I'm trying again to reply to post #9.
Quote:
Originally Posted by simply14prem View Post
Sorry I mean the shunt resistor ,mine is 680r ,,I just short it to ground,does is it make any difference,,what is the benefit in your case,you are having 220uf +2.2k.
Click the image to open in full size.
Omitting the NFB cap omits some dynamics and some safety. 22k input load, 22k feedback, omitted NFB cap, only looks balanced but is actually pretty far off the mark because of the missing cap; and, although, one could find a balance with trimmers, this would vary per each chip if the NFB cap is omitted.

Using the NFB cap helps keep the amp in balance (lets me choose some wild resistor values like whatever sounds good), prevents amplification of DC, makes for more durable amplifier and is part of the audiometric bass tuning. Like any signal cap, the NFB cap can be somewhat time consuming to select, but generally "contest of 5 peers" routine will work rather quickly (for NFB cap and Input cap selection). For treble helper, it is possible to give the NFB cap a little bypass cap (in parallel to the larger cap) with 0.47u or smaller electrolytic and/or 22n or smaller polyester.
Quote:
Originally Posted by simply14prem View Post
Power caps near the ic is 220uf +200nf.What is your 2u unpolarised doing there?
2u rail to rail? Standard noise filter technique for clearer, less foward, less congested midrange. Its for more fun when playing loud. RCR filter also works.
Whatever you do with the power circuit, make sure the results are "cooler and clearer" which indicates the chip really liked the change.
Quote:
Originally Posted by simply14prem View Post
I have 220uf as bootstrap,,is it too large?
A 220u bootstrap would be normal for TDA7294S/TDA7293 if run three or four paralleled by the "modular" schematic. I would suggest reducing that to 47u~100u range for the single chip amplifier shown in the schematic.
Question:
What audio difference is there between 100u vs 220u bootstrap cap?

P.S.
22u standby cap. I like this! You can use 10u~47u range for standby, whereby larger value is larger delay, which avoids turn-on thumps. It also delays turning off, so too large would make for turn-off thumps, but those have less current and thus less significance. Anyway, I like your 22u because it is in the middle of the range and just makes sense.
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Last edited by danielwritesbac; 20th January 2013 at 05:40 PM.
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Old 20th January 2013, 08:41 PM   #13
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Quote:
Originally Posted by simply14prem View Post
Plz have a look at this one.It follows your advice....Plz tell me any change in the values for Bass and mid +tweeter sound.
Click the image to open in full size.
Wow, right on the mark for audiometric feedback-shunt area! Kudos!
That should have really good bass!

You can change the input cap to 1u if you want to.

Clean bari: You can "double up" the 220u power caps by 220u||220u (makes very clean 440u caps). We wouldn't like the individual power caps larger than 220u; however, we would like the total values to be almost as big as the NFB cap, so a great way to do it is: 220u||220u (parallel pairs).
That's simply an additional 220u on both rails.

Arbitrary bypass may be an error in some cases: Bypass caps, such as the 200n on your power rails should be specifically sized to help the larger caps that they're teamed up with. Need varies per model of cap (and PCB) and can't be predicted by either a schematic or a crystal ball. If the bypass cap size is too large or if they are suddenly much more efficient than the 220u electrolytic, then a ringing peak (bad quality power) results. Polypropylene of 200n would do badly because of efficiency mismatch. However, some wima polyester, ceramic disc, cheap polyester dip cap or fractional value electrolytic, could work okay. The task could be easier if the bypass cap were smaller. Showing a bypass cap on a schematic is gambling, but we could make a safer bet with a smaller cap. The longer the traces between the 220u and bypass cap, the larger the bypass cap. In my photos, the traces are minimal length and so the bypass caps are 3n3 polyester.

Cleaner power for prettier midrange: Try one 2u Or one 3u polyester cap at rail to rail (per the schematic in post 1) and observe to see if you get nicer sounding midrange. This cap can be installed right where the DC cable attaches to the amplifier board. Polyester dip caps, 100v electrolytic and 250v electrolytic are likely candidates to test drive. The actual range is 1u~4u7, so you can "test drive" to find what works for you. RCR can be used, if desired. A smaller size cap could be used if directly attached from pin 13 to pin 15.

Treble: The 470u NFB cap may have less inductance with a smaller body cap, such as a 16v computer grade 470u cap, and that could help. Your amplifier looks quite good on bass; however, the highest treble might be a bit quieter than the midrange. Of course the rail to rail cap should help for quieter&clearer midrange and you should try that first; But, after you've done all you can with clean power, it is time to try some small signal fine tuning.
Try adding a treble bypass cap in parallel to your 470u NFB cap. Little bypass caps suitable for treble clarity boost include 0.47u and smaller electrolytic and/or 22n and smaller polyester. I can't predict what a random model of 470u would like as its partner, so if the schematic showed a treble bypass cap, that would be a non-polar of 4n7 which is too small to conduct audio but it is big enough to dampen/remove inductance of the 470u. Actually, I would leave the bypass cap value blank on the schematic, because, of course, you should experiment to find the perfect size bypass cap for pretty treble.

Your feedback current is far higher than mine, and I've got a 1/2w 60k resistor. You could use a 33k 1/2w comp resistor (high end) or you can make a high end 34K resistor out of a parallel pair of 68K ordinary 1/4w carbon film or metal film resistors. Anyway, there is enough current going though your 33k so that resistor quality matters, and needs to be assured.

Your input load of 22k does less snubbing than my 10k. You snubbed less RF, less HF and less audio. Okay, snubbing less audio was good, but the RF could be a problem. To do a more complete job, add a 330pF (or nearby value) cap in parallel with your 22k input load resistor.

P.S.
I like your design. Starting out with good bass should be quite satisfying.
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Last edited by danielwritesbac; 20th January 2013 at 08:50 PM.
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Old 21st January 2013, 01:49 AM   #14
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Thank you Daniel.
I didnt know that feedback resistor should of higher wattage,,last time my 1/4w 22k for tda2040 blew,but i didn't know the reason.
Yes I put a filter at the input.I have a big unpolarized 10u for bass amp and .4u for front speakers.I have been running 5.1 amp with my old configuration.I think my front amp needs a buffer.Becoz i think it is not loud enough .My bass has a buffer+LP filter,my surround amp(tda2040) at the rear has surround circuit+LP filter,,they all sound louder,So I think i will use a buffer+ that two transistor (classA) for my front Amp.
I dont know if introduction of that buffer will make the tda7294 noisy or not.
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Old 21st January 2013, 02:28 AM   #15
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The greater the resistor current, the worse the resistor quality, so that's why I put a better quality resistor for feedback.

If you want the front channels louder, you'd need either a preamplifier or a higher gain setting. Preamplifier has gain from clean regulated power, so that is a better place to put gain.
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Old 21st January 2013, 02:47 AM   #16
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This is my input ,I use 1nf in parallel With 22K for front,and 4.7nf for woofer as Rf filter,I separate SG and PG with 10R.Plz throw some light in this config.
Plz also tell me the value of NFB cap for a mid,tweeter amp configuration.
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Last edited by simply14prem; 21st January 2013 at 02:53 AM.
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Old 21st January 2013, 04:27 AM   #17
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I don't know what that is, but you might want a buffer in front of it, so that it doesn't apply dampening to other amplifiers run from the same source.

I don't use multi-channel, except for my own TrioPhonic, and the center channel mixer has to have a buffer in front if it, so that piece won't affect the main channels.
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Old 21st January 2013, 05:08 AM   #18
Art M is offline Art M  United States
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Unequal equivalent resistances for bias currents on pin2 (IN-) and pin3 (IN+) for
all circuit examples going to cause unnecessary output offset voltage.
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Old 22nd January 2013, 01:13 AM   #19
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Quote:
Originally Posted by Art M View Post
Unequal equivalent resistances for bias currents on pin2 (IN-) and pin3 (IN+) for all circuit examples going to cause unnecessary output offset voltage.
So far, zero. There's cap in the feedback-shunt, so no DC offset. There's no DC input and it can't amplify DC. It is safety AC coupled for dynamics & durability.

This is an ordinary miller comp amp with a normal caveat:
There is a small signal source pushing at the input load resistor
There is a Power Amplifier pushing harder at the feedback resistor
This current is quite different and trumps the voltage settings.
If the resistor values are identical for a pretty schematic, then loud playback comes with dreadful audio quality. An additional problem is that the protection doesn't work when feedback current is too high.

So, I have chosen a different option for cool, loud, durable and clear, shown at post#1. In a 4 year run of this design and similar amplifiers, there has been zero breakage and zero offset.

P.S.
Some amplifiers of this design are like post 1 except for using the option of an 82k feedback resistor and a 22k||220p input load. This can work well with a buffer or a source that has a buffered output.
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Last edited by danielwritesbac; 22nd January 2013 at 01:20 AM.
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Old 23rd January 2013, 09:18 AM   #20
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My 3rd attempt for front speakers.
plz check this one.
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Last edited by simply14prem; 23rd January 2013 at 09:36 AM.
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