LM3886 variation - filtered without el caps - Page 2 - diyAudio
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Old 8th September 2012, 03:53 AM   #11
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by DF96 View Post
That means that the amplifier circuit has non-linear input impedance - which can happen. However, if the input series resistor has to be small to avoid distortion then this means that the source impedance from which the signal comes must be even lower. If the chip is this sensitive to source impedance it really ought to have a buffer in front of it.
Of course they need it. Which one does not? It is all about how much is audible or not.

Quote:
Originally Posted by DF96 View Post
Smaller feedback resistors (other things being equal) will mean less HF boost due to stray capacitance at the feedback pin, but can also cause greater distortion due to thermal effects if the feedback resistor has highish tempco or too small power rating.

Finally, note that physically large caps can inject more hum and RF into sensitive circuit nodes so better screening may be needed than is necessary with the more usual electrolytics.
The TSSA is a good example to get the feel to work with those issues.

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Originally Posted by DF96 View Post
This sort of thing can't be done by ear, as ears are very easily fooled. Much better to be done by datasheet, calculator and measurements.
Measurement, okay. Datasheet, calculator/simulator? Those are full of assumed numbers. Some ears can be better.
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Old 8th September 2012, 04:06 AM   #12
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by Redshift187 View Post
If you want to do away with electrolytics in the signal path, why not just use a servo?
Few days ago Mikelm posted, I think in the "Simple Symmetrical Amplifier" thread, saying that a (especially when badly designed) servo may increase the level of very high order harmonics. I have never simulated such thing as I have had big doubt with servo but couldn't find a reason (as I don't want to use it).

High order harmonics always have relationship with fatigue. Caps in feedback may color the sound, blur the sound, or whatever, but it doesn't create offending sound. So there is nothing too wrong with using caps. It can be a better option.
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Old 8th September 2012, 04:47 AM   #13
gootee is offline gootee  United States
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The small-signal path might be free of electrolytics. But THAT signal path stops, right after the chipamp input pins.

The real signal path, the one that directly provides the exact signal that makes the sound come out of the speakers, is the power supply.

Virtually ALL of the current that IS the actual signal that you HEAR, comes not past, but directly OUT of, the electrolytic capacitors in the power supply, or out of the ones that decouple the chipamp's power supply inputs.

Get over it. (Then embrace it. Read on.)

In case you still have doubts about all of the load current coming straight out of the electrolytic caps, here is some pretty good proof:

http://www.diyaudio.com/forums/attac...om3a_33kuf.jpg

Which is part of my post #372 at:

Power Supply Resevoir Size

Since that is the case, the best thing to do is to optimize the parasitic inductance and resistance between the reservoir caps and the chip's power pins, and between the decoupling caps and the chip's power pins, which in this case means minimizing those parasitics. (It's not too difficult. Read on.)

According to Terry Givens, a noted expert in the power supply field, one excellent way to do that would be to populate a two-sided circuit board with a matrix (rows and columns) of electrolytics, one circuit board per power rail, with one side of each board being that rail's ground and the other side being that rail's voltage. Leave all of the copper on both sides, removing just enough to insulate around a hole for one lead of each cap to go to the other side of the board. Bend about a half-inch of each lead and solder most of its length to the surface of the board.

For important details about that, see his posts about it, starting at post # 1009, at the link below and reading the rest of his posts up through at least post # 1024.

Power Supply Resevoir Size

If, for some reason, you had to do it with point-to-point wiring, and no PCB, you could try the approach in post # 1004, there.

For the best-possible transient response, I believe that you will still need to also have several (as many as will fit, totaling at least 200-300 uF, but 10x that would be better) small electrolytic decoupling caps right AT each power pin of the chipamp, in parallel with each other, with their other leads taking separate short-as-possible paths to the speaker and zobel ground point, which should also be located as close as possible. (It's very important that those caps provide the shortest-possible round-trip path, and that there are as many in parallel as possible, all with separated leads/conductors in parallel that stay separated all the way to the decoupling points, if at all possible.) Film caps are not easy to recommend, there. You might get away with it but without some serious test equipment (network analyzer and spectrum analyzer and a good oscilloscope) and calculations, and a degree in electrical engineering plus relevant experience, there's too much risk of creating a resonant LC tank that will ring and/or oscillate at very high frequencies, seriously degrading performance or even destroying the chip. ALSO, you "must" have a small X7R-type ceramic BYPASS cap in parallel, there, too, for high-frequency stability. Again, using a "better quality" C0G or NPO type there, or a film cap, is probably too risky. (But, you can certainly experiment with it and might get lucky.)

Cheers,

Tom

Last edited by gootee; 8th September 2012 at 05:14 AM.
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