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Old 16th September 2016, 04:06 PM   #1
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Default flaws of some powerful chipamps

Just my opinion.

TDA2050 seems to be a chip of lo distortion, hi efficiency and hi reliability, if maximum output is limited to 25 Watts. But i need more power from one output.

I read German stories of TDA729* becoming damaged, if Vdd is missing, while Vcc is up, say if chip pins "signal ground" and "standby/mute ground" become lowest chip potential. If this is really so, this is a huge flaw. (TDA7293 seems to be the most powerful but also critical one. For 7294V, 7295 and 7296 "standby/mute ground" is allowed to be Vdd.)

Now the LM3886 has propagated some genes of the former. For one thing muting functionality, and it has a ground pin. An op-amp should have no ground pin.

A true bipolar op-amp has a maximum differential input voltage of +-2.5V, because inputs are connected to a differential amplifier, which consists of two equal transistors with common emitter, and BE junction must not exceed 5V for PNP and -5V for NPN. But datasheets for LM3886 and TDA7293/4 state much more.

LM3886 has emulated DMOS output stage of TDA7273/4 with large shunt resistors (0.45 Ohms). These resistors heat up the chip, when it is driving a heavy load into clipping, and reduce power. On the upside they limit current. So it comes to output protection:

Maybe one should leave protection to passive devices. Small signals can cause large signals but cannot protect from large signals. Large signals must run out of power.

A single rail amplifier with rail ripple shows unsymmetrical clipping, and I(U) functions are different for positive and negative output transistor. This is bad. On the upside, the output capacitor is a great protector, tho i have already seen a shorted electrolytic capacitor. A wire fuse may also be useful. Sure this does not help, when output is short-cut, as transistors cannot take hi currents at hi voltage.

So one could either use that large shunt resistors and that strong or many output transistors, that it can withstand a short-cut, until the wire fuse blows, or employ proper jacks and plugs. Those ones should withstand heavy pushing and pulling and seem better than Speakon.

This seems right.

Last edited by Grasso789; 19th September 2016 at 02:01 AM.
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Old 16th September 2016, 05:14 PM   #2
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I can confirm the 7293 will light on fire if you don't follow the spec sheet mute/stby wiring. It doesn't explain why they use the parts how they do and why, but unless you like flames you should follow it.
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Old 17th November 2016, 02:16 PM   #3
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So one would use foil capacitors, which do not drift as much as electrolytics, for the stby/mute section. And do not use fuses in the power supply rails but only in the mains and the loudspeaker circuits, for if negative rail fails, while positive one is still on, the chip blows.

I used UTC TDA2050 with 46V single rail supply and found that i must lay two 1N4148 diodes antiparallel to inputs, limiting differential input voltage. Tho the latter is stated as +-30V maximum, it is stressed too much within my unusual circuit and way.

Usual circuits and other op-amps should profit from these diodes, too, getting hier reliability. Only catch may be, that the diodes may cause some circuits to get stuck at positive or negative rail: Input stage would not blow, but output could not settle, as negative feedback would like to.

Even with chipamps still not everything has been said and done.
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Old 19th November 2016, 01:11 AM   #4
tomchr is offline tomchr  Canada
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If you look at the simplified circuit schematic of the LM3886 in its data sheet, you'll notice that the ground pin is only used for the MUTE function. I'm not sure why that would ever be an issue for you.

Tom
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Old 19th November 2016, 01:21 AM   #5
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My library system has a Linn Majik and LK100 biamped driving Kelidh's. The Majik's horsepower is driven by a chipamp. The system is over 20 years old and still sounds wonderful.

The only flaw I can see with the chipamps is the absolute necessity that you get the heat sinking correct.

J
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Old 19th November 2016, 03:36 AM   #6
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by jackinnj View Post
The only flaw I can see with the chipamps is the absolute necessity that you get the heat sinking correct.
I know! The laws of physics are so inconvenient. Just say no to physics...

Tom
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Old 19th November 2016, 07:49 AM   #7
ICG is offline ICG  Germany
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Quote:
Originally Posted by Grasso789 View Post
LM3886

[...]

A single rail amplifier with rail ripple shows unsymmetrical clipping
The LM3886 isn't a single rail amp. And why would you want symmetrical clipping? You DON'T want clipping AT ALL!
Playing troll again?
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Old 19th November 2016, 08:14 AM   #8
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Initial post is loaded with so many boldly stated misconceptions that it would take too much effort to help the OP understand things better, I fear.
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Old 19th November 2016, 01:28 PM   #9
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Originally Posted by tomchr View Post
I know! The laws of physics are so inconvenient. Just say no to physics...

Tom
Fortunate to have a Grizzly mini-mill I took some 1/8" stock and fashioned a tortion bar to attach the chipamp to heatsink. Before I did this (when it was just screwed down), I could get the thing to thermally cycle. Not pleasant.

The Majik, which dates from the early 1990's, used a TDA1514.
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Old 19th November 2016, 08:07 PM   #10
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hmmm i can agree with the tda1514 needing some more heatsinking than expected.
that guy can get hot and has some problems with heat dispersion i guess.
used to have them, and love them.
it can deliver quality, i spent quite some hours polishing away the package to provide a better surface for the heatsink, and i used to sandwitch the chips between heatsinks.
made a world of difference, for some reason that thing is sensitive to heat as hell.
at one point i was close to smacking it with a water cooler to be honest.
it does a good job until you can keep it cold.
once it gets hot, the soun is medicore at best.
dynamics are lost as heat is gained.

anyone have a tip howcome? i'm usually not sensitive to stuff like that, i don't have that audiophool thing in my head at all. this is one of the few chips where i can sense a huuuge difference.
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