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Old 19th November 2016, 11:34 PM   #11
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Quote:
Originally Posted by Arty View Post
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.
You have to invest a few sous in Bob Cordell's book "Designing Audio Power Amplifiers". Worth every centime.
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Old 20th November 2016, 12:51 AM   #12
tomchr is offline tomchr  Canada
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+1 for Cordell's book.
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Old 20th November 2016, 05:53 PM   #13
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i ment that chip in particular is more sensitive to heat than others i had a chanse to play with.
noise from heat is far more apparent, and i have no clue why the chip was designed thatway.
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Old 21st November 2016, 02:19 AM   #14
PRR is offline PRR  United States
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> 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.

IF the limit were 5V, then the max diff would be +/-5V (not +/-2.5V).

The "limit" is really closer to 7V, but I agree with not pushing it because device damage increases hiss.

This "limit" is really for "optimum Silicon doping". If you want excellent hFE, and have no other constraints, the emitter junction breakdown will come to about 7V. Old-old Germanium devices were often symmetrical: 30V on both junctions. Lateral PNPs on basic planar integrated circuits are far from optimum dope, and one good side-effect is high emitter breakdown (because it is really doped to be a Collector of an NPN).

> leave protection to passive devices

Power-absorbing (wasting) resistors? Non-linear fuses which the user will replace with tin-foil?

It takes clever, even severe, protection to get 60+ Watts out of one bit of Silicon, and also keep total product cost down.

> employ proper jacks and plugs.

Wires run under rugs and furniture can short-out. Connectors on backs of speakers short out. In general, equipment designers pick lower-cost connections. Speakons etc will never catch-on outside the professional field.

> LM3886 ....output protection:

Yes, the '3886 can do some horrible things when pushed to the limits. But chipmakers have to design for millions of sales to cover their up-front costs. '3886 was not designed for the fussy members of DIYaudio, but for the millions of surround-sound systems sold to the general public. They serve very well in that application. The fact that a coddled '3886 can also be a very good HI-FI amplifier is a bonus for us. The fact that it does not cover ALL cases leaves room for us to play with many-device amplifiers to over-kill our problems.

> with chipamps still not everything has been said and done.

That could be true. But for large-production applications, Class D has massive advantages. I doubt any chip-maker will begin another large linear power amplifier in this market. Not enough potential sales.
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Old 23rd November 2016, 04:09 PM   #15
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Originally Posted by Destroyer OS View Post
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.
Perhaps we should avoid combining all of online lore available into one amplifier; because, such a combination could hinder efficiency?
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Old 23rd November 2016, 05:30 PM   #16
JMFahey is offline JMFahey  Argentina
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Originally Posted by Grasso789 View Post
So one would use foil capacitors, which do not drift as much as electrolytics, for the stby/mute section. .
Talk about overkill

The much feared electrolytic value drift (or are you talking tolerance?) is still way smaller than anything which could cause trouble here.
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Old 27th November 2016, 10:40 PM   #17
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Some TDA7293/4/5/6s blow up together with their stby/mute capacitors. One blow-up was traced to missing negative rail. OSdestroyer traced one to the stby/mute. There is something wrong in the state of Denmark.

LM3886 is prolly better but still not as reliable as it could simply be. I proposed to protect input stage by adding two small diodes. Can you comment on that, founded on your experiences?

In the Revox A78 signal transistors are not diode-protected, and at least in my built +18V supply voltage rang on switching on or off, causing too hi BE voltage, so burst noise appeared.

Protection diodes should be present, unless
* there are no external inputs, and supply voltage is at most a few Volts and guaranteed to not ring (by adding a power diode shortcutting any negative and a resistor-zener combination limiting positive supply voltage)
* or all input voltages, even internal ones (count any transistor base as input, unless you know better) are guaranteed harmless, and supply voltage guaranteed to not ring.

With comparator ICs there is a design dilemma; either good input protection or hi switching speed and hi input resistance. But if designers of linear audio circuits needing negative feedback anyway do not add protection diodes yet state +-30V maximum differential input voltage anyway, then well, i do not get it. Who pays me 198 cents for two blown TDA2050 ICs?

Quote:
Power-absorbing (wasting) resistors? Non-linear fuses which the user will replace with tin-foil?
See Black Devil Improved post 7; give the amplifier positive output resistance, which limits shortcut current. Now fuses should not blow that often, that users start to replace them by tin-foil.

Is this a remote war, in which i have to fight men of my own nation?

Last edited by Grasso789; 27th November 2016 at 10:44 PM.
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Old 28th November 2016, 01:01 AM   #18
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Grasso789 View Post
In the Revox A78 signal transistors are not diode-protected,
Cant you get a more modern and relevant example?
That circuit is 40 or 50 years old
Quote:
do not add protection diodes yet state +-30V maximum differential input voltage anyway, then well, i do not get it. Who pays me 198 cents for two blown TDA2050 ICs?
Not sure what datasheet are you looking at, but the SGS Thompson one Im looking at states +/-15V ... whichb reaqds as "15V either way", not "15+15V=30V" as you seem to assume.
And 15V peak is quite consistent with the roughly 7V per BE junction estimated by PRR .
And even so, damage will be nil if current is limited by a series resistor.
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Old 28th November 2016, 02:37 PM   #19
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Even after 50 years still the same mistakes are maDe.

+- meaning "either way", wEll maybe, thanks! But datasheets of some bipolar opamps state a maximum differeNtial input voltage of only +-2.5V. I know also from first-hand experience, that 7V is too much for a BE junction.

In my circuits input currents Are Limited, positive input by 22KOhms DC and 2K2 AC, and negative one by the standard 22K/680 feedback network. Yet input stages broke.
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Old 28th November 2016, 04:16 PM   #20
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These chips are purpose built for industry. They are ubiquitous in consumer products. They were not designed to be high power op amps. You can actually buy high power op amps and they cost a lot more than the LM and TDA series audio amplifier chips.

They're affordable, simple to use, and respond very well to all the gilding the lily (or polishing the turd) stuff we like to do here. They're so easy and cheap to use that a DIYer is virtually obliged to do something with them.

But maybe you should buy a couple of those power op amps (which are really designed for use in industrial servo systems) and see what you get. Some of them work with higher voltages. I bet you could make a nice power amplifier out of some of them, but they sure are expensive.
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