National Semiconductor AN-125 for LM378/LM379

Good google-fu..

GH

Yeah, I have that one and the 1980 Audio Handbook, neither specifically have AN-125...

John :-#)#

Yep, got AN-125 in my 1980 NatSemi Linear databook. I am away from scanner a few hours - do you want me to scan it?

piglet

onal/archiveorg-an

s noise.

0_SND.png

If you don't mind. I'll send it on to bitsavers too...

It isn't in my 1982 NS Linear databook.

Looks like the LM2879T was the replacement part for the LM379S, looks like I'll be making some small PCBs to be able to update boards where I can't get LM379Ss any more. Of course the LM2879T is obsolete, but available inexpensively - and they added the 2R7 & 0.1uf cap to the outputs, so for now I'll do that too on my LM379Ss.

Thanks!

John :-#)#

Took me less time than expected ... here it is.

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Thanks!

Reading that AN-125 shows not much new from the other data I found in the Power Amplifiers sections of later NS data books, but I will check the gain to make sure that fits withing NS requirements for this device,

John :-#)#

I see AN-125 doesn't show a Zobel network on the outputs, but it's a good idea, you should add it.

--
 Thanks, 
    - Win

Ah, so that is the correct name for the resistor/cap that NS recommended for some of their amp drivers.

I'll check the math C'=L/R(2) when I get access to the speaker circuit and get the DC resistance. For now the speaker is 8 Ohms impedance and if the recommended resistance is 2R7 then C' = 1.1, and I assume that is pretty much 0.1ufd:

Thanks for confirming my assumption,

John :-#)#

** It is the usual name, but in reality it ain't one.

** Waste of effort - the series RC is not there to make the impedance resi stive at all frequencies cos that would be both expensive and unnecessary.

Plus speakers are *bad* inductors with lots of eddy current loss so do not follow the simple formula for inductive reactance. Sure, impedance rises at high frequencies, but it doubles every TWO octave instead of one until par allel capacitance comes into play.

At some supersonic frequency, the reactance of a typical speaker is inducti ve or capacitive enough to cause phase rotation at the output of the drivin g amplifier - sometimes resulting in parasitic oscillations at particular c urrent levels or at idle.

A series RC of a few ohms and 100nF swamps this effect and makes the amp st able again. It would be a nice touch if speaker makers incorporated such RC networks in side every speaker box but AFAIK none do.

..... Phil

Wikipedia articles often pick a sensible argument and ignore the broader knowledge out there. In fact this R+C network is used to cure audio power-amplifier ills that may be unrelated to the speak inductance. Excess amplifier gain and phase shift, at high frequencies near its bandwidth rolloff limit, create instability vulnerabilities. Adding a low-value load resistor, such as 2.7 ohms, serves to lower the amplifier's gain and even pull back it phase shift. A series capacitor insures that this special severe output load disappears at slightly-lower frequencies, e.g. 0.1uF and 2.7 ohms adds a zero in the feedback loop at about 600kHz.

--
 Thanks, 
    - Win

d t

sistive at all frequencies cos that would be both expensive and unnecessary .

t follow the simple formula for inductive reactance. Sure, impedance rises at high frequencies, but it doubles every TWO octave instead of one until p arallel capacitance comes into play.

tive or capacitive enough to cause phase rotation at the output of the driv ing amplifier - sometimes resulting in parasitic oscillations at particular current levels or at idle.

stable again.

inside every speaker box but AFAIK none do.

Huh, I'm not in disagreement. But when driving air coils I use a series RC in parallel with the RL load. That makes the big inductor look mostly like a resistor to the amp driving it... (it's about keeping the amp happy.) The circuit is a VCCS. Speakers are driven with voltage sources, so that would be different, I think.

George H.

Thank you all for your advice and confirmations. I have managed to reduce the quiescent temperature of the device by about 10C adding appropriate filters.

John :-#)#

Wow, full schematics n everything. Holy shit, that LM3809 schematic Lol