National Semiconductor AN-125 for LM378/LM379

[John Robertson]

Does anyone have the National Applications Handbook #2 from 1977 that
contains (I think) AN-125 - LM377, LM378, LM379 Dual Two, Four, and Six
Watt Power Amplifiers?

Usual and even unusual suspects don't have it. (Bitsavers, archive.org,
etc.)

I did find a very nice listing of the application notes that lists not
only the AN-xx(x) number but the title of the AN:

http://www.projects.scorchingbay.nz/dokuwiki/electronic/datasheet/national/archiveorg-an

Having a heck of a time tracking this down. I am checking the Gottlieb
pinball sound/speech sound board design (late 70s) around the LM379 and
have already found at least one error in their circuitry and with AN-125
I hope to see if there is anything else I can do to hunt down stray
parasitics as the audio IC is heating up even when idle and I recall it
always ran a bit warm. Scoping doesn't show any obvious noise.

Schematic here:

https://www.flippers.com/images/Gottlieb/TechTips/LM379S_Gottlieb_Sys80_SND.png

According to the LM378/9 spec sheet the first error was that pin 14
should have had a 220ufd cap to ground, I have now added that. Next we
have an unused half of the 379s so I tied pin 6 via 100K to pin 14.
Should I also ground pin 7?

PDF of the LM379:

https://www.flippers.com/pdfs/LM379_AMP.pdf

Lastly, even though the NS Power Amplifiers section of their 1980 Audio
Radio Handbook says it is not required on the LM378/9 chips, can it hurt
to add a 0.1uf in series with a 2R7 between the output and common? Or is
that just a waste of time installing?

I know this is an antique, but a large part of my business is keeping
antiques going and fixing factory errors...

Thanks,

John :-#)#

 

[John Robertson]

On 2019/08/14 10:56 a.m., George Herold wrote:

On Wednesday, August 14, 2019 at 1:33:49 PM UTC-4, John Robertson wrote:
Does anyone have the National Applications Handbook #2 from 1977 that
contains (I think) AN-125 - LM377, LM378, LM379 Dual Two, Four, and Six
Watt Power Amplifiers?

Usual and even unusual suspects don't have it. (Bitsavers, archive.org,
etc.)

I did find a very nice listing of the application notes that lists not
only the AN-xx(x) number but the title of the AN:

http://www.projects.scorchingbay.nz/dokuwiki/electronic/datasheet/national/archiveorg-an

Having a heck of a time tracking this down. I am checking the Gottlieb
pinball sound/speech sound board design (late 70s) around the LM379 and
have already found at least one error in their circuitry and with AN-125
I hope to see if there is anything else I can do to hunt down stray
parasitics as the audio IC is heating up even when idle and I recall it
always ran a bit warm. Scoping doesn't show any obvious noise.

Schematic here:

https://www.flippers.com/images/Gottlieb/TechTips/LM379S_Gottlieb_Sys80_SND.png

According to the LM378/9 spec sheet the first error was that pin 14
should have had a 220ufd cap to ground, I have now added that. Next we
have an unused half of the 379s so I tied pin 6 via 100K to pin 14.
Should I also ground pin 7?

PDF of the LM379:

https://www.flippers.com/pdfs/LM379_AMP.pdf

Lastly, even though the NS Power Amplifiers section of their 1980 Audio
Radio Handbook says it is not required on the LM378/9 chips, can it hurt
to add a 0.1uf in series with a 2R7 between the output and common? Or is
that just a waste of time installing?

I know this is an antique, but a large part of my business is keeping
antiques going and fixing factory errors...

Thanks,

John :-#)#

Good google-fu..
https://www.technicalaudio.com/pdf/National_Semi/National_semi_Audio_Handbook_1976.pdf

GH

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

John :-#)#

 

[George Herold]

On Wednesday, August 14, 2019 at 1:33:49 PM UTC-4, John Robertson wrote:

Does anyone have the National Applications Handbook #2 from 1977 that
contains (I think) AN-125 - LM377, LM378, LM379 Dual Two, Four, and Six
Watt Power Amplifiers?

Usual and even unusual suspects don't have it. (Bitsavers, archive.org,
etc.)

I did find a very nice listing of the application notes that lists not
only the AN-xx(x) number but the title of the AN:

http://www.projects.scorchingbay.nz/dokuwiki/electronic/datasheet/national/archiveorg-an

Having a heck of a time tracking this down. I am checking the Gottlieb
pinball sound/speech sound board design (late 70s) around the LM379 and
have already found at least one error in their circuitry and with AN-125
I hope to see if there is anything else I can do to hunt down stray
parasitics as the audio IC is heating up even when idle and I recall it
always ran a bit warm. Scoping doesn't show any obvious noise.

Schematic here:

https://www.flippers.com/images/Gottlieb/TechTips/LM379S_Gottlieb_Sys80_SND.png

According to the LM378/9 spec sheet the first error was that pin 14
should have had a 220ufd cap to ground, I have now added that. Next we
have an unused half of the 379s so I tied pin 6 via 100K to pin 14.
Should I also ground pin 7?

PDF of the LM379:

https://www.flippers.com/pdfs/LM379_AMP.pdf

Lastly, even though the NS Power Amplifiers section of their 1980 Audio
Radio Handbook says it is not required on the LM378/9 chips, can it hurt
to add a 0.1uf in series with a 2R7 between the output and common? Or is
that just a waste of time installing?

I know this is an antique, but a large part of my business is keeping
antiques going and fixing factory errors...

Thanks,

John :-#)#

Good google-fu..
https://www.technicalaudio.com/pdf/National_Semi/National_semi_Audio_Handbook_1976.pdf

GH

 

[piglet]

On 14/08/2019 6:33 pm, John Robertson wrote:

Does anyone have the National Applications Handbook #2 from 1977 that
contains (I think) AN-125 - LM377, LM378, LM379 Dual Two, Four, and Six
Watt Power Amplifiers?

Usual and even unusual suspects don't have it. (Bitsavers, archive.org,
etc.)

I did find a very nice listing of the application notes that lists not
only the AN-xx(x) number but the title of the AN:

http://www.projects.scorchingbay.nz/dokuwiki/electronic/datasheet/national/archiveorg-an


Having a heck of a time tracking this down. I am checking the Gottlieb
pinball sound/speech sound board design (late 70s) around the LM379 and
have already found at least one error in their circuitry and with AN-125
I hope to see if there is anything else I can do to hunt down stray
parasitics as the audio IC is heating up even when idle and I recall it
always ran a bit warm.  Scoping doesn't show any obvious noise.

Schematic here:

https://www.flippers.com/images/Gottlieb/TechTips/LM379S_Gottlieb_Sys80_SND.png


According to the LM378/9 spec sheet the first error was that pin 14
should have had a 220ufd cap to ground, I have now added that. Next we
have an unused half of the 379s so I tied pin 6 via 100K to pin 14.
Should I also ground pin 7?

PDF of the LM379:

https://www.flippers.com/pdfs/LM379_AMP.pdf

Lastly, even though the NS Power Amplifiers section of their 1980 Audio
Radio Handbook says it is not required on the LM378/9 chips, can it hurt
to add a 0.1uf in series with a 2R7 between the output and common? Or is
that just a waste of time installing?

I know this is an antique, but a large part of my business is keeping
antiques going and fixing factory errors...

Thanks,

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

 

[John Robertson]

On 2019/08/14 11:48 a.m., piglet wrote:

On 14/08/2019 6:33 pm, John Robertson wrote:
Does anyone have the National Applications Handbook #2 from 1977 that
contains (I think) AN-125 - LM377, LM378, LM379 Dual Two, Four, and
Six Watt Power Amplifiers?

Usual and even unusual suspects don't have it. (Bitsavers,
archive.org, etc.)

I did find a very nice listing of the application notes that lists not
only the AN-xx(x) number but the title of the AN:

http://www.projects.scorchingbay.nz/dokuwiki/electronic/datasheet/national/archiveorg-an


Having a heck of a time tracking this down. I am checking the Gottlieb
pinball sound/speech sound board design (late 70s) around the LM379
and have already found at least one error in their circuitry and with
AN-125 I hope to see if there is anything else I can do to hunt down
stray parasitics as the audio IC is heating up even when idle and I
recall it always ran a bit warm.  Scoping doesn't show any obvious noise.

Schematic here:

https://www.flippers.com/images/Gottlieb/TechTips/LM379S_Gottlieb_Sys80_SND.png


According to the LM378/9 spec sheet the first error was that pin 14
should have had a 220ufd cap to ground, I have now added that. Next we
have an unused half of the 379s so I tied pin 6 via 100K to pin 14.
Should I also ground pin 7?

PDF of the LM379:

https://www.flippers.com/pdfs/LM379_AMP.pdf

Lastly, even though the NS Power Amplifiers section of their 1980
Audio Radio Handbook says it is not required on the LM378/9 chips, can
it hurt to add a 0.1uf in series with a 2R7 between the output and
common? Or is that just a waste of time installing?

I know this is an antique, but a large part of my business is keeping
antiques going and fixing factory errors...

Thanks,

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

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 :-#)#

 

[piglet]

On 14/08/2019 8:01 pm, John Robertson wrote:

On 2019/08/14 11:48 a.m., piglet wrote:
On 14/08/2019 6:33 pm, John Robertson wrote:
Does anyone have the National Applications Handbook #2 from 1977 that
contains (I think) AN-125 - LM377, LM378, LM379 Dual Two, Four, and
Six Watt Power Amplifiers?

Usual and even unusual suspects don't have it. (Bitsavers,
archive.org, etc.)

I did find a very nice listing of the application notes that lists
not only the AN-xx(x) number but the title of the AN:

http://www.projects.scorchingbay.nz/dokuwiki/electronic/datasheet/national/archiveorg-an


Having a heck of a time tracking this down. I am checking the
Gottlieb pinball sound/speech sound board design (late 70s) around
the LM379 and have already found at least one error in their
circuitry and with AN-125 I hope to see if there is anything else I
can do to hunt down stray parasitics as the audio IC is heating up
even when idle and I recall it always ran a bit warm.  Scoping
doesn't show any obvious noise.

Schematic here:

https://www.flippers.com/images/Gottlieb/TechTips/LM379S_Gottlieb_Sys80_SND.png


According to the LM378/9 spec sheet the first error was that pin 14
should have had a 220ufd cap to ground, I have now added that. Next
we have an unused half of the 379s so I tied pin 6 via 100K to pin
14. Should I also ground pin 7?

PDF of the LM379:

https://www.flippers.com/pdfs/LM379_AMP.pdf

Lastly, even though the NS Power Amplifiers section of their 1980
Audio Radio Handbook says it is not required on the LM378/9 chips,
can it hurt to add a 0.1uf in series with a 2R7 between the output
and common? Or is that just a waste of time installing?

I know this is an antique, but a large part of my business is keeping
antiques going and fixing factory errors...

Thanks,

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


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.

<https://www.dropbox.com/s/ke2mii94rzblqat/NatSemi_AN-125.pdf?dl=1>

piglet

 

[John Robertson]

On 2019/08/14 12:28 p.m., piglet wrote:

On 14/08/2019 8:01 pm, John Robertson wrote:
On 2019/08/14 11:48 a.m., piglet wrote:
On 14/08/2019 6:33 pm, John Robertson wrote:
Does anyone have the National Applications Handbook #2 from 1977
that contains (I think) AN-125 - LM377, LM378, LM379 Dual Two, Four,
and Six Watt Power Amplifiers?

Usual and even unusual suspects don't have it. (Bitsavers,
archive.org, etc.)

I did find a very nice listing of the application notes that lists
not only the AN-xx(x) number but the title of the AN:

http://www.projects.scorchingbay.nz/dokuwiki/electronic/datasheet/national/archiveorg-an


Having a heck of a time tracking this down. I am checking the
Gottlieb pinball sound/speech sound board design (late 70s) around
the LM379 and have already found at least one error in their
circuitry and with AN-125 I hope to see if there is anything else I
can do to hunt down stray parasitics as the audio IC is heating up
even when idle and I recall it always ran a bit warm.  Scoping
doesn't show any obvious noise.

Schematic here:

https://www.flippers.com/images/Gottlieb/TechTips/LM379S_Gottlieb_Sys80_SND.png


According to the LM378/9 spec sheet the first error was that pin 14
should have had a 220ufd cap to ground, I have now added that. Next
we have an unused half of the 379s so I tied pin 6 via 100K to pin
14. Should I also ground pin 7?

PDF of the LM379:

https://www.flippers.com/pdfs/LM379_AMP.pdf

Lastly, even though the NS Power Amplifiers section of their 1980
Audio Radio Handbook says it is not required on the LM378/9 chips,
can it hurt to add a 0.1uf in series with a 2R7 between the output
and common? Or is that just a waste of time installing?

I know this is an antique, but a large part of my business is
keeping antiques going and fixing factory errors...

Thanks,

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


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.

https://www.dropbox.com/s/ke2mii94rzblqat/NatSemi_AN-125.pdf?dl=1

piglet

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 :-#)#

 

[Winfield Hill]

John Robertson wrote...

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.

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

 

[John Robertson]

On 2019/08/14 3:12 p.m., Winfield Hill wrote:

John Robertson wrote...

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.

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

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:

https://en.wikipedia.org/wiki/Zobel_network#Zobel_networks_and_loudspeaker_drivers

Thanks for confirming my assumption,

John :-#)#

 

[Phil Allison]

John Robertson wrote:

Winfield Hill wrote:

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



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

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

I'll check the math C'=L/R(2) when I get access to the speaker circuit
and get the DC resistance.

** Waste of effort - the series RC is not there to make the impedance resistive 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 parallel capacitance comes into play.

At some supersonic frequency, the reactance of a typical speaker is inductive or capacitive enough to cause phase rotation at the output of the driving amplifier - sometimes resulting in parasitic oscillations at particular current levels or at idle.

A series RC of a few ohms and 100nF swamps this effect and makes the amp stable again.

It would be a nice touch if speaker makers incorporated such RC networks inside every speaker box but AFAIK none do.




...... Phil

 

[Winfield Hill]

John Robertson wrote...

On 2019/08/14 3:12 p.m., Winfield Hill wrote:

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

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:

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

 

[George Herold]

On Wednesday, August 14, 2019 at 8:23:01 PM UTC-4, Phil Allison wrote:

John Robertson wrote:

Winfield Hill wrote:

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



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



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


I'll check the math C'=L/R(2) when I get access to the speaker circuit
and get the DC resistance.


** Waste of effort - the series RC is not there to make the impedance resistive 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 parallel capacitance comes into play.

At some supersonic frequency, the reactance of a typical speaker is inductive or capacitive enough to cause phase rotation at the output of the driving amplifier - sometimes resulting in parasitic oscillations at particular current levels or at idle.

A series RC of a few ohms and 100nF swamps this effect and makes the amp stable again.

It would be a nice touch if speaker makers incorporated such RC networks inside every speaker box but AFAIK none do.




..... Phil

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.

 

[bitrex]

On 8/14/19 1:56 PM, George Herold wrote:

Lastly, even though the NS Power Amplifiers section of their 1980 Audio
Radio Handbook says it is not required on the LM378/9 chips, can it hurt
to add a 0.1uf in series with a 2R7 between the output and common? Or is
that just a waste of time installing?

I know this is an antique, but a large part of my business is keeping
antiques going and fixing factory errors...

Thanks,

John :-#)#

Good google-fu..
https://www.technicalaudio.com/pdf/National_Semi/National_semi_Audio_Handbook_1976.pdf

GH

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

 

[John Robertson]

On 2019/08/14 5:36 p.m., Winfield Hill wrote:

John Robertson wrote...

On 2019/08/14 3:12 p.m., Winfield Hill wrote:

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

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:

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.

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 :-#)#