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Old 22nd May 2003, 01:46 PM   #1
MadMax is offline MadMax  New Zealand
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Default Do I need these

Ive got LM3876T based amp with RC network on input 1KR & 220pf shunt cap also Zobel network on output 2.7r &.1uf & 10r and .7 uh in parallel. can anyone help with info or threads on this
Cheers Max
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Old 23rd May 2003, 07:57 AM   #2
moamps is offline moamps  Croatia
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Default Re: Do I need these

Quote:
Originally posted by MadMax
Ive got LM3876T based amp with RC network on input 1KR & 220pf shunt cap also Zobel network on output 2.7r &.1uf & 10r and .7 uh in parallel. can anyone help with info or threads on this
Cheers Max

Hi,
Please, download datasheet from National, you will find some good information.

From datasheet:

14. RSN (2E7)
Works with CSN to stabilize the output stage by creating a pole that eliminates high frequency oscillations.
15. CSN (0.1uF)
Works with RSN to stabilize the output stage by creating a pole that eliminates high frequency oscillations.
fc = 1/(2pRSNCSN)
16. L (0.7nH)
Provides high impedance at high frequencies so that R may decouple a highly capacitive load
17. R (10R)
and reduce the Q of the series resonant circuit due to capacitive load. Also provides a low impedance at low frequencies to short out R and pass audio signals to the load.

Disconnect all (bypass L,R, omit RSN and CSN), and then if amp stays stable, you don't need them.

Regards

P.S. GC gurus, where are you?? Initial post is almost 1 day old.
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Old 23rd May 2003, 10:12 AM   #3
diyAudio Retiree
 
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Unhappy Calling all Gurus.... dangerous advice at 3rd and Main

"'GC gurus, where are you?? Initial post is almost 1 day old."

Gurus really not required for these basic questions. I do know THE Guru on GC type amps though, and he gave me some good advice from his experience designing them.



"14. RSN (2E7)
Works with CSN to stabilize the output stage by creating a pole that eliminates high frequency oscillations'

20 Megaohms(2 times 10 to the 7th power by that notation.....)! Are you sure?

'Disconnect all (bypass L,R, omit RSN and CSN), and then if amp stays stable, you don't need them."

If will if you're actually interested in what it sounds like rather than the minor achievement of only keeping it from oscillation. Keep the input filter (aim for Fc at about 100 Khz taking cable capacitance and preamp output impedance into account) and use polystyrene cap if possible. Keep the RC Zobel on the output and 2.7 ohms and 0.1 uF is fine. Put the zobel right at the Chip out put and not ay binding post. Use short leads and return Ground connection directly to Local decoupling caps ground connection. Nelson says output inductors don't do much and I would leave it out also. Who am I to dispute the ZEN master Pass about output inductors.


Photo from GC Master ScottyN.-san
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Old 23rd May 2003, 12:01 PM   #4
moamps is offline moamps  Croatia
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Default Re: Calling all Gurus.... dangerous advice at 3rd and Main

Quote:
Originally posted by Fred Dieckmann
[B"14. RSN (2E7)
Works with CSN to stabilize the output stage by creating a pole that eliminates high frequency oscillations'
20 Megaohms(2 times 10 to the 7th power by that notation.....)!
Are you sure?[/B]
When I was young "E" was same letter like ohm-omega, now is "R" correct. My mistake. 2R7 is 2E7 for me.
Anyway, letter for exponent is "e" not "E", isn't?

Quote:
[i]
If will if you're actually interested in what it sounds like rather than the minor achievement of only keeping it from oscillation. Keep the input filter (aim for Fc at about 100 Khz taking cable capacitance and preamp output impedance into account) and use polystyrene cap if possible. Keep the RC Zobel on the output and 2.7 ohms and 0.1 uF is fine. Put the zobel right at the Chip out put and not ay binding post. Use short leads and return Ground connection directly to Local decoupling caps ground connection. Nelson says output inductors don't do much and I would leave it out also. Who am I to dispute the ZEN master Pass about output inductors.[/B]
Why this quote to my post? MadMax use LM3876, slightly different chip then LM3875, on your picture.
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Old 23rd May 2003, 12:24 PM   #5
diyAudio Retiree
 
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Wink maybe I've built mo' amps than you have....... maybe not

I think it is pretty reasonable advice for most of the gain clone chips and solid state amps in general. There is a pretty wide margin between oscillation and good design approaches to fighting RF interference from getting into the amp and getting the stability solid. Getting the Zobel in the right spot is definitely important. You can take up the inductor with Mr. Pass. My own personal Guru has built lots of these and most of the advice is inline with his findings. Sorry about having a little fun with you on this one, but I still stand on my previous post. That's what is fun about simple amps like these is that it easy to try these things and hear the results. I was suprised at how sensitive they are to all this stuff when I built mine.

Bygones?
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Old 24th May 2003, 11:34 PM   #6
MadMax is offline MadMax  New Zealand
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Unhappy Iam confused now

Thanks for replies, Iam a bit confused as to what its safe to try, Iam using these amps for tweets and mids in active setup so no xo between amps and drivers, whats best way to test mods for unstablity.
Cheers Max
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