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Old 14th January 2013, 06:14 PM   #11
Mooly is online now Mooly  United Kingdom
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Also very important.
I'm pretty sure the LM3886 should have a zobel network (a cap and resistor in series) from the output to ground to ensure stability. Typically values around 0.1uf and 10 ohm.
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Old 14th January 2013, 06:38 PM   #12
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Originally Posted by pacificblue View Post
So do I and probably most people with an engineering background.
The snubberised power supply by CarlosFM was hyped a lot in conjunction with the audiosector.com and chipamp.com kits. It is said to improve the sonic quality of those kits.
OK I get the deal!
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Old 14th January 2013, 07:24 PM   #13
blu_glo is offline blu_glo  England
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Originally Posted by mxriddler View Post
Thanks for the suggestion. I was just reading about buffered GCs and trying to determine if I needed that or not. I don't fully understand input/output impedance concepts yet.

Can you recommend a schematic for a buffer? I have lm4562s, opa2107s, and opa2132s on hand.
Input impedance of an integrated amplifier is usually 10-47 K ohms; its so it doesn't unduly load equipment connected to it such as CD players etc.

See attached; it seems all the opamps you specify are unity gain stable so hould be ok, so it's up to you which to use or put a socket and choose afterwards! Make R1 = R2 in attached circuit the other resistor not really needed in audio (short "+ input" to 0V) but if you insist, when R1 = R2, R3= 0.5 x R1 (or R2!)

Suggested values R1 and R2 = 22K R3 if insisted apon, 10K is quite close enough.
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Last edited by blu_glo; 14th January 2013 at 07:27 PM.
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Old 14th January 2013, 08:12 PM   #14
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Quote:
Originally Posted by Mooly View Post
Also very important.
I'm pretty sure the LM3886 should have a zobel network (a cap and resistor in series) from the output to ground to ensure stability. Typically values around 0.1uf and 10 ohm.
I agree its to do with the non-complimentary output stages in power IC design and also mentioned in Mullard transistor circuits books. Especially important if the amp is to be connected to long speaker wires.
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Old 14th January 2013, 09:30 PM   #15
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Quote:
Originally Posted by Mooly View Post
Also very important.
I'm pretty sure the LM3886 should have a zobel network (a cap and resistor in series) from the output to ground to ensure stability. Typically values around 0.1uf and 10 ohm.
I was planning on including the 0.7uH inductor and 10ohm resistor in parallel as shown in the suggested application. Is the resistor/cap to ground config better? Or is it a matter of which is easier? I do have 16aug magnet wire i was planning on using for the inductor.
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Old 15th January 2013, 06:17 AM   #16
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The resistor and cap to ground is a different thing altogether really. Many amps need those to ensure stability with no load (speakers) connected. Its a must have.

The output coil isn't always normally needed in practice. Its there to prevent small capacitive loads from causing instability... such as the long speaker wires blu_glo mentions. The 10 ohm across the coil damps any "ringing" caused when squarewave testing the amp. The coil isn't critical in value, just make sure its air spaced (not wound on a magnetic former) and of reasonably thick wire.

The latest diagram you have posted is a "single rail" configuration. Yes As long as you appreciate the differences there.
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Old 15th January 2013, 06:33 AM   #17
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Quote:
Originally Posted by Mooly View Post
The resistor and cap to ground is a different thing altogether really. Many amps need those to ensure stability with no load (speakers) connected. Its a must have.

The output coil isn't always normally needed in practice. Its there to prevent small capacitive loads from causing instability... such as the long speaker wires blu_glo mentions. The 10 ohm across the coil damps any "ringing" caused when squarewave testing the amp. The coil isn't critical in value, just make sure its air spaced (not wound on a magnetic former) and of reasonably thick wire.

The latest diagram you have posted is a "single rail" configuration. Yes As long as you appreciate the differences there.
Indeed.
- I do tend to still include the coil as it also stops demodulation of strong AM radio channels (yes they still exist) in the output devices. You'd have thought the Zobel network might have prevented this in theory.......
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Old 15th January 2013, 06:39 AM   #18
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AM signals could be an issue in some locations so yes, the coil will help. The impedance of the zobel network isn't low enough to "short" any AC hash that come in via the speaker leads. If you connect a good and sensitive scope across an open circuit speaker and its cable its surprising what you see. A 10 ohm shunt does nothing to dampen it.
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Old 15th January 2013, 11:37 AM   #19
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Quote:
Originally Posted by blu_glo View Post
.............. I do tend to still include the coil as it also stops demodulation of strong AM radio channels .................... You'd have thought the Zobel network might have prevented this in theory.......
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Originally Posted by Mooly View Post
AM signals ................ so yes, the coil will help. The impedance of the zobel network isn't low enough to "short" any AC hash that come in via the speaker leads......................
Use Dr Cherry tapping point for the NFB back to the -IN input.
That adds an extra pole of attenuation for RF injected into the speaker terminal.
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Old 15th January 2013, 12:01 PM   #20
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Originally Posted by AndrewT View Post
Use Dr Cherry tapping point for the NFB back to the -IN input.
That adds an extra pole of attenuation for RF injected into the speaker terminal.
'gis a clue .... who/whats Dr. Cherry???
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