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Old 1st May 2004, 05:06 AM   #1
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Default Gainclone output resistor

Greetings,

I did a search on this and came up empty, so I'm posting it
here. Why do all the diagrams that I see call for a 0.22
ohm (or R22) resistor on the output, after the feedback
loop? From the descriptions, it is there to control HF
oscillation, but that doesn't make sense to me.

NS's data sheet suggests a 10 Ohm resistor in parallel with
a 0.7 mH choke. Simpler yet is to take a length of insulated
wire (magnet wire works best,) and coil it around a 10 ohm
resistor, and then solder it to the resistor at both ends.
This arrangement passes low frequencies through the coil,
but damps out high frequencies better than a R22 alone would.

Any ideas on why this is used? Any one out there tried an
A/B with each one? Any differences? Or could one just
drop the whole thing if the feedback loop has a lowpass
somewhere in it?

Regards,
aeronaut
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Old 5th May 2004, 08:10 PM   #2
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No wonder you are confused

You need to "protect" your power amp from capacitive loads that would cause instability (typically "ringing" ).

This is normally done with a 10 ohm resistor in parallel with a choke. This is what you see most of the times.

A other way to do it, is to put a small resistor, like 0,1 ohm at the output instead. You then don't have to make a small choke. However with the use of a single small resistor compared to the 10 ohm//choke you do lose some efficiency (=output power). If you keep your resistor small this will never be a problem.
Your so called Damping Factor will also only be "so big" (also not a problem)

You can NOT "substitute" this form of output resistor/choke with a lowpass filter in your feedback path.

But you can try to completely remove your output resistor/choke and see if the combination of your amp, speakerwire and speaker are OK (but you are likely to still have som "ringing"). If yes, you can leave it out, but if you change anything you have to "test" again (I find it much easier just to use a small resistor (like 0,1 om) in the output and leave it in at all times.

If you start paralleling your amps you also need a small resistor in your output.


Hope this could help you.

Have fun


Thomas
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Old 6th May 2004, 08:12 AM   #3
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Hi.

I have a question about that choke.
How dop you make it?
I have just taked a trafo wire and roll it over a pencil 20 times.
I realy doubt that choke is 0,7mH...
any formula or tutorial how to make the right choke?
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Old 6th May 2004, 09:45 AM   #4
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The inductance value you think you have got is very much likely and not very critical I would say.

I have made a couple of inductors with this look:

http://home5.swipnet.se/~w-50674/hif...0_overview.jpg

14 turns, 1.5 mm Cu wire, diam. 8 mm => 0.7 uH
14 turns, 1.5 mm Cu wire, diam. 10 mm => 1 uH
17 turns, 1.3 mm Cu wire, diam. 8 mm => 1 uH
17 turns, 1.3 mm Cu wire, diam. 10 mm => 1.4 uH


The wire is the same type as for motor and transformer windings.

I used a plain philips screwdriver for the winding.

If anyone want this type of inductors, with 1.3 or 1.5mm wire and 8 or 10 mm diam, drop me a note.
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Old 6th May 2004, 09:48 AM   #5
djQUAN is offline djQUAN  Philippines
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is that inside diameter or outside diameter?
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Old 6th May 2004, 09:54 AM   #6
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Whats the wattage rating on the resistors if using <0.5 Ohm? 5W? 3W?
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Old 6th May 2004, 09:59 AM   #7
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you could easily calculate it from the power rating of the amp. for about .1 ohm, you might be able to get away with a 5 watt for a 250 watt amp......
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Old 6th May 2004, 10:04 AM   #8
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Hi guys,

Just curious, which one is better? Small resistance resistor (0.1-0.22), a 10 ohm resistor with magnet wires around it, or just a magnet wire formed in a typical screwdriver?

JojoD
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Old 6th May 2004, 10:30 AM   #9
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Quote:
Originally posted by djQUAN
is that inside diameter or outside diameter?
Inside
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Old 6th May 2004, 10:34 AM   #10
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Quote:
Originally posted by JojoD818
Hi guys,

Just curious, which one is better? Small resistance resistor (0.1-0.22), a 10 ohm resistor with magnet wires around it, or just a magnet wire formed in a typical screwdriver?
You MUST have resistors if you connect the LM3886 in parallel but if you only have one IC you can have nothing or the inductor. This depends of your load and cable lenght. You can have this resistor also if you wish but start with as little as possible at first. My examples gives you a DC resistance of 5 mohms.
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