Gainclone output resistor

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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
 
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
 
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/hifi_pics/hifi_100pr/qrp01r0_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.
 
JojoD818 said:
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.
 
i ran a multisim bode plot (1Hz to 100kHz) using 10R and 0.1R resistances with 0.7uH inductor -- the load was the ESP simulated speaker, with simulated inductances, capacitance and resistance for 10 feet of Monster Cable -- the "gain" is the difference in gain between the two versions:
 

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Hybrid fourdoor said:
Whats the wattage rating on the resistors if using <0.5 Ohm? 5W? 3W?


Since most of the current is going through the choke at low/mid freqs, the resistor can be small wattage. If you use the resistor as 'body' for the coil just chose the resistor from that pov. The resistor DOES carry the current for high freqs, but these signal levels are very low (avarage) anyway, so again the resistor choice is more determined by the required physical dimentions. A 1 W res would do anyway.

Jan Didden
 
peranders said:

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.


Hi peranders,

I am currently building my latest stereo GC project that uses only one IC per channel and already bought a pair of 0.1 and 0.22 ohm 5W resistors and several 10 ohms 2 watts. I also have some spare magnet wires from a passive crossover network project so I have the things that I need. I am seriously considering the inductor.

My last question is, for capacitive loads (such as cables), is the inductor only any good?

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

This small value resistor at the output doers nothing to protect the amp from capacitive loads.
GC is very sensitive to this kind of loads.Take a pair of speaker cables that measure more than 1000p (there are many on the market) and connect "not easy" speakers withe them to GC.It will crack /distort like hell no matter the resistor is there or it isn't.
Only Zobel fixes that ,but it makes the amp sound worse.

Bartek
 
This small value resistor at the output doers nothing to protect the amp from capacitive loads.

I am sorry "zygibajt", but a small resistor (0.1 - 0.5 ohm) does indeed protect your amp from capacitive loads.

Try the following experiment. Take a amp with no kind of output resistor or inductor. Connect it to a resistive load, like 8 ohm. So far you should be fine. Now connect a capacitor at about 100 nF across the resistive load. If you are "lucky" you can see evidence of unstability as sign of oscillation in the 100-500 kHz area. You might have to "adjust" capacitor to make it happen.
(with your "crack /distort like hell " it should not to to difficult)

When you have the "oscillation" put the small resistor between the amp and load. You "oscillation" are completely gone.:)

Only Zobel fixes that ,but it makes the amp sound worse.

Hmmmmmmmmmmmmmmmm, how surprisingsly it may sound, the Zobel network "protects" the amplifier from inductive loads and I have a very hard time seeing how it can make the amplifier sound worse, so please explain.:xeye:

Thomas
 
All I'm saying is based on my experiments with GC,not theory.
GC doesn't like high capacity speaker cables,and with some speakers going with the cables higher than some 400-500p makes GC crack and distort.I have some audophile quality cables that measure 1,4nF for 3meters long.It's impossible to listen to the GC on that cables,it distorts like hell.In every case of GC distorting small series resistor made no diffrance.Only 100n in series with 1ohm resistor to ground fixes this.With Zobel at the output even those high capacity cables sound fine,but that Zobel makes the amp sound worse,checked it many times.


Bartek
 
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