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Old 1st July 2008, 01:05 AM   #1
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Default Function of Output Inductor

I am new to amplifier circuit design so please excuse my ignorance.

I noticed that some amplifier circuit have an output inductor parallel with a 8 to 10 ohm resistor. Could someone educate me as to the function of the inductor? What is the benefit of this part? For a high current amplifier driving a low impedence load, this part would seem to contribute considerable amout of DC resistance or cause the inductor to go into saturation. I may be mistaken about this saturation stuff since it is an air core inductor.
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Old 1st July 2008, 01:09 AM   #2
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air core inductors cant saturate. Thats why they are used in passive Xovers (and a few other kinds of circuits). I don't do much in the way of SS amps outside of gainclones so I cant tell you what the circuit does or what it is for.
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Old 1st July 2008, 03:26 AM   #3
Eva is offline Eva  Spain
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The inductor is there to isolate the internal feedback loop of the amplifier from external RF signals picked up by speaker wiring, crossover coils, driver voice coils, etc.. and from the own RF resonances of these elements.

Above audio frequencies, the internal RC dummy load should always dominate against external loads.
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Old 1st July 2008, 04:12 AM   #4
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Inductor isolates the capacitive loads from the feedback loop.

There goes to show that opinion is like an A** hole, everybody have one!
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Old 1st July 2008, 04:47 PM   #5
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Thanks to all who replied. Okay, I am beginning to understand better now. The inductor is to prevent amplifier oscillation when driving "difficult" loads that have high capacitance.

I would conclude that amplifiers with small or no feedback would be less susceptable to such speaker load and could do away with it.

Anyone know what a recommended value of inductance it should be?
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Old 1st July 2008, 05:41 PM   #6
fotios is offline fotios  Greece
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Quote:
Originally posted by caesar148
Thanks to all who replied. Okay, I am beginning to understand better now. The inductor is to prevent amplifier oscillation when driving "difficult" loads that have high capacitance.

I would conclude that amplifiers with small or no feedback would be less susceptable to such speaker load and could do away with it.

Anyone know what a recommended value of inductance it should be?
In the quoted graph bellow, appears the ringing caused from a resistive 8 dummy load paralleled with a 2F capacitor connected and damped via a 8H inductor in parallel with a 4,7 resistor in the output of amplifier. The test signal is a square wave.

Click the image to open in full size.
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Old 1st July 2008, 08:06 PM   #7
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Quote:
Originally posted by Eva
The inductor is there to isolate the internal feedback loop of the amplifier from external RF signals picked up by speaker wiring, crossover coils, driver voice coils, etc.. and from the own RF resonances of these elements.

Above audio frequencies, the internal RC dummy load should always dominate against external loads.


RF picked up by the speaker wiring ?
How on earth is a 4 ohm impedance going to pick up RF ?

Yes have an RF filter on the high impedance input but never on the output.

You can get oscilation on the output due to capacitance and inductors ringing hence adding an inductor to decouple it from the feedback loop.
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Old 1st July 2008, 09:01 PM   #8
kevinkr is offline kevinkr  United States
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Actually RF very frequently couples effectively into the low impedance speaker wiring, your amplifier output may not look like a low impedance (and your speakers don't either) at a couple of MHz and above where even a few inches of wire may have an effect. Often rf picked up on the output wiring will couple through the feedback network particularly if there is a cap across the feedback resistor right back into the input stage which in most cases will rectify it, resulting in unwanted interference with the audio signal at the output.

Jfet based inputs generally don't exhibit this problem to the same degree as bipolar inputs. (no intrinsic diode junctions and higher slew rates, etc.)

This was an all too common problem at the large mid-fi manufacturer I spent most of the last decade at. Extensive testing was done in RF shielded chambers to mitigate this problem.

The inductor is there primarily to isolate capacitance from the output and feedback loop, but it may stop some rf from getting back to the input stage of the amplifier.
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Old 1st July 2008, 10:30 PM   #9
CBS240 is offline CBS240  United States
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I agree.

For a typical amp, 100Wrms, I just wrap some 22WGA magnet wire around a 10 Ohm 1/2W resistor and solder it at the ends. Use a 1/2W metal film resistor in series with the 100nF cap for the Zobel before the inductor. Don't think AC current can't flow through the 100nF cap because its value is small. I used a cheap 1/4W 11 Ohm carbon resistor in series with 100nF at the output of a test amp I built. I was fiddling with it once while it was on and it broke out into full RF oscillation and the 1/4W resistor suddenly burst into flames and burned up. . Once the funky smoke cleared, I replaced it with a more suitable component.
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Old 1st July 2008, 10:43 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by CBS240
I agree.

<snip>

Don't think AC current can't flow through the 100nF cap because its value is small. I used a cheap 1/4W 11 Ohm carbon resistor in series with 100nF at the output of a test amp I built. I was fiddling with it once while it was on and it broke out into full RF oscillation and the 1/4W resistor suddenly burst into flames and burned up. . Once the funky smoke cleared, I replaced it with a more suitable component.

Me too! (and more than once) Quite exciting...
Don't use fusible metal oxide resistors either... You could loose your zobel network at an inopportune time..
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