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Old 2nd September 2013, 10:21 PM   #1
danny92 is offline danny92  Portugal
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Default Questions about output inductors

Hi everyone,

I've some questions to do about the output inductors, I think that they are used in conjuction with Zobel networks to prevent oscillations at the output of the amplifier, right?
Why they are used with a resistor in parallel? What is the mathematical expression used to calculate the values of both the resistor and the inductor, for some frequency cutoff value?
I only know this expression but it's for resistor in series with the inductor (f-3dB = R/(L*2*pi) [Hz]). What is the cutoff frequency that should be used to still have a good sonic quallity at 20kHz?
Some claim that the output inductor should be avoided, they are right or wrong?
The output coil must have no ferrite bar, right?

Best regards,
Daniel Almeida

Last edited by danny92; 2nd September 2013 at 10:25 PM.
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Old 2nd September 2013, 10:31 PM   #2
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danny92,

The output inductor basically serves to prevent the amplifier from 'seeing' a capacitive load. Its actual value is relatively unimportant, often cited as being between about 1uH to 7uH. I like to use about anywhere from 12 to 16 turns of #14 magnet wire.

The parallel resistor is there to damp any potential resonance and its value is often loosely specified in the 4 to 10 ohm range and is often chosen to be close in value to the speaker's nominal impedance.

To avoid saturation issues the coil needs to be air cored.

Some seem to prefer a design without an output coil but IMHO it serves to help keep an amplifier stable in the face of an adverse load. At the very least a Zobel should be fitted and sometimes a Pi network is seen, which is a Zobel on both the amplifier and speaker side of a damped coil.
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Old 2nd September 2013, 10:52 PM   #3
danny92 is offline danny92  Portugal
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Hi jkuetemann and thank you for your clarification about output coils,

Could you give me a good / common specific value (in inductance) for an output coil?
Another question if you have an inductor (that is by itself a low pass filter), why you need a zobel network after that, near to the speaker connection?

Best regards,
Daniel Almeida
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Old 2nd September 2013, 11:16 PM   #4
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Quote:
Originally Posted by danny92 View Post
Hi jkuetemann and thank you for your clarification about output coils,

Could you give me a good / common specific value (in inductance) for an output coil?
Another question if you have an inductor (that is by itself a low pass filter), why you need a zobel network after that, near to the speaker connection?

Best regards,
Daniel Almeida
I did, about 12 or so turns of #14 enamelled solid magnet wire, which yields about 1uH or so of inductance. More isn't really beneficial and from my reading on the subject generally 1-2uH is all that's needed.

The Zobel at the amplifier terminals ensures that a predictable impedance is presented to the amplifier at high frequencies and the Zobel at the speaker terminals is used to 'catch' or shunt HF interference that arrives via the speaker cables and is supplementary to the coil. The second Zobel isn't strictly needed, just sometimes seen in practice.
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