output LR filter

[MarcelvdG]

If you want the amplifier to be small-signal stable with any passive load:

1. Determine the amplifier's output impedance without the L and R
2. Look for frequency regions where the real part of the output impedance goes negative. I'll assume there is a negative peak at frequency f_neg.
3. Choose an R that is greater than two times the opposite of the most negative real part of the output impedance, for example four times the opposite of the most negative real part of the output impedance
4. Connect an L in parallel with a reactance equal to R at f_neg
5. Determine the impedance of the amplifier with the L and R included and fine tune their values as needed to make the real part of the impedance positive at all frequencies

I took a more pragmatic approach when I designed my amplifier: I came up with a target for the magnitude of the output impedance at 20 kHz, divided that by 2 pi * 20 kHz to find the inductance, put a resistor in parallel with a value close to the nominal load impedance (8.2 ohm) and shunted the output with a capacitor with value C ~= L/R^2 to turn the whole thing into a first-order series filter. (Making it a first-order series filter was recommended in an article of A. N. Thiele, as it helps to keep RF signals out of the amplifier.) I have no idea if it is stable with any passive load, but it certainly is with all loudspeakers and cables I ever connected to it.

[kokoriantz]

Thank you for details. A standard amplifier exhibits resistive output impedance at low frequencies up to OLBW, than it becomes inductive. If a capacitive reactance comes on the output, the two opposite reactances in series provoke resonance. The serie LR will come in series with the output inductance to lower the resonance and damp it. The question is if I know my internal LR how to estimate the external one. Also to note that there is the Zobel which comes into account. If I understand, the Zobel, internal LR and the external LR must be link to each other, but how?

[OldDIY]

As a result, get a compensated active circuit. At least at critical frequencies.
All together is called Zobel

[MarcelvdG]

Originally Posted by kokoriantz

Thank you for details. A standard amplifier exhibits resistive output impedance at low frequencies up to OLBW, than it becomes inductive.

True, but at yet higher frequencies, the real part of the output impedance will usually go negative, which may cause oscillations with the wrong load impedance. That can be solved with either an RC series circuit across the output or an LR parallel circuit in series with the output. I haven't a clue why designers often do both.

[kokoriantz]

Most Japanese, McIntosh, put the Zobel after the LR. The Zobel is mandatory for high impedance current outputs as the OLG depends on the load. If the load is inductive, the amp oscillates. All explanations I found about Zobel by google, relate to speaker impedance. An amp is universal impedance.

[JMFahey]

Originally Posted by hifi

BTW: i once read an article on magnetic coupling between output coils in an amplifier....but i dont remember the conclusions or where i read it, Electronics World? ....

anybody?

/micke

Not sure there is any mutual coupling between these coils.

Too small, too few turns, too far from each other.