When an output inductor is needed???

[Leolabs]

Depends on the output stage or the load????

[lineup]

Originally posted by Leolabs

When is an output inductor is needed???
Depends on the output stage or the load????

We can say it depends on BOTH.

1. how stable the amplifier is, into capacitive loads
2. if the load is considerably Capacitive

Most loads: loudspeakers + loudspeaker cables are Inductive
This is why a Zobel Filter ( f.ex: 10ohm + 100nF in series )
is always a good recommendation.

To be safe, if somebody some time happens to attach a Capacitive load
usually, at least in commercial amplifiers,
one ~1 uH inductor is also attached across and in parallel with ~10ohm resistor.

See my attachment, of the output of Professor Leach Super Amp.


This inductor is many times not needed.
Nelson Pass has once told, he almost never have needed to use inductor at output.

Regards
lineup

[lumanauw]

Even if the amp is stable from capacitive load, output inductor provides another advantage. That is to "filter" anything HF garbage from entering to the base of inverting differential.
HF garbage will make intermodulation, even if the amp is stable, it will affect the sound. Output filter will not "allow" this HF garbage entering the feedback system.

[AndrewT]

Hi,
the full Thiel network should be fitted every time unless the designer can demonstate to himself that some or all parts can be deleted.

In the Leach link r50+c25 can be fitted either before or after the R//L pair.

[ilimzn]

Amps that have a default resistive character to the output, most often don't need the inductor (but the Yobel is always recomended). Musical Fidelity A1 comes to mind, as well as some Naims - the trick is that there are effectively or literally (Naim!) series resistors in the output so current rise is limited for capacitive load.

[lineup]

Originally posted by AndrewT

In the Leach link r50+c25 can be fitted either before or after the R//L pair.

Professor Leach have a comment to this.
He is too clever ( professor ) to do things by chance.



Regards to AndrewT
lineup

[thanh]

Hi Lineup and AndrewT!
Zobel circuit can cause ocssialtion! I have experiented this thing.
Now, my amp have two zobel circuit which is place into both side of inductor but my out transistors are't violently hot
Do a AC sweep simulation by a sofware simulation to view voltage on both side of inductor, I agree with lumanauw.
My amp include : inductor is paralell with 1K ohms

[lineup]

Originally posted by thanh

Zobel circuit can cause oscillation!
I have experiented this thing.

Now, my amp have two zobel circuit which is place into both side of inductor but my out transistors are't violently hot
Do a AC sweep simulation by a sofware simulation to view voltage on both side of inductor, I agree with lumanauw.
My amp include : inductor is paralell with 1K ohms

I wonder what can make a Zobel cause oscillation ....
Are you sure Zobel did ... because there are usually other things
that cause oscillation


My experience is, ONLY this :
Zobel can help to make my amplifier stable.


Regards
lineup

[thanh]

oh! Leach also said
" On the first amplifier I built, I had these on the circuit board where they caused violent oscillations because the current through R50 and C25 generated positive feedback when it flowed through the circuit board signal ground lead. When the oscillations occurred, the heat sinks would get very hot. Moving R50 and C25 to the loudspeaker output binding posts solved this problem.
"
http://users.ece.gatech.edu/~mleach/lowtim/output.html

[AndrewT]

Hi,
adding a Thiel network will cause a damped ripple on test square wave. This is a natural consequence of the inductors and capacitors on the output.
IT IS NOT AMPLIFIER OSCILLATION.

put the zobel in the wrong place on the PCB and it may contaminate a low signal, thus causing amp-lifier oscillation as Leach has described and many other warn against.

The Thiel network can be R+C // {series (load +R//L)}
or
R//L in series with { R+C}//load,

the network behaves in both forms. The component values are different.

Thiel considers both forms in his equations. Dr Cherry expands on this to show how to scale the components and how to incorporate the whole family of networks onto the output.

attachment of an excel spreadsheet showing the calculation and arrangement of components for the Dr. Cherry treatment of the extended Thiele equations.