Leolabs said:
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
Attachments
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.
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.
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.
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
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
thanh said:
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
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
" 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
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.
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.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.