why is there an inductor on my class ab amp output?

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#22
scoobis said:
maybe someone can clue me into why a coil is otherwise a better solution other then the power loss savings over a resistor. At this point it is the only reason i know and why i think using the right series resistance for a design is the right way to do it.
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I had someone point out the obvious to me on the coil....one of those slap your forehead moments cause its so obvious ... so i had to post it..

simply the coil allows a by pass path around the resistor at lower frequencies .. specifically in the audio spectrum.

so it is just a better way of using a series resistance to prevent instabilities with a capacitive load without the downside of it reducing the audio output power.

brilliant.
 
#25
phase said:
Search for “output zobel”, is put there as a safeguard against oscillation.
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The inductor is separate from with the Zobel network, it is there to isolate the amp from capacitive loads (such as electrostatic speakers or long speaker leads) that could otherwise cause HF oscillation. [ In an active speaker there typically isn't need for this inductor as the leads are short and little capacitance is present. ]

The Zobel network comes before any inductor and makes the amp itself stable under inductive load (or no load), typically preventing VHF instability.

The combination of output circuitry basically presents a mainly resistive load at high frequencies, meaning the amp doesn't have to deal with heavily reactive or varying loads up where oscillation might be an issue.
 
#26
A couple of additional points.
The inductor limits the peak current into a capacitive load with fast rise time signals.

The Zobel directly at the amplifier output helps stability during power up in an unloaded condition. In many cases it worsens the phase margin and may not be included if amp stability is not an issue, or has relay delay. It may be included after the inductor for cable equalization at RF with a another recommended at the speaker.
 
#29
Most of the designs are OK with an inductor.

I always like to think that inductor is there for an "EMP" purpose😀.

If you think about it, it is paralleled with the ground, so it is definitely messing with the amplifier output impedance at much "higher frequency s" 😛 Lagging the signals and amplifying them at certain point in time with a much stronger force 😛

Some amps can sound totally different when an inductor places right near the output transistors (shortest path from emitter connections).
 
#30
What would be the drawbacks if any for using the PCB and trace length to accomplish similar performance?

I have a bit of PCB real estate that I can use to run a trace, folded back even, to create both a resistance and some inductance. Using a 4 layer board, I can even fold the traces back on top of each other.

Any insight on a design like this?
I've come across calculators for spiral traces, but I would prefer something more linear and layered over one another. I can do 5inch strips or longer if necessary.

S
 
#31
You will need to design it to take the full fault current of the amplifier. Also, be aware that the mage fields coming off the output conductor can couple into the amplifier stages and cause distortion - so either you fix this with the correct orientation, or you move it off board. some amplifier locate the output L at the output terminals for example. for the Zobel, it makes more sense to keep this on the amplifier board (although I have some designs with it on the PSU board).
 
#33
Ok

To take this a little further, lets say that we have a SSR at the output as well, providing a resistance of about 10mOhm

Is this SSR better suited before or after the inductor?
Then, at which SSR resistance would we find the inductors usefulness diminished?

In general, I know the rule is to have the inductor at speaker terminal entry, however, is this still the case when incorporating an output relay such as an SSR?
 
#34
Suppose I am whipping on a dead horse here perhaps..

Another idea that I am growing closer to adopting is using a ferrite with a few loops of the speaker wire through it at the connections on the chassis, instead of the inductor at the interior.
Seems to be an even bigger solution and maybe even easier.
 
#35

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