Zobel network/filter

[rex1232]

Hey guys 

I am a little bit confused on why you would use Coil in the zobel network circuit.
I understand that the zobel filter is there to "compensate" for the output impedance.

But why would you put a coil in when you already have the cap?

I have linked the zobel with the coil and one without the coil.

Thank you in advance 

[Gyro]

It looks like somebody's got a bit confused there.

It's normal to have a zobel network (series R and C) directly across an amplifier output so that it always sees a resistive load at high frequencies. This is often followed by parallel R and L, in series with the output to isolate it from capacitive loads. Both can aid stability.

I'm not sure how the network shown would work in practice, it would be hard to optimize the conflicting component values at least. It doesn't look worth it to save one resistor.

[magic]

According to Self

Capacitive loads may put the global feedback loop in oscillation so a coil isolates the amplifier from all the cables, crossovers etc.
Inductive loads may cause local oscillation in the output emitter followers, so a Zobel network provides a low impedance at RF path to ground. Since the output coil is an inductive load too, it has to come after the Zobel.

[Gyro]

According to Self

Bob Cordell too.

[T3sl4co1l]

If the output is coming directly from a low impedance amplifier, the RL is necessary to provide some matched resistance for the parallel capacitor to act against.  Otherwise it's just shorting out the amplifier, and the amplifier will win*.

*For small signals.  For large signals, it will go into current limiting, which manifests as slew rate limiting.  And, assuming it doesn't oscillate, which is a stability issue in poorly made designs.

Tim

[magic]

Bob Cordell too.

So it's settled

For all we know, this schematic could be somebody simulating stability with 150nF load in SPICE

edit
Or a clever trick to save one resistor, yeah, Gyro already mentioned it.

[gbaddeley]

The “with the coil” zobel is a load for an amplifier, as already explained.

The “without the coil” zobel across a loudspeaker is to compensate for its rising impedance at higher frequencies. This would be preceded by a passive crossover network, which works more effectively into a reasonable constant load impedance, hence the zobel. Very common on woofers.