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Old 5th June 2009, 12:21 PM   #1
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Default Zobel networks

Hi all,

just looking at designing in my zobel network to the output stage of my class D amp. A few questions before I continue.

1. Everyone seems to stress how important it is to flatten your impedance in the higher frequency range near the crossover band. What I don't get is, if there is a rise in impedance which causes some phase shift and a bit of additional impedance at the crossover band, how can it possibly result in such huge issues as people seem to say? Keep in mind I've never actually had the chance to compare the differences, so perhaps somebody can put its importance into better terms than "it messes with your crossover".

2. How does one determine the power density of your average audio signal to know how to size the resistor? 1W, 10W? I am designing a 200W/channel amp and have referenced a similar 200W design using a 1W resistor. Considering that if the impedance of the speaker itself rises to say 200-300%, that means the zobel is taking most of that high frequency energy and 1W seems hardly enough.

3. Is another key reason for a zobel network in class D stability on the output filter?

Many thanks in advance.

James
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Old 10th June 2009, 04:04 AM   #2
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Is this a silly question or am I perhaps barking up the wrong forum?

Thanks
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Old 10th June 2009, 07:31 AM   #3
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LC lowpass filters are designed for a given load resistance, so they have a desired frequency response only at a ceratain load R.
If load R is lower, they are just less steep, which is not a big problem, but if the load has too high resistance and/or series inductance (as most speakers do), they have a bump in frequency response.
That's why no-post-filter-feedback class D amps without zobel boost hights, even by several dB.
Zobel is there to provide low enough resistance to ground at audio hights and resonance frequency.
Play a bit with simulating simple LCR ciruits and you will see.
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Old 10th June 2009, 07:57 PM   #4
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LC filters without load result in a big resonant peak, only limited by parasitic losses in L and C.

Some amplifier control schemes require RC networks to provide a worst case maximum load resistance and limit the amount of peaking. The resistive component of the load damps the resonant characteristic of the filter.

Phase response of the filter is also affected. Some post-filter-feedback schemes require a maximum load resistance for proper phase margin (to avoid the phase getting too close to 180 degree at a too low frequency).
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Old 10th June 2009, 11:46 PM   #5
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Thanks guys,

I'll muck about in the simulator.

Any idea on how one determines the power density the resistor in the zobel needs to carry?

By looking at a few speaker impedance curves, it seems not uncommon to expect the real resistive impedance of the zobel to be 1/3 of the inductive impedance in the speaker as it rises. As such, the zobel network is actually taking a fair amount of the 'real power' above and beyond its designated frequency, say 1kHz. How can I make a good estimate on how to size the resistor to handle the power it must dissipate as heat? I am assuming there

In one example I saw, a 1W resistor was in the zobel for a 200W amp, which implies the power density outside of the woofer range is really quite small. 1W just seems to small to me.
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Old 11th June 2009, 02:31 AM   #6
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Note that you only need the RC network to act at and above filter resonance. In practice this means that the imopedance of the C only becomes low above 10Khz or so and the resistor only starts to dissipate substantial power above that frequency too.
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Old 14th June 2009, 03:18 AM   #7
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That seems to disagree with some literature I have read on the subject. The literature emphasized that you try to keep the impedance flat 'after' the woofer's resonance peak. The inductive rise of the mid-high range drivers seems to always pick up very shortly after that.

Perhaps this literature was aiming to use the zobel more for the passive crossover than it was the LC low pass on the amplifier output?
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Old 15th June 2009, 08:04 AM   #8
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The Zobel network that you might place at the output of a class-d amplifier serves a different purpose from the inductance-compensation Zobel that you might put at the input to a passive loudspeaker crossover network. (Which is simply to flatten the load impedance).

On a pre-filter NFB class-d amplifier the network reduces the Q of the filter at resonance, which reduces the peaking of the filter into a high-impedance load, thus providing a flatter frequency response.

In a post-filter NFB UCD amplifier, the Zobel may still be necessary to ensure that the oscillation remains stable into a high-impedance load (such as you might get with a transformer-coupled 100V output amplifer with only a few speakers connected at low-power tappings). In this case the filter phase response without the Zobel might have a very sharp transition from 0 to 180 degrees at the filter resonance frequency. A UCD amplifer could go into a much lower frequency of self-oscillation if operated like this. The Zobel slows-down the rate-of-change of phase when operated into a high impedance. This helps the overall loop response to provide a steeper slope on the phase as it passes through the 180 degree point (the desired self-oscillation frequency) A typical Zobel here might be 10R + 220nF.
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