Why a Zobel network across OPT primary or secondary? - diyAudio
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Old 4th March 2005, 08:44 AM   #1
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Default Why a Zobel network across OPT primary or secondary?

I've tried to find out why some amp designs use a Zobel N/W across the output (e.g. HK Citation II, Alan Kimmel's UL amp), but I've been unable to find an explanation. Could someone please explain why such a network is needed?
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Old 4th March 2005, 08:50 AM   #2
Gluca is offline Gluca  Italy
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I belive they are there to damp unwanted spikes in the frequency response.

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Old 4th March 2005, 10:50 AM   #3
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zobel is C& R to the ground? to filter out all high frequency which is inaudible but can pass through (AM radio, for example)?
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Old 4th March 2005, 02:05 PM   #4
JoshK is offline JoshK  Canada
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Someone correct me if I am wrong, but typically Zobel networks are used in speakers to tame either uneven impedance curves or a mismatch in impedance. I believe, as Gluca said, that big spikes in impedance will cause uneven frequency response at the output of the amp and the zobel network evens out this effect.
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Old 4th March 2005, 02:24 PM   #5
rdf is offline rdf  Canada
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From another thread:

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Old 4th March 2005, 07:54 PM   #6
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It dampens resonance (seen in the squarewave response), particularly in gNFB amps where the transformer's inductance and capacitance contribute to loop phase shifts, causing a damped rise.

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Old 4th March 2005, 10:26 PM   #7
EC8010 is offline EC8010  United Kingdom
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If you choose the values correctly, a series CR circuit in parallel with a series LR circuit (O/P transformer DC resistance in series with leakege inductance) produces a purely resistive load at all frequencies. Since valve amplifiers use output devices with significant output resistance (ra), if the load resistance is not constant, it makes the frequency response deviate from flatness with frequency. This is usually tested by using a square wave to look for ringing (by far the quickest) or sine wave frequency response. Thus, as other posters have said, a Zobel network is there to prevent square wave ringing or peaks in the sine wave frequency response. The values have to be found by experiment.
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