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Old 12th May 2006, 05:48 PM   #1
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Default the zobel

The zobel is just the RC circuit or the RC + LC?

I was recently doing a pcb layout but had the problem of too tight space. I had to go thru the LC first and then the RC. Does that matter?

As far as I can understand, the whole circuit was not affected but the order they are in might affect their own operationi. Is that true?

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Old 12th May 2006, 06:21 PM   #2
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It could matter.
The normal purpose of the RC is to present the amplifier with a resistive load at high frequencies so that it does not become unstable. If you put an L between the amp and the RC the amp will see an inductive load at HF.

This might not be a problem but you'll need to check the circuit is stable.
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Old 12th May 2006, 06:43 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
the combination of series L//R with the load and R+C parallel with the load is a Thiel network. The Zobel is just the R+C parallel part.

Dr Cherry did a good paper on the Thiel and a whole family of circuit values that can be used.

The RC part of Thiel can be on either end of the inductor and the amp still sees the load since it can pass the signal through the R of the LR pair to the RC part.

Although not discussed by Cherry I have seen a few using a double RC locating an RC at both ends of the inductor. It appears that reducing the C to C/2 and putting both in gives an acceptable network. This Pi network is also better at rejecting speaker cable interference from the NFB loop entry point.

BTW there is no PCB space issue. Many recommend mounting either the Zobel or the Thiel on the back of the speaker terminals. Most also recommend that the ground leg of the Zobel should NOT be taken to the signal ground. It is OK. to take it to the speaker return.
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Old 12th May 2006, 07:39 PM   #4
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Quote:
Many recommend mounting either the Zobel or the Thiel on the back of the speaker terminals.
Assuming that the inductance of the wiring to the speaker terminals is not significant.
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Old 12th May 2006, 07:52 PM   #5
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Zobel just another name for a type of conjugate matching circuit. Can be either series or parallel, mainly used by adding the dual of the reactive element to be absorbed to give the desired impedance usually real or resistive.
Andrew is correct in regards to PA outputs. The inductor resistor network first to isolate the NFB loop from loads that could cause instabilities. The series RC after to provide a load at higher frequencies. The series RC network is technically the zobel.
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Old 13th May 2006, 07:17 AM   #6
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Traderbam is correct in saying the amp will see an inductive load at HF. Since this is high impedance at HF and you must present the amp with a low resistance at HF the order is important.
The R-C network slugs the output to compensate for the poor HF response of the PNP transistor in the output stage of IC power amps like the TDA2030 and LM3886 It is vital for stability.
Discrete designs have better PNP's so do not need so much compensation.
If you do full power testing at 20KHz or higher the power dissipation in the zobel resistor on IC amp designs can become an embarassment!
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Old 13th May 2006, 09:14 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi Consort,
have another look at the Thiel Network.

there are two parallel routes for current out of the amp.
1. through the inductor (L) to whatever follows.
2. through the resistor(R1) to whatever follows.

route 1 has a rising impedance with frequency.
route 2 has constant impedance (if low inductance resistors are used)

At high frequency route 2 becomes dominant.

After passing through the R//L the next choice is
3. through the speaker to ground.
4. through the Zobel R2+C to ground.

At lower frequencies route 3 is your sound output. At high frequency route 4 becomes the high current escape route.

For stability the amp may need a high frequency load. This will consist of route2 + route4. i.e. R1+R2+C
This high frequency load is just as valid as the alternative Thiel option with the R+C before the inductor.
Thiel discusses both options in his paper.
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Old 13th May 2006, 10:51 AM   #8
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Andrew,
I had in my mind the more common output circuit where the shunt RC is followed by a series L. If you put the series L first followed by the shunt RC you have the situation described in my post. The L of course is to isolate the amp from capacitive loads
Wouldn't the Thiele network be part of the speaker design rather than the amplifier? I must admit to not having read Thiele,s or Cherrys paper, even though I am a fan of both their work. There was a popular 2 transistor feedback amp called a Cherry pair which worked very well for RF type circuits
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Old 13th May 2006, 02:23 PM   #9
AndrewT is offline AndrewT  Scotland
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But the series L is parallel to a damping resistor.
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Old 13th May 2006, 03:56 PM   #10
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I think the key is to analyze the output L//R and zobel at the frequency where the PA's closed loop gain crosses zero. Optimize closed loop phase margin for required range of reactive loads.
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