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Old 9th November 2008, 08:34 AM   #1
chev350 is offline chev350  New Zealand
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Default Amp Stability techniques: Zobel et al.

Hi,

Solid State Amplifier stability


I was wondering if anyone here could share their knowledge on Audio Amplifier design; in particular the design of Zobel (aka boucherot cell – not sure why this term is used as this guy seemed more concerned with Power Systems and their stability) and series Parallel RL networks that are used to ensure amplifier stability.

I’m a student studying Electrical Engineering and I’ve got a project that looks at techniques to stabilise solid-state amplifiers. I decided to focuses on the design of Zobel Networks (series RC network across the output) and the use and specification of the series-parallel RL network; the network that is typically placed between the output and the speaker terminals of the amplifier.

I’ve read Self’s latest book on Audio Amplifier design and also glanced at others such as Randy Sloane's books.
Other than offering empirical advice, none provided a ‘scientific’ method or approach to selecting the right values, nor in particular the kinds of components to use. Comments like “…is about right…” is not really that helpful and don’t stand-up to scrutiny in the Academic environment that I’m in.

Sure, there are general comments like use a film capacitor and an air-cored inductor but, there are many different types of capacitor, and many different ways to make an air cored inductor that affect the Q and other characteristics.

I just wondered if someone here might have some insight on designing these kinds of stabilising network and could offer that advice.
It would be really interesting to understand the processes that are used to design and than test and verify that the selected components are right or suitable?

For my project I’ve got to right up the purpose and effects of these networks, a discussion on the tradeoffs with designs (e.g. impact on performance and what down-sides might exist or appear) and then the processes that are used to arrive at the right values.
If I get a chance I might even do some lab work….

From surfing around the web and looking at schematics of commercial amplifiers, there are a load of variations and around how these networks are applied.

Thanks in advance.


Paul.
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Old 9th November 2008, 11:34 AM   #2
AndrewT is offline AndrewT  Scotland
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try to find Dr Cherry's paper on his development of the Thiele Network.
It was in Electronics World jan95 & jul97
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regards Andrew T.
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Old 9th November 2008, 12:36 PM   #3
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There was a thread here not so long ago where this was discussed. Something that was shown was that you can make the output LR combination to not have a big input impedance peak with capacitive loads and then the series R-C on the input is used to counter the rising impedance of the LR network and provide a somewhat defined load at the critical frequencies around 1MHz.

With good component selection you get an impedance that varies very little between open load, shorted load and any capacitive load.
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Old 10th November 2008, 07:01 AM   #4
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Here's a big thread on the zobel...

Zobel Or No Zobel ?

many different opinions..

And the math (for Academia)..
http://users.ece.gatech.edu/~mleach/...oads/zobel.pdf
Very difficult thesis because most depends on the physical world
(the loudspeaker) I think self generalized it because it would of
added 200 pages to his book.

OS
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Old 10th November 2008, 01:37 PM   #5
chev350 is offline chev350  New Zealand
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Hi AndrewT,

Thanks for the info - I'll try and track that info down.

Hi megajocke,

Thanks also for your advice.

I too read a bunch of posts here on the use of the Zobel network and the series-parallel R-L network on the output.

I'm not sure which amplifier you had in mind that has a critical 1MHz frequency. Several amplifiers I recently played around with, one all FET, had a natural oscillation frequency of 80MHz. This amp burst into oscillation with a 1.3nF cap on the output and the RL network removed – I did try other values but this value seemed to give the best peaked response.
The other amplifier, a BJT design from Randy Sloane seemed to have a natural oscillation frequency around 10-15MHz. The range of frequency dropped as the amplifier output stage got hotter. With the R-L network removed placing a 100nF or larger cap across the output worked.

Another National GAINCLONE IC based design with a natural oscillation frequency of around 2.7-3.0 MHz. This was probably the most sensitive design – as soon as I removed the R-L network and excited the input with an impulse this circuit took off.

In all cases I excited the amplifiers with a step impulse of 0.01s pulse duration that varied between 0.2V IC, 0.65V for the BJT and 1.0V peak for the MOSFET design.

The FET amplifier has a series-parallel R-L combinations of 7.1uH and 25 Ohm. The BJT design has a the same network topology but with values of 0.7uH and 1 ohm. On the other hand the National Semiconductor design uses a 2.2uH and 5 ohm design.

All amplifiers had a Zobel network installed, in the case of the FET amplifier this had too networks, one either side of the series-parallel R-L combinations. For testing purposes I removed one side of the resistors from the circuit, effectively isolating these networks from the output stage of each amplifier.

I guess I can say that there isn’t one fit or necessarily one solution. Hence, my initial questions…

I really don't think that the selection of these parts or the 'right' or 'best' fit is obvious with empirical rules of thumb?


Thanks.

Paul.
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Old 10th November 2008, 01:54 PM   #6
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Here is the thread I was thinking of:

Function of Output Inductor


I was thinking of the unity gain crossover that often is around 1MHz and you'd want a pretty defined load there to make the loop stable. 80MHz sounds like local output stage oscillation. If you have a series RC on the amp side of a parallell LR you can (and probably should) design it so that impedance as seen by the amp doesn't depend much on load in the 1MHz region. Capacitive loads will resonate with the L and the parallell R has to be chosen to dampen this resonance.

If it is designed like this the impedance at >10MHz where you can get local oscillations will be practically load independent.

I wonder if a low inductance resistor is really needed in the RC-network, an emitter follower shouldn't do anything stupid with lightly inductive loads.
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Old 10th November 2008, 07:44 PM   #7
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Try to find information on power factor correction. A zobel network is supposed to make the load look resistive at all times, so there is no phase lag between voltages and currents.

A single zobel could be quite useless and you often find historical values of 10 ohms in series with 100 nF.

A single loudspeaker should be inductive, so is the cable feeding it, this inductor is a constant unless you remove windings from the speaker or adjust the cable length. The capacitance in the zobel is intended to counter the inductive reactance and make the load appear purely resistive and therefore you get maximum power transfer.
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Old 10th November 2008, 07:49 PM   #8
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Quote:
Originally posted by Nico Ras
[...]The capacitance in the zobel is intended to counter the inductive reactance and make the load appear purely resistive and therefore you get maximum power transfer.

That is not the reason.
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Old 10th November 2008, 07:59 PM   #9
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Quote:
Originally posted by megajocke



That is not the reason.
Then what is? If you say it is to stop the amp oscillating then design a better amp.
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Old 10th November 2008, 08:02 PM   #10
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Please explain how it even in theory could increase power transfer.
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