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Old 20th September 2007, 02:39 PM   #1
gni is offline gni  United States
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Default Zobel Values in output stage

I've seen a number of Zobel networks [RC] in the output
of solid state amplifiers shunted to ground

I was wondering how the values are determined.

Elliot Sound Website almost always uses them with 10ohm/100nF.
The chip amps seem to use smaller resistors but the same
100nF cap. Seems like it would produce high impedance at most
frequencies and drop to close to 10ohms at some high frequency.

There also seems to be an RL in series with the output. . .seems
like it would create more output impedance (generally bad?) at
higher frequency.

Would these circuits prevent oscillation at high frequency. . .especially
if using high amounts of negative feedback?
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Old 20th September 2007, 02:46 PM   #2
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The values are determined by experimentation, but common 10R and 100nF works well.

The inductor is a different thing, that protects against excess capacitive loads.

Both will influence the stability. Instability can be caused by high feedback levels, but also by very wide bandwidth.
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Old 20th September 2007, 02:52 PM   #3
gni is offline gni  United States
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Therefore. . .could you add the circuits to any amplifier
and gain some sort of protection?
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Old 20th September 2007, 03:57 PM   #4
Dzsoni is offline Dzsoni  Hungary
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Check this attachment out, gni
Attached Files
File Type: zip zobel.zip (4.9 KB, 94 views)
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Old 20th September 2007, 07:07 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
it would be nice to credit the author.
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Old 20th September 2007, 07:17 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
the Thiele network is there to provide a load for the amplifier at very high frequency.

Consider the component values at low frequencies.

The Zobel cap (~100nF) is effectively an opencircuit.
the series L (~2uF) is effectively a short circuit.
The result is that at audio frequencies the output of the amp passes straight through the L to the speaker and the Thiele network has little effect.

Now consider high frequencies above the audio band.
The C of the Zobel is now effectively a short circuit and this leaves the series R (~10r) as a load to ground.
The Series L is now a high impedance and the parallel R (~4r) bypasses the L.
The amplifier now has two parallel loads connected to it's output
1. the 10r + a cap.
2. the 5r feeding the speaker cables and the speaker (effectively in parallel).

The resulting High Frequency load is about 3ohms and gives the amp some lossy load to dissipate any VHF oscillations that might try to start. But this load damps them out, if the components are correctly chosen to suit the amplifier circuitry. That's the tricky bit that most builders avoid. They choose 10r+100nF and omit the inductor.
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Old 20th September 2007, 07:39 PM   #7
gni is offline gni  United States
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"...the Thiele network is there to provide a load for the amplifier at very high frequency..."

I think that answers the question. The speadsheet helps
visualize the changing values.

With all that in mind. . . what types of capacitors should be used?

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Old 20th September 2007, 07:54 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
Low inductance caps are necessary.
Probably film types are best. I did have a bad experience with a Wima fkp4, but maybe it was a rogue. I replaced with a PES 150V and that was better.
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Old 20th September 2007, 08:50 PM   #9
gni is offline gni  United States
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That is helpful. Still working on bad M-45.. . .
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Old 21st September 2007, 12:25 PM   #10
AKSA is offline AKSA  Australia
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Best cap to use is self-healing polyprop. It can take a thrashing and remain reliable, like motorstart caps. I use Arcotronics, 100nF 160V.

Dielectric is not important sonically, however.

The 10R resistor in series with the cap should be a minimum of 1W, and preferably flame proof.

This network also keeps the amp loaded down at all frequencies, which is important to the phase margin, since with the inductance of voice coils their impedance increases with frequency, unloading the amp.

Cheers,

Hugh
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