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Old 4th April 2008, 10:22 AM   #11
GK is offline GK  Australia
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Quote:
Originally posted by Bonsai
Even Self seems not to be able to explain precisely why the universally optimum values are 10 Ohms and 0.1uF for the Zobel network .

However, I can tell you from practical experience that you should always use a Zobel network.

It should run from the output node of the amp - so directly at the junction of the emitter degeneration resistors in a typical EF bipolar output stage to the main filter capacitor 0V junction.

Don't forget the out put inductor either - 2-5uH with 2.2 Ohm 3W resistor in parallel.

If you leave out the Zobel network, you are likely to see parasitic HF oscillations in the 2-5MHz range. If you omit the output inductor, you can expect oscillations of 200Khz to 500KHz with a capacitive load - and real world capacitive loads come from typical speaker cables and speakers themselves.

The two simple techniques - Zobel + output inductor networks - will go a long way to creating a stable, reliable amp.

There are no generally optimum values for a Zobel RC, it's just a high frequency load for the amplifier. 10 ohms and 0.1uF are just convenient enough values that work in most instances. The only requirement is that the Zobel RC is a sufficiently 'heavy' load for the amplifier.

Also, there are many design factors involved, but it isn't always particularly difficult to compensate a design for unconditional stability without a Zobel RC.
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Old 4th April 2008, 11:55 AM   #12
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One of the reasons for adding a series inductor//resistor is to prevent the amplifier from detecting AM radio signals. But when your amplifier doesnít receive Radio Russia or whatever, can one conclude after reading the previous posts, that an amplifier doesnít need a series inductor//resistance when it passes the following test?

Load the amplifier with an 8 Ohm load and a capacitor in parallel, ranging from a few pF to letís say 2uF (I have a decade capacitor). Apply a 10 kHz square wave at the input and vary itís amplitude for each chosen capacitor, from mVís till onset of clipping at the output. Observe that, apart from some ringing at higher capacitances, no oscillations do occur.

What do you guys think? Especially in High End audio often less is more. So itís interesting to know if you really need that series inductor//resistance at the output of your amp, otherwise leave it out.

Peter
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Old 4th April 2008, 12:17 PM   #13
timpert is offline timpert  Netherlands
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Hi,

I'd like to warn against capacitive loading of your amp as an experiment. If you really must, start out with a small one and increase it gradually, and stop at any sign of distress. Many amplifiers aren't designed to remain stable with capacitive loads as high as 2 uF. In addition, 2 uF @ 10 kHz is about 8 ohms, but the maximum current is at the zero crossing of the signal, as a result the output stage is likely to overheat if you drive things too far.

The amplifier from Rod Elliott's site (P3) at http://sound.westhost.com/project03.htm seemed to perform better without the inductor, but since this is just one of many possible designs, I would be very hesitant to just remove components from an arbitrary amp which are there for a good reason. Experiment carefully, and without an oscilloscope, don't!

Jurgen
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Old 4th April 2008, 12:17 PM   #14
GK is offline GK  Australia
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Quote:
Originally posted by pietjers
One of the reasons for adding a series inductor//resistor is to prevent the amplifier from detecting AM radio signals. But when your amplifier doesnít receive Radio Russia or whatever, can one conclude after reading the previous posts, that an amplifier doesnít need a series inductor//resistance when it passes the following test?

Load the amplifier with an 8 Ohm load and a capacitor in parallel, ranging from a few pF to letís say 2uF (I have a decade capacitor). Apply a 10 kHz square wave at the input and vary itís amplitude for each chosen capacitor, from mVís till onset of clipping at the output. Observe that, apart from some ringing at higher capacitances, no oscillations do occur.

What do you guys think? Especially in High End audio often less is more. So itís interesting to know if you really need that series inductor//resistance at the output of your amp, otherwise leave it out.

Peter

The RC Zobel won't do any harm, even if it isn't really needed.
The best output network for RFI supression is the constant impedance network described in an IEEE publication by Nevile Thiele (that I can't remember the name of ATM) in which the speaker terminals are directly shunted with a parallel capacitor, isolated from the amplifier output with a parallel LR.
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Old 4th April 2008, 12:35 PM   #15
timpert is offline timpert  Netherlands
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Hi,

RFI picked up by the speaker wire is a common-mode signal, the same signal that is injected in the "hot"speaker wire is injected into the neutral one. From an RF point of view, both wires are very close together, and as a result, tightly coupled. If you wish to do anything against RFI, then the only really effective means are provisions to block common mode signals. A bifilar inductor is often used for this.

Problem is, in a typical stereo setup, one speaker wire goes one way and the other one the other way. This forms a very good dipole antenna for AM signals, with the amplifier in the center of the dipole. Any effective RFI suppression boils down to preventing the amplifier from loading the dipole by making its RF impedance as high as possible.

The output inductor is only there to keep the impedance seen by the amp itself within manageable bounds. It might help a bit in suppressing RFI, but it doesn't do anything against RF currents flowing through ground. That's why you need a bifilar inductor for RF suppression.

Jurgen
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Old 4th April 2008, 12:48 PM   #16
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Quote:
Originally posted by timpert
Hi,

RFI picked up by the speaker wire is a common-mode signal, the same signal that is injected in the "hot"speaker wire is injected into the neutral one. From an RF point of view, both wires are very close together, and as a result, tightly coupled. If you wish to do anything against RFI, then the only really effective means are provisions to block common mode signals. A bifilar inductor is often used for this.

It is common mode, but the L+R/C network with the cap across the speaker output (for example, as this is the only really effective RFI supressing output network) ensures that the speaker output is at RF ground, at least preventing the developed RF voltage from being fed back to the amplifiers inverting input via the feedback network.
The rest is combated with proper ground lead routing and cabinet/enclosure earthing/shielding.
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Old 4th April 2008, 01:05 PM   #17
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Hi,
the Thiele network is examined by Dr Cherry.
I think his paper was published 1995 & updated 1997.
I covers the two original Thiele options of the cap before and after the series inductor.
He also looks at the whole family of values between the Thiele extremes.

Most competent designers now advocate reducing the inductor value to between 0uH to <1uH. But only folk like Glen and his ilk can achieve unconditional stability with 0uH.

Glen,
can your write a tutorial on how to achieve Nirvana?

I'll try to compress my Excel sheet and post it later.

Compression is tested and excel file opens using winRAR. It was created with 7zip.
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Old 4th April 2008, 01:31 PM   #18
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Quote:
I'd like to warn against capacitive loading of your amp as an experiment. If you really must, start out with a small one and increase it gradually, and stop at any sign of distress. Many amplifiers aren't designed to remain stable with capacitive loads as high as 2 uF. In addition, 2 uF @ 10 kHz is about 8 ohms, but the maximum current is at the zero crossing of the signal, as a result the output stage is likely to overheat if you drive things too far.
Off course an oscilloscope is needed for close observation!

Quote:
Load the amplifier with an 8 Ohm load and a capacitor in parallel, ranging from a few pF to letís say 2uF (I have a decade capacitor). Apply a 10 kHz square wave at the input and vary itís amplitude for each chosen capacitor, from mVís till onset of clipping at the output. Observe that, apart from some ringing at higher capacitances, no oscillations do occur.
But when an amplifier is stable under the conditions I described above (perhaps 2 uF is to much and 1 uF is more realistic) can one safely dismiss the output inductor//resistor network? Or are there still load conditions that can drive amplifiers in oscillation?

Another way to put it is: What are realistic test conditions to test the stability of an amplifier? Does one of you have a standard recipe after which you can be quite sure an amplifier is "stable under all conditions"?

Peter
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Old 4th April 2008, 01:36 PM   #19
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FWIW, if you're building anything along the lines of Self's blameless amps, with very low THD, don't try to save space by putting anything inside the output inductor. It should be an air coil of about 3/4" diameter or greater, of sufficiently heavy wire to keep the low frequency damping factor where you want it. When working on an ultra low distortion amp, every coil I tried that had anything even remotely magnetic in it, increased the THD. I was hoping to save space with ferrite, but the thing just has to be physically large for maximum performance.
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Old 4th April 2008, 01:51 PM   #20
timpert is offline timpert  Netherlands
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Peter,

One standard recipe is to make a single ended triode amp without feedback. That thing is stable, no matter what you connect to the output transformer's secondary. But I guess this is a bit off-topic in this forum ;-)

Saying that amplifier is stable under all possible conditions is like stating you've built a ship that cannot sink. And we all know what happened to the Titanic... But seriously, I don't know of any topology that uses feedback that is unconditionally stable. A good amp is stable under all practical loudspeaker loads, but I am sure that even the best amps can be made to go bananas if some outrageous load is connected to it. In practical situations however, the zobel and output inductor do a pretty good job. But the poles and zeroes of the amp itself determine it's ultimate stability, and there's no easy way out of that!

G. KLeinschmidt,

I see how the Thiele network works, but I fail to see why this is the best you can do. Perhaps I should look a little deeper in the matter, as it is certainly an interesting subject, but I don't have time for this right now. I have a basement to clean...

Jurgen
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