guypettigrew

Zobel network

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I have just bought some 'speaker zapperators from Russ Andrews.

These are, apparently, Zobel networks.

They have changed the sound of my 'speakers. This is the technical section, so it's not appropriate to say whether they've made it better, worse or just different.

What is a Zobel network, what does it do and how does it do it, please?

I've looked on Wikipedia and got flummoxed by the article. The photo of the old BBC studio with it's variable Zobel network is great, though!

So, an easy to understand reply, suitable for a non electronicy person would be appreciated.

Thanks.

Guy

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DEALER

The job of your amplifier is to provide voltage (measured in, er, volts) and current (measured in amps) to your speakers. Amplifiers can generally deliver a wide range of voltages and currents but they tend to work best for a certain ratio of voltage to current. This might be 8 volts for every 1 amp. Or it might be 4 volts for every 1 amp. The ratio will have been chosen by the amp's designer. Whatever he/she chose, you're stuck with it unless you can change, say, transformer connections or some other electronic setting. You can't usually do this while you're playing music. The ratio of volts to amps is called impedance. It's measured in ohms and it's controlled by your speakers.

When we buy speakers the handbook will tell us what their impedance is. It might say 8 ohms (8 volts per amp) or 4 ohms or something else. The problem is that the number is fairly meaningless because in fact the impedance depends very strongly on the frequency of the sound we're playing. There's a concrete example here http://www.troelsgravesen.dk/vintageBC1.htm. Going down the page, the red curve in the fifth graph (the first one that says 'Sinusoidal' above it) shows the actual impedance of the Spendor BC1 speaker. The impedance, in ohms, is shown up the left hand side and the frequency, in hertz, is shown across the bottom. So at 100Hz for example the red curve shows us that the impedance is a bit less than 10 ohms. But at a frequency just above 1kHz the speaker's impedance exceeds 40 ohms. And between 10kHz and 20kHz it goes as low as 6 ohms. So what is the impedance of this speaker ? We can't really say because it varies so much.

This variation in impedance is a problem for the amplifier. If the amp is set to match a 10 ohm speaker then it will work well at a frequency of 30Hz, and between 100Hz and 200Hz, and around 10kHz. But at other frequencies it will work less well.

So, to get to your questions:

What is a Zobel network ? It is a circuit made up of electronic components which can be wired to your loudspeaker. It does a specific job.

What does it do ? It makes the impedance of the composite assembly - Zobel network plus speaker - vary much less with frequency than the impedance of the speaker on its own. Since the amplifier is now delivering voltage and current to the composite assembly the effect of the Zobel network is to make the amp's job easier.

How does it work ? This is a technical question and it isn't easy to answer if you don't have a technical background. Basically the added components in the Zobel network draw current at frequencies where the speaker itself tends not to. To go much beyond that we're going to have to start speaking mathematics I'm afraid.

Hope that helps some,

VB

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DEALER

http://ccs.exl.info/calc_cr.html

(scroll to near bottom) - sure sorted my Eminence 15" Low Freq II's and Eminence Kappa 15's, doing sub/tapped horn and mid bass duty.

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What does it matter? If I play a tone generated sweep from 20Hz to 20Khz and measure the output and it's almost flat. Why would you need such additives?

If the amplifier is unstable with the speaker load and as a result oscillates about the audio band, the effects can be very audible or, worse, damaging. In this case, your frequency sweep may very well not show up any problems.

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I have just bought some 'speaker zapperators from Russ Andrews.

These are, apparently, Zobel networks.

They have changed the sound of my 'speakers. This is the technical section, so it's not appropriate to say whether they've made it better, worse or just different.

What is a Zobel network, what does it do and how does it do it, please?

I've looked on Wikipedia and got flummoxed by the article. The photo of the old BBC studio with it's variable Zobel network is great, though!

So, an easy to understand reply, suitable for a non electronicy person would be appreciated.

Thanks.

Guy

Amplifiers that need them already have them, so adding another won't do anything.

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DEALER

As Wikipedia succinctly puts it:

"When used to cancel out the reactive portion of loudspeaker impedance ... only half the [Zobel] network is implemented as fixed components, the other half being the real and imaginary components of the loudspeaker impedance."

So in the case of the OP's "speaker zapperators" the network should, presumably, have been designed to work with the speaker, not with the amplifier.

VB

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I have used Zobels to flatten the rising HF impedance of bass units to make crossover design easier. If you add one to an existing design then the results will be unpredictable. Actually they WILL be predictable, but only if you understand the maths. I suspect any perceived improvements will be variable.

C'mon Serge, this is your cue!

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Yes, using zobels in my current speaker project for the reasons given above. Sounds like a typical piece of Russ Andrew's technology built on the imagination of the listener and not the validity of the technology.

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I have used Zobels to flatten the rising HF impedance of bass units to make crossover design easier. If you add one to an existing design then the results will be unpredictable. Actually they WILL be predictable, but only if you understand the maths. I suspect any perceived improvements will be variable.

C'mon Serge, this is your cue!

Not quite sure what you're expecting from me. Zobel networks do exactly as they're supposed to do, and flatten off the rising impedance at HF. They're pretty much essential in valve amplifiers, as valves don't like to be operated unloaded, and the rising impedance of most loudspeakers at HF effectively unloads the amp at high frequencies.

S.

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Not quite sure what you're expecting from me. Zobel networks do exactly as they're supposed to do, and flatten off the rising impedance at HF. They're pretty much essential in valve amplifiers, as valves don't like to be operated unloaded, and the rising impedance of most loudspeakers at HF effectively unloads the amp at high frequencies.

S.

Interesting. My system's all valve, using 300Bs in PSE configuration in the power amps.

As akwardbydesign suggested, though, I did wonder how an "all pupose" Zobel network could be built without the builder knowing the characteristics of both the amplifier and the 'speaker. Perhaps there's a sufficiently large window of acceptability, though, for a general network to be developed.

Guess we'd have to ask Ben Duncan that. 'Twas he who designed the Zobel networks sold by RA.

Guy

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Interesting. My system's all valve, using 300Bs in PSE configuration in the power amps.

As akwardbydesign suggested, though, I did wonder how an "all pupose" Zobel network could be built without the builder knowing the characteristics of both the amplifier and the 'speaker. Perhaps there's a sufficiently large window of acceptability, though, for a general network to be developed.

Guess we'd have to ask Ben Duncan that. 'Twas he who designed the Zobel networks sold by RA.

Guy

In terms of an "all-purpose" zobel network, I would use something like a 10 ohm resistor and a 1.6 uF capacitor in series, as this will apply a load from 10kHz upwards where the inductance of the loudspeaker takes the impedance above the 8 ohm nominal. It really isn't critical, but I wouldn't use it when driving electrostatics which have a falling impedance at HF.

S.

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In terms of an "all-purpose" zobel network, I would use something like a 10 ohm resistor and a 1.6 uF capacitor in series, as this will apply a load from 10kHz upwards where the inductance of the loudspeaker takes the impedance above the 8 ohm nominal. It really isn't critical, but I wouldn't use it when driving electrostatics which have a falling impedance at HF.

Total component cost a couple of pounds tops. What does RA change for their Zobel network? I have just looked on their site and it is £148! albeit for a stereo pair. I must be in the wrong business. :nup:

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