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Thread: Zobel network

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    Zobel network

    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
    Last edited by guypettigrew; 23-07-2012 at 07:47 AM.
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    In the trade Wammer Valvebloke's Avatar
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    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
    Last edited by Valvebloke; 22-07-2012 at 09:24 PM.

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    Thanks very much. The explanation and the link are very helpful. Much appreciated.

    It makes me realise just how important the amp/'speaker combination is. Each is dependant on the other.

    Guy
    Know what's important, go for it, and value it when you get it.

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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?
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    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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    Quote Originally Posted by drummerjohn View Post
    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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    Quote Originally Posted by guypettigrew View Post
    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.

  9. #9
    In the trade Wammer Valvebloke's Avatar
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    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!

  11. #11
    Malvolio Evans
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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.

  12. #12
    Too much time on my hands SergeAuckland's Avatar
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    Quote Originally Posted by awkwardbydesign View Post
    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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    Quote Originally Posted by SergeAuckland View Post
    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
    Know what's important, go for it, and value it when you get it.

  14. #14
    Too much time on my hands SergeAuckland's Avatar
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    Quote Originally Posted by guypettigrew View Post
    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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    Quote Originally Posted by SergeAuckland View Post
    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.

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    Wammer awkwardbydesign's Avatar
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    Quote Originally Posted by guypettigrew View Post
    Interesting. My system's all valve, using 300Bs in PSE configuration in the power amps.
    Guess we'd have to ask Ben Duncan that. 'Twas he who designed the Zobel networks sold by RA.
    Guy
    Interesting factoid. My active xover is his design from the early '80s. Still going strong with some updates.

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    Wammer awkwardbydesign's Avatar
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    Quote Originally Posted by SergeAuckland View Post
    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.
    So if I use my little EL84 amp to drive ribbon or electrostatic (Shackman) tweeters, and use a fairly large cap to block LF, do I need to protect the amp as it would now see a high impedance at low frequencies? And how would I implement that? I am using an active xover, BTW. The MF driver doesn't need protecting.
    See, I said we needed Serge!

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    Too much time on my hands SergeAuckland's Avatar
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    Quote Originally Posted by awkwardbydesign View Post
    So if I use my little EL84 amp to drive ribbon or electrostatic (Shackman) tweeters, and use a fairly large cap to block LF, do I need to protect the amp as it would now see a high impedance at low frequencies? And how would I implement that? I am using an active xover, BTW. The MF driver doesn't need protecting.
    See, I said we needed Serge!

    In your case, using the EL84 amp just for HF duties, I'd put a 25 ohm resistor ( 3 watt will be ample) across the output of the amp. That will maintain a load, but isn't enough to give the amplifier any sort of a hard time driving the resistor as well as the tweeter. The resistor value isn't at all critical, anything from 20-30 ohms will be fine, enough to maintain a load, not enough to load the amplifier.

    S.

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    Fascinating! So, Serge, does it matter what value capacitor Mr Awkwardbydesign is using to block his low frequencies? Or, as it will be in series with the driver, is it irrelevant to this Zobel discussion?

    Guy
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    Wammer awkwardbydesign's Avatar
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    Quote Originally Posted by guypettigrew View Post
    Fascinating! So, Serge, does it matter what value capacitor Mr Awkwardbydesign is using to block his low frequencies? Or, as it will be in series with the driver, is it irrelevant to this Zobel discussion?

    Guy
    What he said! So the resistor would just go across the terminals, pos to neg?
    If I use a ribbon, I would xover no lower than 3kHz, (24dB/octave, active) but the Shackman could go down to 500Hz, allegedly, so a different cap for each. I haven't yet worked out yet what value I would use in each case to block LF, it would have to be low enough to not interfere with the xover slope. Ribbons, or planars, tend to be about 6-7 ohms and fairly flat, but the Shackman is transformer coupled. Pretty poorly IMO, so if it was worth persevering with I would probably buy something better, maybe Sowter, as the originals drop to about 1 ohm at 20kHz! I would put a couple of ohms in series with it to start with.
    Last edited by awkwardbydesign; 02-08-2012 at 10:25 PM.

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