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  1. #1
    Hifi Freak
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    Amplifier stability and speaker cable - a cautionary tale

    Alan Shaw eier og utvikler for Harbeth Audio UK ansees av mange å være en pålitelig og kunnskapsrik mann. Han sier oft ting som: Cables/Harbeth interface is a non-issue by good design.

    Men det kan likevel være greit å ha med seg det som skrives med små skrift. Her advarer produksjonsavdelingen ved britiske Harbeth om mulig ustabilitet ved enkelte kabel/høyttaler laster. Dette har vært nevnt flere ganger i ulike tråder her på sentralen, men kanskje verd å nevne her i egen tråd på tech-hjørnet.


    Production dept. asked me to provide them with a new small power amplifier to be used on one of the test rigs. Only a few watts were needed, just enough to sweep-test every drive unit listening for buzzes by ear. The previous B&K oscillator (now faulty awaiting repair) had a speaker drive output stage, but the newer B&K doesn't have an inbuilt power amp.

    As It happens, I'd half assembled into a case a commercial "DIY" power amp module comprising of heatsink, power supply and circuit supplied in one neat resin-potted housing. It's a design that's been available for years and sold to the disco/pa market to drive long speaker lines.

    Assembly complete (wiring mains, phono input socket and psu transfomer) and hooking up to an oscillator and speaker and all seemed well. Until I turned the volume up more at which point there was a tell-tall audible distorted character to the pure tone: the character of circuit instability. I clipped the 'scope across the speaker feed to see, as expected, that one half of every sine wave had a burst high-frequency supersonic noise, which whilst inaudible itself, manifest as distortion in the audio band. Reduce the audio input and stability returned.

    Disconnecting the speaker so the amp was not actually generating power into a load, and again, stability. The solution was a 'zobel network' comprising a small resistor and capacitor in series soldered directly across the output terminals. Cost about GBP 0.10 (ten pence) and commonly fitted to amplifiers to partially-cancel the inductive/capacitive reactive load of a speaker cable and/or loudspeaker itself. But not this one.

    So the message from this is .... we want to assume that audio power-amplifiers are well designed and stable into the sort of loads that typical cables + speakers present, such as this combination. But that may be over optimistic on our part. Unless we have test equipment we cannot look for instability and if we identify it by ear, the damage may have already been done. There may indeed be latent instability which is only triggered on signal or transient peaks, depending upon drive level as in this case. The more exotic the speaker cable and/or the loudspeaker load the more we tempt instability. That's why I design Harbeth speakers to present as benign a load as possible to the amp and advise you to use simple, standard cable that looks like simple standard cable.


    https://www.harbeth.co.uk/usergroup/...oc-cable/page2


    Navn:      speaker-zobel.jpg
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  2. #2
    Bransjemedlem Audiomix's Avatar
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    Alan Shaw eier og utvikler for Harbeth Audio UK ansees av mange å være en pålitelig og kunnskapsrik mann. Han sier oft ting som: Cables/Harbeth interface is a non-issue by good design.

    Men det kan likevel være greit å ha med seg det som skrives med små skrift. Her advarer produksjonsavdelingen ved britiske Harbeth om mulig ustabilitet ved enkelte kabel/høyttaler laster. Dette har vært nevnt flere ganger i ulike tråder her på sentralen, men kanskje verd å nevne her i egen tråd på tech-hjørnet.


    Production dept. asked me to provide them with a new small power amplifier to be used on one of the test rigs. Only a few watts were needed, just enough to sweep-test every drive unit listening for buzzes by ear. The previous B&K oscillator (now faulty awaiting repair) had a speaker drive output stage, but the newer B&K doesn't have an inbuilt power amp.

    As It happens, I'd half assembled into a case a commercial "DIY" power amp module comprising of heatsink, power supply and circuit supplied in one neat resin-potted housing. It's a design that's been available for years and sold to the disco/pa market to drive long speaker lines.

    Assembly complete (wiring mains, phono input socket and psu transfomer) and hooking up to an oscillator and speaker and all seemed well. Until I turned the volume up more at which point there was a tell-tall audible distorted character to the pure tone: the character of circuit instability. I clipped the 'scope across the speaker feed to see, as expected, that one half of every sine wave had a burst high-frequency supersonic noise, which whilst inaudible itself, manifest as distortion in the audio band. Reduce the audio input and stability returned.

    Disconnecting the speaker so the amp was not actually generating power into a load, and again, stability. The solution was a 'zobel network' comprising a small resistor and capacitor in series soldered directly across the output terminals. Cost about GBP 0.10 (ten pence) and commonly fitted to amplifiers to partially-cancel the inductive/capacitive reactive load of a speaker cable and/or loudspeaker itself. But not this one.

    So the message from this is .... we want to assume that audio power-amplifiers are well designed and stable into the sort of loads that typical cables + speakers present, such as this combination. But that may be over optimistic on our part. Unless we have test equipment we cannot look for instability and if we identify it by ear, the damage may have already been done. There may indeed be latent instability which is only triggered on signal or transient peaks, depending upon drive level as in this case. The more exotic the speaker cable and/or the loudspeaker load the more we tempt instability. That's why I design Harbeth speakers to present as benign a load as possible to the amp and advise you to use simple, standard cable that looks like simple standard cable.


    https://www.harbeth.co.uk/usergroup/...oc-cable/page2


    Navn:      speaker-zobel.jpg
Visninger: 136
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    Fancy kabler sammen med på kanten stabile / ustabile forsterkere bør unngås hvis en vil ungå slike problemer som nevnt over.
    Hans - DIY'er

  3. #3
    Hifi Freak elefantmannen's Avatar
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    Hva er fancy kabler? Alt annet enn standard to eller fireledere hvor lederne ligger tett sammen?

    Hva om man bruker to strekk av forskjellig kvadrat pr. høyttaler til bi-wiring f.eks.?
    “This band is about devils and race cars, why the fuck are you talking to me about Ronald Reagan?”
    - J. Yuenger

  4. #4
    Hifi Freak
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    Zobelfilters in action.

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    Navn:      Zobel-Network-Photo-of-Actual-Network-Closeup.jpg
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    Siste redigert av Hedde; 31.01.2019 kl. 11:00.

  5. #5
    Hifi Freak
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    Burde sidde i forstærkeren og tilpasset denne.

  6. #6
    Hifi Freak
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    Sitat Sitat fra Drossel Se Innlegg
    Burde sidde i forstærkeren og tilpasset denne.
    Det burde høyttaleren også....

  7. #7
    Hifi Freak
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    Sitat Sitat fra Drossel Se Innlegg
    Burde sidde i forstærkeren og tilpasset denne.
    Det sitter en slik krets på utgangen i min Tandberg TR-1010 ser jeg.

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