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© November 2009 Ben Duncan Research - UK & Russ Andrews Accessories Ltd - page 7 / 11





7 / 11

Slew Limit - An awakening after 30+ years

To understand another dimension of the benefits of Zapperator technology, in today's world, consider slew limits for audio - that a circuit capable of (say) 1.25v/µS (one and a quarter volts per microsecond) can barely handle 20kHz at 10v peak, as this is at the onset of plainly visible, hence serious, 'wave bending', and so distortion becoming extreme.

Next, consider that the relationship is linear. So for signals at 1GHz*, we can see that slew-limiting will likewise onset visibly and grossly, with a peak signal value that is 50,000 times smaller than whatever voltage is safe at 20kHz. So 10v becomes 200µV. For a lower acceptable degree of non-linearity (this means sonic degradation), we might reasonably set a slew rate limit, the maximum rate in the signal applied, of a 1/10th or a 1/100th of this, as being 'safer'. For a start, the harmonic structures arising at different percentages of and below the limit, are unknown and unexplored. So conservative measures are valuable to be sure there is no distortion, hence no intermodulation, hence no extra noise generation, hence a lower noise floor.Then in using the 1/10th criterion, the 1GHz limit would be 20µV. And with the 1/100th, the maximum peak value for signals around 1GHz is 2µV (two micro-volts).This signal level is typical of weak RF broadcast signals. Considering most audio cables are not hermetically shielded at RF, nor are able to be, RF voltages at these levels - or higher - are ‘not unlikely’ to be found induced into the associated conductors, in most places today. Accordingly, it is valuable to have other means to reduce not just RF amplitudes, but equally - or even more preferably - to reduce the high rates of change, of RF signals that have frequencies up to thousands of times higher than the highest audio- relevant harmonic (which means dealing with them needn’t affect sonic quality).

The results of RF causing slew limiting are surprisingly long established, although the cause is infrequently identified. Critical operational DC levels inside circuits may be altered, including amplifier biasing, and in turn, the stability and poise of electrolytic capacitor dielectrics can be upset. Sharp edged pulses can also give rise to trauma to some materials inside equipment optimised for audio, notably the plastic dielectrics of cables, capacitors and semiconductors. Of course, direct effects may well prove invisible.The structure and audio quality of ’impulsed’ materials may take several days to return to normal. Such might explain the sometimes noted sudden loss of sonic quality - possibly after a flare-up of RF spikes arising from sun-spot activity or lots of thunderstorms, for example. Such vast noise sources may induce spikes directly into the audio path, via speaker voice coils, crossover parts, speaker wiring, or amplifier wiring, inductors - let alone via line-level connections, and the surrounding AC power wiring.

  • *

    1 GHz(One Giga Hertz) is a frequency 50,000 - fifty thousand - times higher than

20kHz (Twenty kilo Hertz).

© November 2009 Ben Duncan Research - UK & Russ Andrews Accessories Ltd

Russ Andrews Accessories Ltd, 2b Moreland Court, Westmorland Business Park, Shap Road, Kendal, Cumbria, LA9 6NS, UK. Call UK Local Rate 0845 345 1550 Mon-Fri 9am-5pm, from abroad +44 1539 797300 Email info@russandrews.com Website www.russandrews.com


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