Alan Schneider wrote:
> Norman Wootan <normw@fastlane.net> and others wrote quite a bit about
> the MRA...
>
> Norm, you wrote that the resistance of the "primary" circuit is very
> critical, suggesting silver tinned stranded wire to get resistance down
> as low as possible. Have you considered using litz wire? This is used
> extensively at lower radio frequencies to reduce "skin effect" and thus
> increase "Q". I've seen the concept used in some switchmode power supply
> transformers as well. You're running ultrasonic so skin effect will
> certainly be a factor.
>
> You also mentioned using a transformer coupled amplifier to drive the
> MRA. Possibly a bad idea as the DC resistance of the output transformer
> secondary will be in series with your tuned circuit, thus reducing "Q".
> Also, the inductance of the winding would significantly alter the tuning
> of the MRA. As far as I can see, you'd need an amplifier with the lowest
> possible output resistance and, at the same time, *very* low inductance.
>
> Thirdly, the bridge rectifier you were using. At the frequencies
> concerned, this would have to be done with schottky or other fast turn-off
> diodes or you would have major losses in the secondary circuit.
>
> An idea which you may find worth looking into is to build two identical
> MRA's with the same resonant frequency and have the piezo elements "coupled"
> by placing them in a rigid tube with the spacing set up so they reinforce
> each other, acoustically speaking. This *may* enhance the effect but it
> would probably make tuning the system considerably harder.
>
> I'm curious about the piezo transducers you used. Are you able to give more
> details, ie manufacturer, part number and what sort of transducer they are
> (piezo tweeter etc)? When I first heard about the MRA, I did some looking
> around over here and none of the catalogs I checked even mentioned what
> material was used for the piezo element. Likewise the composition of easily
> available magnets.
>
> The fact that only one in five transducers displayed the effect implies that
> the MRA relies on some parameter which is normally considered parasitic and
> hence able to be largely ignored by the manufacturer. Do you have any idea
> as to what this key parameter(s) is/are?
>
> Cheers,
> Alan Schneider
>
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