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Inside the PM-175. The PM-350 looks very similar, having a larger power transformer and a smali cooling fan to the right of the heatsink extrusion. The triac and capacitors are at lower right.

magnetic field amplifiers have bcen de-velopcd and refined from the original “magie cube” M-400 model, and the modcls PM-175 and PM-350 shown herc are from the latest rangę. Thcy^łre nomi-nally rated at 175W per channcl and 350W per channcl rcspcctively, into 8 ohm loads and with both channels driven. Howcver the PM-175 can also delivcr 300W per channcl into 2 ohm loads, or 500W into a single 8 ohm load in mono bridging modę. Similarly the PM-350 can delivcr up to 450W per channcl into 2 ohm loads, or no less than 900W into a single 8 ohm load jn bridging modę.

All this from boxes which, although they^łrc rather morę conventional look-ing than the first Carver amp, can still be lifted in one hand — although in the case of the PM-350, not for very long!

How^łs it all done? Weil, one aspect of Carver^łs “magnetic field” principle is that it uses a spccial power transformer, with a dclibcrately high leakage induct-ancc in the 240V primary winding. A

— but taking this principle still further. In this case the transformer secondary

Then about eight ycars ago, the audio and hi-fi magazines all began buzzing _[    ' with news of a radically new “magnetic

field” amplificr design that was capable of producing incrediblc amounts of audio power, fiom a tiny box. The new design had bcen devclopcd by Bob Carver, a US cnginccr-entrcprcncur who had prcviously bcen the founder of Phase Lincar Systems (wherc he had developcd the PL super amps, and a noise icduction system    bascd on auto-

f    corrclation).

At first the details of the new ap-proach were very sketchy, and sccmed to consist of littlc morę than marketing , “hypc”. But gradually the design details emerged, and it was scen that Bob Carver rcally had madę    a highly innova-

\    tivc contribution to the    tcchnology. His

company Carvcr Corporation flour-ished, and sińce then it has diversified into other areas such as FM tuners and CD players — all with the innovative I    Carver touch.

Along the way the firm’s original

triac dcvice is conncctcd in scrics with the primary, and is fed with a control    i

signal derivcd from the amplifier’s audio input signal (via an opto coupler for isolation). W hen thcre’s littlc or no signal, the triac is barely turncd on, and    l

the power drawn from the mains is quite Iow — hence the cool operation.

But w hen the signal inereases, the triac

turns on and delivcrs morę power.    ^ł

A bank of capacitors also conncctcd    )

in the transformer primary Circuit are uscd to create a icsonant Circuit. The purpose of this is to make use of the energy stored in the transformcr^łs leakage inductance, to allow the power sup-ply to delivcr pulses of power extremcly rapidly in response to audio signal peaks. And this is esscntially the “mag-nctic field” principle, as far as we havc bcen able to dcterminc.

However along with this tcchniquc,

Carvcr also uses the idea of dynamie

*

supply yoltage switching on the DC sidc

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ELECTRONICS Australia. September 1987