Moving-Coil Step-Up Roundup (Vol. 2, No. 3, Spring - Fall 1980)

Moving-Coil Step-Up Roundup: Audire 'Poco' , Fidelity Research FRT-3G , Nagatron Ag 9200; PS Audio (interim report) , RWR Audio MCT-1 (follow-up) .

By the Staff of The Audio Critic

We're unable to report any major breakthroughs in either transformers or pre-preamps, but the average quality seems to be getting a little better.

We keep hearing and reading more muddleheaded misinformation on the subject of moving-coil pickup trans formers and pre-preamps than anything else in audio, with the possible exception of loudspeakers. We're hoping that the applicable chapter in our forthcoming Handbook will systematically cleanse impressionable minds of all that pollution; meanwhile we wish to state a few basic points here for the sake of at least temporary decontamination.

Transformers vs. pre-preamplifiers.

A moving-coil phono cartridge in most cases has higher energy output than moving-magnet, moving-iron or other moving-field designs. The trouble is that the energy is in the form of relatively high current at relatively low voltage, whereas the typical phono preamplifier stage in an audio amplification chain needs somewhat higher voltage to drive it. The logical thing, therefore, would be to perform an impedance transformation by means of a transformer and change the current into voltage without significant losses, rather than to throw away the current and boost the voltage by introducing outside energy from the power supply or battery of an added pre-preamplifier stage. Remember, other things being equal, a passive signal path is cleaner than an active one. The question is, are other things really equal? A transformer, in addition to effecting an impedance transformation, also acts as a bandpass filter. Bandpass filters whose low-frequency and high-frequency ''corners'' are correctly located and contoured for audio purposes are inaudible in a signal path, as long as they also possess sufficient dynamic range so that the filter characteristics remain unchanged regardless of signal amplitude. A lot of MC pickup transformers, even some fairly expensive ones, have insufficient bandwidth and/or funny group delay characteristics on account of the order and Q of their filter equivalents and/or poor dynamic range accompanied by hysteresis distortion as a result of core saturation and kindred causes. None of this is inherent in the transformer concept; it's the implementations that are usually inadequate. A truly excellent trans former is necessarily costly, whereas a fairly decent pre preamp can be built quite cheaply. Cultists will then conclude that the latter sounds better because its bandwidth extends from 0.1 Hz to 1MHz, when the truth is that it sounds better because the transformer it's being compared against is a piece of junk. Never trust a DC-to-light freak on any audio subject.

A further advantage of a transformer is that the DC resistance of the primary can be made sufficiently low so that it's the cartridge impedance (10 ohms or less nowadays) that dominates the total noise level of the system. It would require cryogenic techniques or similarly heroic measures to duplicate that condition with an electronic circuit.

None of this means, of course, that the very finest pre-preamp available at a given moment may not be superior to whatever exists in transformers at the same moment. All we claim is that Mother Nature's deck is stacked in favor of the transformer.

Watch your input stage acceptance level.

One thing that can easily confuse the reviewer of MC step-up devices is the inability of so many preamplifier phono stages to handle high-speed RIAA-preemphasized signal peaks of large amplitude. For example, the Koetsu cartridge through the Cotter MK-2L transformer can generate fast-rising waveforms at sufficient energy levels to make a lot of preamps go ''crunch.'' The reviewer might then blame the unsatisfactory results on the transformer, when it's really the phono input stage acceptance level that's inadequate, and the cartridge/transformer combination is actually pro viding the most advantageous energy transfer and signal-to noise conditions imaginable. The same can also happen with pre-preamps; peak overloads can occur almost anywhere along the signal path, up to and including the main pre amp's phono stage.

This may be one reason for the obnoxious but wide spread practice of loading a moving-coil cartridge down with a low-value resistor. The signal-to-noise ratio is shot to hell as a result, but by shunting off a significant amount of energy the resistor brings the signal peaks within the acceptance range of the circuitry. The damping effect of the resistor may conceivably be of significance in certain specific cases (again, the Handbook will go into greater detail on this), but by and large we don't believe that could be an important influence on the behavior of typical moving-coil mechanisms. Choose a correctly damped cantilever/generator design in the first place and a preamp input stage with high acceptance level, connect the best step-up device you can afford (preferably a good transformer) without any cockamamie resistors, and let her rip. That's our philosophy.

The step-up devices reviewed below were all tested in the manner described on p. 43 of the last issue (Vol. 2, No. 2).

Audire 'Poco'

Audire, Inc., 9576 El Tambor Avenue, Fountain Valley, CA 92708.

Poco pre-preamp for moving-coil cartridges, $175. Three-year warranty. Tested #8098, on loan from dealer.

Here's another relatively inexpensive battery-powered pre-preamp a la Marcof, and it's another good one. In some ways we like it more, in some ways less.

The Poco comes in two versions, each with a different choice of input impedances; the one we tested was the so called low-impedance version. Compared with the Marcof PPA-1 (not the very latest modification, which we didn't have, but the ''improved'' one reviewed in the last issue), the Poco sounded distinctly more transparent (i.e., less veiled) but also more aggressive in the upper spectrum and ultimately more fatiguing.

This was a frustrating test, resulting in a general lack of enthusiasm for our previous Reference B choice (namely the PPA-1), since it appeared to be no longer the undisputed winner in its category, and a no less halfhearted feeling about the new challenger, the Poco, since it didn't turn out to be clearly better, either. Luckily the PS Audio pre-preamp (see review below) managed to extricate us from that impasse. Of course, the "'late late'" PPA-1 may turn the situation around once more. We shall see.

Fidelity Research FRT-3G

Fidelity Research of America, PO Box 5242, Ventura, CA 93003.

Model FRT-3G Toroidal Step-Up Transformer, $250. Tested #067007, on loan from distributor.

It would have been nice to find a really good, clean transformer for only $250, especially since the FR cartridges have always been among our favorites and this transformer was obviously designed to mate with them. Unfortunately, even though the FRT-3G has the open sound quality one can immediately discern when a reasonably good transformer is being used in a system of high resolution, it doesn't quite satisfy us with its reproduction of the treble range. There's something strained, metallic and unpleasant about it up there that makes us want to stop listening. And that's not the purpose of sophisticated audio equipment, regardless of price.

Obviously, toroidal construction isn't nearly as important as the criteria discussed in the preamble to these reviews above.

Nagatron Ag 9200

Nagatronics Corporation, 2280 Grand Avenue, Baldwin, NY 11510. Nagatron Model Ag 9200 'Z Coupler', $325. Tested #4086, on loan from owner.

Originally designed for the 3-ohm Nagatron ribbon cartridge, this transformer works very nicely with a variety of low-impedance moving-coil cartridges. Smoothness and euphony are its chief sonic virtues; we heard none of the hard, edgy, over bright quality that bothers us in so many other step-up devices. On the other hand, the Nagatron introduces a certain amount of veiling: or opacity, which eliminates it from consideration as a reference transformer. Both the Cotter MK-2 and the RWR Audio MCT-1 sound distinctly more transparent.

The "'Ag'' in the model designation of the Nagatron is the chemical symbol for silver, to alert you that this is the transformer with coil windings of ''chemically pure silver (purity better than 99.99%).'" The quotation comes right from the blurb printed on the bottom of the unit. We're still waiting for the silver cultists of the audio world to present us their scientific rationale (or even a scientifically controlled demonstration of the superiority of silver wire); meanwhile we notice that the cores of the Nagatron transformer coils are pitifully small. Wouldn't it have been better to spend the money on that instead?

PS Audio (interim report)

PS Audio, 3130 Skyway Drive, #301, Santa Maria, CA 93454.

Moving-coil pre-preamp, $180. Tested prototype, on loan from manufacturer, We can't make a definitive evaluation here, since the production model that PS Audio ended up selling to the public is different in some details, though not in basic design, from the very early version they had sent to us for testing.

What's more, consistency in production doesn't look to us like PS Audio's long suit, even if their circuitry always shows considerable insight into what makes audio electron ics sound good. So-be prepared for possible surprises and/or disappointments.

That said, we can unequivocally state that the AC powered unit we tested was sonically superior to the competition (moderately priced pre-preamps such as the Marcof PPA-1 and Audire 'Poco') in every way. PS Audio's one transistor class A circuit without feedback sounded cleaner, smoother and more transparent than the others; it made a good showing even in comparison with the better transformers, though by no means surpassing them. This is the kind of sound you can accept at face value without any complaints, until a step-up device with even better resolution and focus comes along, making you realize that a little something was still missing.

We're therefore making the PS pre-preamp our tentative Reference B recommendation, pending a final test of the actual production model and reliability reports from the field.

RWR Audio MCT-1 (follow-up)

RWR Audio Ltd., Box 3080, Station D, 340 Laurier Avenue, Ottawa, Ont., Canada K1P 6H6. MCT-1 Moving-Coil Transformer, $450 (direct from factory) or $600 (suggested list price in U.S. stores, if and when available). Five-year warranty. Tested #18006038, on loan from manufacturer.

Now that we've tested a production sample of the RWR transformer, we can report that the ranking established in our last issue on the basis of two preproduction units still stands. This is a very fine transformer indeed, but it's still audibly second to the Cotter MK-2 (or to the Cotter-designed Verion MK-1, for that matter) in the various ways discussed in our original review. As a matter of fact, the differences have become more distinct with the improved MC cartridges of recent months.

The production version also exhibited slight anomalies in its square wave response, but somewhat different in appearance from those we had observed in the earlier samples.

We measured a rise time of 2% microseconds, 5% overshoot (but with the second ripple of larger amplitude than the first) and a total ring pattern of approximately 12 microseconds before complete damping. Is this audible? We can't be sure.

Again, the two channels weren't absolutely identical; no two RWR channels seem to be, at least so far.

On top of it, the price has gone up 50%, but you didn't really expect it to remain at the original bargain level, did you? Recommendations We aren't too happy with the following two choices, since the first is extremely costly and constantly rising in price, the second somewhat different in production from what we tested. On the other hand, these are the best we've found so far in their respective categories, and we can't in good conscience recommend second best.

Best step-up device for moving-coil cartridges, regardless of price: Cotter MK-2 transformer (see also Cotter System 2 in this issue, under preamps).

Best MC step-up device per dollar: PS Audio pre preamp (tentative rating-see review).

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[adapted from TAC, Vol.2, No.3 ]

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Also see:

Box 392: Letters to the Editor

Various audio and high-fidelity magazines