Analog Corner #284: Air Tight PC-1 coda phono cartridge, Zesto Andros Allasso step-up transformer Page 2

A step-up transformer has two coils of wire—a primary and a secondary—wound around a magnetically permeable core. (Among some transformer experts, cobalt-based amorphous alloys and ferrite-based nanocrystalline amorphous alloys are considered the best core materials.) The coils themselves are usually of copper wire, although silver is used in some of the more costly ones. (Those who claim copper and silver can't sound different from one another haven't listened.) Voltage gain is largely determined by the turns ratio between the two coils: If the primary and secondary have the same number of turns, a volt in would result in a volt out. If the secondary coil has fewer turns of wire, voltage output would be reduced, and if the secondary has twice the number of turns as the primary, the input voltage would be doubled—and so forth.

Transformers have some disadvantages, the biggest being cost. The very best are expensive, but there are also some very good ones at reasonable prices. The other big disadvantage has to do with transformer specifications and cartridge loading. Unlike with head amps, loading and gain can't be set independently with an SUT.

Or, as Ypsilon Electronics' Demetris Backlavas, who winds his own coils and is a leading authority on transformers, once told me: "I know it's confusing with transformer specs. It's confusing even for engineers. This is because a transformer itself has no intrinsic impedance. It simply reflects impedances modified by the square of the turns ratio, from one winding to another."

This is where transformer life gets difficult for the mathlexic, especially me—but the basics are important, and have wide application throughout audio.

When you connect any source component to a preamp, or any preamp to an amplifier, the device you're plugging into—the load—should have an input impedance that's at least ten times that of the source impedance. Otherwise, you get voltage loss. That's why the rule of thumb for phono cartridges is that the source (the cartridge) should see a load of at least 10 times its internal impedance. If the cartridge's internal impedance is 5 ohms, the load impedance should be at least 50 ohms.

Here's where it gets difficult: A transformer with a turns ratio of 1:10 will produce 10 times the voltage at the secondary but because transformers are passive devices, the increase in voltage results in a decrease in current Twice as many turns of wire on the secondary results in much higher impedance there, and much lower impedance at the primary. To calculate the load that an SUT presents to a cartridge, divide the resistance (in ohms) connected to the secondary with the square of the turns ratio expressed as a single number—ie, a 1:20 ratio is expressed as 20, and its square is 400. (I'm leaving inductance out of this discussion.)

And this is where my eyes glaze over—although I have done the calculations needed to load the secondaries of my Ypsilon MC-10L (1:10 turns ratio) and MC-16L (1:16 turns ratio) transformers, to better match them with my cartridges. (For this I use high-quality Vishay resistors, soldered into similarly high-quality RCA plugs; by doing this, the additional resistance is in parallel with the 47k ohm input impedance of the phono preamp to which the SUT is connected.) For instance, after adding 15k ohm resistors, the cartridge plugged into the MC-16L sees 44.4 ohms (combined in parallel, 15k ohms and 47k ohms produce 11,371 ohms—which, divided by the square of the Ypsilon's turns ratio of 16, works out to 44.42 ohm); for a cartridge with a super-low internal impedance of between 2 and 4 ohms, that's close to ideal. Without those resistors, the input impedance of the MC-16L, connected to a 47k ohm load, is 200 ohms.

And that is the problem solved by Zesto Audio's Andros Allasso step-up transformer ($2995, footnote 2): Transformer-coupled phono preamps that offer no loading options can produce nonflat frequency response depending on the cartridge used—which, if your goal is the best possible sound, limits your choice of cartridge.

Zesto's George Counnas designed the Allasso (Greek for change or transform) to be both incredibly flexible and easy to use. Inside are two transformers of unspecified manufacture (Lundahl? Jensen? someone else?) that use four taps to produce, at the twist of a knob, a gain of 12dB (1:4), 16dB (1:6), 18dB (1:8), or 22dB (1:12)—as those settings are labeled on the Allasso's front panel. Each setting, of course, changes the input impedance, so in addition to those four choices there's a 10-position loading knob on the secondary for a total of 40 options ranging from 20 to 2375 ohms, depending on the combination of gain and loading selected.

Best of all, rather than overload your primary and secondary coils, Counnas says that "it's about the best sound, not the math." He suggests beginning with the middle load position (5). I'd still use math to set the gain, based on the cartridge's output, but you're free to experiment while listening. Turning the Andros Allasso's knobs produced no thumps or bumps through my speakers.

In addition, there's a Stereo/Mono knob. Used in conjunction with two ground-lift toggle switches on the rear panel, one each for the left and right channels, this is extremely useful for eliminating hum. Also in back are single-ended (RCA) and balanced (XLR) inputs, and a single set of output jacks (RCA). Set to Mono, the Allasso amplifies only the left-channel signal. The supplied ground wire proved useful when connected to the Allasso and my Ypsilon VPS-100 MM phono preamp.

Dead-Quiet, "Flat" Performance: I used several cartridges whose sound I know well with the Zesto Andros Allasso: everything from the Lyra Atlas (standard 0.5mV output) and Atlas SL (0.25mV) to the Ortofon MC Century (0.2mV). Each was easily tweaked to perform as expected in terms of frequency response. Normally, getting the best out of these cartridges with my reference Ypsilon transformers requires swapping between the MC-10L and MC-16L and sometimes inserting or removing resistor plugs, depending on the cartridge's output.

The Zesto's Stereo/Mono switch was very useful with Miyajima Laboratory's Infinity Mono cartridge, which in my system produces a hum in one channel, unless I flip my darTZeel NHB-18NS preamplifier's Mono switch. If your preamp produces hum with mono cartridges but lacks a Mono switch, the Andros Allasso could be the solution.

I played the spectacularly natural-sounding Yuko Mabuchi Trio (2 45rpm LPs, Yarlung YAR88157-161V). The sound quality of this set by a jazz piano trio, recorded live to tape with an AKG C24 stereo tube microphone, reveals all. The sounds of Mabuchi's piano, Del Atkins's double bass, and Bobby Breton's drums is stellar in every way, but especially the piano, an instrument whose sound is very hard to capture convincingly. The overall transparency and sense of three-dimensional space is intense and sensational. And Mabuchi is tremendously talented—this is one audiophile recording in which the talent is commensurate with the sound quality.

Did the Zesto Allasso's transformers sound as good as the separate, far more costly Ypsilons? No. The Allasso's transformers softened somewhat and slightly blurred instrumental attacks and textures, and somewhat reduced this recording's you-are-there transparency and intense three-dimensionality—but I heard these differences only in direct comparisons with the Ypsilons. Otherwise, I didn't know what I was missing—until I swapped out the $2995 Zesto for my $12,400 worth of Ypsilon transformers.

As a replacement for the "fixed" (and thus compromised) step-up transformers built into your favorite MC preamplifier, or to add MC capabilities to a high-quality MM phono preamp, Zesto Audio's Andros Allasso is a smartly designed, reasonably priced piece of analog kit!

Footnote 2: Zesto Audio, 3138 Calle Estepa, Thousand Oaks, CA 91360. Tel: (805) 807-1841. Web:

PeterPani's picture

This is the first time I can follwo the calculation of ratio and loading, so I am able to calculate the right windings for my (unusual types of) cartridges.
Q: with the Zesto, did your calculated values give the best sound results or did you end - after trial and error - with other switsch positions?
(the Yuko Mabuchi Trio I got on r2r)