June 2003

The "ZERO" Audio Autoformer 
Loudspeaker Impedance Multiplier
Review by Dick Olsher
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  Paul Speltz is a hobbyist turned manufacturer, who has produced a nifty device that ought to be seriously considered by tube amplifier owners of the output-transformerless (OTL) variety, as well as those fanatical about transformer-coupled, low-power, single-ended triode (SET) designs.

What Exactly Is A ZERO?

No, it's not the Japanese fighter plane that bombed Pearl Harbor. A ZERO is an auto-transformer or autoformer that connects between the loudspeaker and power amplifier and allows the end user to adjust the effective loudspeaker impedance. The impedance seen by the amplifier can be multiplied by up to a factor of four, for example, turning a 4-ohm loudspeaker into a 16-ohm loudspeaker. What do you get when you put a ZERO in an exotic, solid-wood, box with Cardas binding posts? The answer: a ZERO Box!

Unlike a conventional output transformer with primary and secondary windings, an autoformer uses only a single winding for all connections. The impedance transformation at the amplifier end is accomplished by tapping off winding sections larger than those used by the loudspeaker. Unlike conventional tube output transformers with large primary to secondary impedance ratios, the ZERO's design and mild transformation ratios make for an extremely extended bandwidth of 2Hz to 2MHz.

Why Should I Bother?

The ZERO operates right in the middle of the amp-loudspeaker interface allowing greater flexibility in fine-tuning this critical boundary between electrical and acoustical domains. One immediate benefit is control over the loudspeaker's current demand.

The physics are quite simple and dictate that for a given sound pressure level (SPL), the current demand decreases with increasing impedance. A 16-ohm loudspeaker requires 50% of the current drive of a 4-ohm loudspeaker - for equal power delivery. So why the shift from the 16-ohm spec prevalent in the 50s to a multitude of 4-ohm designs in the last two decades? The answer is that for a given setting of the volume control, a 4-ohm loudspeaker will exhibit a 3 dB increase in SPL relative to an 8-ohm design, and a 6 dB increase relative to a 16-ohm loudspeaker. This is reflected in the sensitivity rating of the loudspeaker, which is referenced to a voltage of 2.83 volts across the loudspeaker terminals. Using the well-known relationship, Power = V2/R, and substituting V=2.83 volts and R = 8 ohms we obtain 1 watt. However, for the same input voltage, and R = 4 ohms the answer is 2 watts. That means that for a given setting of the volume control, more current rushes into the 4-ohm load, raising the power delivered to 2 watts. That is precisely why sensitivity ratings are misleading since they do not denote SPL for identical power inputs. In fact, a loudspeaker that sucks more current and power out of an amplifier is rewarded with a higher sensitivity rating. In an age dominated by high-power transistorized designs, "short-circuit' loudspeaker loads may be no problem, but for us guys enchanted by the siren call of OTL and SET, such loads are most definitely problematic.

Do not confuse loudspeaker sensitivity ratings with true efficiency. They are not synonymous. Two 4-ohm, 90dB sensitive, woofers connected in parallel result in a broad impedance minimum of about two ohms. The overall sensitivity of such a loudspeaker would be 93 dB and might tempt someone to consider matching such a load with an OTL or SET type amplifier. Don't do it! You won't be happy with the results. The situation may even be worse, in terms of current demand, when the effect of complex crossover networks is considered. The impedance will dip even further, possibly as low as 1 ohm, around the crossover region due to the effect of shunt capacitance in the woofer's low-pass network. Assuming you already own an average audiophile loudspeaker with the ubiquitous 4 to 8-ohm nominal rating, the ZERO offers a chance of sonic redemption. There have been many field reports (as documented on Paul Speltz's website) from Atma-Sphere and Transcendent Sound OTL users extolling the virtues of the ZERO. Ramping up the loudspeaker impedance to 16 ohms with the ZERO is a no-brainer in these cases, as it increases power delivery and reduces distortion. I am sure that the late great Julius Futterman is presently smiling upon ZERO users from OTL heaven.

What If My Tube Amplifier Provides
A Full Complement Of Impedance Taps?

OK, that is a reasonable question. After all, the purpose of providing 4, 8, and 16 ohm output taps is to maximize power transfer to a given loudspeaker load. For example, if I were to connect a 4-ohm load to the 4-ohm taps, wouldn't that obviate the need for the ZERO? Not necessarily. The best sounding taps are often the 8 and 16 ohm ones. Inserting the ZERO into the chain makes it possible to multiply the loudspeaker impedance by a factor of two or four and then experiment with the higher impedance taps. It is always a good idea to experiment with a higher impedance tap, since it's not an easy matter to predict the optimum connection. The problem is that a loudspeaker's impedance magnitude varies by at least a factor of two from its nominal rating over the audio spectrum. An 8-ohm rating could mean 4 ohms - or even 2 ohms - in the lower midrange. Playing strictly by the numbers is no guarantee of success. Let the ZERO help.

What About The Sound?

First, the system details. The specific amplifier-loudspeaker interface during the review period was the T-Rex 300B SET, a DIY design project in the works (still unpublished) using Plitron output transformers with only 8-ohm taps, and the Lowther DX4 BassZilla - a 97dB efficient loudspeaker with an 8-ohm nominal impedance rating. Some of you may already be wondering if the added complication of the ZERO is really necessary in the context of a loudspeaker system combining high-efficiency and benign impedance magnitude. The possibility that the ZERO may simply get in the way suggested itself, but I was eager to experiment and explore higher impedance territory. Neither was I initially concerned about impedance miss-matching issues, as factor of two differences are non-critical in terms of maximum power delivery. What I was after were the benefits of reduction in current drive and distortion, and enhanced damping factor.

Installation was a breeze, using short sections of Audio Note silver cable. The ZERO benefits from an extended break-in period. For the record, the recommended break-in period is 50 to 100 hours. Starting at X2 impedance multiplication, I settled in for an extended "first impression" listening session. At the time I was in the process of tuning the T-Rex's power supply B+ regulation, which in its early version, lent a bit of brightness to harmonic textures. From T-Rex's perspective, the ZERO made the BassZilla "feel like" a 16-ohm load, the result being better control over resonances in the upper midrange. Harmonic textures became smoother and richer; the overall sound being very relaxed and eminently listenable. Finally, bass lines were more tightly defined. Next, I experimented with impedance multipliers of X3 and X4, and found that these settings over damped the sound. By that I mean that microdynamic nuances were squelched, robbing the music of its drama. It was time to retreat to the X2 setting for the duration of the evaluation. So far so good.

After I finished tweaking T-Rex's shunt regulator, I decided to undertake another round of comparative listening - with and without the ZERO. T-Rex has been a living and breathing thing for almost three years, during which time I've become sensitive to small changes in its sound quality. In other words, consider it to be a sonic "microscope" able to magnify sonic differences. At his point, it became clear that although bass lines still benefited from the ZERO, overall the T-Rex - BassZilla interface sounded better with the ZERO out of the way. Small, but noticeable errors of commission crept in. Soundstage transparency diminished, which reduced the music's intimacy. Microdynamics now sounded slightly over damped, releasing some of the music's tension. The midrange, which for me paints a window onto the music's soul, became slightly cloudier. On balance, I would rather give up 20% of bass definition for a 10% increase in midrange clarity.

Conclusion

The ZERO, in my opinion, represents an important new product category that allows music lovers to actively modify and control the amplifier-loudspeaker interface. It is not a panacea, and as you can see from my experience, it does not always pan out. However, if you're the proud owner of an OTL or a low-power tube amplifier and presently driving a 4-ohm loudspeaker load, you owe it to yourself to give the ZERO a try. Even some so-called 8-ohn loudspeakers present significantly lower impedance in the lower midrange and in the vicinity of complex crossover networks. The ZERO is available direct at reasonable cost and comes with a 30-day trial period.

Specifications

Pricing (shipping included): 
ZERO with 18-inch long magnet wire leads with spade connectors $450/pr.

ZERO Box: $850 per pair to $1,050 per pair depending on wood finish

Company Information

Paul Speltz
Voice: (651) 735-0534
E-mail: PaulSpeltz@hotmail.com
Website: www.zeroimpedance.com