The ZERO's Story
by Paul Speltz

Low impedance speakers are not a good match for small amplifiers, especially vacuum tube Output Transformer Less (OTL) designs, but that was the combination I found myself living with a few years ago. The basic problem at hand was that the woofers were electrically under damped which caused a bloated sounding bass. After considering and trying a few obvious solutions, I used an autoformer to transform my low impedance speakers to high impedance speakers. This increased the amp to speaker damping factor. I found that in addition to the expected results, some unexpected benefits.

Mismatch Troubles
As a veteran speaker builder, with the experience of roughly 10 different speaker designs built from scratch, I had essentially settled into a speaker design I was happy with.  I was running a 3-way system using Dynaudio and Peerless drivers making a nominal 4.7-ohm load speaker system. Being content with my speakers, I started exploring amplifiers, which lead me to tube amps, and then OTL tube amps. By definition, my OTL amp's output tubes need to directly handle the low 4.7-ohm impedance load of my speakers. I would have been set if I had previously designed and build a 16-ohm speaker system that would have been more OTL friendly.

Isn't this just how it goes some times; I spent years designing, building, changing, tweaking, and then finally settling in and loving my DIY speakers. Then I borrowed a friend's OTL tube amps. Even though the OTLs didn't properly dampen the 4 ohm Peerless woofers, music poured out of these amps and brought life to the music in a way that there was just no going back to a more traditional/popular amplifier design. I knew I had to build myself a kit pair of Atma-Sphere M-60 OTL mono blocks (no longer available in kit form).

I considered kicking off another speaker project, this time a 16-ohm design, but I was never able to build up enough steam to take it past the concept stage. I had already experienced the benefits of no feedback in tube amplifiers, so I was unwilling to increase the damping factor of the amps that way. I instead decide to address the overly blooming bass with a bigger OTL amp. This would lower the amp's output impedance and raise the damping factor. I did this by building up my M-60 kit using larger output power supplies, and loading up each amp with 8ea 6336B tubes per channel in place of the stock 6AS7 tubes. Each 6336B tube is roughly equal to two stock 6AS7 tube so I had essentially doubled the amp. With the "hot rod" amp biased at 1 amp, I achieved the woofer control as expected. The ability to flow twice as much current gave me 4 times more output power as well. This worked out well all winter, but with about 1200 watts of heat being dumped into the room, I wanted a different solution for the summer time.

Autoformer Ideas
After hearing that autoformers had been used to drive extremely low impedance (1-2 ohm) speakers, I tried a wild experiment. I had sitting in my basement a pair of 300VA 50-60Hz toroidal power transformers. I figured out how to configure them into a 2:1 winding ratio autoformer. This would give me a 4:1 impedance ratio and enable me to transform my 4.7-ohm speakers into 18.8-ohm speakers. I was quite surprised at how well they worked. Running them full range I could hear an increase in bass control, and the high frequency was only a bit squirrly. I felt that the bass could get even better if I could get a lower DC resistance value on the autoformer, and the highs would clean up if the autoformer was actually designed for audio. I also tried the same experiment with a 50-60 Hz EI core power transformer, but the results were poor. The EI transformer had no frequency extension. It liked to run only in the 50-60 Hz region that it was designed for.

Further pursuit of autoformers gained me a used pair of Atma-Sphere Z-Music (Auto) Transformers. New units were no longer available, so I felt lucky to get them. These really worked out well. I used them to make my speakers high enough impedance that now during the summer months I could run the amps with the stock tubes instead of my double sized version. One of the hardest pills to swallow for an OTL amplifier lover, like myself, is adding "iron" to the system. But since I liked them, I left them connected to my speakers through the summer.

Autoformer Basics
In general, a power transformer is used to transform voltage and current. An audio transformer is used to transform impedance, but both are really doing the same thing. The winding with more turns will have more voltage, less current, or higher impedance. The winding with fewer turns will have less voltage, more current, or lower impedance. A typical audio transformer has a "primary" winding and a "secondary" winding. For example, a tube amp transformer may be used to convert a 10k ohm plate-to-plate impedance to a 4-ohm impedance for driving a 4-ohm speaker. To achieve this 10k:4 impedance ratio, a 50:1 winding ratio is required. This is because there is a square root relationship between the impedance ratio and the winding ratio in a transformer. This typical transformer also provides electrical isolation from primary to secondary so that the output can be electrically floated.

An "auto-transformer" or "autoformer" is the simplest type of transformer. An autoformer has only one winding with multiple taps available. The impedance conversion is achieved by bringing the audio signal out on a different set of taps than the audio came in on. An autoformer can be used anytime electrical isolation is not required. The same impedance ratio to windings ratio rules apply, that is the winding ratio is the square root of the impedance ratio required. So an impedance ratio of 4:1 (16 ohms:4 ohms) would be achieved with a winding ratio of the square root of 4, so 2:1 winding ratio.

Another way to think of an autoformer is to imagine a typical tube amp output transformer with the primary windings removed. There is only the output secondary winding with 16 ohm, 8 ohm, 4 ohm, and common taps. Now connect a 4-ohm speaker on the 4-ohm tap, and it will be reflected as a 16-ohm load on the 16-ohm tap. I tried using a typical tube amp transformer as an autoformer, but as expected, there was a serious lack of low frequency energy transferred since the transformer was not designed to be used in this manner.

The Autoformer Advantage
There are a few general advantages an autoformer has over a typical transformer. I usually feel that simpler is better, and I think it applies in this case. With only one winding, the music has the advantage of coming out on the same winding that it goes in on, instead of having to totally pass from the primary winding to the secondary winding. I believe that removing the need for the entire audio signal to leap over the electrical isolation of a typical transformer allows for a much greater transfer of the low level information resolved in the music.

Second, the autoformer is "dry"; meaning it is not "soaked" with DC bias current. Accommodating DC current in general limits the frequency extremes. More "iron" needs to be added to keep the core from saturation and a transformer needs to be even larger to do bass well. This larger size tends to limit high frequency extension due to an increase in inductance, so some middle of the road compromise is decided upon. I am not a transformer engineer, but I think I am safe in stating these generalities. As an example, a SET transformer will typically have a more limited frequency response than a P-P transformer, because it is dealing with greater DC current demands.

Third, the impedance ratio is very small. There is only a 16-ohm to 4-ohm conversion, not a 10,000-ohm to 4-ohm conversion. This makes the autoformer's job much simpler, making things like frequency extension, linearity, transparency, etc. even easier.

The ZEROs Story
The summer went by and still feeling good about the used autoformers, I responded to a call for help from an OTL'er with the same OTL amps connected to low impedance speakers. Since the Z-Music Transformers were still not available, I pitched out the idea on the Atma-Sphere Owner's Group (ASOG, visit ) that if a few guys are interested, I would "roll my own" autoformer for them. There was enough interest, so I took on the challenge. From my earlier experiments, I decided the autoformer would have a toroidal core. The ability for audio to pass through a toroidal transformer that was designed for 50-60Hz power impressed me. I also knew I was only capable of specifying the design requirements.

After two or three weeks of investigation, I found an audio transformer engineer willing and capable of the task. I specified the frequency response, DC resistance, impedance of each tap, maximum power, etc. The transformer engineer needed to find the proper combination of core size, core material, wire size, number of windings, etc., to fulfill my design requirements. I was intuitive enough to set the requirements high so that there could be a large fluctuation in operation conditions and have it still function properly. The transformer engineer was smart enough to implement ideas such as using multiple smaller gauge wires wound together to gain the lower skin effect of the smaller wire, but still have the low DC resistance of a larger wire. A few weeks later I had my prototypes and nick named the pair of big round autoformers the "ZEROs".

Evaluating the Prototype ZEROs
I needed to make sure that first, the prototypes achieve the sonic benefits of multiplying the impedance of the speaker load, and second, they do this without showing any sonic signatures of their own. For three days I simply ran them on my system, played a lot of music and listened for any sonic clues that told me that I added "iron" to the system.

I invited myself over to my friend Joe's house and ran them on his excellent system which include Atma-sphere MP-3 preamp and MA-1 mkII amps, and a pair of top of the line Magnepan MG20 speakers. Joe was hoping for better bass from his Maggies. Using the ZEROs as an impedance doubler (2x) to make the 4-ohm Maggies feel like 8-ohms to the amps, it took about 3 seconds to hear that the bass had come out of the background and became a balanced part of the rest of the music. Joe was thrilled and commended, "I'm sold, count me in".

What we started noticing next, was an overall improvement top to bottom. A nice drop in distortion and an increase in resolution made it easier to hear a lot more of everything that was happening in the recording. This was not subtle either. We both quite quickly noticed and described to each other the same things.

After some time, we tried using the ZEROs as an impedance quadrupler (4x) to make the 4-ohm Maggies feel like 16-ohms to the amps. This time the improvement was a little more of the same. If the 2x gave us 5 steps forward, then the 4x gave us just 1 step more beyond that. Yet it was enough for Joe to say that he plans on using them that way. In the 3 hours of listening with them on Joe's system, we were unable to hear any problems with the "ZEROs".

In an attempt to find out if the ZEROs were truly transparent, I listened to music on my system back and forth with them in and out, but compensating for the impedance change of the ZEROs by pulling output tubes. In other words 8ea 6336B tubes per amp biased up to 1 Amp directly connected to my 4.7 ohm speakers, compared to, 2ea 6336B tubes per amp biased at 250mA with the ZEROs used as a 4x speaker impedance multiplier. The thought here is that 1/4th the tubes at 1/4th the bias current driving what feels like 1/4th the load should sound exactly the same if the autoformers are truly transparent. Well, not only were the ZEROs utterly transparent, I have to say the music with just 2 tubes running through the ZEROs sounded a little cleaner in the bass, and the highs were better focused.

I declared the prototypes a success, and had enough ZEROs made up for all the guys that wanted a pair.

The ZEROs Stretch Their Legs
The first batch of ZEROs hit the streets, and everybody seemed to love them. Then a second wave of interest came, so I ordered a second batch. It seemed as though people liked them so much they told their friends about them and also tried them on many different types of amplifiers. Just 16 months since the prototypes and I had shipped six batches of ZEROs all over the United States and other countries including: Canada, England, Turkey, Sweden, Scotland, Australia, Portugal, Holland, Puerto Rico, Malaysia, Italy, Greece, Spain and Thailand.

The benefits of driving a higher impedance load seem to go beyond OTL amplifiers. I have been told by ZEROs users that they gained sonic improvements on Atma-Sphere (OTL), Transcendent Sound (OTL & P-P), VAC (P-P), BAT (P-P), Audio Research (P-P), Bel Canto (SET and SS), LAMM (P-P), Joule-Electra (OTL), Meitner (solid state), Wright/Sound (SET), Cary (P-P and SS), Tenor (OTL), Carver (SS), Jolida (P-P), SONY (SS), NAIM (SS), Rogue (P-P), Futterman (OTL), Graaf (OTL), Decware "Zen" (SET), and some DIY (SET) VTL (P-P), McIntosh (P-P), Forte (SS), Manley (P-P), McCormakc (SS), Odyssey (SS), Monarchy (SS), Wolcott, Dyna, (P-P), Sonic Frontiers (P-P) and Welborne (SET).  It just seems as though most amplifiers sound better when they are driving a higher impedance load.

Although the ZEROs have been used with many amplifiers, I don't consider the ZEROs an amplifier tweak; I consider them a speaker tweak. The ZEROs have been used to increase the impedance of a large variety of manufactured speakers including: Magnepan, nOrh, B&W, Dynaudio, Carver, PBN, Merlin, Martin-Logan, QUAD, Klipsch, SoundLab, Silverline, Vandersteen, Audiostat, Soliloquy, Cabasse, N.E.A.R., Waveform, Shamrock Audio, Audio Physic, Spendor, JM Lab, InnerSound, KEF, Medowlark, Alon, Galo, EV, Montanna, Triangle, Wisdon, Quadrature, Foxtex, Spendor, Omega, Infinity, VMPS, Newform Research, Dayton/Write, Parsifal, Horn Shoppe, Omega, Definitive, Thiel, Bertagni, Oskar, Sonus Faber, Dakini, Swan and Wilson Audio.

Typical reported improvements from people who used the ZEROs to increase their speaker's impedance are:

  1. Lower distortion
  2. Firmer bass
  3. Higher resolution
  4. More extended and better-focused high frequency
  5. Natural, effortless, and organic

The combination of lower distortion, higher resolution, and more extended / better-focused highs often resulted in a larger soundstage with better imaging. Greater dynamics and an increase in the instrument's tactile texture seemed to improve as well. Click here for a complete list of testimonial field reports.

I have to ask myself why does increasing the impedance of a speaker lead to such globally common reported improvements. It goes against my audio philosophy that adding an extra component in the signal path can bring the listener closer to the original musical event. It must be the component added can provide greater benefits than "penalty". I believe the ZEROs are very transparent so the penalty is small. Imagine the power of "magically" changing the impedance of your speakers. What's left are the gains the amplifier and wires provide when driving an easier load. I can not prove anything, but I suspect it is simply easier for an amplifier and speaker wire to generate and transfer voltage than current. One watt into 4 ohms requires 2 volts at amp of current. One watt into 16 ohms requires 4 volts at 1/4 amp of current. That's twice the voltage and half the current for the same amount of power.

Running high voltage / low current lines are a necessity for both the power utilities (high voltage power lines) and commercial audio installers (70v paging systems) where high voltage / low current is generated, distributed, and then transformed to lower voltage / higher current when it has arrived to its destination. The advantages of running low current lines are well understood, so it makes sense to apply it to our hobby as well.

Add a Resistor?
If amplifiers and speaker cables like to drive higher impedance loads, why can't I simply add a large 10-ohm power resistor in series with my 4-ohm speaker? That approach will lighten up the load on the amplifier, but bring back the original problem of an electrically under damped woofer which caused a bloated sounding bass, and over 2/3 of the audio power will be lost as heat into the resister as well.

Some Final Thoughts
I will never tire of hearing the enthusiasm of fellow audio hobbyist as they report the sonic improvements gained by simply increasing the impedance of their speakers using the ZEROs autoformers. Even 8-ohm loads are not optimal. Half of the ZEROs owners used them to increase the impedance of their 8-ohm speakers. Multiplying the reported best sounding ZEROs multiplication factor times the original speaker's impedance, and then averaging all the results, I found the average reported optimal impedance to be 14.6-ohms.

Paul Speltz, 2002


The Industry Mistake
I feel the speaker industry has made a mistake migrating from the vintage norm 16-ohm speaker to today's short circuit designs. (A couple of obvious offenders are the Apogee Scintilla 1-ohm design and the Apogee Duettas Signatures with a stated manufacturer's impedance of 3/4 ohms) I think this has happened because a 4-ohm speaker will draw four times more current from a solid state amplifier than from a 16-ohm speaker at the same volume (voltage) setting, thus sucking four times more power from the amplifier (again at a set volume setting). This helps make the speaker play louder in the showroom when it is going up against competitors. I think "perceived" efficiency helps sell speakers, and this is why speaker manufactures have gone astray.

This has gotten so out of hand that the speaker industry stopped rating a speaker.s "efficiency" and started rating it as speaker "sensitivity", which is how loud a speaker will play at a given voltage rating no matter how much current (and power) it is sucking out of the amplifier.

This aspect of the speaker industry has got to change back. After meeting with and then writing a letter to the Editor of Stereophile on this issue, John Atikson has noted in his "Measurements" sections of low impedance speakers, that the speaker is actually drawing more power than the 1 watt standard. It's a start, so good for him. Below are three examples:

1) July 01, p.65, Martin Logan Prodigy review:

"....indicates it to be, as specified, basically a 4 ohm load through much of the audio band. This will mean the speaker will actually draw 2W from the amplifier from that voltage level, not 1W."

2) August 01, p.70, Krell LAT-1 review:

"However, as shown by its impedance plot, it is a 4 ohm design, drawing 2W rather than 1W from the partnering amplifier to reach this measured sensitivity."

3) August 01, p.79, Audio Physic Avanti III review:

"...revealed it to a 4 ohm design; ie, it actually draws two watts from the amplifier to raise the rated sound pressure level."

Speaker Efficiency vs- Sensitivity
Speaker efficiency is not the same as speaker sensitivity. Years ago, all speakers were specified as having an efficiency rating, now speakers seem to be specified with a sensitivity rating. I think that the speaker industry needs to get back to rating a speaker's efficiency, or at the very least, we as Audiophiles need to understand the difference.

Efficiency is the true measure for speakers. It is like miles/gallon to the automobile, but speaker sensitivity is like saying how many miles an automobile can go on a tank of gas. If we don't know how many gallons of gas are in the tank, it is meaningless in terms of efficiency. This is the speaker industry's way of making 4-ohm speakers with 2-ohm impedance dips sound as if they are efficient.

Speaker efficiency is specified as sound pressure per watt (like miles per gallon of gas). In other words, how much acoustical power is put into the air, for how much electrical power is put into the speaker. The rating is typically given as dB per 1 watt measured at 1-meter distance.

Speaker sensitivity is specified as sound pressure per volts (like miles per tank of gas). In other words, how much acoustical power is put into the air, for how much voltage is put into the speaker, regardless of its impedance. The rating is typically given as dB per 2.83 volts measured at 1-meter distance.

It is true that 2.83 volts RMS into 8 ohms is equal to 1 watt of power, but now the "sensitivity game" is played and a speaker is said to put out 90dB at 2.83 volts. The truth is that a 4-ohm speaker at its 2-ohm impedance dip, is requiring 4 watts of power, not 1 watt like they would like you to believe.

It is true that using an autoformer to transform a 4 or 8 ohm speaker into a 16 ohm speaker will decrease the speaker's sensitivity since it will take more voltage (but less current) to achieve 1 watt of power, though the speaker will still remain just as efficient. Actually some systems will have the ability to play louder if the amplifier being used has a maximum power band curve that peaks around 16 ohms. Some OTL examples are the 60-watt Atma-Sphere M-60 that will do 80 watts into 16 ohms, and the 25-watt Transcendent Audio Stereo T8 that should do 40 watts into 16 ohms. Sure most solid state amplifiers can provide more power into a 4-ohm load, but do they sound better that way? My SONY ES series receiver sounds a lot better when I increased my speaker's impedance to 16 ohms. I am also able to listen to the music at louder levels without feeling like I want to turn down the volume. Others have reported similar results with their solid state amplifiers.

A word of Caution
The ZERO autoformer holds the speaker's multiplied impedance charter down to 2Hz, yet the low 0.3 ohm DC resistance may cause trouble if using a Solid State amplifier with excess offset voltage. As little as 300mV of amplifier offset voltage will cause 1 amp of DC current to flow. Also, if the SS amplifier's offset voltage changes dynamically with the music, a large amount of DC current can flow as well. Because of this, I recommend that a fast blow fuse (probably about 3-5A) be installed in series between any SS amplifier that is not fault protected, and the autoformer. The autoformer will protect the speaker from DC; the fuse is there to protect the amp from itself if it does try to push DC.


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