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 ½ 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 http://www.otlamp.com/ )
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:
- Lower distortion
- Firmer bass
- Higher resolution
- More extended and better-focused high frequency
- 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.
Why?
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 ½ 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
