The basic concept of the ZERO Audio Autoformer is to effectively increase the impedance of any speaker so that it becomes the most effective load for the amplifier used. For example a M-60 (8ea 6AS7 OTL) puts out the most power into about 16 ohms and does so with less distortion than when driving a lower impedance load. The ZERO can be used to double (2x) an 8 ohm speaker, or quadruple (4x) a 4 ohm speaker to make either of them 16 ohms. All of the impedance multiplication factors available with the ZEROs are as follows: (2x), (3x) and (4x). Virtually any speaker can be made to feel like the optimal load for the amplifier used, thus operating the amplifier in its maximum power transfer region.

Important: Keep all unused taps isolated from each other. Touching any two taps together is essentially the same as shorting out the amplifier.

Using the ZEROs will not affect a speaker's efficiency, and it has been found that when paired with an OTL amplifier, the speakers will play at about the same (if not greater) volume level with the ZEROs installed and the volume control left unchanged.

The ZEROs have a very low DC resistance value of about 0.3 ohms. This allows for a very efficient transfer of power and has the added benefit of protecting the speaker as it will bypass almost all of the DC current in the unlikely event of a tube short. This low DC resistance will alter the offset voltage meter reading (Atma-Sphere OTL amplifiers) in a similar way, but to a greater extent, as having a low impedance speaker connected. I recommend that you set the amplifier's bias and offset voltage prior to installing the ZEROs, and then view the bias and offset voltage reading again after the ZEROs are installed, so you can see the effect. Be aware that the ZEROs do not change the bias nor the offset voltage , just the meter's ability to display the actual readings. I have installed my meter differently than the factory does. My way displays the correct bias and offset voltage meter readings under any condition. For more details visit: http://www.otlamp.com/tweaks/ps/dis3/index.html.

The ZERO autoformer holds its impedance charter down to 2Hz. But still 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 3A fast blow fuse be installed in series between the SS amplifier and the ZERO. The ZEROs will protect the speaker from DC, the fuse is there to protect the amp from itself if it does try to push DC.

I recommend that you install the ZEROs close to the speakers so that there is essentially no speaker wire run from the ZERO to the speaker. This is the low impedance / high current side of the autoformer. The longer speaker wire run should be from the amplifier to the ZEROs, which is the higher impedance / lower current side of the ZEROs.

This may seem to cause a problem if your speakers are bi-wired. Yet the advantage of using the ZEROs to reduce the current in the speaker wire run may be comparable to bi-wiring. You may want to first install the ZEROs at the amplifier and continue to use the bi-wire run to your speakers. After using this configuration for a few weeks, re-configure the set up so the ZEROs are at the speakers and a single speaker run goes from the amp to the ZEROs as described in the above paragraph for comparison. I get better sound in my system this way, so I have abandon using bi-wires and I was able to upgrade my speaker wires for half the cost.

Finding the best speaker impedance multiplication factor is not difficult. Start by using the 2x and then the 4x multiplication factors to get the feel of the ZERO's effect. As the speaker's impedance is increased you will notice a decrease in distortion, an increase in resolution, and a tightening of bass control. Start with a low multiplication factor and step up to higher multiplication factors. You will know when you have gone to far when the system starts sounding dry. This happens when the amplifier has over damped the speakers. When this happens, the multiplication factor needs to be reduced again. Trust your ears, and when you achieve the best sound possible, you are done. It is likely that you will find that the best sound is achieved when running the OTL amplifier in its maximum power transfer region. This will be loading an Atma-Sphere MA-1 with 8 to 10 ohms, Loading a M-60 with 14 to 16 ohms, and loading a S-30 with 22 to 26 ohms.

After you have found how best to utilize the ZEROs for your system, you can now effectively change the number of output tubes without affecting the optimum impedance relationship (and optimum sound) that you have established. For example if you have found that you get the best sound using the ZEROs as a 4x impedance multiplier, then you can try removing half of the output tubes, but compensate for it by using the ZEROs as a 8x impedance multiplier, so that half as many tubes are presented with half as difficult of load to drive. The system should sound about the same but will play at 3dB less volume.

A number of ZEROs owners have asked if there are any other multiplication factors available. In fact there are. The Yellow and Blue leads are designed to be connected to either the speaker or the amplifier, which opens up a variety of interesting configurations. I have pulled together all available configurations into 3 different groups. Many of them are ridicules multiplication factors, and will not sound good at all. Others are quite interesting possibilities. For example, if you are currently using 2.75x, you will find a 2.9x in "Group 3 Configurations" that you may want to try. There is also a 2x configuration in all three groups. They may sound slightly different from each other, so find which one works best for your system.

Paul Speltz
651 735 0534
PaulSpeltz@hotmail.com