What About ZERO Autoformers?

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For the last few months I've been seeking to find what might be possible to help my Bob Latino M-125 tube amps be more comfortable with driving my Expressions such that the upper frequencies above 7kHz would be better represented. To this end I've replaced a resistor as suggested by Bob's former partner (Bob recently retired) Roy, and also per Roy changed the driver tubes. These changes helped with some things including a really tiny amount of tilt towards extra treble, but across a larger band of frequencies than I really want - but I kept these changes in play for the other attributes they bring which include cooler operation of some key components and some extra "sparkle" I've been wanting in the sound.

Enter Anticables' ZERO Autoformer. I've looked at these for a few years but never got around to doing anything with them until now. With the small amount of actual customer information out there about what these really do for someone like me with my amps and my speakers, I decided to take the 60 day challenge.

The length, gauge, and type of wire I've come to use for speaker cables has all come about via trial and error listening. I've been happiest with what I've been using. I go where the sound quality leads me.

The instant I connected a ZERO there was a noticeable difference in treble. I began with the 2X impedance multiplier, then tried the 3X. This was with using the speaker cable I've been using. So, some good stuff happening, but maybe some stuff - while not bad, was not what I wanted, namely a lot less output. The whole reason the speaker cables are the length they are is because shorter cables of this design don't sound as good for this combo of amp/wire/speaker.

So I cut out the middle man and connected the amps directly to the ZERO's and started over with the 2X connection and voila', just what I wanted! The sound is the same but better. Still have the dynamics I've been used to, a lot more output from cymbals and other high frequency percussion, and the power transformers run cooler. When I ran REW it confirmed what I heard. About 1dB more output from the Expression's XO of 300Hz on up, but there is a tilt from 6kHz on upward such that there is 4dB more output at 10kHz which extends out to losing only 1dB from there to 12kHz.

As far as I'm concerned this is great! There is one caveat. There is about 3dB less output overall of the frequencies below 6kHz. What happens is that the output below 6kHz is less while above 6kHz it's 1dB more, which makes the relative difference +4dB in the higher frequencies.

I've read some forum posts saying these are a bandaid. I don't see it that way. These are tuning devices, exactly the same as using a different speaker cable. They also are not transformers. They won't work with every combo of amp/wire/speaker. The user must experiment to find out what might be the correct combination of factors, or only to discover the ZERO's aren't needed in the first place.

For me, this is a winner. I'll put up with less output overall when I get to keep the dynamics I'm used to below 6kHz, and get more output AND more dynamics in the top frequencies I knew were lacking, and also keep the fantastic soundstage these amps are known for.

I created this thread to get this info out there all on its own so it might benefit others. Maybe in a few days or next week I will also post about using the ZERO's with another amp where I'm finding some interesting improvements where I expected there to be none or a definite degradation of sound quality.

 

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I measured the impedance of a ML 13A Expression, first without, and then with, a AntiCables ZERO Autoformer connected so the impedance is 2X (2 times the impedance). And it pretty much is double, plus or minus a bit. I'm not using UL quality laboratory testing equipment or anything, but it's close enough for hobbyist level accuracy. I used a Krell DUO XD amp at a very low volume level. For the lowest frequencies I was forced to use a higher Input Level to keep the Current high enough above the noise level of the probe I used, otherwise it wouldn't be accurate at all. I created a text file to import into REW for the graphing, so disregard the fact that it shows SPL in the top left corner. I entered the values for Impedance into the part of the text file normally reserved for dB numbers. It's just a quick way to make a graph is all.

This plainly shows why we need amps that can double as much as possible. Just look at the Current as the impedance falls below 3Ω.

Using the ZERO pushes the impedance drop much higher in the frequency sprectrum. I tested to find the exact frequency where the impedance drops to 2Ω, and it is 12.9kHz. The plot below doesn't show this because I didn't graph any points between 10kHz and 16kHz, so it's just a straight line.

The Red impedance trace is without the Autoformer, and the Blue trace is with the Autoformer. The Red trace follows very closely to the drop in frequency response when I used my tube amps with these speakers prior to getting the ZERO's. Instead of dropping output level at around 7-8kHz, the tube amps would drop output level at around 11kHz when the ZERO's were used, so this puts the difficult impedance load at about 2.5Ω at which point the output level would drop at a rate of about -2dB per kHz up to 15kHz and then drop very steeply from there.

I don't really understand how ML calls these speakers 4Ω. I thought it is generally accepted that the rating of a speaker is stated such that the impedance is never less than 80% of the rating from 20Hz to 20kHz. That would put the absolute minimum value at 3.2Ω, which is quadruple the actual minimum impedance. ML states the impedance to be 4Ω, and 0.7Ω @ 20kHz, so how does 4Ω enter into the spec? It's only "compatible with 4Ω rated amplifiers" up to 6kHz. Anyway, I lost track here.