Slagle Autoformer Volume Control Modules And My First DIY
This is my first completed DIY project #I01 or I owe one. A while back I found a 1930's Phillips battery box and had refinished it for some
project in the future. Ok I knew what I wanted it for as I was already
a Slagle AVC fan. There are no power supplies or resistive parts which made it a good first project. Dave Slagle
has cured me forever wanting or needing a traditional preamp. The AVC has been one of the biggest improvements I've made to my system.
I really dig the 80 years of attitude the box has picked up and glad it still shows. Problem is now I want to use this look again but I need bigger boxes for amp building.
This is the best shot I got of the Slagle autoformer modules as I wanted to get them loaded ASAP. 
These Autoformer
modules operate in a slightly different manner than traditional linearly
switched volume controls. Rather than using a single switch, two
discrete switches are used to control the level. The switch on the left
has 12 posiitons that are in 3.75dB increments. The switch on the right
has 3 positions allowing +1.25dB, 0dB and -1.25dB modification of the
overall level. By using this dual switch arrangement 1.25dB steps are
possible. The total attenuation range goes from +1.25dB to -41.25 dB in
32 steps. The nature of the device skips the -3.75dB tap so the steps
are +1.25, 0, -1.25, -2.5, -5, -6.25, -7.5...1.25db steps to -41.25 and
off.
Q
- Why use an autoformer volume control? I've been happy with a good
quality potentiometer.
A
- The simple answer is that most people find that the autoformer volume
control sounds significantly better than even the best potentiometer
or stepped attenuator. And it's not a subtle difference; most people
report that the autoformer has such an open, effortless sound that they
would not consider going back to a resistive device.
Exactly
why they sound better is still under debate, but one theory is that
it's because an autoformer doesn't attenuate by wasting energy. A fairly
good analogy can be made to the transmission in a car. If you need to
drive at a slow steady speed which method would you choose: (1) leave
the transmission in high gear and apply the brakes to keep from going
too fast, or (2) downshift into a lower gear that will allow the car
to go the desired speed with minimum effort?
An
autoformer is essentially an electronic gearbox that operates without
wasting significant energy. Potentiometers and stepped attenuators adjust
the signal level by literally turning the excess signal into heat. On
the other hand, when an autoformer is adjusted for low volume level
it actually makes things easier for the source, much like a low gear
makes things easy for your car engine.
It
becomes quickly apparent that the reflective load can be ignored in
this case since it is many multiples of the inductance in parallel with
it. Inductance gives you a impedance = 2*pi*Frequncy*L(inductance) or
Z=2piFL. Its this simple formula that tells you what you need to know
about the impedance presented ot the source.
Here you can see the acid burns from the old batteries that leaked in the box. This was before and after wiring these up with magnet wire. I think the green wire sounds best ;-). Fortunately Dave's modules makes them very easy to install so even I couldn't screw it up.
Here are the left and right sides. 
I tried many different new knobs on this box but they just didn't look right. After purchasing the wrong sized vintage knobs three times I finally got these and pulled the big selector from a Bud national radio I had. Lesson learned vintage box gets vintage knobs.
There will be no labels. Perhaps that will keep party guests off my knobs.
