Elma 47 Position Autoformers
Clamor for a higher quality switch version of the autoformers lead me to designing a custom circuit board with hard gold plated contacts that integrates to an elma switch mounted directly on the board. The top of the line unit is a 47 position stereo attenuator that goes seamlessly from +5dB to -51.25dB in 47 1.25dB steps. This stereo unit can also be wired as a mono balanced unit. For the ultimate, a version in all silver is available. (Dimensions 2.9” wide by 2.25” tall and 3.3” deep with the knob being offset and 0.6" from the edge). Units are available wound in copper and 0.9999 pure silver.
Blore Edwards 23 Position Autoformers
Filling in the middle ground between the Elmaformer and the 23 position are these units that use a metal shafted Blore Edwards rotary switch. The stereo unit measures 2.75" X 3.5" and is 3.3" deep. The mono version is 2" X 2.5" and 2" deep. The standard taper has the following steps: (0, -3, -6, -9, -12, -14, -16, -18, -20, -22, -24, -26, -28, -30, -32, -34, -36, -39, -42, -45, -48, -53 and Mute). Custom tapers are possible please email to discuss.
Speaker Level Autoformer Modules
These turn-key tapped units make a perfect replacement for resistive L-pads. They allow for 20 attenuation positions in 0.5dB or 1dB steps. The 49% nickel core can handle 15V input signal at 100hz. Every doubling of frequency doubles the maximum input voltage 200hz allows 30V and 400hz allows 60V of input. The nature of an autoformer transforming the excess voltage into current drive makes this an ideal solution for matching driver efficiency. Four versions are available with different attenuation ranges and step size. The "Rshunt" calculator can be found below and you can find the full story at this link.
Low ratio speakerformers become problematic because with each 1dB click the reflected load from a fixed shunt resistor varies enough to cause a substantial change in the crossover behavior. These units have 6 steps from -1dB --6dB and the required shunt resistor is adjusted for each step so the crossover sees a constant 8Ω or 16Ω with each attenuation setting. The resistors are sized to handle 20W of input power. Custom 6 step ranges and impedances are possible, please email me to discuss.
23 Position Autoformers
The basic building block of the family is a compact 23 step autoformer that measures 2.4" X 1.6" and is 1.8" deep. They can have any taper desired and the standard version comes with the following dB steps. (0, -3, -6, -9, -12, -14, -16, -18, -20, -22, -24, -26, -28, -30, -32, -34, -36, -39, -42, -45, -48, -52 and Mute)
One of the biggest complaints about the $200 autoformers is that they are a real pain in the you know what to wire up. In one of my many discussions with John Chapman of Bent Audio we came up with a simple switching arrangement to make the units a breeze to install. They are dual mono units with 1.25dB steps and give a maximum attenuation of 41.25dB. Complete details are available Here
The $300 Autoformer Volume Control (AVC)
Pictured above is our first off the shelf device. It is intended as a high bang for the buck entry into the magnetic volume control arena. It allows attenuation from 0dB to -42db in -3dB steps. It has an 80% nickel core, all brass hardware and sounds every bit as good as our custom units. For more information please check out the FAQ, Wiring Directions and visit the Forum.
Price: $300 a pair
28 Step Autoformer Volume Control (AVC)
The Demand for more positions and finer attenuation steps has brought about a second product. This device has 28 steps and provides attenuation from 0dB to -52dB in 2dB steps.This uses a similar approach as the $200 kit but has a 28 pin bobbin in place of the 14 pin. When wiring this device to a switch with fewer than 28 taps you simply assess your gain structure and choose the appropriate steps.
Price: $500 a pair
Custom Magnetic Volume Controls.
For those of you who need a bit more control or a special taper, we can offer the perfect MVC to fit your needs. We can help you choose between transformer and autoformer and help you select the proper values of inductance and attenuation to best fit your system. Please check out the $200 autoformer FAQ and join us on the forum to discuss your needs. Please note that the leads of the custom volume controls should not be cut under any circumstances!