Showing posts with label Autoformer. Show all posts
Showing posts with label Autoformer. Show all posts

Sunday, June 29, 2014

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.
Thank you Dave Slagle aka Intact Audio.

Monday, June 6, 2011

Intact Audio - Slagel AVC - Is Less More?

I have been wanting to experiment with a passive preamp or transformer based volume control for a few years now. In fact doing research about passive preamps and autoformer volume controls I found many of my old posts asking about them. Recently Dave kindly let me use one of his autoformer volume controls which was unexpected but heavy boxes at your doorsteps are wonderful things. Out of the box visually it does have the drug through the Hudson look although I would call it steampunk. Very cool looking device and not your ordinary boring high dollar black box solution.
I'm not an "audiophile" reviewer but just a guy with a new piece of equipment to listen to for a few weeks. As I told Dave and Jeffery I'm not great at putting into words what I hear. I can't discern the crinkling of Diana Kralls nipples as I'm listening to a piece of music nor will I delve into audiophile techno babel verbal masturbation. I hate that crap. Please shoot me if I start to do that.
Now with that out of the way I'll try to tell you what I noticed as best I can. This is in my system in my room so your mileage may very. It took me a little while to find out what this box is and does. I read the FAQ numerous times. The less is more approach does appeal to me but this thing doesn't even have a power cord LOL. The speakers I use are very revealing and efficient so I am accustomed to detail. This is where I may struggle a little. Now when I added the Slagle AVC it took me a while but I knew something was different. After being in place a few days I noticed there seemed to be even more detail and more information on songs I knew by heart. It was like getting a more powerful microscope but for music and system. When I switched sources it was apparent which source was superior. Listening to cds was not as enjoyable as vinyl. The system with cds sounded pretty good but the vinyl was king in this setup. My 301 suddenly asked me for a better cartridge and my George Wright AG Phono preamp asked me to dial down the gain. Once dialed down it was very nice. I also noticed the needle was dirty faster than normal due to the noise but once removed the music shone through. These were just a few very simple but telling observations. There are times when you add a piece of equipment to your system and it raises the bar. This certainly happened with the Slagle AVC. Adding the AVC reminded me of my first experience with highly efficient speakers. They are very honest and true to the sources and source material. That says more than anything I can think to convey about the AVC. It has been a while since I changed anything in my system so I know it very well. I've been blissfully listening to music since getting my speakers together. Now it seems I may be on a new path. How can so few parts make such a big difference? I don't know but it does. In this instance less is more. Take the less resistive path.

Photo from the first five minutes of listening. Sorry for the iPhone quality pic.
System components:
Klipsch Jubilee LF clones.
K403 protorype last hf horn PWK designed with Roy Delgado.
ALK ESN 120DB slope crossover
Cambridge Azur 640C CDP
Garrard 301 Jelco 12" Ortofon 2M blue
George Wright AG Phono Telefunkens and square halo Mullards.
Pioneer 707
Tube Luxman KMQ60/SQ38 50CA10 based 30wpc

This post originated at retro vintage modern hi-fi