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Old 4th October 2003, 02:17 AM   #1
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Lightbulb Autoformers (again??!)

Do the autoformer volume controls such as:

http://www.megathon.com/home_theatre...e_controls.htm

do the same thing as a normal potentiometer ? Can I use one as the volume control in a passive pre-amp.

I am an old (30 years?) Alps pot from an old amp, set up to shunt a resistor between my cdp and some wave av-8 monos.

It doesn't quite sound the same as when I had a cheaper (yamaha) cdp hooked up without the pot. (the yamaha had volume controls, the nad cdp dosen't)

Has anyone tried to use an auto former like the ones in the link above? how much attenuation should I use in a passive preamp between the cdp and the amp.

Thanks, I really hope this isn't a stupid question.

Adam
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Old 4th October 2003, 03:23 AM   #2
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Default Re: Autoformers (again??!)

Quote:
Originally posted by adharvey
Do the autoformer volume controlsdo the same thing as a normal potentiometer ? Can I use one as the volume control in a passive pre-amp.
Adam

well the simple answer is yes.... they are three terminal devices, they do not behave exactly the same way as a pot, but they are close.

the autoformers you refernced can work very well at line level under the proper circumstances, and like any other device can fail miserably when put into the wrong situation.

the speaker level controls you mentioned are cheap ($15 is a more common price without the high end packaging) and designed for power on an 8 ohm speaker. this means they have limited primary inductance and you need a low source Z to drive them, but i have heard of many situations at line level where people have had great results driving them with the low Z-out of a CD player or transformer output tube stage.

the good news is their power handling at 8 ohms buys them headroom in voltage so higher output sources tend to fare pretty well with them, but its a case by case ears on experience... there are no rules.

the only other real issue is the attenuations are pretty limited and coarse... 10-12 steps 3-6db per step with less than 40db of max attenuation... if you have a proper gain structure this isn't an issue... and i find 3db of autoformer attenuation to be much less to my ears than 3db of resistive attenuation... even with 3db steps level matching with mono autoformers is often a task for me, but with mono davens and 2 db steps its easy... still don't understand why.

dave
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Old 4th October 2003, 03:30 AM   #3
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Default z ?

What exactly is the z out of a cd player, and where would I find this out?

Adam

(thanks for the rapid reply)
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Old 4th October 2003, 12:00 PM   #4
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While these will work you will be unhappy with the response that you get from the transformers. They may claim a response down to 20hz but what they don't tell you is how many db down you will be at 20hz. If you purchased a high dollar VC with much better transformers the response will be better but by this time you would be spending enough money to warrant purchasing a nice stepped controls.

This type of control is meant to maintain a constant load on an amplifier/s. Most are switchable from 2,4, or 8ohm in order to best match loads. Just my .02

Joe

I will look for a link on making a control and post it.
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Old 4th October 2003, 12:09 PM   #5
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Some of those controls actually did give specs on response with dB limits. But these are not the right sort to use at line level. Yes, with enough "stuff" around them, one could make them work, after a fashion, but why? There are high quality autoformer volume controls out there (expensive, alas) and plain old ordinary potentiometers which are much more suitable for line-level attenuation.

For $1.49, a Radio Shack-sourced Alps potentiometer will outperform most of those designer controls. For $25, you can get even better pots, and throw a buffer in. Don't think about autoformers until you're ready to part with $150+.
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Old 4th October 2003, 12:28 PM   #6
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http://www.siteswithstyle.com/VoltSe...ure_Shunt.html

Try this...

Like SY pointed out there are much better ways to go. The Radio Shack controls do deliver very good performance for the money. If your adventurous check the site I posted. It will provide step by step instructions on fabrication of a control.


Joe
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Old 5th October 2003, 01:58 AM   #7
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Default I've tried the shunt...

I really want to try a transformer based volume control on a passive pre. Apart from sowter and s&b what are my options...

Dave how expensive are autoformers to purchase? Are they difficult to make well?



Thanks

Adam
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Old 5th October 2003, 05:00 AM   #8
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I'm using a pair of autoformers as my linestage. They were built by Dave Slagle. I'm not sure if he's still making them or not.

I paid $250 for a pair of autoformers, and I think they're significantly better than any active preamp I could have built for the same amount of money. My total cost was about $300 - $350, because you'll need a pair of rotary selector switches (one for volume and one for source selection), RCA connectors, chassis, wire, solder, etc.

Here's a picture of my unit in its current incarnation (it's been through a few enclosures):

Click the image to open in full size.
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Old 5th October 2003, 03:23 PM   #9
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Sowter makes a autoformer for controling line level volume. It can be found here:

http://www.sowter.co.uk/attens.htm
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Old 5th October 2003, 08:21 PM   #10
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G

Those look like they are well worth the time and money. The response looks good also. Quite a bit different from the off the shelf speaker volume control.

Joe


Any ballpark idea on the cost?
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