intro to Autoformers

discussion of magnetic volume control design for both line and speaker levels.

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dave slagle
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Re: intro to Autoformers

Post by dave slagle »

I'll answer these question sin line.
marsattacks wrote: -what will be the ideal output impedance of the phono stage output(to go into the autoformers preamp)?
The lower the better and typically under say 3kΩ but I have had people go as high as 6-8k and report great results.
-what will be the ideal output impedance of the autoformer preamp to go into the power amp?
worst case is full volume where the output impedance will be roughly that of the source. As you attenuate the output Z goes down and at -6d it will be 1/4 the source Z and at -12dB it will be 1/16th the source Z. This is one of the plusses of using an autoformer.... as you attenuate you turn excess voltage into more available curent.
what will be the input sensitivity needed for a perfect signal going into the power amp?
the source must be able to drive the amps to the loudest listening level required. the numbers around this always relate to full power or clipping so a 2V source into an amp with a 1V input sensitivity will require 6dB of attenuation to avoid clipping. Starting with a situation where the source output is higher than the amps input sensitivity will assure a good match but in many cases may result in a "hair trigger" volume control.
-Do i need extra volume controls on the phono stage and/or the power amp?
No. they only become necessary if there is way too much gain in the system. This is a band-aid and the proper solution to this situation is to adjust the system to the proper gain required for your listening habits and not to some mythical spec.
-Is it possible to add an XLR output to the autoformer preamp output(to go into the power amp) and does the balanced output has any benefits sonically?
This topic is a long one and I could write pages. For a balanced signal that adheres to the professional standard a single autoformer will do balanced to balanced attenuation but will not provide SE to balanced conversion. To both do the conversion and provide attenuation a transformer attenuator is needed but my experiences show that approach is not optimal. If you have balanced sources or some of your gear does not adhere to the standard then two autoformers can be used for balanced >> balanced and balanced >> SE.
-What is the best enclosure for the autoformer preamp? i see a lot of wood enclosures... for instance is corian, brass ,copper any good or is it a matter of extra shielding inside the enclosure?
Steel provides an extra layer of shielding which is always a help.

much of this is covered in the FAQ linked below.

http://www.intactaudio.com/atten_FAQ.html

dave
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