Autoformer module in balanced configuration

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

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accessdenied
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Joined: Sat Jul 06, 2019 6:29 am

Autoformer module in balanced configuration

Post by accessdenied »

Is it possible to make a configuration of autoformer modules in such a way that there is 1 balanced input and i.e. 2 unbalanced inputs and 1 unbalanced output and 1 balanced output? How many autoformer modules are needed for such a setup and how should the wiring be done?

With regards, Bert.
dave slagle
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Post by dave slagle »

hello,

for only balanced use a single autoformer can be wired to work but it requires a very specific set of circumstances to avoid ground loops. For a "universal" setup two autoformers are used with the + and - signal halves attached to each end and the grounds tied together. From this setup connections can be made for:

Balanced in / Balanced out
Balanced in / SE out (in phase or inverting phase)
SE in / Se out.

This setup will not convert a single ended input to a balanced output.

In order to do this you would need the elma switched versions since they allow for multiple decks to be controlled with the same knob. Using the less expensive single deck units would work but you would have to adjust 4 knobs to adjust volume level.

The basic wiring is simple since only 6 connections to the autoformers needs to be made per channel.

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In the right situation a single autoformer can be used for balanced and the basic idea can be seen in the image below. The key for this to work is how the ground references are handled on each side and the circuit needs to be floating on both sides as shown with transformers or only achieve a ground reference on one side or the other. The exact hookup that works seems to vary between devices and takes some experimenting to work out. This setup will only do balanced to balanced or SE to SE attenuation. An input switching arrangement could be done so the same autoformer could be used either SE or Balanced.


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accessdenied
Posts: 2
Joined: Sat Jul 06, 2019 6:29 am

Post by accessdenied »

Thanks for the provided information.

Bert.
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