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Using a Transformer as a Variable Choke

(aka Variable Choke as Speaker Tone Control.)

kensKenS
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    In my teens I was always building quickie, cheap, DIY Speaker Cabinets. One particular one I built needed a Tone Control because there was too much Treble. Thinking out side of the box, I came up with the idea of a Variable Choke in series with the speaker to control the high frequencies. It worked great, and I used it for years after, even taking it camping where it got some rough and tumble abuse.

    We all know that a Choke will limit high frequencies.  A Transformer can just be considered a Choke with an additional Winding.  If you put the low impedance side of a Transformer in a circuit and leave the high impedance side unconnected, the transformer will act exactly like a Choke; as if it had no additional winding.  If you short out the high impedance side, the transformer will act like a straight wire (like it isn't there.)  And here's the cool part: if you put a Variable Resistor across the high impedance side of the transformer, you can vary it between open and shorted making it operate like a Variable Choke.  When put in series with a Speaker it will operate like a Tone Control to variably limit high frequencies.

Hack_A_Day_Component_Abuse_Challenge_Transformer_as_Variable_Choke.png

Using a Transformer as a Variable Choke for a Speaker Tone Control

Portable Network Graphics (PNG) - 215.54 kB - 10/05/2025 at 19:59

Preview

  • 1 × Step Down Transformer I used one from the output of an old tube amplifier.
  • 1 × Variable Resistor Value found by trial and error.

  • Project Log

    KenS • 10/05/2025 at 20:03 • 0 comments

    * Built in an afternoon, used for years.
    * This a cheaper way to manufacture a variable choke than to physically vary the magnetic core.
    * And it should be possible to use a Digital Potentiometer making a Computer Controlled Variable Choke a reality.

View project log

  • 1
    Instructions

        It's all down to trial and error to find the right transformer to use, and the right variable resistor for that transformer to work the way you want.  Start with putting the low impedance side in series with your speaker.  Then see how it sounds with the secondary open and shorted.  When you find the right transformer it will limit the high frequencies coming out of the speaker when open, but not excessively so.  Then try different variable resistors till you find one that gives you the range you want.

        I went to a Vocational School for Electronics in High School (like 55 years ago!)  At the end of the 2nd year, the teachers took us aside one by one and asked us questions on how we like the course, suggestions to improve it etc.  One question was if we've done anything unusual or unique in electronics; so I explained the Transformer used as a Variable Choke to them and they went silent and seemed to be deep in thought since it followed all the theory they gave us for chokes and transformers, but they had never heard of one being used in that way.  I'm sure at least one of them must have gone home and tried it.

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John Clayton wrote 10/08/2025 at 19:02 • point

Leakage inductance is usually quite low, and I have no doubt that it works!  It's an interesting and valid approach, it seems!

  Are you sure? yes | no

KenS wrote 10/08/2025 at 17:15 • point

It worked great on the low power equipment I had available to me at the time (i.e. battery powered cassette recorder,) but I'm sure if you tried it on high powered equipment you'd smoke that variable resistor.

  Are you sure? yes | no

Gianluca wrote 10/08/2025 at 16:25 • point

Inductance doesn't change! The resistor is reflected by the transformer in parallel to the inductance!

  Are you sure? yes | no

John Clayton wrote 10/08/2025 at 16:52 • point

I would venture to propose this refinement in wording to the gentleman Gianluca's comment:  "Leakage inductance doesn't change appreciably!  The resistor is reflected by the transformer in series with the leakage inductance!"  I'm hoping that this will make sense, and help bring everyone into better alignment here.  And I welcome further corrections.  For example, am I correct about the leakage inductance being in series with the reflected impedance?  Do tell!

  Are you sure? yes | no

KenS wrote 10/08/2025 at 18:36 • point

https://en.wikipedia.org/wiki/Leakage_inductance You lost me there!  I just know that it works.

  Are you sure? yes | no

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