Film-Tech Forum ARCHIVE: Matching transformers for Audio

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Posted by Matthew Taylor (Member # 2310) on 06-15-2005, 06:15 AM:

Reading through the archives, I see that the best way of connecting a stereo cell to an amplifier is via matching transformers.

Does the output from a matching transformer still have to go into a preamp?

What benefit is a matching tranformer?

Posted by Josh Jones (Member # 447) on 06-15-2005, 04:05 PM:

Yes, it still has to go to a preamp. The purpose of the impedance matching is to reduce the signal loss caused by the missmatch. Along with signal loss comes non-linearity in frequency response, namely, loss of high end (treble)

JJ

Posted by Gilbert Travin (Member # 2138) on 06-16-2005, 01:56 AM:

Hi ! [Big Grin]

a matching transformer has several advantages :

- balanced input eliminates noises (particulary noises produced by Ac power @ 50 Hz or 60 Hz, etc.)
- the transformer eliminates cell DC component (*)
- it works at low impedance

(*) You can see also Gordon's answer in an earlier post

BUT a good transformer MUST be shielded to avoid interference inductions !

Posted by Pete Naples (Member # 761) on 06-16-2005, 04:24 AM:

And good ones aint cheap either!

Posted by Gordon McLeod (Member # 33) on 06-16-2005, 06:29 AM:

http://www.sowter.co.uk/
was the supplier to Dolby for the older CP100-50-200 optical preamp transformers as well as the MPU

Posted by Sam D. Chavez (Member # 1841) on 06-16-2005, 10:50 AM:

Those old Dolby transformers were just good enough to do the job shielding wise. The Dolby MPU was always touchy about where it was mounted in relation to hum fields.

There are better shielded transformers, but prices go up rapidly for good electrostatic shielding.

Even though the original question was about matching transformers, they are most often used today for absolute isolation from one piece of audiovisual equipment to another.

Posted by Stephen Furley (Member # 1277) on 06-16-2005, 10:59 AM:

Sometimes also used to connect a balanced line to an unbalanced device; more often done with valve (vacuum tube) devices, I think more modern inputs tend to be transformerless.