line level autoformers

[mikep]

I don't know of any off the shelf parts for line level operation. I tried connecting a 1:1 Lundahl output TF as an autoformer (all coils in series) and it sucks, high odd order distortion that seems to go UP at lower levels. FR is flat to 100kHz though. I guess the ideal transformer to use this way would be built up from 2 coils only, no interleaving and no bifilar, etc. any suggestions?

mike p

 

[NewYorkDave]

An autotransformer for line-level audio is indeed a rare thing. But I'm aware of these:

http://www.edcorusa.com/classx/matching/ex/ex102.htm

http://www.edcorusa.com/classx/matching/ex/ex103.htm

 

[Brian C.]

Also these :

http://intactaudio.com/atten.html

Brian.

 

[gyraf]

..make sure you have plenty of drive-current and a low drive impedance, or you will get low-end distortion and -level loss..

 

[moamps]

Hi:

I have some experience with designing and building autotransformers, which were mostly used for passive volume control in HIFI systems. Their quality is, IMO, even better than that of most run of the mill potentiometers. Here is an example:

Regards,
Milan

 

[NewYorkDave]

That looks nice. How many taps does it have, spaced how many dB apart?

 

[moamps]

Hi:
You can check out my web site for more info. The text is in Croatian but the pictures are self-explanatory. If you need more info, just holler.

http://www.moxtone.com/passive.htm

Regards,
Milan

 

[CJ]

I think Sowter makes a xfmr based attenuator.

 

[kevinc]

Sorry to rain on your parade, Mike, but does it sound like it sucks or do you only look at your distortion analyzer when you judge sound quality? In this business, good sound is a good result, but good measurements are only good measurements.

 

[moamps]

[quote author="CJ"]I think Sowter makes a xfmr based attenuator.[/quote]

You're right (Sowter types: 9336, 9337, 9338, 9339, 9150, 9155, 9395). However, these are transformers, not autotransformers, although they can be set to autotransformer mode.
Another well-known and widely used transformer is TX102 from S&B.
http://www.stevens-billington.co.uk/page102.htm

Regards,
Milan

 

[mikep]

thanks for the replies

[quote author="kevinc"]Sorry to rain on your parade, Mike, but does it sound like it sucks or do you only look at your distortion analyzer when you judge sound quality? [/quote]

not at all, I listen first, measure second. (tannoy reveals and AKG-240M) I listen to the residual quite alot as well (more than looking at it on a scope). IMO you can tell alot this way. I was measuring something like .5% THD at -10dBu. this was a LL1517. hooked up as per the data sheet (as a transformer) it is at least 2 orders of magnitude better. I think the problem might be with the muti-section interleaved windings, perhaps if I connect up the coils in a different order it would be better.

edit: nope, it was an issue of drive impedance. when driven from a very low-z buffer things improve drastically. and it needs oodles of drive current. Jacob was spot on.

Those multi-taped parts look interesting, but for my application I need +6dB voltage gain only. so I was hoping to be able to use an inexpensive 1:1 line transformer wired up as one center-tapped coil and use it as an autoformer. I *think* it should be possible to get slightly better performance this way than using it as intended.

mike