Line input transformer for unbalanced to balanced conversion
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 Chip Amps Amplifiers based on integrated circuits

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 22nd December 2007, 02:36 PM #1 okapi   diyAudio Member     Join Date: Aug 2004 Location: Buffalo Line input transformer for unbalanced to balanced conversion I am under the (mistaken??) impression that it is possible to use a line input transformer to convert a unbalanced signal to balanced. i would like to do this so that i can bridge my chip amps. however, when looking at the typical implementation of line input transformers: cinemag line input transformer it is not obvious to me how to do it. i can see how i could use the transformer to invert (or conserve) the signal but i don't see how to get the original signal and the inverted out of the transformer at the same time. would i just pull it from the input?, use two transformers (one to invert and one to conserve)? attach the output of the transformer to the chip amps in opposite phase? thanks for your help.
 22nd December 2007, 06:55 PM #2 tiltedhalo   diyAudio Member   Join Date: Sep 2005 Location: Melbourne Florida unbalanced to balanced depending on the specific transformer, you need two for each channel if they are 1:1 ratio, both transformers are wired together on the input side making sure of identical polarity, then the output of one trans can be the non-inverted in 1 = out leg one, in 2 =out leg two. the other trans you will wire the out two your amp opposite, in 1 = out two in 2 = out one. Basically the trans will invert your signal 180 degrees when wired opposite the other trans.
 23rd December 2007, 02:36 AM #3 okapi   diyAudio Member     Join Date: Aug 2004 Location: Buffalo thanks for the help. that definitely confirms what my oscilloscope and function generator were telling me. it seems to me that using transformers for single ended to balanced conversion is a expensive option.
Peter Daniel
diyAudio Member

Join Date: Jul 2002
Quote:
 Originally posted by okapi it seems to me that using transformers for single ended to balanced conversion is a expensive option.
Not really. That one is cheap and produces very satisfactory results: http://www.edcorusa.com/products/tra...sm10k-150.html

I tried it briefly with unbalanced to balanced signal conversion for LM4780 based amp and was not disappointed; actually the transformer produced cleaner signal than modified ML380 preamp.

I do conversion on S&B TX102 and it doesn't matter if I connect unbalanced or balanced inputs or outputs, the conversion works both ways with single windings on primary and secondary (no CT).
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Burnedfingers
diyAudio Member

Join Date: Nov 2002
Quote:
 it seems to me that using transformers for single ended to balanced conversion is a expensive option.
Actually you are correct. A good performing transformer that doesn't loose out on the low end gererally runs about \$90 a pop.
The Edcor transformers are a budget transformer that will come close in performance but cannot quite keep up to the more expensive brands.

I've tried about everything in large systems to balance or to combat noise problems and issolate when needed.

Added* I would try to find some Altec 15335 octal transformers.
They aren't made anymore but can be purchased reasonably and will deliver better performance than some of the cheaper transformers on the market. They are a 1:1 transformer and have a CMRR of 92db@60hz.

 24th December 2007, 12:47 PM #6 KSTR   diyAudio Member     Join Date: Jul 2007 Location: Central Berlin, Germany Take any cheap and small xformer and hook it up in a balanced "zero field"**) circuit (2 op-amps in instrumentation style, or a discrete equivalent). Gives stunning results and is used to a good extent in many pro gear... **) look at this Lundahl datasheet for an unbalanced example of this: http://lundahl.se/pdfs/datash/6404.pdf - Klaus
okapi
diyAudio Member

Join Date: Aug 2004
Location: Buffalo
thanks everyone.

i have been able to follow/understand just about all aspects of the comments.

peter i am a little confused about:
Quote:
 I do conversion on S&B TX102 and it doesn't matter if I connect unbalanced or balanced inputs or outputs, the conversion works both ways with single windings on primary and secondary (no CT).
i am having trouble resolving your comment with:
Quote:
 you need two for each channel if they are 1:1 ratio
klaus, i think i will give the "zero field" circuit a try. thanks for pointing it out. once i think i have it figured out, i'll post a schematic here that is specific to the components i am using to see what everyone thinks.

thanks again for everyone's feedback. i am at the "in laws" now in buffalo, ny then i'm off to toronto.

happy holidays.

Burnedfingers
diyAudio Member

Join Date: Nov 2002
The transformer is the best way to balance because it provides a higher CMRR than an op amp or other SS circuit is capable of.
You can of course use either transformers to balance/unbalance or a op amp circuit or a number of other ways.

If you are interested in a circuit just to bridge to identical chip amps or other amps period I will try to attach something here.
Attached Images
 bridging adapter.gif (2.9 KB, 2015 views)

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