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Old 17th May 2010, 10:24 PM   #1
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Default Basics on transformer-coupling dacs ?

Looking for basic guidelines regarding use of a Transformer in the *output position* of a given DA converter... I'm interested in modding a specific Dac, but I find I don't quite have a grasp of the names of things, the relevant values, etc ..

There are lots of posts about specific implementations, but here are 5 generic questions I hope to get some information about ....

* What information is critical --about the Dac--- in order to specify an appropriate transformer ? Output impedance, voltage, what is the important info to know ?

* This one for beginners, like me -- What, exactly, is the name of this transformer-- Interstage ? Line-level output (preamp out) ? Isolation ? Matching ? I see posts recommending all sorts of different tx types in the output position of typical Dacs; is this a case where the specs tell the tale and the name of the thing can vary ?

* What are the minimum values & info you need to find an appropriate audio transformer in a manufacturer's catalog and have it match the output of your dac ?

* In most cases the pre-existing output cap is removed and the transformer is wired into it's place. Anything non-intuitive about that, any exceptions, or "don'ts" -that apply ?

* Once a transformer is in place, I've seen posts discussing adding a loading resistor (on the secondaries, if I recall) to trim values... rules & guidelines there ? And is there any consideration necessary for the downstream linestage that will be driven by the Dac ? (or unnecessary, if a standard linestage...)

Okay, that's a lot -- guess I'm basically asking for a FAQ on tx-coupled-dac-output. Please presume a standard 2V dac output signal, and thanks for any of the requested info.

intermitten t
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Old 18th May 2010, 01:10 PM   #2
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The first thing you have to do is pick a DAC chip, there is no standard solution because they are all different. Is the output current out or voltage out. What output level do you have to work with. Is it a single ended output or differential output. Does it have a DC component you have to deal with. Yes, some are easy to deal with, some are not, so generic questions and answers are difficult.
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Old 18th May 2010, 07:35 PM   #3
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Some aspects of chosing an I/V transformer are discussed around the end of this thread:

choosing best output I/V transformer for PCM1704 DAC
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Old 18th May 2010, 10:09 PM   #4
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Thanks, here's a simple Q: in the context of the circuitry of a Dac, the output tranformer is referred to as an I/V transformer, current-to-voltage transformer.

What is it called in the pages of the transformer catalog ? (again, I'm a beginner at this)-- Is it an interstage, output, matching, transformer ? I don't see I/V listed as a type. Forgive this for being extreme novice question, but that's where I'm starting...
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Old 19th May 2010, 12:12 PM   #5
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There are very few trafo makers that list I/V converter usage for their units. The Jensen website has their white papers listed, a good source for info. Sowter has a list of their units used as i/v converters. A study of the chip makers app notes would be helpful too.
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Old 20th May 2010, 09:27 PM   #6
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My interest is in substituting appropriate output transformers-- replacing output coupling-caps--- in a Teradak Nos Chameleon that I just got. It's chip complement is 16 paralleling TDA1543, Philips manufactured. Output is redbook standard 2v rms, single-ended, and-- that's all I see in terms of the output stage of the dac...

So am I just after some kind of high-spec, line-level, matching audio transformer that has good freq response figures ? Should it be the default 6oo : 6oo type ? Core type... ?

And I'm interested in shielding, too, since it would be entirely appropriate to fit this inside the enclosure of the Chameleon. (these are built to mod & so have lots of room to accommodate varying kinds of coupling strategies, etc).
Since size is relevant, something in the Lundahl or Sowter ranges seems to make sense.
But there are so many choices......

That's about what I know on the specifics...
Thanks for any pertinent info, whether generic or specific.
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Old 21st May 2010, 08:05 AM   #7
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I'm not too sure if inserting a transformer would improve the sound on that dac, But it certainly would change the sound.

Why not try another caps?

Or a Pass B1 buffer.
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Old 21st May 2010, 09:46 AM   #8
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You should contact Doede Douma and ask him if he's tried it. He has gone as far as 60 parallel 1543s. 16 chips puts out 35ma or so, quite a large signal to deal with. A center tapped 600 using half of the primary might work but I'm not going to guarantee it.
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Old 21st May 2010, 10:48 AM   #9
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FWIW, I have built a DDDAC with 60 TDA1543's in parallell and used Lundahl LL1676's as output transformers in a parafeed arrangement. I kept the I/V resistors in place and connected a cap between the two primaries of the transformer. This option give me the chance to use both SE and balanced cables. I really like the sound of this DAC.

Geir
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Old 21st May 2010, 11:13 AM   #10
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That's a slick idea, what is your load resistor? Have you determined what output impedance you have?
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