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Old 22nd April 2010, 08:32 AM   #1
Marek is offline Marek  Poland
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Location: Katowice
Default choosing best output I/V transformer for PCM1704 DAC

Hi,

I'm trying to choose best possibe transformer for my audio digital to
analog converter based on PCM1704 chip.Transformer will work as passive output stage I/V. PCM1704 has current outputs of max amplitude +/- 1,2mA (no DC) and output impedance od 1kOhm and it is designed to drive almost zero impedance (like OPAMP input) to achieve best THD. Using transformer as output stage it is important to ensure lowest possible load (impedance) seen by DAC chip (preferably 10ohm o less). So important are: high turns ratio
(1:16 or even 1:20), low DCR of primaries.

My first choice was LL1674, LL7903/5.
After some additional research I relized that I need step-up trafo with
higher turns ratio (16 or even 20 so secondary load will transform to
pimaries to very low value) and very low primary DCR.

So there are following types satysfying those requirements:
TYPE CORE TURNS RATIO effective prim.DCR PRICE NOTES
LL1636 amorph 1:20 2,5ohm $83 small core
LL1678 amorph 1:16 4,5 or 1,1ohm $78 small core
LL1681 MU 1:26 2,4ohm $105
LL1931 amorph 1:16 0,9ohm $200
LL1933 MU 1:16 0,75ohm $175
LL9206 amorph 1:20 2,5ohm $78 small core
LL9226 amorph 1:20 1ohm $86 small core
LL7905 MU 1:11,2 7,5ohm $135

Now, which one should I choose?
My problem is that there is no info in datasheets about inductances of
windings - I'm afraid that low inductance will result in bad low
frequency response.
And what about small core transformers - I'm afraid that bass will sufer
too....


Please help!
Marek
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Old 22nd April 2010, 09:02 AM   #2
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Quote:
Originally Posted by Marek View Post
Hi,

I'm trying to choose best possibe transformer for my audio digital to
analog converter based on PCM1704 chip.Transformer will work as passive output stage I/V. PCM1704 has current outputs of max amplitude +/- 1,2mA (no DC) and output impedance od 1kOhm and it is designed to drive almost zero impedance (like OPAMP input) to achieve best THD. Using transformer as output stage it is important to ensure lowest possible load (impedance) seen by DAC chip (preferably 10ohm o less). So important are: high turns ratio
(1:16 or even 1:20), low DCR of primaries.

My first choice was LL1674, LL7903/5.
After some additional research I relized that I need step-up trafo with
higher turns ratio (16 or even 20 so secondary load will transform to
pimaries to very low value) and very low primary DCR.

So there are following types satysfying those requirements:
TYPE CORE TURNS RATIO effective prim.DCR PRICE NOTES
LL1636 amorph 1:20 2,5ohm $83 small core
LL1678 amorph 1:16 4,5 or 1,1ohm $78 small core
LL1681 MU 1:26 2,4ohm $105
LL1931 amorph 1:16 0,9ohm $200
LL1933 MU 1:16 0,75ohm $175
LL9206 amorph 1:20 2,5ohm $78 small core
LL9226 amorph 1:20 1ohm $86 small core
LL7905 MU 1:11,2 7,5ohm $135

Now, which one should I choose?
My problem is that there is no info in datasheets about inductances of
windings - I'm afraid that low inductance will result in bad low
frequency response.
And what about small core transformers - I'm afraid that bass will sufer
too....


Please help!
Marek
FORGET the Lundahls. Get a Sowter!
www.sowter.co.uk/dacs.php
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Old 22nd April 2010, 09:06 AM   #3
Marek is offline Marek  Poland
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Join Date: Apr 2003
Location: Katowice
Yes I know that Sowter has specialized trafos for DAC output, even more: 8347 and 9055 are espiecially suited for PCM chips from BB, but I read many times in net that pepole don't like sound of Sowters...
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Old 22nd April 2010, 09:20 AM   #4
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Quote:
Originally Posted by Marek View Post
Yes I know that Sowter has specialized trafos for DAC output, even more: 8347 and 9055 are espiecially suited for PCM chips from BB, but I read many times in net that pepole don't like sound of Sowters...
Yes that's the problem of Internet so many opinions.
I tried the Lundahl LL1636 as Andrea Cuffoli used it... Now I am glad I looked further and sold it
I don't believe ANYTHING untill it tried it myself!

(Also tried LL1681)

Last edited by QSerraTico_Tico; 22nd April 2010 at 09:22 AM.
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Old 22nd April 2010, 09:37 AM   #5
Marek is offline Marek  Poland
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Location: Katowice
Thats very concrete answer!
Which one Sowter you use and with which DAC chip? balanced DAC output or SE?

And please give me some info about experience with LL1636 and LL1681 (used configuration, sound quality, low freq response) and ofcourse Sowter.

I really want to have a chance to try every possibility by myself....
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Old 22nd April 2010, 09:54 AM   #6
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Quote:
Originally Posted by Marek View Post
Thats very concrete answer!
Which one Sowter you use and with which DAC chip? balanced DAC output or SE?

And please give me some info about experience with LL1636 and LL1681 (used configuration, sound quality, low freq response) and ofcourse Sowter.

I really want to have a chance to try every possibility by myself....
AK4396: best solution for output stage
and below
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Old 22nd April 2010, 10:03 AM   #7
kimk is offline kimk  China
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Note impedance matching transformer primary should not be less than 1KOhm(Audio transformers are usually relatively large inductance, in order to ensure low-frequency response. So their AC input impedance is usually large.)

Do not care about
Quote:
Originally Posted by Marek View Post
Using transformer as output stage it is important to ensure lowest possible load (impedance) seen by DAC chip (preferably 10ohm o less).

PCM1704 or do not apply to..Because it is small output current ..
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Old 22nd April 2010, 10:35 AM   #8
Marek is offline Marek  Poland
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Location: Katowice
kimk,

no, every current-output DAC "wants" to see almost zero impedance (like opamp) to achieve best THD.
every voltage-output DAC respectively infinite impedance .
Thats why datasheet says ex: "at least 1kohm load" or "maximum load of 1kohm"

PCM1704 is a current-output DAC so you must load it with lowest possible impedance.
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Old 22nd April 2010, 10:57 AM   #9
kimk is offline kimk  China
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Marek,

I agree with you those points.

However, the reference manual to be traced, and the passive I / V, output current is too small in the small resistance can not give you more voltage output. Voltage transformer can improve the final, but the distortion of the transformer parameters such as how to compare with pure resistance?

In addition, the small level of the input transformer primary voltage, induced voltage and signal noise ratio will become worse ..
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Old 22nd April 2010, 11:48 AM   #10
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Anything else (besides an OpAmp) on the analog output will deteriorate the performance of the DAC, because it will not provide that zero-impedance needed. Even more, a transformer with no proper RC damping will induce ringing and overshoot. And it will have poor low-frequency response.

It will work, sure, and many cannot tell the difference anyway, but it will cripple that DAC.

Last edited by SoNic_real_one; 22nd April 2010 at 11:50 AM.
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