choosing best output I/V transformer for PCM1704 DAC

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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
 
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
 
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)
 
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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....
 
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
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 ..
 
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.
 
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 ..
 
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.
 
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kimk,

when you use for example LL9226 transformer in 1:20 configuration -> effective prim DCR=1ohm, sec DCR=260ohm. then load SECONDARY windings with for example 10k you will get:
impedance seen by DAC less than 20ohm - not bad
output voltage about 1V p-p - quite usable

so if youuse pure resistance of 20ohm you will get only 48mV output - definetly not directly usable. When you use larger resistor you get horrible THD...
moreover trafo used with balanced outputs greatly eliminate quantization noise and do PP->SE conversion with great CMRR.

Have you ever compare sound of resistor I/V to those with transformer + resistor?
 
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.

My first output stage for my DAC was based on OPAMP (the one of the best OP627BP an AD797) but when i tried not suited for this purpose LL1517 (1:1) I knew I will never come back to OPAMP again. with OPAMP DAC was in most possible comfort situation but sound sucks. Why, i think because of NFB->IMD.
I think when I use well suited trafo I'm sure it will be even better.
 
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.

OpAmp will deteriorate as D / A Performance
Only a few people talking
(Example: IC internal, interface, transfer of distortion, transient intermodulation distortion, etc.)
 
kimk,

when you use for example LL9226 transformer in 1:20 configuration -> effective prim DCR=1ohm, sec DCR=260ohm. then load SECONDARY windings with for example 10k you will get:
impedance seen by DAC less than 20ohm - not bad
output voltage about 1V p-p - quite usable

The case of square wave Ring How?

How many flat frequency response?

In fact D / A chip output AC current, your Connection to the D / A chip, its load resistance has been greater than 1KOhm ...

so if youuse pure resistance of 20ohm you will get only 48mV output - definetly not directly usable. When you use larger resistor you get horrible THD...
moreover trafo used with balanced outputs greatly eliminate quantization noise and do PP->SE conversion with great CMRR.

I totally agree!But I would like to 1KOhm load impedance for the PCM1704 is not too large (this is the standard manual for, THD is a good level)

Have you ever compare sound of resistor I/V to those with transformer + resistor?

As you mentioned the "Using transformer as output stage it is important to ensure lowest possible load (impedance)"

Can be seen, the larger the output current D / A will be more consistent with this principle .. I was in line with the vacuum tube to complete the last required current drive and voltage swing ..
 
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In fact D / A chip output AC current, your Connection to the D / A chip, its load resistance has been greater than 1KOhm ...

No its not! transformer is not the same thing as inductor! you cannot treat primary winding as a stanalone inductor with impedalne Z=2*pi*f*L.
Transformer transforms impedances form one winding to another. Impedance from secondaries is transformed to primaries with 1/N^2 and impedance connected to primary is transformed to secondaries with N^2 relation.
So if you connect for example 10kohm on secondaries of 1:20 trafo you will "see" from primary side load equal to:
10000/400=25ohm, ofcourse DCR of windings not taken into account.

I totally agree!But I would like to 1KOhm load impedance for the PCM1704 is not too large (this is the standard manual for, THD is a good level)

I'm not sure if i understand correctly.
this link treat about load of PCM63, the older brother of PCM1704
Digi Scoop 1
 
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No its not! transformer is not the same thing as inductor! you cannot treat primary winding as a stanalone inductor with impedalne Z=2*pi*f*L.
Transformer transforms impedances form one winding to another. Impedance from secondaries is transformed to primaries with 1/N^2 and impedance connected to primary is transformed to secondaries with N^2 relation.
So if you connect for example 10kohm on secondaries of 1:20 trafo you will "see" from primary side load equal to:
10000/400=25ohm, ofcourse DCR of windings not taken into account.

According to your formula, the secondary load is less than their copper resistance, and then mapped to the primary, whether this resistance will be less than the resistance of the copper?

The following are LL9226 manual to the data:
Primary no load impedance @ 0 dBU, 50 Hz, all in series:
3 kΩ typically


I think there is no parallel primary "energy release resistance" or called "matching resistance" (the transformer inductance and distributed capacitance are energy storage devices) the transformer input inductance and capacitance is very big, for AC voltage Said. (Especially high frequency)

So, I think your formula and in practical application here is not appropriate!

In addition, (your link is given on the PCM63), D / A chip that is quantifiable output AC current. Only a 5V supply voltage of the D / A, its internal high-speed CMOS switch that, in the appropriate voltage to guarantee the correct current output, so the high output voltage will destroy the conditions necessary! Therefore, D / A chip of the Iout terminal, matching the appropriate resistance appeared to be very important for the THD.
 
no load impedance means that secondary winding is open so OFCOURSE we can assume that secondaries not exist and we can treat trafo as a "ferrite bead". But we don't speak about this.

teach yoursefl: http://www.analog.com/static/imported-files/application_notes/AN_912.pdf

Thx..Marek,

when you use for example LL9226 transformer in 1:20 configuration -> effective prim DCR=1ohm, sec DCR=260ohm. then load SECONDARY windings with for example 10k you will get:
impedance seen by DAC less than 20ohm - not bad
output voltage about 1V p-p - quite usable

I still stick to their views! its load resistance has been greater than 1KOhm ...
 
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