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Output transformers for DACs

To Zoran:

Different output currents - different i/v resistors for the same targeting Vout - simple story. Regarding 1794. Simply use the same values of both poles i/v resistors - and there will be almost no DC-offset across the Pos and neg poles (as the offsets for neg and pos poles has the same value including sign). Connect the primary coil between pos and neg then - that's it.


Regarding specs. I have some technical data, but for some particular trafos versions only. Measuring and collecting info such you described is boring process for me in most cases, so I prefer to interact with the customer on the level of the real project implementation. In such a case the knowledge of electrical parameters you have described is really fades into the background. Parameters are just parameters. They matters for sure, but in most cases within my own transformers, for my own internal needs. I don't want to fight windmills, I have no time for such.
 
There are -6.2mA of DC component x 4 times, creating offset at the Riv @ direct output?
This produce -0.62Vdc off-set @ 100 ohm Riv resistor. And this -0.62V will run trough the primary of the transformer. If the transformer core is from high permeability (probably is) this can produce core saturation and magnetization.
This is specific only for this dac chip model...
 

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Zoran, you should understand that there is no offset between pos and neg poles. Primary coil has no deal with the gnd, so the real offset that can be found between the pos and neg poles (between Ioutp and ioutn) relates to the internal imbalance of the DAC chip between the two poles output currents. In real it is less than 5mV with the Ra/Rb iv resistors with 1% tolerances. You can make it even 0.0mV if needed by adjusting the Ra/Rb values.
 
original_63fe05d2-ffad-4368-adbc-f8363c59fbfc_PXL_20231204_203734964.jpg


Here is an example of how to "scale" my trafos/buffers boards (using iancanada's DAC II for example, but it works excellent with any other current output type DACs) to allow DAC chip to work on to the very small i/v resistor with the high enough voltage output level after my trafos (boards). Active stage is mandatory (+/-15V) in this case.
 
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Hello Werner,
Please use 100R across the primary with ~13k across the secondary coil instead of 4k12 across the secondary. 100R resistors should be as good quality as possible. Wirewound or AN tantalum/niobium or Z-Foils, etc high quality audio resistors.
Sure you can use trafos with D3 or DDDAC.

Ivan.
Hi Ivan,

did not had time to tinker with my setting. Which is working fine btw. I use 4 Kohm on the sec. on your trafos right now (balanced to preamp).
I was planning to go with only about 16 Ohm on prim. (1/60 of 100 Ohm (or lower)).
Now I see you recommend to split load between prim. and sec. What is the best way to go? and how to split? I wish to lest my setup with "normal" resistors to get the right volume for my chain an then update to "quality" resistors.

thank you
Branko
 
My cores are not nickel-based. FINMET if in two words. You can try to use a separate primary coil to compensate the DC-offset btw (gap - is really not my story with such cores). Or you can compensate the DC using extra circuit on jfet. But I prefer to avoid such extra difficulties because...
I have done some measurements couple years ago using 1:44 trafos with TDA1541 (1541 sees 10 Ohms, output impedance was ~20k).
sine 20Hzsine 1kHzsine 20kHz
-2dB0dB (0,52Vrms)-0,7dB
THD 1,42%THD 0,0023%THD 0,0033%

As for me results was good enough without extra coils, compensating circuits, extra PSUs, etc... 🤷‍♀️
 
I thought I read somewhere a long while ago that there was some modulation of the DC current that would make a strip would core unhappy even with a counter coil. Maybe that is why we don't see it implemented often? I have heard about the implementation but see very little evidence it gets implemented with great success. I am not trying to stirr any hornets nest, just wondering if it would be worthwhile to have a small gapped unit for the TDA. The LF 20hz distortion in the numbers you posted certainly is higher than at 1k but if it is an ungapped core it's not as bad as I would expect and then maybe the thinnest of gap or even a polished cut may be enough to help the LF saturation.
 
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The LF 20hz distortion in the numbers you posted certainly is higher than at 1k but if it is an ungapped core it's not as bad as I would expect
PCM63 in the same conditions gives very similar results to the TDA1541. Diff mode for 1541 gives 0.66% at the same output level. So I can conclude that the DC-offset not affecting at the chosen configuration, or affecting slightely.
 
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Hi,
as I am in the process of buying (high quality) resistors for the primary side of my transformers I measured them today for the first time. Left channel 1-->2 -- 2.4 OHM; 9-->10 -- 110.2 OHM. Right channel 1-->2 -- 2.0 Ohm; 9-->10 -- 88.2 OHM (Fluke 177).
Do I have to change anything or could I use same resistor value across booth sides. At the moment I use 1000 OHM at the secondary and would like to go with 16 OHM on primary. Or even less (how low could/should I go?)

Thank you
Branko
 
Hello Ivan,

got my resistors today. As I have the older version of your PCB for Ian's DAC, there are only soldering points for resistors at the secondary R1/R2. So I would solder the 16R directly between the receptacles of the primary? And the 10K at R1/R2. Would that be OK?

BR
Branko