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In Reply to: RE: Lundahl LL1545A, best dac connection posted by eirper@ath.forthnet.gr on October 10, 2015 at 13:52:42
You didn't mention whether your DAC has balanced outputs or single-ended outputs. In any case, look at the LL1545A datasheet, which is at the link. There are balanced (B) and unbalanced (U) diagrams for wiring.
I suggest R2B/R2U : L2B/L2U for 1:1 and R4B/R4U : L2B/L2Y for 2:1. These connections are from the table as the bottom of page 1 of the data sheet and the wiring is shown on page 2. No 15K resistor is necessary...
Kevin Carter
K&K Audio
www.kandkaudio.com
Follow Ups:
Thanks a lot for the respond, Kevin.It is true, I forgot to mention what type output is!
It is an SE output, but I would like to have and a balanced too.Sorry about my ignorance, but I am a newbie at the line audio transformers.
Allready, I have tried the two options that you proposed to me. With a FFT Analyser that I have used, it seems that the L2B/L2U (2:1) has a cleaner capture.
But I have some concern about Zo.
With R4U/L2U (2:1) the resistance output (Zo) for the 234 of direct dac output goes to 364 ohm.
With R2U/L2U (1:1) the resistance output (Zo) for the 234 of direct dac output goes to 536 ohm.
It is normal? This isn't an advantage for Zo of dac?Additional, at the R2U/L2U (1:1) setup, I had to join the pin10 & pin15 to signal_ground to have cleaner FFT capture.
I hadn't time to explore the balanced output ...
Edits: 10/18/15 10/18/15
The DC resistance of the transformer windings are added to the source impedance (and transformed by a step-down ratio, if that is used) to get the output impedance of the combination, so your results make sense. If you want lower output impedance, then you must use a transformer with lower resistance windings. The LL1545A was not designed to be an output transformer. Something like the LL1527 would be a better choice.
Kevin Carter
K&K Audio
www.kandkaudio.com
Thanks for your advices, Kevin.
Your help are much appreciated.
Kevin,
How much dc-offset can removed this type of transformer, without any problem?
Very little DC offset is tolerated by this transformer. I think that mVs is too much.
Kevin Carter
K&K Audio
www.kandkaudio.com
The one dac channel output has <3.5mV and I don't measure any dc-offset at the lundahl output.
Is there any way to see the limit in this case?
For example, if the output of LL1545A remains 0mv, this is a sign that enough or created other problems?
Since it's a transformer, it won't pass DC, so you won't measure any offset at the secondary connections. When the offset is too large, the core is saturated, and bass frequencies are essentially not passed by the transformer. Run a frequency response sweep or just listen. It will be obvious.
Kevin Carter
K&K Audio
www.kandkaudio.com
Thank you for the very nice explanation, Kevin.
I forgot to say that the load was 10K at the output of LL1545a, at the Zo test.
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