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Lundahl LL1545A, best dac connection

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Posted on October 10, 2015 at 13:52:42
eirper@ath.forthnet.gr
Audiophile

Posts: 7
Location: GR
Joined: October 10, 2015
 Hi,
I have a voltage output dac with 1.9Vrms output and 230 ohm Zo about.

What is the best way to join a LL1545A at the output.
What is most preferable an 1:1 or 2:1 with the 37.5 ohm output and a half voltage output (but a twice current).

I have seen a schema http://www.4tubes.com/SCHEMATICS/LUNDAHL-TRANSFORMERS/Input-transformers/1545a-1.jpg
that uses a balanced output from unbalanced input.
The 15K resistor btw Pin1 & 3 at the output is necessary or it is optional?

 

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RE: Lundahl LL1545A, best dac connection, posted on October 12, 2015 at 06:54:04
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 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

 

RE: Lundahl LL1545A, best dac connection, posted on October 18, 2015 at 06:21:40
eirper@ath.forthnet.gr
Audiophile

Posts: 7
Location: GR
Joined: October 10, 2015
 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 ...

 

RE: Lundahl LL1545A, best dac connection, posted on October 18, 2015 at 06:56:08
eirper@ath.forthnet.gr
Audiophile

Posts: 7
Location: GR
Joined: October 10, 2015
 I forgot to say that the load was 10K at the output of LL1545a, at the Zo test.

 

RE: Lundahl LL1545A, best dac connection, posted on October 14, 2015 at 04:52:47
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 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

 

RE: Lundahl LL1545A, best dac connection, posted on October 14, 2015 at 05:05:47
eirper@ath.forthnet.gr
Audiophile

Posts: 7
Location: GR
Joined: October 10, 2015
 Thanks for your advices, Kevin.
Your help are much appreciated.

 

RE: Lundahl LL1545A, best dac connection, posted on October 18, 2015 at 00:30:29
eirper@ath.forthnet.gr
Audiophile

Posts: 7
Location: GR
Joined: October 10, 2015
 Kevin,
How much dc-offset can removed this type of transformer, without any problem?

 

RE: Lundahl LL1545A, best dac connection, posted on October 18, 2015 at 09:10:46
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 Very little DC offset is tolerated by this transformer. I think that mVs is too much.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: Lundahl LL1545A, best dac connection, posted on October 19, 2015 at 22:51:46
eirper@ath.forthnet.gr
Audiophile

Posts: 7
Location: GR
Joined: October 10, 2015
 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?

 

RE: Lundahl LL1545A, best dac connection, posted on October 20, 2015 at 05:29:58
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 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

 

RE: Lundahl LL1545A, best dac connection, posted on October 20, 2015 at 14:09:59
eirper@ath.forthnet.gr
Audiophile

Posts: 7
Location: GR
Joined: October 10, 2015
 Thank you for the very nice explanation, Kevin.

 

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