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Are there any "issues" with the use of a transformer in place of an output coupling cap (to stop DC)?

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Posted on December 18, 2012 at 15:18:07
andyr
Manufacturer

Posts: 12548
Location: Melbourne
Joined: September 2, 2000
I am considering using coupling transformers, in place of coupling caps, on the output of a phono stage. But I have a few Qs: :-))

The spec sheet for the Lundahl LL1527 says it has a 0.4dB loss at the mid-band.
Q1: Does this imply the loss is different at different frequencies?

The spec sheet also specifies some particular input & output impedances for the loss & frequency extension figures it quotes - source 800ohms, load 4Kohms. This seems to me to not be compatible with the real-world situation which a phono stage finds itself in ... a phono stage feeds a line-stage preamp ... which is likely to have a Zin of 10-50Kohms?

Q2: Will this different load change the behaviour of the transformer? In particular, will it worsen its performance - in terms of loss & frequency extension?

Q3: The source impedance, also, is around 25ohms (not 800ohms) - so will this affect the performance of the transformer?


Thanks for any input,

Andy

 

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RE: Are there any "issues" with the use of a transformer in place of an output coupling cap (to stop DC)?, posted on January 15, 2013 at 08:32:10
tesla
Audiophile

Posts: 3180
Location: San Diego County, California
Joined: October 25, 2000
Perhaps you should post your question to Lundahl. I would think they are the expert on their transformer.
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Yes, I did ..., posted on January 15, 2013 at 11:06:31
andyr
Manufacturer

Posts: 12548
Location: Melbourne
Joined: September 2, 2000
And their answer suggested these loading issues make life difficult. So I will continue to use a coupling cap.


Regards,

Andy

 

RE: "issues" ...coupling cap (to stop DC)?, posted on February 17, 2013 at 05:05:01
1. Some transformers can't tolerate any DC in their primary winding, so you would have to use a coupling cap to keep DC out of the transformer so you would not replace the coupling cap. Transformers in audio really are AC devices and DC in their winding requires special construction, ie gaps to prevent saturation. Even in AC, saturation is a problem on peaks. There are many different types of transformers each for a particular application so you need to direct this question to the manufacturer.

2. In audio, you want a low output impedance feeding a high input impedance by a factor of 5 or more to insure good frequency response. It's not like an AC power delivery system where you are concerned about power efficiency at only one frequency. An audio signal is broad band. Ray

 

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