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Old 18th December 2012, 06:25 AM   #11
andyr is offline andyr  Australia
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Quote:
Originally Posted by hazard500 View Post

pass band is 10 hz to 150 kHz +/- 0.2kB. You can really hear 0.2dB drop at 150kHz?
I would say no! I had thought the "pass band" meant the -3dB frequencies, not the 0.2dB frequencies.

BTW, the spec sheet shows a 0.4dB loss at the mid-band. Does this imply the loss is different at different frequencies? The reason I am interested is that I can save a bundle if I can use LL1527s instead of output coupling caps, to stop DC passing, in my phono stage. I have been using $350 a pair caps!

One point, though ... the spec sheet specifies some particular input & output impedances - source 800ohms , load 4Kohms. This seems to me to not be comparable to the real-world situation which a phono stage is in ... eg. a phono stage feeds a line-stage preamp ... which is likely to have a Zin of 10-50Kohms?

Regards,

Andy
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Old 20th December 2012, 02:21 AM   #12
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Quote:
Originally Posted by andyr View Post
I would say no! I had thought the "pass band" meant the -3dB frequencies, not the 0.2dB frequencies.

BTW, the spec sheet shows a 0.4dB loss at the mid-band. Does this imply the loss is different at different frequencies? The reason I am interested is that I can save a bundle if I can use LL1527s instead of output coupling caps, to stop DC passing, in my phono stage. I have been using $350 a pair caps!

One point, though ... the spec sheet specifies some particular input & output impedances - source 800ohms , load 4Kohms. This seems to me to not be comparable to the real-world situation which a phono stage is in ... eg. a phono stage feeds a line-stage preamp ... which is likely to have a Zin of 10-50Kohms?

Regards,

Andy
Andy, I have always assumed that the 0.4dB loass at mid band implies a constant loss across al freq due the DC resistance of the TX. IE if you had a perfect 1:1 tx then the gain is 1, but real world losses mean that this tx has a gain of -0.4dB at mid band. Then the pass band is referenced to this point, ie Lundahl quote 20hz to 150kHz +/- 0.2dB referenced to the mid band.

In answer to your question about replacing caps with the Lundahl LL1527 - I would not do this if your amp is single ended. If you look at my circuit you will see that it has a balanced output so there is NO DC passing through the tx. If you have a single end output, and connect the tx between output and ground, there will be a DC voltage across the tx and hence a DC current - not a great outcome.

If your line stage has a Zin of 50k, then you can provide the 4k load for the tx by putting a resistor across the output of the tx - that's the way I do it. 4K resistor would be OK, this will give a total resistance of 3.7k. As long as the output of your phono stage is 1k or less then you will have no problems (as longs as its balanced!!)
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Old 20th December 2012, 06:59 AM   #13
andyr is offline andyr  Australia
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Quote:
Originally Posted by hazard500 View Post

In answer to your question about replacing caps with the Lundahl LL1527 - I would not do this if your amp is single ended. If you look at my circuit you will see that it has a balanced output so there is NO DC passing through the tx.
Thanks for this additional info, mate. Yes, my phono stage is single-ended, not balanced. I obviously had it wrong - I thought it was possible to use a series coupling transformer as an alternative to a series coupling cap, to stop DC offset passing though.

Oh well, I'll have to go back to using expensive 1uF caps!

Regards,

Andy
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