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Old 20th July 2007, 01:29 PM   #1
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Default Impedance matching; phono > pot > A/D convertor?

I'm going to run the line out of a phono-stage straight into a A/D convertor for recording vinyl, the A/D (flying-calf 24bit) has no level control, so I need to use something between them to set levels.

The question is, do I need to worry about impedance matching? I can't remember where I read it, but I think the Calf has a buffer chip on the analogue-in, so is it safe to assume that it would be quite happy with the high-impedance output of a potentiometer, say a 50K Alps?

thanks.
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Old 23rd July 2007, 08:59 PM   #2
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Depends on the input to the A/D. Is it buffered? High impedance? If so, then you can likely get away with a simple volume pot.

jh
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Old 25th July 2007, 12:15 PM   #3
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Quote:
Originally posted by hagtech
Depends on the input to the A/D. Is it buffered? High impedance? If so, then you can likely get away with a simple volume pot.

jh
The specs for the A/D's line-in (which are 1/4" jacks) simply state "greater than 10K ohms", which I guess doesn't qualify as high-impedance.

As I said, I recall reading somewhere that the input is buffered, but I can't be sure.

Does that ">10Kohms" figure indicate anything one way or the other?

BTW, Jim Hagerman, I presume? Thanks for your interest!
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Old 25th July 2007, 05:56 PM   #4
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Might be able to stick a DVM on that input to measure a dc resistance. If it is 10k, then there is an old trick you can try. Use a 25k or 50k LINEAR pot for volume. The 10k on the wiper to ground (return) will change the transfer function to something that almost appears to be log. At least over the useful range you'll need.

The problem is that doesn't help you much if the phonostage can't drive a 10k load (8k at full volume).

jh
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Old 25th July 2007, 06:29 PM   #5
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Well, coincidentally I bought an Alps blue 50K linear a week ago!

The phono is a (cough) Gram Amp 2 - I emailed Graham Slee about it's output a while back, he was emphatic that it would have no problems driving a 10K load, so I could try what you've suggested.

Would a 10K DC resistance on the A/D's input imply it uses a buffer chip?
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