Can resistor i-v voltage be compared to MC output? What transformer specs? - diyAudio
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Old 17th October 2006, 01:56 PM   #1
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Default Can resistor i-v voltage be compared to MC output? What transformer specs?

After much thread searching , I'm going to try resistor based i-v conversion in lieu of op-amp , in my Denon DCD-2560 player. The player uses the AD1862N-D dac chip , with 2 per channel. Appears to be in differential mode. I'm having a difficult time determining which transformer to use for step-up. I would like to be able to use the trans as the output stage also. The AD1862 datasheet states ouput current as +-1ma. I'm thinking I should begin with a 100ohm resistor(?). My thought is , since this voltage out will be in millivolts , can I look at this output as stepping up the voltage of a MC phono cartridge? I would seem a lot easier to determine which transformer to use. Are there other considerations I should be aware of? What transformer specs should I look for when chosing one for I-V step-up (pri imped, sec imped)?
Thank-you
Paul
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Old 17th October 2006, 03:21 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

it sounds like it will not be able to drive the output cable capacitance.
The higher the output voltage, the longer the cable, the worse it will get.

/sreten.
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Old 17th October 2006, 04:18 PM   #3
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Quote:
Originally posted by sreten
Hi,

it sounds like it will not be able to drive the output cable capacitance.
The higher the output voltage, the longer the cable, the worse it will get.

/sreten.

Is there a practicle way of doing this resistor i-v with a passive(transformer step-up) output stage? Is itthe AD1862 DAC chip(s) or is this the usual problem with transformer based output?
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Old 17th October 2006, 04:48 PM   #4
phn is offline phn  Sweden
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An "easy" way is to use a tube stage and step-down transformers.

How much of a problem is step-up transformers? People are using passive transformer attenuators seemingly without problem.

Since you will end up with something like 5kOhms with step-up transformers, your pre/amp will need to have higher input impedance than that. That should be no problem.
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Old 17th October 2006, 05:11 PM   #5
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My tube pre-amp has an input imped of 300k. At this point , I need some help in chosing a transformer . Do I want very low pri. imped.(<100ohm)? Does it need to be center-taped? And does this whole thing become more problematic as I increase the ,turns-ratio'? I'm willing to purchase good(expensive) transformers for this , but I would really like to do that just once .
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Old 17th October 2006, 06:08 PM   #6
phn is offline phn  Sweden
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I hear you. I'm planning to build a SET with interstage transformers. Once the transformers are bought, it's final.

I think 250-Ohms primary will do it. Something like these: http://www.lundahl.se/pdfs/datash/1538_8xl.pdf But this is nothing I know. I don't even know what kind of gain you need.

Hopefully somebody else will jump in. I think you do the right thing. But most people tend to shun transformers for reasons I don't understand.
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Old 22nd October 2006, 12:29 PM   #7
phn is offline phn  Sweden
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This is what I meant with, "But most people tend to shun transformers for reasons I don't understand." Not much interest.
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Old 23rd October 2006, 08:41 AM   #8
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look at sowter:

http://www.sowter.co.uk/

search for the dac page on the left
and than the 9762. Specially built for the 186* family.

I have the 9545 with the PCM 1704 sounds very nice

Good luck
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Old 10th August 2007, 01:58 AM   #9
wobbly is offline wobbly  New Zealand
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Hey Robbie b, what I/V resistor did you use with PCM1704/9545 - before or after the sowter.
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Old 20th August 2007, 04:16 PM   #10
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Dear Wobbly,

nice to give a new life to the discussion.

i used the Caddock MK132 on the primary side of the transformer, 100 ohm's to start with. It will give a low output of about 600 mV in avarage, if I remember well. Later I doubled the resistor to get 50 ohm, it sounded better but the output was twice as low.

Now I use a AudioNote tantalum resistor of 100 ohm and it sounds more natural than the MK132.

I heard that it was possible to put the resistor on the secondary side only a few weeks ago, when I checked the sowter site I read it. At the time that I build my player it was not known so i never experimented with the secondairy resistor.
i can not tell you what the effect will be. But this is the nice world of DIY, try and see, it will not destroy your transformers so,...
I would like to hear the results though.

Good luck

Rob
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