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Old 13th December 2003, 12:12 AM   #1
Lyra is offline Lyra  Norway
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Default Need help with this I/V stage replace...

Hello. Trying to attach schematics of the DAC/ IV-stage of my Denon DCD-2560. (hope it works...)
What I really could need some help with is:
If You look at the left channel, there are two AD1862 DAC's, each one is feed into its own corresponding OP-amp (IC315/IC317) (buffer?), and further into one OP-amp named IC311. (the right channel is like)
Is it possible to take the signals from the AD1862's, and feed it directly into the I/V converter from the D1. (Schematics at www.passlabs.com ), and get a balanced output.
I am aware some of you don't like CAP's in the signal path, but if htis could work, I am pretty sure it would SOUND pretty amazing compared to the original Denon I/V !!!?
My problem is that i am not sure if the DAC's is originally balanced, or just some strange (?) form of OS....

All the best....and please...HELP :-)
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Old 13th December 2003, 12:18 AM   #2
Lyra is offline Lyra  Norway
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And here is a picture of the DAC's and before this (this is why I am not sure if my DAC's are balanced....)
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Old 13th December 2003, 02:14 AM   #3
Pedja is offline Pedja  Serbia
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Hi Lyra,

The IC315/IC317 are I/V converters (it is strange the dual opamps are put inside but both have one half left unused; these converters also have strangely high value I/V resistors and then the signal is attenuated). The IC311 converts the signals to SE.

I am not very familiar with AD1862 (nor with PCM63 nor D1) but looking briefly I’d say it shouldn’t be a problem to replace this I/V with that from D1. For the AD1862 you’ll need a higher value I/V resistors than those used in D1.

Pedja
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Old 13th December 2003, 02:55 AM   #4
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Be very careful when working with the '1862s. They are very static sensitive, and if you operate them without a load, they will fry.

If the MSB trim is properly set, it will be good for he life of the part.

They are not cheap, so don't ruin them!

(I like them.....in some ways easier to work with than B-Bs.)

Jocko
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Old 13th December 2003, 11:22 AM   #5
Lyra is offline Lyra  Norway
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Quote:
Originally posted by Pedja
Hi Lyra,
I am not very familiar with AD1862 (nor with PCM63 nor D1) but looking briefly I’d say it shouldn’t be a problem to replace this I/V with that from D1. For the AD1862 you’ll need a higher value I/V resistors than those used in D1.

Pedja
Which resistors are you referring to ? (R26 and R33??), and what should the value be ??

All the best!
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Old 13th December 2003, 03:56 PM   #6
Pedja is offline Pedja  Serbia
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I/V resistors are R27 and R34. However, raising their values the operating points will be changed. It will take some time to find the best new operating point. Using the circuit as it is with AD1862 you’ll have about 1V RMS out per phase. You did not tell what you’ll do further with this balanced signal, but it might be enough as well.

Pedja
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Old 13th December 2003, 04:12 PM   #7
Lyra is offline Lyra  Norway
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If i could get it working, i will feed the signal into the balanced input of my Electrocompaniet EC 4.5 preamp. Dont you think the signal-swing would be enough?

Lyra
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Old 13th December 2003, 04:45 PM   #8
Lyra is offline Lyra  Norway
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And....
-when making a new powersupply to this I/V-stage....a 15-20VA toroid transformer would be enough, wouldn't it?.
How much current do i have to expect/ calculate for with the use of two of theese cirquit's?

-As for the I/V resistors in the original Denon drawing, is this R811, and R813 (9k1) ?

-What kind/size of resistors would you recommend for the D1, and what kind of results will this accomodate ?

-The JFET's in the D1-stage have to bee matched, but if i buy matched JFET's (e.g. 2sk389 dual Jfet's), and use this ones so that Q2, and Q5 are matched, and also Q3, and Q6 are matched, this would be good enough or ??

PS: I don't feel to confident that i have figured it all out (or even enough.. ..) yet, regarding theese kind of cirquets. I don't have an intension of reinventing the wheel, just to get the maximum out of my sound-system.


All the best
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Old 13th December 2003, 09:34 PM   #9
Pedja is offline Pedja  Serbia
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Expect about 15mA per phase i.e. 30mA per channel.

Yes, 9.1k are I/V resistors and these give quite high output voltages that are attenuated with 4.3k/2.2k dividers and then with 6.8k/3.3k divider and... (signal goes out of the sch).

610 are IRF610 MOS-, not J-FETs.

No idea about Electrocompaniet. But if you are not sure what to change and how, it would be best to make the circuit as it is, I guess it is well tuned to its sweet spot. It is not a problem to make the circuit working with higher value I/V resistors (generally you should alter R26/R33 to ensure proper biasing and scale C15/C16 to match the new I/V resistors) but you’ll have more to do to make it give its best.

Pedja
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Old 14th December 2003, 12:36 PM   #10
Lyra is offline Lyra  Norway
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Thanks Pedja!
I think this will help me a lot.....i think i build the D1 as is, and try it. Then if the signal is too low i try to modify it.

As for Jocko Homo: "If the MSB trim is properly set, it will be good for the life of the part" i'll try to find out about this, but i don't think i have the right tools needed for this adjustment, but then again i am not sure what tools needed. Maybe it's not all that much tho....

Thanks!
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