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Old 24th June 2010, 06:49 PM   #11
novec is offline novec  Norway
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SoNic_real_one: Like awpagan says, increasing the Iref resistor would decrease Iref and thus Iout to a level the zaps can handle. And I am annoyingly aware that'll increase S/N at the same ratio... Right now I'm torn between accepting that fact and using the zaps that I know and like, and have paid good money for, or start working on another output that may or may not be better.

jcx: Your solution looks very interesting, the only caveat is the dreaded T&M - time and money. Do you have any idea when you might have a working and tested circuit i can "borrow"? Despite being stubborn and annoying, I am open to other solutions, as long as they're better ;-)
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Old 24th June 2010, 11:05 PM   #12
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Quote:
Originally Posted by novec View Post
SoNic_real_one: Like awpagan says, increasing the Iref resistor would decrease Iref
You said 'Iref', not 'Iref resistor', just making an observation
Personally I think that good OpAmps are all you need for a good DAC.

Last edited by SoNic_real_one; 24th June 2010 at 11:08 PM.
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Old 25th June 2010, 01:33 AM   #13
novec is offline novec  Norway
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That's me - writing half a sentence and thinking the rest

I tend to think a discrete output stage has the potential of being the best, but has a greater potential of being a complete failure. The only opamps I have any kind of experience with are the LM4562, which have fared pretty well with the PCM1794 for a friend of mine. And as far as good goes, those tend to come at a price, and I need 10+ channels for five channel active XO. But let's not worry about that just yet...

jcx: I'll start ordering samples right away! With 50-ish opamps it's gonna take a while
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Old 25th June 2010, 04:25 AM   #14
jcx is offline jcx  United States
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I suspect the ADA4898 in the single op amp transimpedance I/V per the DAC app note will be very good even at the PCM 1794 current levels - I should think a significant improvement over the 5534 - the low Vin noise allows the noise gain input mod that shaves a little off the peak error by directly shunting switching glitch energy to gnd

the ADA4898 multi-tanh linearizes the input to 1 ppm gm change at ~ 4 mV diff Vin

the multiloop Hawksford I/V would be a beast - I'd probably use a composite for the final follower to reduce its errors too

but op amp expense is only the start - you will be spending $5-10 ea for bulk metal foil resistors - its the only right thing to do when the active devcie distortion is driven so low
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Old 25th June 2010, 11:44 AM   #15
novec is offline novec  Norway
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Ok, now you're getting me seriously interested... Potentially broke, but interested... Where do I sign up? :-)

Do you have a rough estimate on when you'll have tested the Hawksford with ADA4898? I'm leaning towards making a simple opamp output for now, and go for a better one once I've played with the DACs some more. I'm still not sure whether the SRC will be necessary with the Lynx AES16 as the only source.

Would you recommend ADA4898 for summing too, or just I/V? after a lot of back and forth I've decided to go single ended.
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Old 25th June 2010, 02:24 PM   #16
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I did have a great experience with LM4562 in summing/filter, so good that I replaced all the OpAmps in my Denon AVR3805 receiver, DVD2930 universal player, Toshiba universal player (SMD's) and even the I/V stage in a Phillips CD 374 (8DIP) with those...

I am interested in the ADA4898, as a matter of fact I am waiting for a couple samples right now. But in Denon's I have PCM1791A, that are voltage-output.
But already the LM4562 is so good that I cannot envision making a big improvement - but who knows?

Last edited by SoNic_real_one; 25th June 2010 at 02:29 PM.
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Old 25th June 2010, 03:37 PM   #17
novec is offline novec  Norway
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I actually have some LM4562 DIPs laying around, so I guess the first batch will use them Do you have your circuit (with values) handy?
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Old 25th June 2010, 06:17 PM   #18
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Use the one from the reference PDF page 20,21 - resistor values are ok for 4562. Just ditch the 22pF capacitors, 4562 is internally compensated (unity gain stable).
You also can take a look at how it is done for PCM1792 in Denon DVD-5910...
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Last edited by SoNic_real_one; 25th June 2010 at 06:21 PM.
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Old 26th June 2010, 02:15 AM   #19
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Quote:
Originally Posted by SoNic_real_one View Post
You also can take a look at how it is done for PCM1792 in Denon DVD-5910...
Take a look by all means, but don't try that at home. This is quite a good example of how not to do a DAC output stage - in particular the decoupling. Are all those thick (power) traces ground fills?
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Old 26th June 2010, 03:22 AM   #20
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Quote:
Originally Posted by novec View Post
... considering the awesome REG113...
I'd not come across that part before, certainly does look high performance. But then, buried deep in the datasheet I read this :

The REG113 uses an internal charge pump to develop an
internal supply voltage sufficient to drive the gate of the
DMOS pass element above VIN. The charge-pump switching
noise (nominal switching frequency = 2MHz) is not measur-
able at the output of the regulator over most values of IOUT
and COUT.


Are you really sure sensitive analog circuits are going to be better off with multiple built-in 2MHz oscillators? Especially as each reg has its own and they won't be synchronised.
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