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Old 17th October 2008, 10:50 PM   #1
konut is offline konut  United States
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Default DACs and Passive Pre Question

I am considering 2 DACs. Both are very highly reviewed recently introduced products by highly regarded companies. DAC A has an output impedance of 560 ohms, 3V out. DAC B has an output impedance of 50 ohms, 2.3V out. Both DACs have discrete output sections. Both DACs are SS. My mono-blocks have an input impedance of 100k ohms, and a sensitivity of 1.5V for rated power. Amps are SS. The IC from the DAC will be 2' of Signal Cable Silver Resolution, 18.9 pF/ft , 0.12 H/ft , going to a Creek OBH-12, 0-50K ohm output impedance, 50k ohm input impedance. From the OBH-12 are 1.5m of Eichmann Express 4 IC going to the amps. Are there any other specs you would need to answer the question, which DAC will have the greatest gain?
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Old 17th October 2008, 11:56 PM   #2
kevinkr is offline kevinkr  United States
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DAC A will give you the greatest output voltage, but in any event the difference is just a couple of dB..

I am assuming (from your description and no research) that the Creek is passive in which case the output impedance is 0 - 12.5K not 50K as you indicated. (Maximum output Z is at -6dB or half the overall resistance 25K||25K is 12.5K.) Thevenin is your friend.. (see wikipedia)

There is no real need for the "DAC A" versus "DAC B" here - you might get some other useful insights by being more forthcoming. Diyaudio.com is not a forum where flaming is tolerated so have no fear.
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Old 18th November 2008, 11:00 PM   #3
konut is offline konut  United States
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FWIW, I was finally able to measure the SPL level of the 2 DACs at max volume of the Creek. Output was from a SB3 playing files from a Stereophile disc for the 1000hz tone, and a XLO Reference disc with the 315hz tone. Playing a 1000hz, -20db test tone, the Rat Shack meter read 86db for DAC A and 94db for DAC B. A 315hz test tone, at unknown level, measured 90db for DAC A and 98db for DAC B. I was not expecting this result, and kevinkr didn't either. It seems the lower impedance trumps the difference in voltage level. I'm really shocked at the 8db disparity.
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Old 18th November 2008, 11:06 PM   #4
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This cannot be, something else was amiss, otherwise Ohms Law just went out the window.

Cheers George
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Old 19th November 2008, 05:17 AM   #5
konut is offline konut  United States
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You're probanly right. DAC A is going back to the manufacturer, out the door.
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Old 28th November 2008, 12:24 AM   #6
konut is offline konut  United States
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The latest.........for your consideration. I sent the unit back. I also noted, to the manufacturer, noise at full output. This is the reply.

"We revised your DAC . It tests out great and does indeed swing plus and minus 3 volts (6 volts peak to peak) with a test CD will all the bits turned on (max possible) into a 10K ohm dummy load.

We replaced the transformer and actually changed two resistors per channel that would affect the gain into a low impedance load. The series output resistors were changed from 560 ohm to 100 ohm. Also the reference resistors for the film output coupling capacitors were changed from 22K ohm to 47Kohm Both changes make the unit less load sensitive. We would have to drop the logic board again to make further changes and I think you will like it as is.

I would like to keep the output resistors at 100 ohms to provide good protection from excess capacitive loads and stray shorts and transients."

The unit will be shipped back to me this Monday. Thoughts?
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Old 28th November 2008, 02:12 AM   #7
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Tell us if now with the 100ohm output resitor if it has picked up in level, if it has, I would then say it has pittyful output driving current, <10mA.

Cheers George
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Old 28th November 2008, 02:31 AM   #8
konut is offline konut  United States
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Will do.
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Old 28th November 2008, 11:56 PM   #9
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The difference between 100 ohms and 560 ohm source impedance into 100K load even allowing for the change in shunt resistance from 22K to 47K at the output should not make a big difference in levels. (A few tenths of a dB at best)

Given the 100K input impedance the output stage is not called on to deliver more than a few tens of uA pk-pk even at 0dBFS.

I am not entirely sure what relevance that <10mA output current comment has, in no instance is anywhere near that kind of output current required to drive that input to 3Vrms or any other reasonable value. (In fact we are talking 84uApk-pk to do this into a 100K input impedance)

Op-amps capable of sinking or sourcing 10mA linearly into 1K would deliver around 7Vrms into that load. My tube based line stage can drive a 600 ohm load to about 4Vrms which apparently is also quite wimpy.
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Old 29th November 2008, 12:48 AM   #10
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Agreed, it was too late to edit out, was was thinking when I had a output transformer coupled dac here for repair and it could not drive anything below 20k yet it could give 3v out into 500k. But these are solid state.

Cheers George
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