[konut]

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, 2V out. My mono-blocks have an input impedance of 100k ohms, and a sensitivity of 1.5V for rated power. Are there any other specs you would need to answer the question, which DAC will have the greatest gain?

[JoshK]

DAC A has the highest gain, since it has 3V output.  The output Z in both these cases is sufficient for both these cases.  Ideally you'd need to know how much current both can drive to fully understand the ability of the DACs to drive a given amp. 

I'd guess that DAC A is a tube output DAC and DAC B is SS.  That is just a guess, but 50 ohms is hard to do with tubes without feedback.  560 ohms could be SS too I guess, but seems more typical of tube SRPP type or CF output stages.  One could hazard a guess at how much current DAC A has from the tube choice (rough ballpark).  I am not comfortable/familiar enough with SS circuitry to hazard a guess there.

How long are you IC's?  Is the input stage of your amp SS or tube?  (SS tends to have more input capacitance requiring more current drive from the source or preamp; miller effect in tubes).

[TomS]

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, 2V out. My mono-blocks have an input impedance of 100k ohms, and a sensitivity of 1.5V for rated power. Are there any other specs you would need to answer the question, which DAC will have the greatest gain?

It sounds like you're going direct to amp, so do the DACs and/or the amps have attenuators?

[konut]

Both DACs are SS. The IC from the DAC will be 2' of Signal Cable Silver Resolution, 18.9 pF/ft ,   0.12 µH/ft , going to a Creek OBH-12, 0-20K ohm output impedance. From the OBH-12 are 1.5m of Eichmann Express 4 IC going to the amps. Amps are SS.

[JoshK]

Are the dac's opamp based? (opamps are sometimes slew limited, i.e. not enough current).  Do you know if the amps have a bipolar or fet input?  (I think bipolar usually has more input capacitance).   Your cables aren't very long, so that shouldn't be an issue. 

[konut]

Both DACs have discrete output sections. Unknown if amps are bipolar or fet.



CORRECTION: The Creek has 0-50k ohm output impedance. Input impedance is <50k ohms.

[JoshK]

Well some other folks more knowledgeable than me may be able to tell you definitively, but I think you should be good to go with either. 

[jrebman]

Either will work, but I think you'll get better results with the DAC with the lower output impedance.

50 or 500 into a 50k passive controller sho0uldn't be all that different, but if it were a 10k passive, then the 500 would be getting to the point where I'd start to get concerned about upper frequency rolloff and loss of dynamics.

However, and it's a big however, with a resistive based passive pre, the real issue is on the other side -- from the passive to the amps, and this is where you'd want to keep the distances the shortest -- ie - lowest capacitance, etc.

At 1.5 meters on this side of the passive, the 50 ohm output DAC is most likely going to have an advantage.

It's what I would choose myself with the same setup -- assuming the DACs both sound close enough to what I'm looking for.

FWIW,

Jim

[konut]

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. It seems the lower impedance trumps the difference in voltage level. I'm really shocked at the 8db disparity. Jim, you really nailed it!

[konut]

Update

 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?

[jrebman]

Konut,

Thanks for making the effort to do the actual experiment.  Every once in a while I get one right .  I don't think I would have predicted that much of a difference though.

-- Jim

[konut]

I received the revised DAC A last week and finally had a chance to test it today. The disparity in output has been changed. The 315hz tone, at full volume, now reads 94.5, a difference of 3.5db instead of 8db. This is still perplexing. The only logical explanation I can think of is that the manufacturers measurement methods differ significantly wrt voltage output. I have avoided comparing the 'sound' of the 2 units due to the subjective nature of said comparison and the purpose of this particular circle and thread. Suffice to say both DACs have rightfully earned the reputation of top flight units.