Gmood1,
yes, 10K input imp of the next stage vs. the driving stage is not written in stone. Usually the rule of thumb is 5X-8X. However, I've found that is not high enough & that 10X works 99% of the time successfully.
OK, from your 2nd lengthier post I think I understand better where you are coming from. Let me see if I can summarize: Hudsonhawk & I were musing why these non OS DACs based on TDA1543 DACs sounded (really) bad on bad recordings. He & I were hypothesizing that it's the lousy jitter performance from the recovered clock.
NOTE: both Hudsonhawk & I have non OS DACs that use a Burr-Brown opamp buffer that drives the RCA outputs. From the spec sheet of this opamp, it has very low harmonic distortion over 20Hz-20KHz + it can drive large capacitative loads - we are talking 5nF & still have a gain-bandwidth product greater than 1MHz + it has very good settle time. So, it seems that this buffer does the job of the BVAudio SR10 & similar after-mkt buffers. Thus, I see very little advantage in further attaching an after-mkt buffer. If there is an improvement in these 2 specific non OS DACs, I surmise that it is most likely to be very little. I wish I had one on hand to give your theory a try (I would be double buffering).
Now, in your Audio Sector Premium non OS DAC that uses passive buffering, the ball-game is entirely different. As an aside, if you look @ the TDA1543 data sheet, you'll see (in Fig 1) that they have suggested the use of buffer opamps that have some bandwidth limiting (that parallel cap in the feedback). It is not the only way to "terminate" the TDA1543 output i.e. one could also use passive buffering. However, the passive buffering will rely on the TDA1543 to drive the interconnect parasitic C + the preamp input. It'll do the job (as your ears have discovered) but you know that the sound could be better (again, as you have discovered). In your particular case, the BVAudio SR10 & similar products work & show the difference since the passive buffered TDA1543 has to drive 100K & very little parasitic capacitance of the active circuitry, a much easier load. I do not think that it'll be quite the same for my SN DAC or the DAC-AH.
There is nothing magical about the 50Ohms output impedance of the BVAudio SR10. It is a standard impedance used in test & measurement equipment & by the RF engineers. It is low enough where it'll work w/ 99.9% of the equipment in the market-place no questions asked. For that matter, 600 Ohms would have worked just as well (would have been an easier load actually) as it would have been low enough to work w/ all the preamps out there.
So, you have to pay the Piper - now (opamp buffer as part of overall DAC in the same chassis) or later (use after-mkt buffer).
So, in your particular case, it appears you have 2 issues affecting the sound: insufficient drive from the Audio Sector DAC & poor jitter performance from badly recorded CDs. IMHO.
yes, 10K input imp of the next stage vs. the driving stage is not written in stone. Usually the rule of thumb is 5X-8X. However, I've found that is not high enough & that 10X works 99% of the time successfully.
OK, from your 2nd lengthier post I think I understand better where you are coming from. Let me see if I can summarize: Hudsonhawk & I were musing why these non OS DACs based on TDA1543 DACs sounded (really) bad on bad recordings. He & I were hypothesizing that it's the lousy jitter performance from the recovered clock.
NOTE: both Hudsonhawk & I have non OS DACs that use a Burr-Brown opamp buffer that drives the RCA outputs. From the spec sheet of this opamp, it has very low harmonic distortion over 20Hz-20KHz + it can drive large capacitative loads - we are talking 5nF & still have a gain-bandwidth product greater than 1MHz + it has very good settle time. So, it seems that this buffer does the job of the BVAudio SR10 & similar after-mkt buffers. Thus, I see very little advantage in further attaching an after-mkt buffer. If there is an improvement in these 2 specific non OS DACs, I surmise that it is most likely to be very little. I wish I had one on hand to give your theory a try (I would be double buffering).
Now, in your Audio Sector Premium non OS DAC that uses passive buffering, the ball-game is entirely different. As an aside, if you look @ the TDA1543 data sheet, you'll see (in Fig 1) that they have suggested the use of buffer opamps that have some bandwidth limiting (that parallel cap in the feedback). It is not the only way to "terminate" the TDA1543 output i.e. one could also use passive buffering. However, the passive buffering will rely on the TDA1543 to drive the interconnect parasitic C + the preamp input. It'll do the job (as your ears have discovered) but you know that the sound could be better (again, as you have discovered). In your particular case, the BVAudio SR10 & similar products work & show the difference since the passive buffered TDA1543 has to drive 100K & very little parasitic capacitance of the active circuitry, a much easier load. I do not think that it'll be quite the same for my SN DAC or the DAC-AH.
There is nothing magical about the 50Ohms output impedance of the BVAudio SR10. It is a standard impedance used in test & measurement equipment & by the RF engineers. It is low enough where it'll work w/ 99.9% of the equipment in the market-place no questions asked. For that matter, 600 Ohms would have worked just as well (would have been an easier load actually) as it would have been low enough to work w/ all the preamps out there.
So, you have to pay the Piper - now (opamp buffer as part of overall DAC in the same chassis) or later (use after-mkt buffer).
So, in your particular case, it appears you have 2 issues affecting the sound: insufficient drive from the Audio Sector DAC & poor jitter performance from badly recorded CDs. IMHO.