Audio buffer output

I have a multipart question for purposes of a self-tutorial.

Some of this "esoteric" audio information can drive you nuts and often times, is contradictory.....hence, my questions. I have this little USB DAC chip that I've breadboarded......the PCM2902 from TI.

It's really a neat device, taking my computer's WAV files through it's USB port and ultimately outputing a stereo analog signal....all in one chip.

Works great.....and for now I just have the output feeding (2) 47uf capacitors and some bleeder resistors right into my amplifier.

First of all, the chip shows a Vout peak to peak of .6v and an center voltage of .5v and an impedance of 10k ohms.

(Q) Does that mean the output swings between .2v and .8v? I'm not clear on that.

Secondly, I'd like to learn more about the "importance" of a buffer stage.....("unity gain" or otherwise).

Many of the circuits using this chip are taking the output into high quality opamps (analog devices and the like)....but I'm not clear on exactly "why". (This chip sounds damn good with just the caps...right into the amp). (and "maybe", less is better?)

(Q) Based on the .6v p-p figure (low?) and the 10k output Z -- would that mandate a buffer stage? Is there some loading going on that is affecting performance...(that I may not hear?)

(Q) Or are the opamps used to "tailor" the sound of the chip...using the various opamps available and coming up with your own preferred "recipe"?

(Q) Or I suppose, a combination of the two?

Thanks kindly for any education on this....I really appreciate it.

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It is a bit confusing, but there are the letters, VCCCI below the numbers, so I think that the .5 refers to half of whatever VCCCI is. Likewise .6 ,means 60% of VCCCI.

So what it VCCCI? According to page 2 (Maximums) it can't be more than 4 volts. Near the bottom of page 5, we see it refers to the output of an internal regulator that produces a nominal 3.35 VDC. So, evidently, the DAC produces a signal based on this voltage, and using half that voltage as a zero signal value. And that makes the zero signal voltage a nominal 1.68 volts, and the peak to peak (full scale) swing a nominal 2.01 volts.

The data sheet doesn't say what the output impedance is (I hate Texas Instruments data sheets for stuff like this), but only that he specs given apply with a load impedance of 10k attached. I would assume that thew output impedance is something quite a bit lower than this (but possibly not well controlled), so that if you keep the load at or above 10k, it will work as speced. But they don't say this, explicitly.

I think this has to do with three things. One is the relatively high output load impedance spec (compared to cable capacitive impedance at the highest audio frequencies). If this thing acts strangely with loads below 10k ohms, a long cable could do that. Any ordinary buffer amplifier is likely to be able to drive a load with a lower impedance than 10k.

The second thing is that there is not a very sharp low pass filter in the chip to remove the digital hash from the DAC output, to keep the device as general as possible. If you add some low pass filter to your buffer, you can get a cleaner signal. So, you decide what bandwidth you need, and include filtration that removes stuff higher than that and include it in your buffer.

And then there is the issue of levels. If you require a signal with peaks higher than +-1 volt, you will need to add voltage gain to your buffer.

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John Popelish

I'll remember that and for kicks I'll look at it on my scope. I thought anything below a volt was too low. I need to make a chart with datasheet "abbreviations" like VCCCI.....(I gave up when no one could make their mind up on Vss and Vcc.)

Most amps inputs are usually higher so that's good.

I think this was the concern I saw when it came to these opamps on the output. I see the "stop band" attenuation is only -43db....pretty lousy, I guess.

I'll just have fun playing with sure sounds good to audio I've heard really, I'm hearing detail in some songs that I've never heard before.....and I don't really "hear" any high freq digital noise whatsoever.

So many audio nuts seem to design for test equipment rather than the (limitations?) of the human maybe just raw caps will sound better to me and that (should be) the bottom line.

I think when it comes to's all about how your brain processes it.

John......thanks very much.

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Vss is the Source supply for a Field-Effect Transister (FET). Vcc is the Collector supply for a Bipolar Junction Transister (BJT). These specify not only the terminal, but the technology involved as well.

Are you not finding this to be the case?

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Ok thanks.

I've run into documentation in the past that had IC's marked with Vss pins as well as Vcc and Vdd used and even seen some newsposts as to what it all means. (Of course, maybe the IC actually had an FET in it, so they designated Vss?).

I've even seen an IC marked with Vss that was really a ground pin.

Like I say, I'm going to make a list of what they "should be".

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That is quite normal. Look at some more spec sheets.

Having Vdd (Drain supply) and ground called out is more common, but it's possible to have: Vdd and Vss (both being non-zero potentials) Vdd and ground or a negative-value Vss and ground.

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