Differences in sound: DAC vs. DAC + Pre-amplifier

[Matias]

2 hours ago, The Computer Audiophile said:

Technically a preamp could improve the signal to noise ratio with a powerful output stage, correct?
 

That’s a serious question, no snark. 

I don't think that is possible, because that would mean only the signal gets amplified and the noise floor does not, which is not possible. Unfortunately the best thing an analog preamp can do is not to raise the noise floor further, or prevent adding more distortion (if that is what one wants, thinking of people looking into deliberately adding distortion = tubes), while hopefully adding more current output and lower impedance.

 

But then there are reviewers who prefer adding preamps after their dCS DACs, which have the lowest output impedance I have seen, so in theory preamps should not be necessary. I guess some people just like the sound of adding a little harmonics. Nothing wrong with that.

[Matias]

2 minutes ago, The Computer Audiophile said:

That doesn’t make sense to me. Setting the output of my DAC to 6 volts raises the signal to noise. It should do similar with a preamp. 

Don't ask me, it is what I read on reviews. Again maybe those reviewers prefer the "flavor" that their preamps add? I don't know.

[Matias]

On 3/1/2020 at 3:29 PM, barrows said:

With preamp, higher noise floor, a little less transparency, but better dynamics and fuller, more robust sound.  I preferred with preamp, but wanted to figure out DAC direct as it was clear there was an advantage in transparency with DAC direct.

 

Then, the designer of the Legato output stage changed it, tripling its output current capability.  I now tried DAC direct vs. preamp again:

 

DAC direct: lower noise floor, better detail retrieval, and now, no loss of dynamics either.  So now that the DAC was driving the input stage of the amplifier optimally, there was only loss by adding the preamp.

 

After about 3 weeks of testing, i sold my Ayre preamp, and have not looked back.

 

It is clear to me that DAC direct, for those who do not need source switching. results in the best fidelity to the source, as long as the DAC's output stage is capable of driving the amplifier input stage robustly.  Additionally, most DACs have the capability to drive most amplifier input stages entirely adequately.  Look for a DAC with low output impedance (less than 100 ohms is nice, but up to a couple hindered ohms is going to be OK for most amps) and enough output voltage.  Most IC opamps in DAC output stages can drive amps directly with no problem.  Occasionally there might be a DAC with a bit weaker output stage, but these are rare.

 

I am no expert so correct me if I am wrong, but from what I have researched, looking the system as a whole, I would like to add that there is a 3rd option.

 

1. Fixed gain(s), digital attenuation on DAC.

2. Fixed gain(s), analog attenuation (potentiometer or resistor ladder).

3. Variable gain.

 

Usually preamps are a buffer (gain=1) and/or have a fixed gain (>1), and add analog attenuation. This should add noise and distortion.

 

A few preamps have variable gain output stages (Accuphase and Mola Mola Makua for example), which on lower volumes (gain <1) do not amplify the signal needlessly, preventing noise and distortion. Still the DAC has an output stage with some fixed gain before it, so 2 gain stages in series.

 

Makua goes further by having a top performing DAC inside it, and I think (not sure) it does not duplicate output stages and gains needlessly (DAC+preamp). It only ads the gain it needs (variable gain) and once. So I think this is a very clever system approach.

 

An all in one system (Devialet, Bel Canto Black, digital active speakers, etc) could in theory have only 1 gain stage, ideably variable, between DAC and the output stage of the power amplifier, to have the lowest noise floor and distortion possible.