DAC/Preamp upgrade - novice question

[bluesman]

You may be confusing a "preamp" with a gain control.  A true preamplifier increases the signal voltage generated by a low level analog input like a phono cartridge so that it can drive a "power amplifier", which then increases the power of the signal (i.e. wattage output, which equals EMF in volts x current in amperes) so it can drive speakers.  There is no preamplifier in a digital signal chain because there's no voltage in need of amplification.  The line output from your DAC in volts is in the same range as the line output from an analog preamplifier, which is why you can connect the DAC directly to a power amp.  But there has to be some way to control playback volume.  If it's not in the DAC or the amplifier, you need a third device.  An integrated amplifier has a volume / gain control - it's called "integrated" because it has both preamp and amp.

 

Your DAC has an output level control, which is absolutely necessary unless your power amp has an input level control.  A DAC with no volume control will pump its full rated output into the amplifier uncontrolled,and you do not want this to happen.  So if you want to drive a power amplifier with no volume control using a DAC that also lacks one, you have to add some kind of adjustable voltage control stage between the two.  These can be called many things - a "zero gain preamplifier" (which, to me is an oxymoron), a unity gain buffer stage, a plain old volume control, etc. The way volume is controlled can affect sound quality, and you can read up on digital vs analog volume controls etc.

 

If you want to add other source devices that have line level outputs, you need a DAC that also has a preamplifier stage with analog inputs (like my Emotiva Stealth DC-1).  But if you're purely digital, you don't need a preamp at all.  Some DACs have "better" volume controls than others - search AS and other web sources and you'll find out a lot about it.  Most current DACs with gain controls (like yours) do it well, so I'd be surprised if you could improve your SQ without spending a lot more money.  Personally, I'd stick with your Topping and enjoy your new amplification.

[bluesman]

3 hours ago, al2813 said:

Key reason for hesitating was that the ADI-2 is described in forums as having a much better pre-amp analogue section

Once again, I have to suggest that you explore your concern over the preamp issue.  As I recall, the ADI-2 does not have analog inputs.  Do you have any sources with analog outputs that require a line level input into your system, e.g. disc player, phono stage, tuner of some kind etc? If you do, you’re clearly not using it/them with your Topping because it has no analog input.  You’ll need a true DAC / preamp to connect them, so the Topping will have to go - but not for an ADI.
 

If you don’t, the only “preamp” function you have and need in a pure DAC is the circuitry between the DAC chip and the analog outputs, which is usually in the chip and often even includes an integral headphone amp.  Apart from the question of variable vs fixed output level (i.e. with or without a “volume“ control), there’s actually little attention paid to this specific function in comparison with almost every other part. Debate rages over which chip is best, which implementation of each chip is best, which filter sounds best etc just as it does over the best sounding approach to output volume control. 

 

If you do not need analog inputs, I’d spend the money on music and enjoy your excellent system as it is now. I doubt that you can improve your DAC’s sound quality without spending a lot more than you want to spend. And I’m not at all convinced that you’d notice any clear and definable sonic benefit even then. You may notice differences similar in magnitude to those among digital filters.  But any qualitative judgment would be pure personal preference. I suspect that differences attributable to the analog output circuitry of most DACs, apart from implementation of variable level control if used, are also minuscule and largely preferential.  
 

Parenthetically, the output stage of most DACs is a simple op amp, so it certainly could affect SQ in a meaningful way - but it seems that no one chooses a DAC on this basis. I’m surprised we don’t see more discussion about this, apart from use of discrete op amps rather than chips.