Number 3 on our list of 9 tasks for the output analog stage of the DAC isto include any additional low pass filtering the designer may wish to add.
If you'll recall the chain we described, there's the DAC chip with its stepped output signal which goes into the current to voltage converter and then into a filter to remove all the steps and finally into the analog output stage. Not all designers place the entire filter between the current to voltage converter and the output stage. Many prefer to use the output stage as an active filter, meaning the amplifier's feedback gets involved in the filtering process.
We've never been fans of this active approach. In fact, you may recall that even our phono stages are passive, rather than active, in their EQ duties.
I am not going to argue one way or the other but I think it's important to see the two different approaches and I'll try and share with you the ideas behind both and what we think of the results.
In the active filtering method, by far the most popular amongst designers, the analog output stage becomes the actual filter itself. Designers do this because an active filter provides the ability to make the rolloff steeper and with better control. In older DAC designs this active approach was almost a requisite because digital filtering techniques in DACS were pretty crude and placed demands on the analog filter. But today's DACS need very little filtering and in our experience, keeping the output amplifier linear and without phase altering filters involved gives a much more open spacious sound. My guess as to why this is so prevalent is simple: it's the way it has always been done.
In the passive filtering method, certainly least popular but with a growing following, the analog stage has no EQ around it at all. Instead, all the filtering is handled before it ever gets to the amplifier. About all one can expect to achieve is a couple of "sloppy" 6dB/octave filters in series, but properly done and with a modern DAC, this should be more than adequate. The benefits to this approach are many: linear gain and feedback plus the ability to restrict the overall feedback amount are among the most important.
As you can see, there are a number of choices designers have to make and each has a fairly significant impact on how the DAC will sound in your home.
