Paul McGowan writes .....
One of the hotly debated issues amongst designers is current capabilities of an output stage on a DAC or preamplifier. It’s number 8 on our list of the duties of an output stage. On the one hand it can be argued that modern interconnects placed between the DAC and the preamplifier are an easy load – after all, they are just a wire. That wire is connecting the relatively low output impedance of our stage with the higher input impedance of the preamplifier or power amp. One shouldn’t have to pay much attention to the drive capabilities in order to do that. Of course cables shouldn’t sound differently either, but …..
What we’ve found seems to be quite the opposite of this conventional wisdom. In fact, PS Audio’s output stages are designed in the same way that a power amplifier’s output stage is – a complimentary pair of transistors with a constant bias running through them and extremely low output resistors – all to be able to properly drive a pair of interconnect cables between two products.
The whole notion of high output drive capability for preamps, phono stages and DACS entered into my view years ago when we were first investigating the audible effects of feedback in a line stage. The effects of feedback on the audio signal has been (and still is) a hotly debated topic amongst designers. I would venture to say the prevailing opinion is that judicious use of feedback is probably quite acceptable and even desirable in a high end product – but back in the late 1970′s not many were being judicious with its use and even fewer of us understood the ramifications of its use. It was a time of experimentation and learning.
Part of the experimentation we were involved in was comparing zero feedback circuits with “normal” feedback circuits of the same gain and topology to hear the differences. We noticed two major changes between the two: zero feedback was much more open sounding yet wimpy in the bass and slam than that of the feedback version. What would be awesome is if we could maintain the openness while gaining back the bass and slam.
One of the side benefits of feedback is lower output impedance which, as we’ve discussed before, equates to better drive capabilities at the output. Lower output impedance means that whatever you attach to the output of your amplifier stage has less impact on that stage’s performance – the lower the impedance, the fewer problems driving a cable. So, for example, if you use a long interconnect cable with all its challenges, the circuit with lots of feedback won’t be bothered by the cable and the opposite would be true for the zero feedback circuit.
In our setup at the time we were running a 15 foot cable between the test amplifier circuits and our power amplifier driving the loudspeakers. So obvious was the zero feedback stage’s loss of bass and slam that it was hard for any of us to imagine we couldn’t actually measure something causing this loss. We placed a scope on the output of our stage and noticed a change in the waveforms when we connected the cable or disconnected the cable. Aha!
To fix this problem we designed a simple power amplifier-like output stage that could deliver plenty of current and had low output impedance on its own without the use of feedback. We attached this to the output of our zero feedback amplifier stage – verified the connection of the interconnect cable had no effect – and gave it another listen. Bingo, bass and slam were back in spades and the openness we were hoping to maintain remained intact.
This was some thirty years ago but the principals we learned back then about drive capabilities, managing feedback to respect openness and musicality while giving us much needed qualities difficult to attain any other way remain to this day.
You have to make sure there’s plenty of juice available, even if the chore seems really easy.
