As you probably are aware TAS (the magazine) is publishing a major review on the PerfectWave system in their September issue that starts mailing out today. Since part of the core idea behind the PerfectWave system is using the DAC as a preamp connected directly to your power amplifier, I thought it might be instructive to start another mini series of posts on DACS as preamps.
We’re all interested in music and its reproduction and the preamplifier has been central to most of us for most of the music we enjoy. The day of the preamp is over and I want to explain how we got to where we are and why.
Perhaps in today’s post we’ll cover something very basic: what is a preamp? Yeah, I know, that sounds stupid because YOU already know what a preamp is, so I am talking to the OTHER guy who isn’t completely sure exactly what’s inside.
Let’s start with the name: preamp. As the name implies, it’s “pre” meaning something that comes before the amplifier. But other than the order of the signal chain it also means it is amplifying or conditioning and controlling before the amplifier. So, depending on what our preamp is it can be both a pre-amplification device or it can be a controller or both. For example, a passive preamp has no amplifier inside and it is only a control device: selecting the input and controlling the volume level.
Most preamplifiers we are familiar with are both: a controller and a first stage of amplification before the power amplifier. Which brings us to the insides of a preamplifier. What’s inside?
Actually preamplifiers are quite simple having only 3 elements to them: an input select switch, a volume control and a voltage amplifier. Here’s the signal flow: of the multiple sources plugged into your preamplifier’s inputs, one is selected and fed into the volume control. The level is adjusted (as well as channel to channel volume or balance between the two channels) and then fed into the amplification stage. This stage amplifies the signal between 5 and 10 times larger and then provides enough drive capability to get the signal properly transferred to your amplifier through an interconnecting cable. Note: many preamplifiers also have a phono stage built in to amplify the output of a turntable but that’s not really a part of the definition of preamplifier as it is concerned only with making a source come up to the level of all other sources. The phono preamplifier is more appropriately placed inside the turntable but that’s another story.
So, the preamp selects what we want to listen to, turns the volume up and down and then amplifies the signal to a level acceptable to a power amplifier.
Preamps are nearly always analog and made a lot of sense for many decades when nearly all sources were also analog – tuners, turntables, tape decks etc.
When the digital era began in the early 1980′s CD player manufacturers naturally built analog preamplified outputs into their players so they would be compatible with the analog preamplifiers of the day. In other words, CD players were originally designed as just another analog input for one reason only: to be compatible with existing analog preamplifiers (and receivers which consist of an analog preamp, amp and tuner in one box).
But, today, most sources are digital and not analog anymore so the notion of adding an analog output to every digital source just to be compatible with analog preamplifiers doesn’t make any sense anymore. So the day of the preamp has long passed and it’s time to rethink the entire genre.
Tomorrow we’ll work through more of this.
Paul McGowan – PS Audio, Intl