The Truth About Passive Pre-amps

Are passive pre-amps “real” pre-amps?
There are people who will try to convince you that passive pre-amps aren't proper pre-amps and can't perform as well as active pre-amps. This simply isn't true and is based on some very dubious logic. First let me explain how the term pre-amp came to be. Back in the 1950's what we now call power amps were usually refered to simply as amplifiers. The thing which came before the amplifier in the reproduction chain was called either a "control unit" or a "pre-amp". It was very common for the control unit to feature inputs for pick-up cartridges, inputs for microphones, bass and treble controls etc. These days control units tend to be much simpler devices with the only essential features being input selection and control of volume. The Indus selects between the various inputs and controls the volume as well as being placed before the amplifier (more commonly called the power amplifier these days), so it is indeed a pre-amp. Does it matter that it is passive?

What about gain?
Do you really need voltage gain? Lets consider what a pre-amp from the 1950's would have to do. The inputs from the various sources would have to be amplified sufficiently to give enough voltage to drive the power amp, some of the inputs would require equalisation (RIAA standard for turntables for example), and tone controls would usually be included. All this can only be done with active electronics. However, modern source components like CD players have sufficient output voltage to drive a power amp without further amplification from the pre-amp, so voltage gain isn't necessary. In fact, voltage gain in the pre-amp can be problematic in a modern system - that's why we sell so many In-Line Attenuators.

So where did the myth about passive pre-amps come from? Here's the real truth. An ideal pre-amp would have infinite input impedance and zero output impedance. If it managed to achieve these design goals the infinite input impedance would present no load to the source component and the zero output impedance would be able to maintain its output voltage regardless of the cables or power amplifier(s) connected to it. A passive pre-amp can't achieve both these goals because raising the input impedance would raise the output impedance, and lowering the output impedance would lower the input impedance - its simply impossible to have one high and the other low. So, doesn't that mean that the critics are right? Passive pre-amps can't work, can they? Yes they can. The theory about the failings of passive pre-amps falls down when you actually put some real world figures into the equations.

Firstly - infinite input impedance. That would be impossible anyway, but 1M (1 megohm or a million ohms) would be quite feasible. In fact, many valve pre-amps do achieve this, and when amplifying things like electric guitars (which have high output impedance) it is a very good thing, but most modern hi-fi sources have a very low output impedance of a few hundred ohms at most - often a lot less. When using these source components, the pre-amp's input impedance should be large compared to the source's output impedance. A factor of 10 is usually considered adequate. If the source's output impedance is 300 ohms, then the pre-amp's input impedance should be 3000 ohms. The input impedance of the Indus is 15000 ohms, so that is more than adequate. In fact, many active pre-amps have an input impedance of only 10000 ohms, so they are inferior to the Indus in that respect.

“Passive pre-amps can’t drive the cable”
This is the myth which is commonly quoted. It stems from a mis-understanding of the output impedance of passive pre-amps. This is where most of the criticism of passive pre-amps comes from - the idea that they don't have any circuitry to drive cables. It’s nonsense. The reason why low output impedance is a good thing is because the output impedance of a device will form a potential divider with whatever impedance is connected to the output (the load impedance). If the output impedance was zero, then the divider would have no effect and there would be no loss of signal. If the output impedance was higher than zero, then some of the signal voltage would be lost. The worst case is when the load impedance is reactive rather than resistive. Most commonly, the capacitance of the interconnecting cable forms a reactive element, and that capacitance, combined with the non-zero output impedance forms a low pass filter. ie the high frequencies are rolled off. In PA applications where cable runs may be over 100 metres (from stage to mixing desk, for example) the capacitance of the cable can be considerable and mustn’t be overlooked. The total capacitance of 100m of good cable would be about 10nF (nano farads) -maybe up to 30nF for cheaper cable. 30nF of capacitance loading a passive pre-amp would indeed limit the high frequencies significantly, however, a 1 metre length of hi-fi interconnect cable would be more like 100pF (pico farads). That is a much smaller amount of capacitance. Loading the Indus SE with 100pF doesn’t affect the audio bandwidth. In fact the bandwidth will extend to well over 100kHz. This is because the maximum output impedance of the Indus is 3750 ohms.
Compare that output impedance to the many valve pre-amps which follow minimalist design principles. They tend to have very high output impedances - certainly higher than 3750 ohms and quite commonly 10k or 20k or even more. They have HF roll off problems much worse than a passive pre-amp. The truth is that when using a passive pre-amp the source component (eg CD player) is the thing that drives the cable. It drives the cable through the (passive) pre-amp, with the pre-amp adding only insignificantly to the output impedance. A valve pre-amp (or any other active pre-amp) with a high output impedance has less ability to drive cable than a well designed passive.

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