Passive pre-amplifiers, also called passive control amplifier, line stages and so on, are actually not amplifiers at all despite the name but rather a replacement for a control amplifier (pre-amp). They are passive in the sense that there are usually no active electronic parts in the devices and therefore it requires no electricity to operate and thus no type certification or electrical testing from the authorities. However this is not a firm rule since there are a few passive preamps that actually have some active components in them even if they are not always in the signal path, so it is better to define a passive pre as a utility/control amplifier that has no signal gain or equalisation capabilities.Basically a passive pre-amp controls a power amplifier, in the simplest such devices you get nothing but an attenuator that allows you to control the strength of the signal that is delivered to the power amplifier and thus the volume coming out of the loudspeakers. In more complex devices you get a number of switches that allow you to choose between multiple inputs and in a few rare cases you get some passive signal processing such as high frequency cuts or bass boosts or even tone controls that feature coils rather than active parts. However that is rare and in some ways goes against the basic idea behind the modern passive, but that is to have as short and as simple an analogue line level signal path as is possible and to get rid of possible side effects from the typical power supply such as hum and other modulation issues & in more complex systems avoid ground loops and related problems.
The attenuator (volume control) in a passive amp can be a variable resistor like we find in a normal hi-fi amp but it is more common to see a rotary bank of switched resistors or a transformer as a variable resistor is in ultimate terms a somewhat problematic item, its specifications and performance will change with age, and when you have a stereo pair the changes will differ between each channel, it does also need regular cleaning to perform at its best. This means that despite the lack of active electronics on the inside a passive preamplifier can be just as costly as an active one and in the case of models that use heavy duty specially wound and/or silver wire transformers they can in fact be more expensive that the average preamp.The passive pre-amp makes sense because all source components that you use today will feature line level outputs and thus do not need amplification or equalisation prior to being sent to a power amp, the only exception to that being the turntable but since most hi-fi enthusiasts with such use a separate phonographic pre-amp that puts out a line level signal anyway that is not a problem in the modern world.
A few models of passive preamps exist out there that use an active circuit side by side with a passive one, that arrangement allows the unit to feature for instance a headphone amplifier and outputs while at the same time maintaining a passive signal path from the source input to the input of a power amplifier.The extreme simplicity of the passive pre makes it the ideal DIY project, since there is no electricity involved no real knowledge of electronics is required, merely a little bit of logical thinking, and there is no danger of you making anything that will hurt either yourself or the equipment that you hook up to it. Since all you need are 4 cinch sockets, 2 cheap potentiometers, a case and a little bit of wire to make a basic passive you can even build one for less than 10 UKP/USD/Euro that will outperform quite expensive pre-amps. Even if you choose to go with ganged variable resistors, audiophile quality connectors and so on, this is still one of the few DIY products left in the audio field these days where you can actually save money on versus a fully made item.
A passive is not exactly a new idea since at the beginning of hi-fi in the 1920’s and 30’s almost all switching and volume controls featured no active components and there were indeed a few passive preamps sold as early as the 1920’s for basically the same purpose as we use them today, but at the time a typical audio setup was a radio receiver and if it had any inputs at all it featured a single external signal input that could accept a “phono level” signal from an auxiliary device such as an electric gramophone or a wire recorder.
The more adventurous (and well off) audio buffs of the day that had more than one source signal component could therefore buy or more commonly have made for them a control box that allowed them to control which input source was used and what signal strength the radio got, the attenuator in this case was usually not there for volume purposes but was needed since some manufacturers had somewhat different ideas of what constituted the correct input level and the attenuator allowed you to decrease the signal so that radios with high gain at the input did not distort when being fed a higher level signal than expected, volume was usually controlled by the radio itself. It is actually from this “phono level” where we originally get the standard for line level from, even though the modern line level is slightly higher than the phono level of yesteryear.By the 1940’s there was an increasing desire for a control unit to actually perform some house-holding tasks that either required active electronics, such as gain or tasks that were more easily implemented using active parts such as in the case of equalisation. By that time in the western world electric recording of discs had been the standard for more than a decade and electric gramophones were outselling acoustic ones by a considerable margin. However there was never implemented an international standard for the compensation of the recorded sound.
A gramophone record is by no means a linear devise when it comes to frequency response so to make them sound as good as was economically possible at the time, the higher frequencies where heavily emphasised between the microphone and the disc cutter. The gramophone disc has particular problems reproducing the high frequencies of an audio signal and by emphasising them over the lower tonal registers when they were being written to disc, more of the high frequency part of the signal can be reconstructed on playback. However this required either a de-emphasising circuit on the playback end or more ingeniously a transducer such as a crystal that inherently performs such de-emphasis by virtue of the material behaviour. These compensation schemes where kept simple on purpose so that playback devices could in worst case scenarios perform the de-emphasis with fairly simple and inexpensive passive circuits.However since there never was an international standard, records made in one country or state would sound odd on players made in other countries since the details of the compensations schemes differed from one to another even though they were all performing the same basic task. To add insult to injury practices differed not only between countries but also between individual record companies and between individual production teams inside each record company. Even after international compensation standards such as RIAA or FFRR were introduced with the advent of the LP, record companies continued to “adjust the tonal balance of each record individually, as to suit the program material” for more than a decade, or in other words you could not expect any record you bought regardless of make, to have absolutely the correct tonal balance on playback. This made user controlled variable equalisation not only desirable but in real terms necessary from the late 1930’s until the early 70’s and such equalisation is really only cost effective if active components are used.
This explains the Bass and Treble controls on a modern hi-fi amplifier but it is really a hangover from the time of 78 Rpm. records, but the sound in the mid-range was usually not heavily different from one format to another but the low and high end were problematic, leading manufacturers to incorporate equalisation circuits at the edges of the tonal spectrum. This explains why the bass control is so high up at around 100 Hz where it actually only has a limited effective action in a modern system, the treble tone control was initially around 7kHz but in modern system this has been upped to 10kHz.It also became more common to see amplifiers sport specific equalisation circuits and gain structures for devices such as moving magnet and moving coil phonographic pickups that became more popular with the introduction of the LP. At the same time the introduction of the home audio tape recorder became a reality and unlike the wire recorders that they replaced it was more common to see tape recorders sold as transports and heads only so that people and companies could integrate them into their music consoles, this meant that the more upmarket amplifiers also had to have compensation circuits and gain cells for tape recorders in their tape loop facility, this was more complex than with phonographic playback since each tape speed required a different equalisation curve. The more upmarket amplifier units continued to feature tape equalisation circuits up until the early 70’s even though tape recorders without compensating circuits disappeared in the mid 60’s.
This meant that by the mid 50’s it was primarily budget products that offered passive front ends, with the exception of very few specialist makers that insisted on a passive signal path up until the power circuit and these can be thought of more as curiosity items, there was one attempt at modernising the passive preamp with the introduction of a unit that was like a mash-up between a pre-amp and a passive mixer. These units featured multiple input but instead of a switching action features an attenuator at each input and a passive summing circuit and a tape loop before the volume control, this was presumably done so users could have more flexibility in input levels and have more than one devices hooked up to the same input transformer but these never sold and had died a quiet death by the time stereo was introduced in the marketplace.The separate passive pre hung on for a few years primarily as specialist devices that often has some unusual features such as loads on the inputs so that they could be used with inputs signals that were already amplified, i.e. they were intended to take any input signal even though the source in question only had outputs for loudspeakers.
The advent of the transistor and the cost savings they represented for active circuits versus thermionic valves meant that by the mid 60’s even budget audio products featured active front ends and the passive died out completely, with separate passive pre-amps having disappeared from the market a few years earlier. The interest in minimalistic hi-fi in the 70’s bought back a few attempts to sell people on to the advantages of passive fronts particularly in the latter half of the decade, but it also brought with it hybrid devices such as RIAA preamplifiers that featured an attenuator at the end so that the phono amp could act as a control amplifier in a single source audio system, but neither type of device sold.
It was the advent of the CD in the early 80’s that changed things, it meant that a number of people got rid of their turntable setups and since the other source components such as tuners and cassette recorders were all line level devices and thus required no gain to exist in the preamp but only attenuation, a passive pre amplifier became a practical device once again. By the mid 80’s we see a small number of manufacturers making them, with products ranging from quite complex devices with dual tape loops and multiple inputs to extremely simple devices that had only a single stereo input and 2 variable resistors on the inside to control the volume.The problematic high frequency behaviour of most CD players at the time meant that a number of these passives featured some sort of high frequency damping in the signal path, primarily shunt capacitors or something simple like that, and some featured a specific CD input that had damping while the other inputs did not. This also gave birth to another hybrid device, a buffer amplifier with volume control for those that only had a CD player hooked up to the system.
But the better behaved DAC’s of today mean that most passive pre-amps sold currently do not feature any damping and in fact most manufacturers make a virtue out of having as simple a signal path as possible. While not a mainstream product there are approximately 30 makers of such devices today, the bulk of them being very small specialised concerns at the fringes of the hi-fi world but a few bigger companies started offering passive devices in the early to mid-90’s and they have remained in their catalogues since.
Promethean CD Director
Basically a dual mono configuration intended for those that had CD players as their only source component, hence name, the CD Director features no input switching and only has one input for each channel, the only controls available are a volume controls for each channel. Internally the unit is has two switched resistor arrays with a couple of capacitors for shunting, it is a popular mod with owners to simply remove the capacitors since they do more harm than good.