The term passive preamplifier is actually a contradiction in terms since it has no amplification. Unlike active preamplifiers, which use a power supply and active components (such as operational amplifiers, transistors, or vacuum tubes) to increase signal strength, passive preamplifiers use exclusively passive components. A passive preamplifier can be described as a pure attenuator with an integrated volume control and input selector. Its main function is to regulate the voltage of the signal without adding any gain or affecting the signal's tonal characteristics.
The primary advantage of a passive preamplifier is its lack of
active components. This means that it theoretically does not introduce
noise, nor total harmonic distortion (THD) or intermodulation distortion (IMD),
given that it does not require an
external power supply. At the same time, this also reduces the risk of
phase errors and overshoot that can occur in active circuits. No power
supply means no electromagnetic interference (EMI) or grounding issues,
resulting in cleaner signal paths.
In the figure, the resistor RS represents the source resistance from the Tuner, CD etc. in addition to the potentiometer's output resistance. In my own preamplifier I am using a 10 kohm potentiometer. From this potentiometer alone, the maximum setting is 1/4 of 10 kohm: 2.5 kohm.
So this is also the value of the source resistance RS if the output resistance of your Tuner, CD etc is approximately 0 ohms. However, this output resistance will typically be between a few ohms to perhaps 1 kohm. In the latter case the preamplifier will attenuate the signal by about 10/11 times if the power amplifier has an input impedance RL high enough to be neglected. This also means that the source resistance RS may be a maximum of about 2.8 kohms.
To avoid the power amplifier's input impedance also attenuating the
signal too much, one can require that the power amplifier input
resistance RL is not much lower than 10 times the maximum RS,
i.e. approximately 30 kohm.
Modeling the cable is not an easy task, but the easiest thing to do is to look at the capacitance per unit length: pF/m. It seems that typical, this is about C = 50 pF/m. A quick calculation with our RS about 3 kohm (with a high RL, a lower RL will reduce the resistance), will give us a cutoff frequency of approx.:
f1 = 1/(2πRSC)
= 1 MHz
This is well above the audio spectrum and is in large contrast to
claims that passive preamplifiers limit high-frequency reproduction
and/or reduce the dynamics of the signal. Even with a 10 meter cable, I
would assume that cable capacitance is not a problem if your source
have a reasonable output impedance. It is of course true that the
cutoff frequency is volume setting position dependant, but we are talking of
very, very high frequencies far above audio frequencies. However, if RL
is to small compared to RS, the signal may be too much
attenuated.
The figure below shows the simplified schematic diagram for my own passive preamplifier (one channel).
An old-fashioned solution with an input selector and a tape monitor function is used. The input called Record is connected to my Streamer. The volume potentiometer has a value of 10 kohm. The input impedance of my power amplifier is about 33 kohm, but this has not been a problem. Neither has lack of amplification been a problem.
Below are pictures of the preamplifier.
The complete schematic of the amplifier is shown in the figure below.
The phono connectors J1-3 is for the source inputs: Phono, Tuner and CD, while J4 is for the Record input. The resistors R1-R4 is isolating the input grounds from the output ground. The selector SW1 choose between the source inputs. The outputs from this is fed to the source/monitor switch SW2 and to the selectors J7 and J8. These are used to choose between the output from the volume potentiometer RV1 or from the output of switch SW1. The switch SW2 selects between the sources from SW1 and the Rec input. The midpoint of this switch has a mute function (the output is grounded). From SW2 the signals are fed to the volume potentiometer and further to the preamplifier output connector J6. Please observe that it is possible also to use the output connector J5 as a parallel preamplifier output.
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