The quality (sonic purity) will be largely dependent on the volume potentiometer used. The finest discrete resistor pots will sound more accurate than a variable one, for example, since the signal travels through only one resistor at any given setting and is therefore not dependent on volume position as is the case with a variable pot. Many believe that transformer volume controls can be superior to even the best discrete resistor pots, since the TVC is inherently passive while the latter still requires the audio signal to traverse a resistor. The TVC is more costly to properly implement since the best transformers are very expensive.
Type, length and quality of the internal wire will also affect the sound as will the connectors being used. The inclusion of a selector switch will compromise the ultimate transparency of the sound to a greater or lesser degree compared to a passive eschewing this switch.
Logic would seem to dictate that the active preamp must adulterate the signal to a far greater degree than the passive by virtue of all those additional components. Further, since the active has the additional disadvantage of requiring a power source, tubes or transistors it must not only impose far greater noise, it is also more costly and inconvenient to use. Why would anyone want to use the active in a system? Theoretically, it would seem nonsensical to use anything but a passive preamplifier.
The Imposition Of The Real World
The length of the cables between the source and passive and then to the amplifier must be strictly attended to. Too long a cable and roll offs at both frequency extremes will occur. Impedances between the volume control and the input impedance of the amplifier must be compatible in order to avoid sonic degradation. Furthermore, since a passive has no gain, the output voltage of the source component must be sufficient and the amplifier and the speakers need to be of high sensitivity or not enough ultimate volume can be achieved.
The purest, simplest but admittedly most inconvenient method of passive attenuation is to have discrete resistor pots installed at the amplifier input. This method avoids the extra cable interaction and achieves the desired amplifier input impedance.
To summarize, in order for a passive preamp to work properly the following conditions must be met:
1. Source component must have sufficient output voltages;
2. Short, low capacitance cables must be used between the source to the passive and from passive to the amplifier;
3. Output impedance of the passive's volume pot should be low (less than 25kOhms) while the input impedance of the amplifier should be significantly higher (100kOhms);
4. Amplifier's input sensitivity should be high (minimum of 0.75v for full output).
5. Speakers should be high sensitivity (minimum of 90dB/W/m).
However, when all of the above conditions are met, there are very few active preamps on the market that will yield better sound overall. For an active to provide superior sound, will require an amazing unit designed and executed to the highest level. (translation- it will not be cheap- but do not confuse excessive price with high performance- however high performance will be expensive).
1. The finest passive and active components available connected with the shortest signal paths possible all being hardwired;
2. A separate power supply that is impervious to voltage fluctuation or current demands;
3. The finest form of signal attenuation which means transformer coupling;
4. The lowest amount and purest gain devices which involves transformer coupling inout and output stages;
5. Low output impedance;
6. A highly rigid chassis that is not prone to resonances.
The inherent practical advantages of an active over a passive is that due to output buffering, interaction between the source, connecting cables and amplifier input impedance ceases to be an issue. Further, due to added gain, much greater flexibility in system assembly is possible.
The other huge advantage presented by an active line stage is that it amplifies the source signal to a higher level, unlike a passive design which has no gain, so that the power amplifier is able to more accurately pass this signal to the speakers. This is similar to what occurs when inserting a moving coil transformer or head amp into a low gain phono stage. The low level cartridge signal becomes amplified so that the phono preamp can transmit this incoming signal more accurately to the amplifier. While the system now has an extra component in the signal path which would intuitively create the impression that purity would be compromised, the opposite results -- greater fidelity is achieved. Not only is the sound more dynamic, harmonically more complete with superior rendering of the frequency extremes, there is a noticeable enhancement in detail retrieval.
Coincident has up to the advent of our Statement Line Stage recommended the use of the best passives when all the system conditions were optimized. The sonic compromise inherent with all previously auditioned active units in terms of imposed colorations were too obtrusive to ignore, despite their other advantages.
Coincident Statement Line Stage
The enhancement of musical enjoyment wrought by the insertion of the Coincident Statement Line Stage into an audio system is of enormous proportion. As reviewers and owners of the unit have confirmed, the superiority of the Coincident over passives and existing active preamps is not subtle. A short audition is all that is required to hear this level of advancement.
Our reviews of Coincident Speaker Technology Preamplifiers: