passive preamplifier with switchable active stages
The purist PRE was my first project for the label purist.
This device consequently tackles the problems of classic preamplifiers.
If You look at the signal levels of most source devices like CD-players, etc. it will immediately become clear that we only need to dampen the signal. Additional amplification is typically not needed.
Still though You will regularly find amplifiying stages with a gain of 10 (+20dB). The only desirable characteristic of these amplifying stages is their high input impedance and their low and constant output impedance.
The amplifying stages could be omitted with alltogether in most cases and we would speak of a passive Preamp-stage.
In the vast majority of cases the volume control works with the simple Pot-circuit. This circuit offers the source side a constant and high impedance but it offers the load side a non-constant and partially high output impedance. They discard as a high quality solution.
But there are differing circuit structures, that offer a constant and low output impedance. They exhibit a non-constant, but sufficiently high input impedance though. Such a circuit, built as switched discrete resistor network using relais as switches, forms the heart of the Pre.
The number of switches would quickly become impractically high, the finer the resolution and the higher the volume step number is chosen.
Therefore I chose a bit-wise weighting of the switching stages.
The fist stage switches by -1dB, the second by -2dB, the third by -4dB, the fourth by -8dB, etc. With only 7 stages I could realize 128 steps of -1dB value from 0dB down to -127dB. Unfortunately this technique requires tightly tolerated resistor values. The Pre makes useage of specially manufactured Bulk Metal Foil resistors from Vishay, tolerated to the 4th digit.
With this level of tolerance the volume control circuit of the Pre achieves the precision of a measurement instrument. The linearity difference in volume level remains below 0.02dB over the full range. The interchannel differences is down in the low promille-range.
With typical listening volume levels the audio signal passes just 2-4 extremely highquality resistors. Its close to impossible to achieve less noise, lower distortion values and getting closer to the proverbial piece of wire.
Still though You will find active stages within the Pre in form of plug-In modules. A Input signal symmetryzing stage with switchable gain factors of 1 or 2 (0dB, or +6dB) can be switched via IR-commander into the signal path ahead of the volume control network as wanted and on demand.
Unbalanced sources and/or sources with too low signal level may be useable with the Pre.
As an option a Buffer stage can also be plugged-in. It too is switchable into the signal-path by IR-commander at any time.
Cable lengths up to app. 30ft present no problem for the pure passive volume control.
Also as an option a very flexible Phono-stage can be acquired.
More about this module in the ´PlatINA´ section.
We named this possibility to configure the signal path after Your fashion and needs the ´scalable signal path´. If I remember correctly then the Pre the only preamplifier in possession of such a feature.
On pressing a button of the IR-commander it can change from the ultimate sound machine to a full fledged ´classical´ preamplifier.
So far the Pre has not lost a single contest. Playing more neutral, more dynamic and -simply put- more authentic it always won hands down, regardless of the competitor´s reputation.