Friday, December 26, 2008
DIY Passive Preamplifier
The DIY passive preamplifier is a spin-off project of my T-amp adventure. I was (and still am) so mesmerised by the sonic quality of my T-amp with full discrete parts and fitted with a 24-steps discreet Dale resistors attenuator which I find as much more transparent and natural sounding than even the SimAudio Moon i3 integrated amplifier in my main system. This led me to embark on the DIY project by copying and in the hope of attaining an equivalent performance from the Audio Synthesis Passion at just a fraction of its cost with use of a good quality attenuator, Teflon-insulated high purity (99.99%) silver internal wires, 4% silver solder lead, good quality connectors and an input switch. The DIY passive preamplifier is purely passive and has no gain but otherwise performs all the functions of a conventional preamplifier (it has 3 inputs) with minimal degradation to the delicate audio signal. However, just like the T-amp, it may not be everyone’s cup of tea as most people actually prefer active preamplifiers with more drive capability and dynamics. Nevertheless, if properly matched, any passive pre-amplifier is capable of producing a far more transparent and natural sound if it is compatible enough with other components of our system right from the source, cables and power amplifiers.
To ensure full compatibility, the output impedance of the CD player/phono stage must be low, ideally 600Ω or less. The output voltage from these components ought to be high since this (together with the power amplifier and loudspeaker sensitivity) will determine how much volume would be available. Passive preamplifiers tend to exhibit an output impedance that changes with level and is sometimes higher than that of a conventional active preamplifier, hence the absolute need to use cables of short length, screened and have a low capacitance at the output of the passive preamplifier. As for power amplifier, a little more gain of 30dB or more may be required and this could alternatively be stated as the power amplifier's sensitivity for full output - 1V for maximum output is ideal. The power amplifier's input impedance is also relevant due to its loading effect upon passive preamplifier’s output impedance and hence the source's load driving capability and requirement of the minimum input impedance of 20KΩ attenuator. Audio Synthesis Passion uses a 15KΩ attenuator. The aim here is to raise the input impedance and lower the output impedance as much as possible.
The Elma stepped attenuator fitted with Shinkoh resistors (http://www.hificollective.co.uk/components/shinkoh_stepped_attenuator.html) really stirred up the interest in me and I may opt to replace my existing unit with it. Read the description on sound through it from paragraph 5 onwards under 'Testing the Attenuator', an event, although to a lesser extent, that I have personally experienced before and could readily relate to.
An interesting read on passive pre-amps: http://www.rothwellaudioproducts.co.uk/html/the_truth_about_passive_pre-am.html
to be continued: the internal of the DIY passive preamplifier