A sophisticated grandson of the SIMPRE

I've been thinking and working around a good level pre-amplifier for a long time. Various SIMPRE configurations were experiments related to this research. I wanted a really good pre-amplifier, not too complex and expensive, but potentially at the same level or better than any kit I knew of, at least at the design level.
I was not looking for an esoteric pre-amplifier – in accordance with the TNT tradition, these kinds of components are not really appreciated... due to their price, at least. The pre just had to be very good, affordable, not too difficult to build, with scarcely any set-up required (as far as possible), made of standard easy to find and not too expensive components but it was not to be designed down to a preset cost.
I was also not interested in studying a completely new circuit. Tubes have been used since the beginning of this century and there are very few people alive today that can reach the design level of the best old time designers. Moreover, according to listening tests, it really seems that the best tube configurations are also the simplest. The single ended triode allows us to build very good amplifiers with little effort and few components. The finished object has to be complete, reasonably flexible and not too large, with no apparent defect, easy to use and should not require special component matching.
Before and during the design process all of the aforementioned guiding requirements also had to be translated into more detailed technical and or "musical" design requirements... and some had to be modified or deleted. Important warning:

PLEASE NOTE THAT BUILDING THIS OBJECT IS HAZARDOUS COMPARED TO LOW VOLTAGE KITS: YOU WILL BE USING A FEW HUNDREDS VOLTS ON THE AUDIO CIRCUIT: NO AREA OF THE CIRCUIT IS SAFE. SOME COMPONENTS ARE POTENTIALLY DANGEROUS EVEN USED AT LOW VOLTAGES. IN THIS APPLICATION THEY CAN BECOME DEADLY DANGEROUS: THEY CAN CAUSE INJURIES AND ALSO DEATH IF INPROPERLY HANDLES AND USED.
THIS PROJECT IS NOT FOR BEGINNERS. IF YOU HAVE NEVER BUILT YOURSELF A TUBE CIRCUIT, YOU'D BETTER BEGIN WITH SOMETHING FAR SIMPLER AND WITH A FAR MORE DETAILED INTRODUCTION AND DESCRIPTION. IF YOU HAVE NEVER BUILT AN ELECTRONIC CIRCUIT YOURSELF, THEN TAKE INTO ACCOUNT THAT IN ORDER TO PROPERLY HANDLE THE FOLLOWING INSTRUCTION THE BUILDER IS SUPPOSED TO HAVE A FAIRLY GOOD KNOWLEDGE OF ELECTRONIC DIY. NOTE THAT QUITE A LOT OF PIECES OF INFORMATION ARE MISSING FOR IT TO BE OF ANY USE FOR A BEGINNER. I TOO, AS MY FRIEND THORSTEN, WOULD NEVER SUGGEST ANY TUBE DESIGN FOR A BEGINNER; BUT I WOULD NEVER SUGGEST THIS PROJECT FOR ANY FIRST-TIMER, AS TO SET IT UP CORRECTLY, AVOIDING ALL NOISE AND GROUND LOOP PROBLEMS, IS NOT AS EASY AS IT CAN APPEAR. THERE IS REALLY A SERIOUS DANGER IN ANY TUBE DIY PROJECT: THE REQUIRED DC VOLTAGES ARE REALLY VERY DANGEROUS, AND CAN REALLY BE LETHAL. IF YOU THINK I AM KIDDING, PLEASE HAVE A LOOK AT Workbench Safety di Doc Bottlehead.
IN ANY CASE HAVE A LOOK AT THE GENERAL DISCLAIMER.

Design Requirements

• MM phono input requirements
  • Input sensitivity must be 4mV for 2V output level at 1kHz, or higher if possible, in order to also handle high level MC cartridges
  • Phono stage noise must be masked by disc noise or at least not be audiable
  • That the RIAA curve must be absolutely precise is not a major issue, unless the inappropriate curve has audible effects or the two channels are badly unbalanced
• Line stage requirements
  • Input sensitivity must be such as to get the same output as from the phono stage with a "normal" MM cartridge
  • The output resistance must be as low as possible: low at least compared with the normal input impedance of a tube amplifier
  • No output noise must be audible at "high" output levels when a line input is selected
• Control section requirements
  • There must be an easy and almost continuous level regulation, with one single control for both channels
  • There must be at least three switched line level inputs
  • There must be at least an unswitched recording output (as I do not use recorders, I do not feel the need for a switch....)
  • Balance regulation, or single channel separate level regulation, is required for coping with unmatched or at least not perfectly matched tubes in both stages, if necessary
• Other general requirements
  • A very good dynamic range must be achieved, with a good overload safety margin
  • An excellent tridimensional stage reconstruction is desired
  • No distortion must be audible, or at least not perceived. Hence, the Design must be based on tubes, and all tubes must be triodes used in single ended configuration.

Go to Part 2

© Copyright 1998 Giorgio Pozzoli for TNT-Audio, http://www.tnt-audio.com