Vinyl Pre-Amplifier
Circuit :Graham MaynardEmail :graham.maynard1@virgin.net
Back in the 1970's I became disheartened by audio 'progress', for in spite of them presenting much superior specifications, mainstream transistor amplifiers sounded worse than the tube (valve) designs they were supposed to replace. Ultralinear KT88s made good power amplifiers, but transistor circuitry could be much better for pre-amplifiers. Integrated circuits had also developed well, but these were often thought of as offering a lesser performance when compared to discrete construction.
For those who might wish to 'save' to their computer hard-drive a digital copy of their own vinyl record collection, I have redrawn my 1975 vinyl pre-amplifier circuit. It remains capable of optimising disc playback and offers features still not repeated today.
Many designers opinionate that NFB loop controlled amplifiers are inferior because they degrade the sound, and yet I wonder what it is that these individuals think they actually listen to ? The fact is that most vinyl waveforms and CD pits, as pressed into their own disc collections, have already been mastered through a myriad of NFB loop controlled pre-amplifier and mixer stages - this long before anyone can start listening. Indeed, that Mullard developed TDA1034NB integrated circuit which initially cost me ВЈ67 for ten, went on to be used by the hundred within what became a classic series of world famous Neve mixing consoles.
Properly designed NFB loop controlled IC gain stages can set standards for excellence, and thus I separately list five elements embodied within my 'old' pre-amplifier design.
(1) A moving magnet pick-up cartridge is an inductive transducer that must be resistively damped and reactively tuned to optimise reproduction.
Hence I fitted a sub-miniature twin gang 500pF variable and screen earthed twin gang potentiometers directly to the input circuitry
It is only *after* you have actually used these input damping and tuning controls whilst music listening to optimise your own equipment line-up, that you can understand just how much mind distracting spin has been repeated about fractional 'dB' variation with respect to an ideal RIAA characteristic.
Cartridge to pre-amplifier matching has a much more significant effect upon reproduction than does the achievement of perfect RIAA equalisation !!!
(2) Another factor greatly affecting reproduction relates to NFB loop controlled gain stage interactions and terminations.
For example, it is possible to build a moving magnet stage using just one or two gain stages per channel, and they can measure near ideal under steady sinewave examination, but this cannot guarantee that they will actually sound good when coping with highly dynamic music waveforms. NFB loop controlled equalisation stages should be buffered at input as well as output. The input terminal of a stage that is called upon to output current not retaining a linear relationship with voltage at all frequencies, will itself not respond with amplitude linearly if fed at high impedance, and this is especially so with bipolar input circuitry.
(Stage interaction often arises, and this is why some power amplifier plus pre-amplifier combinations can reproduce less cleanly than expected.)
Interconnects are not the only cause of audible degradation, thus a separate additional NFB loop controlled line output driving stage that does not load previous circuitry whilst providing a lower output impedance can further improve rather than degrade the final sound by its own presence.
(3) The components used for upper and lower RIAA equalisation characteristics are better separated, as in this circuit.
Passive (non distorting) 750 ohm (or 2x 1k5 in //) plus 100nF close tolerance components not only perform the RIAA hf cut between stages three and four, but they also reduce higher audio frequency noise and distortion from the earlier stages.
Here the line driver stage is already operating at good input signal level with falling input input impedance as frequency increases, and the resulting improvement in sound reproduction becomes instantly recognisable.
Additionally, the uncompensated series feedback unity gain error seen on some other vinyl pre-amplifier circuits is automatically covered.
(4) The original RIAA characteristic was, *is*, sub-bass weak, with a low frequency roll-off that introduces notable bass phase distortion.
For this reason I extended the low frequency equalisation to 25Hz instead of 50Hz, with a switchable option for 'standard' reproduction.
Do not try to use the extended bass response for loud real-time playback (via a computer is okay) unless you have solid floors or your turntable is brick wall mounted.
The 22uF capacitors then introduce multi-pole passive roll-off below 20Hz to more sharply cut turntable and pressing rumbles.
(5) At high live playback levels the extended bass response can set up feedback via differentially energised room resonances. This was easily remedied by connecting the primary of a subminiature transistor radio output transformer between channels, thereby mono-ing the sub-bass without affecting other stereo reproduction. Thus this pre-amp offered a new method for bass feedback reduction that has minimal impact upon the overall bass reproduction level, yet which allows higher 'pop-party' sound levels in undamped rooms.
Don't just look at this circuit and think 'Yeah ?' or 'Sure ?' and say to yourself 'Look at all those capacitors !'.
This analogue pre-amp is a tested design providing not just both a cleaner and quieter background to the music we are meant to hear, but also an optimisable clarity of reproduction that few are likely to have heard before, or even imagined could ever have become a realisable experience.
Today there are many internally compensated audio integrated circuits to choose from, several offering fet input devices. Remotely power your construction with at least four 470uF or 1mF capacitors per 15V rail, and place it beside the tone arm with no more than two feet of screened interconnect.
Do please let me know how you can get on, and let me know which ICs performed well so that they can be mentioned here for other constructors.
Unfortunately I do not have any photographs of my original construction.
Good Luck .......... Graham.
PS. After the 1980's I became even more disheartened by the next supposedly superior technical improvement - the CD. Those who construct this pre-amp might come to understand the significance of our loss !!!
Source:www.zen22142.zen.co.uk