Circuit : Graham Maynard
A vinyl pre-amplifier designed by the late Graham Maynard. Using opamp's, as well as the normal RIAA equalisation, the
circuit also offers bass boost.
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 £67 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 !!!
Finished 'ACE' Pre-Amp by Eric from Leicester, United Kingdom
Being a pensioner and not liking to spend my limited loot, I relied extensively on the treasures in my loft
and went with two LM358s for the first two stages and used the only four MC1471s I had for the latter two stages.
I did omit the 'extended bass' circuit, but I compromised a bit by making a small change in the output to the LEVEL pot,
by putting in 680nF, which I had, instead of the 560nF in the design. I reckoned that this was "in the right direction"
for the change to the 'extended bass'.
I built the amp on Vero which I had, and used IC sockets I had, and could do quite a lot of test on my assembly with
my digital meter before inserting the ICs.
My wife Janet found me a nice metal box, which was just the right width to take the Vero, but it meant that I couldn't
get much separation from the PSU with the small mains transformer on, that was on a separate board with two linear
regulators (7815 and 7915 I think) and I mounted this in one end of the tin box.
On initial power up connected to the Turntable and my output Amp (which is our modern digital SONY TV set) I could tell
I had some 'mains hum' before I spun a disc. I tried various things to minimise this, trying a metal "baffle" between
the transformer and the Pre-Amp (which made little difference, but two things did become apparent:
although I had Earthed the incoming mains (because the assembly is not "double insulated") to the tin box, and I
had a wire from the Pre-Amp "Common" or "GND" to the PSU 'centre', and my input from the Turntable (stereo audio
sockets) had the co-ax screen also connected to Common, I found a marked reduction in 'hum' when I put a another
'Earth wire" from the mains Earth connection to the co-ax commoned input screens; so I soldered this in place.
My +15 Volt, -15 Volt PSU board I had made some years ago for another project - long defunct. I had put 4 + 4,
470uF Caps on the Pre-Amp supply rails, but I tried a fifth one of these on the Heatsinks of the linear regulators
and found this reduced the hum as well - to near none - so I took the PSU out again and removed the Electrolytic
Caps that had been in that assembly for some years, and put a total of "2 + 2" more 470uFs with one before the
regulator and one at the output of the PSU, on each supply.
I was then very pleased with the lack of mains hum.
It all works fine and good enough for an output through a TV to the speakers in the TV (I hope Graham is not turning
in his grave!); in fact whilst the LEVEL and the BALANCE work much as I thought they should (and given I have no
signal generator or Oscilloscope) I was unable to determine a setting for either the DAMPING network, OR changing
the Input Resistance' that made any really noticeable effect on the sound.
So we have been able to listen to some discs that we have had for many years, plus some inherited from my Dad,
that we have not heard for many years.
Inside the Case (Click small Images to zoom)
The pre amp Vero shaped to fit in one end of the tin box. Top centre the input terminal for 'left; common; right'
from the turntable; immediately right of that, dual 100k R 'in'; to the left the dual 10k log Damping R; used the 8 bit
piano switch with different capacities to achieve dual 47pF to 438pF in steps of no more than 47pF for Damping; no ICs fitted
in sockets; red links are +15V; near bottom right: output terminals.
230V input on right in white terminal block, yell/greens are mains earth; on the left on the inside of the box, the
'level' and 'balance' controls are on a narrow separate piece of Vero, at right angles to the side of the box; I used al
l screened cables for in and out; the red, yellow and black wires are +15, common and -15V from the PSU at the 'mains'
The small mains transformer is hidden behind a piece of thin stainless used as an attempt to shield the 50Hz magnetic
field; the top of the heatsinks for the 7815 and the 7915 are just visible behind that shield; in the foreground: the
'level' pot has a black knob; the input is on the right: two audio sockets, screens common; the twin screened grey lead
with audio red and white connectors is the output.
The input sockets are above the 'Maynard's' label; the mains driven PSU at the right hand end; the output is the twin
grey audio wire with red and white connectors; (the aluminium foil was an attempt to screen the wires to the 'balance').
You can also see Eric's finished ACE Vinyl Pre-amp in the Audio Gallery.