High-End Audio Modular Preamplifier

  This preamplifier is requirement result of many friends to give a high quality preamplifier, capable to drive high quality power amplifiers with good sound, it's not however difficult to making, it combines simplicity and handiness. It does not allocate a lot of facilities and various filters, but is drawn so the acoustic signal follows the smaller course without it's influenced by stages they do not need, with result the coloration of reproduced sound. It follows the modular philosophy that gives the possibility of changing each module and adapted in yours acoustic taste. Thus it will constitute the base for long-standing listening and experimentations. Use relay in the input selection ensures the smaller way and the better possible quality, because the relay contacts are not influenced easily by temperature changes and environment, offer smaller ohmage and parasitic resistance in the signal course. This of course depends also from the relay quality that you will select; I propose choice of relay small size and very high quality. Enough companies they manufacture such relays. Exists different possibility of choice for the Listen inputs and different for the analogue recording. Thus we can listen other sound source and other recording. Exists the possibility, for those who they judge of suppressing Balance potesometer [RV1], the recording selector and proportional relays making the manufacture simplest and more economic. In the each place module we have big variety of choices. Certain you I will give I below, but can make also your choices. As you will observe exist somebody "facilities" as monophonic [MONO] operation etc. 

  The choice of acoustic signal follows two streets [Fig.1A]. First with the selector S1 that is 1 contact 12 places, from which I use five. Exists the possibility to using also the remainder places adding to them proportional relay and Led. Each place supplies the proportional relay and Led that it shows whom input has been selected. The supply becomes with + 12V [+V4]. Selector S2 functions with the same way as the S1, supplying them proportional relays and Led, so that becomes the source choice that recording. Afterwards recording relay exists the buffer unit round IC1 [Fig.1B]. In the part list exist certain choices for opamp. that will undertake this role, but exists the possibility of selecting somebody of your choice. All the inputs are high level apart from input PHONO [RIAA Filter] that is directed in proportional module PHONO [J11] and by his exit return again in the block selector relay. He is alone module that has stereophonic structure and the two channels that abut to connector [J11].  Those who do not allocate the old turnable [pick-up] and disks collection of 33 turns, they will not need PHONO module. It can easily they change him in line, suppressing connector J11 and you connect the corresponding inputs J1-2 in proportional contacts P1-2 relay RL1-6. The relay team of listening direct the stereo acoustic signal from some input in Buffer module [J12-13]. The gain of this unit is X1, does not provide no gain in the signal. They follow potesometer BALANCE [RV1a-b] and VOLUME [RV2a-b] that regulates the balance and the level of acoustic signal respectively. These two Potentiometers should be high quality also from acquaintance constructor. Afterwards the Potentiometer VOLUME exist the MUTE stage that is constituted by a divider R21-22 and the contacts of relay RL11 that are found parallel with the R21. In normal operation the contact jumper the R21, the signal pass without fall. If it close switches S3, turns on the D22, the RL11 arms, the contact opens and the R21 with the R23 they configure an divider which create a fall of level in acoustic signal [-20dB]. The same logic is also for the other channel where exist the resistors R23-24. In the continuity the acoustic signal enter via the J16-17 in Line module what offers the suitable gain in the signal and this drive all the goods power amplifiers. Then exist the contacts of RL12 that the operation of is described in the delay circuit.

  A good preamplifier should essentially be accompanied from one good power supply. Thus I preferred the solution of separate voltage and stabilisation of each stage, separately. This does not allow in the various units' influences the one the other via the supply, ensuring lower distortions and better transient response. In the input of 230Vac exists the plug JF1 that contains filter of reject parasitic voltages. Transformer T1 it should they are toroidal. Stabilizers IC4,5,6,7 have the possibility of voltage regulation in their exit from the TR1,2,3,4. The choice that appears in the drawing is for output voltage +/-15V but can is also regulated in +/-18V if him it requires the circuit that will be supplied. For PHONO stage exist simple stabilizers IC2,3 with constant out voltage +/-15V. The same choice exists also for the stabilisation of +12V with the IC8. This voltage supplies all the sel. Circuits, RL, delay and Remote. The earth line [0VD] of this stage SHOULD be separate from the earth that concerns audio circuits [0VA] and will be supposed it leads to the common point that leads all the earth sectors, between to capacitors C6,7,8,9 [Fig.5].  Different and accidental arrangement of earth lines will create hum problems. Certain stabilizers should be placed on small heatsinks or in one common bigger it�s enough it's placed between this and the stabilizers suitable silicon mica.

                                                                                     Fig.3                                                                                                     Fig.4

Ground and Supply Lines Block Diagram [Fig.5]

  In photographs appears the prototype, in which base the circuits that exist here. They little differ in the supply philosophy, the quantity of inputs-outputs and in the existence of headphone amplifier that does not offer something in the sound quality.