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. 


Input selector

preamplifier_modular_sel.gif (15674 bytes)


Main Preamplifier

preamplifier_modular_main.gif (47659 bytes)


  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.

Επιλογές για το module PHONO
Arrow.GIF (101 bytes) Phono RIAA Preamplifier 1
Arrow.GIF (101 bytes) Phono RIAA Preamplifier 2
Επιλογές για το module BUFFER
Arrow.GIF (101 bytes) Class A buffer amplifier [My choise]
Επιλογές για το module LINE OUT
Arrow.GIF (101 bytes) Symmetrical Audio Class A preamplifier3 [My choise]
Arrow.GIF (101 bytes) Symmetrical Class A [1]
Arrow.GIF (101 bytes) Symmetrical Class A [2]
Arrow.GIF (101 bytes) Class A Line preamplifiers

Part List for Modular Preamplifier [Fig.1a-b]

R1-2-27=1Kohm metal film 1% C1-2=10uF 25V IC1=TL072 - NE5532 - OP275
R3-4=100Kohm metal film 1% C3-4=47nF 63V MKT 5% S1-2=1X12 step Rotary Selector
R5-6-7-8-9-10-11-12=47Kohm metal film 1% RV1=2X22Kohm Lin. Pot. [Alps] S3=1X2 step mini switch
R13-14-15-16=1Mohm metal film 1% RV2=2X10Kohm Log. Pot. [Alps Black or Blue] RL1....12=12V 2 places 2 contact [Finder -Omron]
R17-18=100ohm metal film 1% D1-2-3-4-5=LED Red 3mm or your selection J1......10=RCA Type Female gold-plated plug
R19-20-25-26=100Khom metal film 1% D6-7-8-9-10=1N4148 J11-12-13-16-17=10pin Gold Female connector 2.54mm step
R21-23=100Kohm metal film 1% D11-12-13-14-15-23=LED Red 3mm or your selection J14-15-18-19-20=RCA Type Female gold-plated plug
R22-24=10Kohm metal film 1% D16-17-18-19-20-21=1N4148 For ICs Techical Data see my Database

Power supply-Delay time and Remote Unit

preamplifier_modular_supply.gif (44717 bytes)

Power Supply and stabilisation.

  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.

Delay Circuit
  An essential circuit is the delay of connection of exit of preamplifier in the inputs of power amplifiers, afterwards the passage of few times. This protects are heard annoying sounds from the capacitors charge and recharge the moment where we open and close the preamplifier. The circuit can be replaced from a transistor, but I believe that solution with the use of IC9 ensures better and steadier behavior. All the circuit is built round the IC9 and functions as follows. When closes switch S4 the voltage in T1 secondary is rectified by D32,34 and via the R32 and D35 is stabilized in +12V. The exit of IC9A becomes High and via the R33 charge the C36 the capacity of that determines also the time of delay, in our case roughly 5 seconds [In order to grows this time should grows also C36]. Hardly charge capacitor C36 the exit of IC9B becomes High and transistor Q1 becomes ON, D37 turns on, is activated the relay RL12, the two contacts in the exit of preamplifier [Fig.1] closes. On the contrary when switch S4 opens then the voltage in secondary of T1 fall cutting off the same become also in the pins of 1,2 IC9A so that are cut-off the circuit and the relay RL12 open, the himself and his contacts, stopping the flow of acoustic signal to the power amplifiers.
  The circuit Remote follows the operation of previous circuit. When capacitor C36 is charge then the exit of IC9C becomes High the transistor Q2 becomes on, diode D39 turns on also the relay RL13 closes his contacts, allowing the AC voltage from the JF1 to pass in the JF2. Simultaneously the supply voltage +12V is presented in exit J20, with which we have the possibility of driving one suitable external relay. In this case if we did not need the relay RL13, we can suppress. The usefulness of this circuit is obvious in the case where the preamplifier drives active speakers, but does not exclude also other uses as the supply via plug JF2 with voltage benefit in the remainder appliances that have relation with the preamplifier and need they open with this, as power amplifiers, Cd player, Tuner etc. The contacts of RL13 IT SHOULD THEY ENDURE the POWER THAT NEEDS the APPLIANCES THAT ARE SUPPLIED BY THEM. Where it exists the 230Vac voltage should insulate very well. It will be supposed I stress that the CONTACT WITH THESE POINTS, NEEDS the PROPORTIONAL ATTENTION, BECAUSE the HIGH VOLTAGE IS DANGEROUS.

Part List for Power supply-Delay time and Remote Unit [Fig.2]

R28-29-30-31=220ohm 1/4W metal film 1% C24-26-28-30-32-33=100nF 63V MKT 5% D35=12V 1W Zener
R32=1Kohm 1W 5% C25-29-27-31-34=10uF 25V D37-39=LED 3mm
R33=1Mohm 1/4W metal film 1% C35=100uF 63V Q1-2=BD678
R34=10Kohm 1/4W metal film 1% C36=4.7uF 25V T1=230Vac//2X15V 50VA Transformer [Toroidal]
R35-38=39Kohm 1/4W metal film 1% C37=47nF 63V MKT 5% RL13=12V Relay 2X2 20A  [Finder -Omron]
R36-37-39=1Kohm 1/4W metal film 1% IC2=LM7815 or LM7818 F1=1A Slow Block Fuse
R40=1.2ohm 1/4W 5% IC3=LM7915 or LM7918 S4=2X2 Switch 10A per contact
TR1-2-3-4=4.7Kohm trimmer IC4-6=LM317 [above in small Heatsink] [See text] JF1=3pin male supply jack with EMI Filter
C5=22nF 300Vac [Y2/X1 Class] MKT 20% IC5-7=LM337 [above in small Heatsink] JF2=3pin female supply jack
C6-7-8-9=10000uF 35V IC8=LM7812  [above in small Heatsink] J20=RCA Type Female gold-plated connectror [isolating]
C10-11-12-13=100nF 63V MKT 5% IC9=4081 [AND C-mos]
C14-15-17-19-21-23=10uF 25V B1=Bringe Rectifier [B80 C5000-3000] For ICs Techical Data see my Database
C16-18-20-22=100nF 63V MKT 5% D23.....34-36-38=1N4002 All the MKT capacitors is polyester
Modular Preamplifier Technical Data Typical
Input Selections 5 stereo sources
Input Impedance 47K ohms
Audio Outputs 1X Stereo Rec, 1X stereo Main
Main Output Impedance 100 ohms
Recording Output Impedance 100 ohms
Output Voltage 1.2 VRMS
Frequency Response DC to 100KHZ +-0.10 dB typ.
Distortion Less than .01% THD and IM typ.
Crosstalk Greater than 90 dB typ.
Noise 105 dB A Weighted @ 1 KHz typ.
Mute Switch Attenuation 0, -20 dB attenuation
Delay time out Protection ~5 sec
Remote outputs +12V [Protected], AC Line
Delay time Remote output ~5 sec
AC Line Voltage 120/230 volts 60/50 HZ
AC Voltage Range +-10%

Alternative proposal for Buffer and Gain module Units

preamplifier_modular_buf_gain.gif (13478 bytes)

                                                                                     Fig.3                                                                                                     Fig.4

  In pictures fig.3-4 appear the alternative proposals for those who they will prefer the opamp choice, from the solution of separately components, in the place of each module. In the Fig.3 exists a typical circuit. In the place of IC1 can use a line from choices of single opamp. The choices that I propose exist in the part list. Certain opamp have pins in which we can place one small trimmer [TR1] with which we have the possibility of regulating DC voltage [Offset voltage] in the exit of IC. If without TR1, this voltage is low, we can omit this. Between pins 5 and 8 of IC1 we can place a capacitor with typical value 22pF, which decreases the open bronchus bandwidth of IC with result better stability. The use the C6-8 is imposed when the amplifier works with X1 gain. If the opamp gain don't suffices you can change the value of resistor R5 following the type of gain calculation [A=R5+R6/R6]. Capacitors C3,4,8,9 should be placed near in ICs pins. Resistances R1,5,8,9 in combination with capacitors C1,2,10,11 create a local filter for the supply stability. Resistors R7,11 protect the opamp exits from short-circuit. In the Buffer unit the IC2 does not present no gain in the signal. Good it’s in the place of IC2 is preferred opamp with FET in his input, because this solution presents more tolerance in the changes of resistance in his input. In the course of acoustic signal, in the exits and in the inputs of units don’t exist capacitors. Thus the units have in the low frequencies, flat response.

Part List for Buffer and Gain Units [Fig.3-4]

R1-2-8-9=47ohm 1/4W 1% metal film C1-2-10-11=47uF 25V IC2=NE5534 - TL071 - OP07C-LF411-OPA604
R3-6-12=1Kohm 1/4W 1% metal film C3-4-8-9=100nF 63V MKT J1-2=10pin pointed male connector 2.54mm step
R4-13=47Kohm 1/4W 1% metal film C5=1.5nF 100V MKT or stypoflex
R5=6.8Kohm 1/4W 1% metal film C6-7=22pF 100V ceramic or stypoflex All the MKT capacitors is polyester 5%
R7-11=100hm 1/4W 1% metal film IC1=NE5534-OPA604 For ICs Techical Data see my Database

Ground and Supply Lines Block Diagram [Fig.5]


preamplifier_modular_block.gif (23544 bytes)


  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. 

Arrow.GIF (101 bytes) Prototype Foto Gallery
Arrow.GIF (101 bytes) High-End Audio Modular Preamplifier in Greek


Sam Electronic Circuits 03/04