EMI TG 12345 Mk.II Handbook Download Page 1

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HANDBOOK OF THE

EMI AUDIO MIXER

TG 12345 Mk.II

RESEARCH LABORATORIES OF ELECTRIC & MUSICAL INDUSTRIES LTD.

HAYES

MIDDLESEX

ENGLAND

Summary of Contents for TG 12345 Mk.II

Page 1: ...HANDBOOK OF THE EMI AUDIO MIXER TG 12345 Mk II RESEARCH LABORATORIES OF ELECTRIC MUSICAL INDUSTRIES LTD HAYES MIDDLESEX ENGLAND...

Page 2: ...4 10 Cue Circuits 3 MICROPHONE CASSETTE 3 1 Channels 3 2 Input Socket 3 3 Condensor Microphone Powering 3 4 Input Circuits 3 4 1 Coarse Input Level Control 3 4 2 Input Amplifier and Fine Gain Control...

Page 3: ...4 3 2 Output Amplifier 4 4 Metering 4 5 Power Supply 5 MAIN CASSETTE 5 1 Contents 5 2 Main Channel 5 2 1 Input Gain Control 5 2 2 Inject 5 2 3 Fader 5 2 4 Poling 5 2 5 Presence Control 5 2 6 Band Pass...

Page 4: ...7 2 5 Pole 7 2 6 Solo 7 2 7 Control Room Loudspeaker Switch 7 2 8 Loudspeaker Circuit Remote Operation 7 2 9 Loudspeaker Circuit Direct 7 2 10 Loudspeaker Line Output 7 3 Record Replay Switches 7 3 1...

Page 5: ...3 5 Artist Manager s Microphone 8 3 6 A M Gain 8 3 7 A M Mixing 8 3 8 R F Suppression 8 4 Alternative Talkback 8 5 Intercom 8 6 Cueing 8 6 1 Operator to Cue 8 6 2 Artist Manager to Cue 8 7 Track Anno...

Page 6: ...pe C 12 6 Amplifier Type E 12 7 Band Pass Filter F 12 8 Amplifier Type G 12 9 Amplifier Type G B 12 10 Amplifier Type H 12 11 Amplifier Type J 12 12 Amplifier Type K 12 13 Amplifier Type M 12 14 Ampli...

Page 7: ...nless otherwise stated 2 1 2 Signal levels Signal voltage levels are expressed in dBv this being the voltage level in decibels with respect to 0 447v this being the r m s voltage across a 200 resistor...

Page 8: ...12 cassettes Blank panels may be fitted if cassettes do not occupy all the channels available Blank cassettes are available having plugs to accommodate the unused sockets on the cableform 2 3 3 Main P...

Page 9: ...recording purposes will always be used with one or more mains powered tape machines there is no need to provide for battery operation of the mixer It is therefore designed to operate from 200 250V 50...

Page 10: ...ich is also the common point of the positive and negative 20V lines is insulated from the chassis thereby facilitating the tracing of earth faults in components or wiring This is also connected to the...

Page 11: ...ters and the correlator the indications of which will be dealt with later 2 4 6 Switching Stud or keyswitches are used for signal switching where these can be used without excessive complication of th...

Page 12: ...ified to 1 The errors in the overall gain due to deviation of resistor values can be cumulative and the unbalance of a pair of channels could be considerable of errors were of opposite sign This is ov...

Page 13: ...ed Signal from any microphone channel is tapped off prior to the fader and fed through a level control to a switch whereby it can be fed to either of the two cue bus lines or both of them simultaneous...

Page 14: ...this supply is connected to the 0 V line in the Control Room Monitor Cassette 3 4 Input Circuit 3 4 1 Coarse Input Level Control The Coarse input level control provides 12 steps of attenuation of 5dB...

Page 15: ...al Path 3 5 1 Band Pass Filter The output of each Amplifier D is connected to the input of a Band Pass Filter the circuit of which is described in 12 7 These contain adjust on test AOT resistors where...

Page 16: ...emove the possibility of earth loops 3 5 4 Presence Controls The circuit of these is described in 12 1 Each channel has two controls One of these is marked dB Presence and provides 0 2 4 6 8 or 10dB b...

Page 17: ...ese conditions This is avoided by using a 3 3k resistor on the first stud and this provides a reasonably low load for the Amplifier B in the off position The remaining ten positions of the channel sel...

Page 18: ...n order that a signal from a channel may be available for cueing purposes when the fader is closed the cue tapping must be taken from the input side of the fader This is the same point as is used for...

Page 19: ...lug This is of the same type as the 7 pin plug used elsewhere but pin 2 is removed and socket 2 is plugged so that they are not interchangeable Soldered connections are made between the 20 0 and 20V o...

Page 20: ...by trimming the band pass filter The a c coupling introduces a bass loss equal to that of the compressor limiter and maintains a similar frequency characteristic whether or not the Compressor Limiter...

Page 21: ...n Amplifier R and is described in 12 17 Together with the output transformer it provides 10dB of voltage gain to give an output of 0dBV into open circuit The output impedance is approximately 200 and...

Page 22: ...ssettes See 3 5 3 5 2 3 Fader Faders with optional ganging are provided as in the Group Cassettes See 4 2 4 5 2 4 Poling In order to maintain correct overall polarity a phase reversal is required so a...

Page 23: ...iers The output amplifiers and output transformer are identical with those used in the Group cassette for the echo outputs see 4 3 2 Since it is common practice to run two tape machines in parallel tw...

Page 24: ...stor is required When the standard internal signal level of 10dBV is applied to the Amplifier U the corresponding VU meter will give a reading of 0VU Push buttons PB1 and PB2 modify the feedback of bo...

Page 25: ...ator position connection is made to the 10dBV oscillator bus line Under these conditions with the oscillator attenuator at 0 the V U meter should also read 0 The frequency characteristics of the Ampli...

Page 26: ...In the auxiliary stereo Track Monitor Cassette the record selector knobs are engraved 5 and the upper level controls are engraved 5A and 5B and the lower level controls are unmarked If due to a fault...

Page 27: ...channel is used for echo return and as described in 3 5 8 this can be switched to certain monitor channels The point of return is the virtual earth input of the Amplifier C2 Connection is made to thi...

Page 28: ...which when pressed reverses the polarity of the signal in the right hand channel The polarity change is effected by using an Amplifier P the circuit of which is described in 12 28 In each channel the...

Page 29: ...als from the two channels are mixed by connecting together the Amplifier P outputs which are built out with 5 1k resistors Thus each provides a termination for the other In position d each channel req...

Page 30: ...y 7 3 3 Follow Replay This also uses 10 poles of a 12 pole change over key Its fixed contacts connect to the movers and replay contacts of the record replay key and its movers connect to the record re...

Page 31: ...tor channels subsequent to the control room monitor level control the low level meter readings are correct only when this control is on zero By use of this control the range of the meters can be exten...

Page 32: ...assette for operating the buzzer and red light The former is non locking and the latter is optionally locking by turning when depressed The push buttons energise 24V relays in the Main Power Unit wher...

Page 33: ...which is a simplified form of Amplifier P Amplifier H is described in 12 10 8 2 4 Loudspeaker Switch This is identical with that used in the Control Room Monitor Cassette section 7 2 7 8 2 5 Studio Ta...

Page 34: ...ront of the respective buttons In order to obtain the loud condition the normal and loud buttons must both be pressed 8 3 5 Artist Manager s Microphone This is a hand microphone of 25 impedance and is...

Page 35: ...ormal line output circuit consisting of an Amplifier R and output transformer the output of which is available on pins 4 and 5 of 5 pin Tuchel socket SK2 for feeding to a power amplifier and loudspeak...

Page 36: ...f the existing signal and also short circuits pins 25 and 35 of PL1 on the Studio Playback Cassette to operate active relay RL1 thereby quieting the control room monitor loudspeakers 8 7 3 Switching M...

Page 37: ...position marked To Tracks The oscillator is fed to the track announce line as in the on position but in addition RL5 is energised to operate the change overs in the Amplifiers N to accept the oscilla...

Page 38: ...ty is provided by the use of steel angle welded to form a rectangular member extending laterally substantially the full length of the frame and vertically from the top of the visible part of the legs...

Page 39: ...cableform to the Main Power Unit 10 MAIN POWER UNIT 10 1 Function This accepts mains supplies 204V to 252V 45 to 65Hz It contains an a c stabiliser and a step down transformer to provide the 50V cent...

Page 40: ...light button on the Control Room Monitor Cassette connects together sockets 2 and 3 of Sk6 thereby operating RL2 and RL3 RL2 applies mains to SK4 for operating a mains red light circuit RL3 short cir...

Page 41: ...h more economical to provide a suitable amplifier than to increase the power to all the Mixer output amplifiers All amplifiers incorporate a considerable amount of negative feedback so distortion rema...

Page 42: ...in the return path serve to prevent r f oscillation With S2 in its central position the feedback path consists of R5 20k in parallel with the total resistance between terminals 1 and 2 on S2 which is...

Page 43: ...s used for the presence circuit The amount of bass is determined by switch S1 which is identical with switch S2 used in the presence circuit 12 2 2 Active Circuitry This is identical with that used on...

Page 44: ...table value for A O T 1 The value 15k given in the table is a typical one Amplifiers A3 and A4 are only used in the operator s and Artist Manager s microphone circuit where a precise value of gain is...

Page 45: ...nity overall voltage gain Type C1 when used in conjunction with a control to Drawing No A40 has unity gain when the control is in the central position This control can increase and decrease the gain b...

Page 46: ...d c coupled transistors d c coupled feedback being provided by connecting the emitter of the input transistor to a tapping on the load in the collector circuit of the output transistor The base of the...

Page 47: ...ines the potential at the output and hence through the d c feedback path the potential of the base of VT1 the input being a c coupled The low frequency time constants are given by C1 C2 with R1 R2 ass...

Page 48: ...ier Type G B 12 9 1 Purpose For convenience of cassette layout it is desirable to have a board with one Amplifier G and one Amplifier B on it 12 9 2 Circuit The circuit diagram is shown on Drawing No...

Page 49: ...its normal setting and also provides three independent quieting circuits 12 11 2 Circuit The circuit diagram is shown on Drawing No AE212 12 11 3 Circuit Description The amplifier transistors VT1 and...

Page 50: ...pled virtual earth amplifier very similar to that of Amplifier H For initial test of this amplifier a short circuit is provided in place of R4 AOT 1 The value for this resistor is chosen on overall te...

Page 51: ...R8 and R9 The 10 dB loss is determined by R6 R7 working into R8 and R9 in parallel the precise loss being adjusted on test by selecting a value for R7 Since the potential drop across R6 and R7 is sma...

Page 52: ...Output The output is d c coupled and is at a potential of about 3 3V It will supply at least 1 4V r m s into a load of 3k 12 15 Amplifier Q Power Unit 12 15 1 Purpose This unit supplies the necessary...

Page 53: ...nged as a transformerless class AB push pull output stage VT5 is driven by an emitter follower VT3 n p n VT6 is driven by VT4 p n p with 100 feedback from the collector of VT6 to the emitter of VT4 Th...

Page 54: ...s and hence has to be of the electrolytic type On test the value of AOT 1 is selected to give a voltage gain of 5dB The output transformer with which it is used has a step up ratio of 1 1 78 thereby y...

Page 55: ...e negative excursions of the base of VT4 to about 0 5V and on positive excursions VT3 is turned on so these are also limited to about the same voltage When VT3 is off current flows from the 20V line t...

Page 56: ...ut stage except that its negative supply is taken through an emitter follower VT31 In the first limiting stage the diode D10 is connected in the reverse direction and so limits the positive peaks and...

Page 57: ...it the meter if the input voltage is more positive than about 0 6V or more negative than about 0 6V VT17 is an emitter follower the emitter of which drives the base of VT18 which provides a gain of ab...

Page 58: ...VT51 positive and giving a positive deflection of the meter If the signals are identical but in opposite phase then the signal in the B path will be switched on when it is positive and negative defle...

Page 59: ...d to be fed from a 600 line terminated in 600 that is from 300 through a 3 6k resistor In order to economise in power in the present case the identical operating conditions for the meter are obtained...

Page 60: ...C8 in parallel This circuit has long charge and discharge time constants due to the high values of R10 and R11 respectively The potential on C7 and C8 is compared with the potential at the junction o...

Page 61: ...fier of the difference between a fraction of the output and the reference voltage provided by D11 The temperature coefficients of VT8 and D11 both of which are prior to the high gain cancel one anothe...

Page 62: ...VT24 The total signal at the collectors of VT5 and VT6 therefore contains the desired signal in push pull together with an undesired control signal in push push It is most important that no appreciab...

Page 63: ...ow being about 3dB the ratio of R46 to R44 In the limit condition R45 with D8 D9 and D10 in series are shunted across R44 thereby increasing the gain to about 20dB The purpose of the diodes is to roun...

Page 64: ...rmines the increase of gain with falling signal level and therefore controls the amount of compression or limiting The earthy lead from the recovery switch is taken to the slider of the hold control s...

Page 65: ...equired to quiet the monitor loudspeakers either if the multi track announce button is operated or if any individual track announce button is operated These cannot however be connected directly in par...

Page 66: ...th the base of VT2 held at about 0 8V from the potential divider R1 R2 R3 connected across the 20V supply R17 connects the base of VT1 to the common emitter point and therefore VT2 is turned on and VT...

Page 67: ...ains 10 identical attenuators the inputs being on two sets of terminals numbered 1 to 5 and the corresponding outputs being on two sets of terminals numbered 6 to 10 Separate 0V lines are provided to...

Page 68: ...s connected between this terminal and 0V AOT1 is selected to set the gain to 20dB with 1 2k connected between Gain Control and 0V 12 27 4 Output The output has no d c blocking capacitor It is in phase...

Page 69: ...No BE238 12 29 3 Circuit Description A transformer B189A as used in the Cassette Power Unit is fed from the 50 V a c supply and the whole of the secondary winding is used to feed a bridge rectifier c...

Page 70: ...ould be connected to the inject plug by twisted screened pair leads 12 30 2 The impedance at the primary of the send transformer should not be less than 3k and the impedance at the secondary of the re...

Page 71: ...ions is detailed in drawing No TG12345 AR008 This drawing should be read in conjunction with TG12345 DE2 The original solo push button was replaced by a latching miniature 2 pole changeover toggle swi...

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