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EMI TG 12345 Mk.II Handbook

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

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Summary of Contents for EMI 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: Table Of Contents

    LIST OF CONTENTS INTRODUCTION GENERAL Terminology 2.1.1 Drawing Numbers 2.1.2 Signal Levels General Drawings Mechanical Components Electrical Components and features 2.4.1 Active units 2.4.2 Mixing 2.4.3 Power Supply 2.4.4 R.F. Rejection and Earthing 2.4.5 Metering 2.4.6 Switching 2.4.7 Frequency Response 2.4.8 Gain 2.4.9...

  • Page 3: Table Of Contents

    3.8.1 Echo Level control 3.8.2 Echo Selector 3.8.3 Echo Bus Lines Power Supply GROUP CASSETTE Contents Group Channel 4.2.1 Input Gain Control 4.2.2 Limiter/Compressor 4.2.3 Inject 4.2.4 Fader 4.2.5 Poling 4.2.6 Presence Control 4.2.7 Band Pass Filter 4.2.8 Output Echo Output Circuit 4.3.1 Main Level Control 4.3.2...
  • Page 4 Inputs Re-Record Monitor Channels 6.4.1 Tape Machine Selection Echo Channels 6.5.1 Echo Send 6.5.2 Echo Return Power Supply CONTROL ROOM MONITOR CASSETTE Function Loudspeaker Channel 7.2.1 Input Key 7.2.2 Input Amplifiers 7.2.3 Monitor Selectors 7.2.4 Level Control 7.2.5 Pole 7.2.6 Solo 7.2.7 Control Room Loudspeaker Switch...
  • Page 5 STUDIO PLAYBACK CASSETTE Function 8.1.1 Synchronous Playback Loudspeaker Channels 8.2.1 Input Selectors 8.2.2 Level Controls 8.2.3 Pole 8.2.4 Loudspeaker Switch 8.2.5 Studio Talkback 8.2.6 Output Talkback 8.3.1 Operator’s Microphone 8.3.2 Gain 8.3.3 Studio Talkback – Normal 8.3.4 Studio Talkback – Loud 8.3.5 Artist Manager’s Microphone 8.3.6...
  • Page 6 MAIN POWER UNIT 10.1 Function 10.2 A.C. Constant Voltage Transformer 10.3 Step Down Transformer 10.4 Relay Power Supply 10.5 Buzzer Operation 10.6 Red Light Operation 10.7 Microphone Powering 10.8 Metering CIRCUIT DESIGN – GENERAL PRINCIPLES CIRCUIT DESCRIPTION – INDIVIDUAL BOARDS 12.1 Presence 12.2...

  • Page 7: Terminology

    Introduction. The mixer has been designed to meet the following requirements: To be suitable for fixed or mobile use. To be light, easily dismantled and transported. To comprise cassettes, which can be assembled in different combinations, to form mixers with up to 24 inputs for up to 8-track recording.

  • Page 8: Mechanical Components

    The Drawing numbers of the block schematics of the six types of cassette are: Microphone Cassette Group Cassette Main Cassette Track Monitor Cassette Control Room Monitor Cassette Studio Playback Cassette Main Power supply unit Mechanical Components 2.3.1 Cassettes There are six types of cassettes, which are of uniform size. These are as follows: Microphone Cassette described in section 3.

  • Page 9: Mixing

    2.4.2 Mixing Although a mixer has to perform many functions other than that of mixing, nevertheless, this function is of predominant importance on account of the multiplicity of input and output channels which may be involved. In the present Mixer facilities are provided for mixing the signals from microphone channels and applying them to group channels, for mixing signals from microphone and/or group channels, for mixing signals from main channels and applying them to the inputs of auxiliary stereo main channels and for mixing...

  • Page 10: Metering

    With this method of supply, the Mixer desk is classed as an “extra low voltage” piece of equipment and it is therefore not legally necessary to bond its metalwork to the mains earth, but this is normally done. 2.4.4 RF Rejection and Earthing Any long lead connected to the mixer is liable to act as an aerial and feed RF into the mixer where, if the level is sufficiently high, it may be rectified and result in audible interference on the programme material.

  • Page 11: Switching

    Since a VU meter is a linear and not a logarithmic meter, the scale becomes very cramped at the lower end of the scale. The range of indication is extended by providing each Main Cassette with two locking push buttons. One of these increases the gain of both VU meter amplifiers by 10dB, and the other increases the gain by 20dB.
  • Page 12 Since each channel contains several separate active units, d.c. blocking capacitors are required at the interconnections. Many of these feed into resistive potentiometer controls, the impedance of which has been limited to a maximum of about 6kΩ in order to avoid excessive impedance at the slider. In order to avoid clicks when the control is operated, it is necessary to use capacitors with very low leakage currents.

  • Page 13: Echo Circuits

    2.4.9 Echo Circuits Each microphone channel has an echo send level control and a switch whereby a signal, which is tapped off after the fader, can be routed to any of four echo lines. Solely for convenience, and not because there is any association, the echo output circuits are located in the Group Cassettes, two output circuits being included in each Group Cassette.

  • Page 14: Channels

    MICROPHONE CASSETTE Channels Each Microphone Cassette contains two identical channels, which can be used either for two mono microphones or for a stereo pair. Input Socket A 5-pin Tuchel socket is used for the two inputs. Each input is provided with a screened input transformer having a step-up ratio of 1: 3.16 the primary winding of which is intended to operate from a 200Ω...

  • Page 15: Re-Record Inputs

    3.4.4 Re-Record Inputs Each coarse input level control has a further five positions in which it can select signals for re-recording via the microphone input bus lines. There are ten of these lines numbered 1A to 5A and 1B to 5B. The A series being available on the left hand channel of each Microphone Cassette and the B series being available on the right hand channel of each Microphone Cassette.

  • Page 16: Presence Control

    the setting is then most easily made by observing the relevant limiter meter and setting the hold control so that the meter reads the desired number of decibels of compression or limiting. If it is found necessary to alter the hold control during a take, the calibration on the panel will be found useful but it is not as precise as the meter reading.

  • Page 17: Output Circuits

    Since the setting of the fader for standard level through the channel is at 5dB loss, it is desirable that this amplifier should have 5dB of voltage gain. The Amplifier E serves this purpose. Its input impedance is 50kΩ and its output is designed to feed a pan pot and echo level control in parallel.

  • Page 18: Cue Channels

    Solo Facility 3.6.1 This facility allows signal from any microphone channel to be fed to the left- hand monitor loudspeaker in the control room without interfering in any way with any recording which is in progress, the right hand loudspeaker being simultaneously muted.

  • Page 19: Microphone Cassette

    particular case it is scaled from –40dB in the penultimate anticlockwise position to 0dB in the fully clockwise position. 3.8.2 Echo Selector The output of the level control is fed to an Amplifier B and thence to the echo selector whereby it may be connected to any of four echo bus lines. There is also an “off”...
  • Page 20 Microphone Cassette and if the fader is set to 0, the output of the Microphone Cassette will be a standard level signal as measured by the limiter meter which is a peak reading meter. Depending on the signal waveform this may be substantially less than a standard level signal as measured on a V.U.
  • Page 21 4.2.6 Presence Controls These are identical with those used in the Microphone Cassette and are similarly positioned. 4.2.7 Band Pass Filter This is similar to that in the Microphone Cassette but two of the components in the part of the circuit which is used for trimming the high frequency response of the cassette have slightly different values.

  • Page 22: Contents

    Power Supply The Power Unit is identical with that used on the Microphone Cassette. The Group Cassette consumes about 57mA at +20V and 93mA at –20V. MAIN CASSETTE Contents Each Main cassette contains two identical main channels, the signal in each of which can also be fed, if desired, to an auxiliary stereo channel and to the two cue channels.
  • Page 23 contacts which connect together the track announce control bus lines. These lines are linked to the studio playback cassette where their connection operates an active control relay RL1, which in turn quiets the control room loudspeakers. The track announce signal bus line also originates in the Studio Playback Cassette and, when the oscillator key is in its central position, receives the amplified output from the operator’s microphone through a pre-set level control.
  • Page 24 5.4.2 Auxiliary Stereo Bus Lines If auxiliary stereo working is provided for, the left and right auxiliary stereo bus lines which are fed from each channel of each of the multi-track Main Cassettes are connected to the left and right inputs of an extra Main Cassette. The main outputs of this feed the auxiliary stereo tape machine(s).
  • Page 25 Amplifier G in the cue channel associated with the main channel under consideration, this being a point, isolated from the main channel, which has the correct signal level at a low source impedance (see 5.3). The “input to main” position derives its signal from the output of the Amplifier C1 at the input to the Main Cassette and therefore measures the signal level after the input gain control.
  • Page 26 level signals and since the normal internal working level of –10dB is required within the Mixer, these transformers have a step down ratio of 3.16 : 1. Re-Record Two re-record selector switches are provided. The left hand one, S1, selects tracks 1 and 2 (or 5 and 6) from Machine 1 (or from Machine 2).
  • Page 27 A Monitor track Level Control (general purpose attenuator) calibrated 0 to –40dB and off An Amplifier B. An Amplifier C2. A Pan Pot. Two Amplifiers B feeding output bus lines. These lines are paralleled in pairs within the cassette and the pairs are paralleled in the track monitor cassette socket of the main cableform.
  • Page 28 7.2.2 Input Amplifiers The bus bars selected by K2 are connected to the inputs of fixed gain Amplifiers C2. 7.2.3 Monitor Selectors These are two five position switches, one for each channel. Each has an “off” position, a normal position, position for Cue 1 and Cue 2 and a synchronous replay position.
  • Page 29 gain of about 2dB giving an open circuit output voltage of –8dBV for solo and –14 dBV for main channel signals. The coupling between the output of each first amplifier (Terminal A), and the input of the corresponding second amplifier (Terminal B) is made through a chain of three resistors in series on the Solo Relay board.
  • Page 30 control (labelled KEY) grouped with the OP and AM pre-sets and which has a range from about 3dB to 23dB. On each side of the loudspeaker quiet key is a spring return push button which mutes the corresponding channel. 7.2.10 Loudspeaker Line Output Each channel has an Amplifier R and an output transformer as used for all other line outputs.
  • Page 31 7.4.2 Correlator The circuit of this is described in 12.18. It gives a positive indication on the meter if the signals at the two inputs are in phase and a negative indication if they are out of phase. If the two signals are completely uncorrelated the meter will remain on its centre zero.

  • Page 32: Echo Output Circuit

    Cue Output The Cue output circuits are identical with the echo output circuits in the Group Cassette which are described in 4.3 Cue to Loudspeakers In order that the cue signals shall be available on the control room monitor and studio playback loudspeakers, the V.U. Meter outputs on the cue output Amplifiers R are both connected to both the input selector switches on both the Control Room Monitor Cassette and the Studio Playback Cassettes.
  • Page 33 Loudspeaker Channels 8.2.1 Input Selectors Each channel has an input selector with positions identical with those of the monitor selectors in the Control Room Monitor Cassette (Section 7.2.3). 8.2.2 Level Controls The outputs of the input selectors are passed through Band Pass Filters and coupling capacitors to level controls.
  • Page 34 8.3.2 Gain The microphone signals are amplified by an Amplifier A4 with 40dB of gain followed by an Amplifier A3 with 20dB of gain. 8.3.3 Studio Talkback, ‘Normal’ The Studio Talkback Normal button PB1 when operated connects the output of the Amplifier A3 (B9) through a 10kΩ mixing resistor R102 to VR9 and R107 in series.
  • Page 35 artist manager have the same facilities. The independent pair of contacts in the Artists Manager’s microphone is used to operate active relay RL3 to quiet the control room monitor loudspeakers and to operate active relay RL4 to switch the Amplifiers N from normal signal to talkback. 8.3.8 R.F.
  • Page 36 8.6.2 Artist Manager to Cue By operating the selector switch in the box marked Artist Manager the output from this microphone can be diverted from the talkback circuit to either Cue 1 or Cue 2. The box contains separate level controls for the two circuits and these controls feed to cue bus lines through Amplifiers B.
  • Page 37 connected to the input of the Amplifier M in place of the track announce signal. The Amplifier M has three outputs. With the oscillator level control set to zero the outputs are as follows: Output No. 1 is at a level of –35dBV which, when the oscillator key is in the “on”...
  • Page 38 Cassette Requirements These are as follows: 1 Microphone Cassette for every 2 microphone channels and every two echo return channels, up to a maximum of 12 cassettes frames should be made to accommodate more if necessary. 1 Group Cassette for every 2 group channels with a maximum of 2 cassettes. Each Group Cassette also provides 2 echo output channels so that it is possible that the number of Group Cassettes is determined by the number of echo outputs required.

  • Page 39: Inject

    This has a multi-pin socket for each cassette and provides all the inter- cassette signal connections. It has two spade terminals on flying leads for connecting the circuit earth chassis to a terminal on the frame. 9.4.2 Power Cableform This has a 6-pin socket of which only two pins are used for supplying 50V a.c.
  • Page 40 Temperature Coefficient Within 0.02% per °C. Maintenance No routine maintenance is required or recommended. In case of failure, a replacement control amplifier should be ordered. 10.3 50 V A.C. Supply This is provided by a step-down transformer T1, the primary of which is fed from the a.c.
  • Page 41 A 60V moving iron meter is permanently connected across the 50V a.c. output. This has a mark at 50V and a black band extending from 48.5V to 53.3V. An hours meter is permanently connected across the output of the a.c. stabiliser to indicate the total use of the unit.
  • Page 42 ratio normally determined sensibly by the input amplifier alone and which is therefore as high; as can be obtained by any technique. CIRCUIT DESCRIPTION. INDIVIDUAL BOARDS 12.1 Presence 12.1.1 Component Parts There are four sections, the complete circuit being shown in Drawing No. BE.200.
  • Page 43 12.1.4 Operation It will be seen that the resonant circuit selected by S1, together with a series resistor R2, can by means of S1, be connected across various amounts of either the forward or the feedback circuit to give 2, 4, 6, 9 or 10dB of boost or cut at the resonant frequency.
  • Page 44 12.3 Amplifier Type A 12.3.1 Types There are three types of Amplifier A. Type A1 had a voltage gain of 10dB and is used in the re-record circuits of the Track Monitor Cassette. Types A3 and A4 have voltage gains of 20 and 40dB respectively and they are used in the microphone circuits of the Studio Playback Cassette.
  • Page 45 12.4.3 Circuit Description Each amplifier acts as a voltage to current converter. In the following description the circuit reference of the “a” – “A” amplifier are used. The base of transistor VT1 is biased to about –12.5V by a potential divider R3, R4 across the – 20V supply.

  • Page 46: Poling

    12.5.4 Output The output impedance is low but varies with the setting of the A40 control in the case of the Amplifier C1. The output will deliver at least 2.4V r.m.s. into a load of 3kΩ. The output of an Amplifier C is in phase with the input of the Amplifier B preceding it.
  • Page 47 12.7.2 Types There are four types designated F1 to F4. These involve small differences in value of two of the components controlling the high frequency cut-off characteristic to enable the overall response of the cassettes in which they are used to be trimmed.
  • Page 48 12.8.3 Circuit Description This differs from the Amplifier E only in minor details. As unity gain is required in the present case, only a low value of resistor is required between the collector of the VT2 and the emitter of VT1. In this case it is an adjust on test component only.
  • Page 49 12.10.3 Circuit Description VT1 and VT2 are d.c. coupled as in Amplifier E. The base of VT1 is held at about –12.3 V by R1 and R2. The d.c. conditions of VT2 are then determined by R4 and R5 whilst the a.c. gain is determined by R4 and R5 shunted by R6 and AOT1 in series.
  • Page 50 potentiometers with screwdriver adjustment. Each of these is a 2-gang unit to control the two loudspeaker channels. The maximum quieting is determined by R13 and R14 and is about 20dB. The minimum quieting is about 8dB. OUT ‘B’ is connected to the correlator input selector switch. 12.11.4 Output The output is intended to work into an impedance of 50kΩ...
  • Page 51 equipment. The associated switching is such that the track announce signal does not get fed to the Microphone Cassettes. 12.13.2 Circuit The circuit diagram is shown on Drawing No. AE215. 12.13.3 Circuit Description VT1 and VT2 are d.c. coupled as a unity gain amplifier. The input base is held at about –3.5V by R2 and R3.
  • Page 52 provided that BT2 is switched off. When a control signal is fed to the base of VT2 to turn it on, R8 (1kΩ) is shunted across the collector load introducing about 20dB of loss. The d.c. coupled virtual earth amplifier VT5, VT6 has a feedback resistor R20 equal in value to the input resistor R10 and the overall gain is therefore unity.
  • Page 53 The high current outputs will give about 15.5 to 18.5V into a load of 1kΩ. The output voltage may fall by up to 1 volt when the load resistance is reduced to 20Ω. 12.15.5 Mechanical The unit is assembled on a plug-in board which is housed in the Main Power Unit case.

  • Page 54: Output Amplifier

    12.16.4 Mechanical The unit is assembled on a plug-in board of the same size as the Q Amplifier Power Unit and is housed in the same frame. 12.17 Output Amplifiers Type R 12.17.1 Purpose This is a line output amplifier which receives signal at a level of –10dB and, in conjunction with an output transformer type B35A it delivers a signal at zero level into a load of 2kΩ...
  • Page 55 reference to a zero level signal at an output of the Mixer. Since this corresponds with a –10dBV signal within the Mixer the actual range of input levels to the correlator unit is –50dBV to –70dBV. 12.18.2 Circuit The circuit diagram is shown on Drawing No. DE221. Board B94/1 carries the circuitry for Forward Path A and Low Level Meter A.
  • Page 56 is 42dB so that the signal level at this point is –8dBV for a reading on the meter of – 40VU. The level at this point is unaffected by AOT8. VT11 is an amplifying stage, the current in which is adjusted by AOT19 and AOT20 to give the required potential at the collector, and the gain of which is adjusted by AOT22 to give the required overall gain.
  • Page 57 12.18.3.5 Inhibitor The inhibitor input (terminal 1) is connected through resistors R9. R15, R21 and R27 to the collectors of transistors in Forward Path A and through R88, R94, R100 and R122 to the collectors of transistors in or associated with Forward Path B. R122 and the associated transistor are actually on the gates board which has not yet been described.

  • Page 58: Power Supply

    amplitude of ±9V at the base of VT51. VT51 is an emitter follower, the output of which is applied to a smoothing circuit R134 C28 having a time constant of about 0.4 second. The output of this passes to a super alpha pair VT52, VT53 and thence to the centre zero meter.
  • Page 59 that under all signal conditions the emitter follower VT27 operates correctly with an adequate current margin. The negative stabilisers are similar but require npn instead of pnp transistors. 12.19 Amplifier Type U 12.19.1 Purpose This receives signal at a level of –10dBV and provides an output to drive a standard VU meter to a scale reading of 0 VU.
  • Page 60 12.20.3 Circuit Description The oscillator is of the Wien bridge type, C1 and C2 being the capacitors, R4 (2.4kΩ) being the resistor in series with C2 and R3 plus the parallel combination of R1 and R2 (= 200 + 2.7k//12k = 200 + 2.2k = 2.4kΩ) being the effective shunt resistance across C1.
  • Page 61 12.21.2 Circuit The Circuit diagram is shown on Drawing No. BE225. 12.21.3 Circuit Description The input transformer receives a 50V supply at 50 or 60Hz and has a centre tapped secondary, the overall ratio being 1:1.04. The centre point of the secondary winding is connected to the 0 V line.

  • Page 62: Function

    12.22 Limiter 12.22.1 Purpose The so-called limiter is provided with a switch which enables it to be used either as a 2:1 compressor or as a limiter with up to 20dB of compression or limiting. 12.22.2 Circuit The circuit diagram is shown in Drawing No. CE226. 12.22.3 Circuit Description The input is coupled by C1 to a pair of transistors VT1, VT2 connected as a...
  • Page 63 The collectors of VT5 and VT6 are connected to the bases of another long trail pair VT7 and VT8, the long tail being the collector impedance of VT9. This stage provides a gain of about 7dB for the push-pull signal and rejects the push- push signal.
  • Page 64 For the purpose of temperature compensation, the base of VT23 is held by the emitter of an emitter follower VT20, the base of which is held at about –15.5V by R47 and R48. VT25 has its base held at –6.7V by R57 and R58 and, for normal signal levels, is cut off.
  • Page 65 12.23.3 Circuit Description The circuit comprises eight transistors connected as four similar but not identical long tail pairs. Taking the simplest of these, VT3 and VT4 are provided with a long tail R7 connected to the +20V line. The base of VT3 is connected, through R6, to a +9V rail, obtained from R22, R23 and R24 connected across the +20V supply.
  • Page 66 In series with the operating push buttons is a section of the talkback key. In the position marked 3 on the circuit diagram (key with lever down) the connection from the buttons to RL4 is broken, so that talkback is routed, by a second section of the key, to the alternative talkback system, without any change in the studio loudspeaker system.

  • Page 67: Track Monitor Cassette

    signal bus is fed with signal from the channel in which the button is pressed, through a 16kΩ resistor. The solo signal bus is connected to the left hand channel terminal B interconnection, between the Solo Relay board and the Amplifier P board. It therefore feeds into the virtual earth amplifier BT4, VT5 of Amplifier P, which has a feedback resistor of 20kΩ.
  • Page 68 as in Active Relays. The operation is as in Active Relays, the outputs being in the present case terminals 1 and 2 which connect to the corresponding terminals of two Amplifiers N. 12.27 Amplifier Type D 12.27.1 Purpose This is used as the input amplifier in the Microphone Cassette. conjunction with a control to Drawing No.
  • Page 69 to realize the required 50kΩ input impedance. The first amplifier is therefore preceded by an emitter follower VT1. R1 and R2 provide the required input impedance and hold the base at about +6.7V. R3 serves to reduce the risk of r.f. oscillation.
  • Page 70 12.29.4 Output The until will deliver up to 20mA at 50V with a total regulation of about 0.2V. The ripple does not exceed 10mV. 12.30 Injection Unit (Specification) Any equipment connected to an inject socket should comply with the following: 12.30.1 Screened Transformers should be used both in the send and the return leads.
  • Page 71 Appendix A Modifications Solo and Cut modification. Abbey Road consoles received a modification to the “Solo” switch and were also fitted with a “Cut” switch on every channel, along with an LED indication of the operation of either the Solo or Cut switch. The circuit of these modifications is detailed in drawing No.TG12345/AR008.

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