ALLEN HEATH 21 Series Compact Stereo Mixers Instruction Manual

Allen & Heath’s new
stereo 21 SERIES

21 Series Compact Stereo Mixers

Allen & Heath proudly unveil their exciting new range of compact stereo mixers – the 21 SERIES. Stylish in looks and superb in performance, these four models offer outstanding value for money.
Available in 6, 12, 18 and 24 input versions, the 21 SERIES boasts the useful feature of a separate mono output derived from the main stereo mix.
Another Plus is the incorporation of an extensive interface facility to ensure accurate compatibility with external equipment, and enables two or more mixers to be linked together for single flexible operation. A separate power supply unit keeps nasty hum fields at bay, letting the sound come through clean and clear.
The 21 SERIES Stereo Mixers by Allen & Heath are available now.
A & H 21 SERIES
Packed with innovative features, this new value-for-money range of quality stereo mixers is ideal for “on the road” PAsystems club installations, stereo recording and community broadcasting.
AUDIO QUALITY
Advanced low-noise circuitry boasts a high slew rate of 13 V/uS, permitting faithful reproduction of transient information. Each mixer is supplied complete with its own external power supply to ensure that hum fields are kept to a minimum. Phantom power is available as an optional extra. As the comprehensive PFL system overrides all monitor outputs and metering, easy checks may be made on all incoming and outgoing signals.
MONO OUTPUT
Invaluable as an extra on-stage foldback mix or as a feed to a mono PA system, permitting the main stereo faders to be used as sub-groups or outputs to a stereo recorder. This feature will be of particular interest to the AV or broadcast user who may need a dedicated mono output for simultaneous video recording, or mono transmission.
PRO AND SEMI-PRO CAPABILITY
21 SERIES mixers are supplied with outputs set for OVU= +4dBm, but external links are provided so that outputs and tape monitor inputs can be adjusted by the user (or the dealer) to ensure compatibility with equipment designed for lower operating levels. A thoughtful facility that will make life a lot easier for the small budget studio. All-steel construction provides strength and good electrical screening.
PLUGABILITY
Rear panel sockets provide bus access points for stereo and mono outputs, cue and echo sends, and the PFL system. Thus, two or more 21 SERIES mixers may be conveniently linked for single operation, greatly increasing the input potential and all-round flexibility of this new range.
INPUT CHANNEL FEATURES

• Electronically-balanced transformerless microphone input on XLR socket, switchable to a high impedance balanced line input when required.
• Effects insertion point that can also be used as a line level input or as a spare effects send.
• 20dB pad switch.
• 3 Band equaliser with mid-frequency sweep from 400Hz to 6kHz.
• Pre-fade cue send and post-fade echo send.
• Channel pan pot and PFL Selector.
• LED peak overload indicator and smooth travel, long throw fader, providing a high degree of operator control.

OUTPUT SECTION FEATURES

• Twin illuminated VU meters with associated PFL override LED indicator.
• Two echo returns, each with PFL, cue send, level and Pan.
• Cue and echo send masters, both with PFL.
• Main Left and Right outputs and mono output on XLR sockets (all featuring PFL selectors).

MONITOR FACILITIES
The two independently-controlled stereo outputs to headphone and monitor amplifier can be switched to Mono. An off-tape return from a stereo recorder may also be selected, enabling A/B monitoring during recording.
TECHNICAL SPECIFICATION
MIC INPUT +20dB to +60dB (PAD -20dB) 1.5kohm
Equivalent Input Noise – 125.5 dBm
LINE INPUT -5dB to +36dB (PAD -12dB) 10kohm
(Further 10dB gain available at fader)
EQUALISATION HF 10kHz (shelving) +/- 15dB
MF 400Hz- 6kHz (Q = 1.4) +/- 13dB
LF 100Hz (shelving) +/- 15dB
HARMONIC DISTORTION typically <0.05%
OUTPUT Unbalanced 30 ohm source impedance
Recommended load greater than 2kohm
Maximum output +21dBv
DIMENSIONS Model 621 458x475x100 Model 1221
674x475x100 Model 1821 891x475x100

STEREO 21 SERIES
OWNER’S HANDBOOK

SPECIFICATION:

Construction: steel main panel and covers, hard stove enamel paint finish with epoxy ink screenprinted legend. Semimodular electronic printed circuit assemblies. IC op-amp and discrete transistor audio system of transformerless design.
Size & Weight:

(14.0)
1221| 673
(710)| 476
(620)| 90
(160)| 14.0
(19.0)
1821| 889
(930)| 476
(620)| 90
(160)| 18.5
(24.0)
2421| 1105
(1160)| 476
(620)| 90
(160)| 23.5
(29.0)

bracketed figures = in shipping carton.
Connectors: Channel input, stereo and mono outputs; 3 pin XLR-type locking. All others 1/4″ jack.
POWER SUPPLY: External, type MPS7 110-120v or 220-240v AC.
Phantom power option +48v MPS7P. (Symmetrical system)
OUTPUT = +15v DC, smoothed and regulated 0.5 amps per rail
INPUT = AC mains voltage at 50/60Hz
220/240v setting = 212v to 250v AC 30VA max
110/120v setting = 106v to 130v AC
PHANTOM POWER available as an option (specify MPS 7-P) = +48v DC, smoothed and regulated at 50mA
CONNECTIONS: MAINS INPUT = flying 3-core lead 1.5m long DC OUTPUT = flying 4-core lead 1.5m long terminated
in a 5-pin XLR female in-line plug.
pin 1 = -15V DC
pin 2 = 0V
pin 3 = 0V
pin 4 = +15V DC
pin 5 = PHANTOM POWER (if fitted)
FUSES:
220/240V setting = 500mA quick blow 5 x 20mm
110/120V setting = 1A quick blow 1 1/4″.

GENERAL

All Outputs:
Normal Operating Level = +4dBv
Maximum Output Level = +2ldBv 20Hz to 20kHz
Output Impedance = 30ohms
all outputs are protected against damage from accidental short circuits.
Maximum Gain From Channel Input to Stereo Output: = +70dB
Frequency Response:
Channel Input to Stereo Output = 30Hz to 30kHz +/-1dB
Equaliser Characteristics:
HF = 15dB continuously variable at 10kHz
MID = +/-13dB continuously variable at any frequency between 400Hz and 6kHz.
Q factor = 1.4
LF = +/-15dB continuously variable at 100Hz
Total Harmonic Distortion:
Channel Input to Stereo Output typically 0.05% THD
Noise:
20Hz to 20kHz MEASURED AT STEREO OUTPUT. Figures quoted are RMS
noise referred to system operating level.
Microphone Equivalent Input Noise = -125.5dBv
Output with Master Faders Closed = -94dB ref 0VU
Output with Master Faders Raised, All Inputs Closed better than
-66dB ref 0VU
DIN RMS
621 . . . -68dBm
1221 . . . -66dBm
1821 . . . -63dBm
Output with One Input Raised, mic source = 200ohms, EQ flat, gain minimum = -61dB ref 0VU
Crosstalk:
Stereo Separation – better than 56dB at 100Hz
better than 75dB at 1kHz
better than 66dB at 1OkHz
Channel Inputs:
XLR sockets. Electronically balanced suitable for safe connection of external phantom power supply of up to 48VDC.
XLR pin 3 = hot
pin 2 = cold
pin 1 = screen
Input Impedance – mic = 1000 to 1500 ohms
line = 10kohms
Gain – mic = continuously variable from +20dB to +60dB
line = continuously variable from -5dB to +35dB
Pad – mic = -20dB
line = -12dB
Insertion Points – unbalanced 1/4″ stereo jack sockets
tip = return
ring = send
outer = earth
normal operating level = +4dBv
Peak Overload Indicator – turns on at 3dB before clipping.
Echo Return Inputs:
mono 1/4″ jack sockets
Maximum Gain to Stereo Outputs = +16dB
Input Impedance = greater than 4kohms
Stereo Tape Input:
stereo 1/4″ jack socket
tip = left
ring = right
outer = earth
Input Impedance = 40kohms
Gain to Monitors – 0dB or +12dB selectable by an internal connector for either +4dBv or -8dBv normal operating level.
Stereo and Mono Outputs:
unbalanced XLR sockets
pin 3 = signal
pin 2 = pin 1 = earth
Normal Operating Level = +4dBv or -8dBv selectable by an internal connector.
Cue and Echo Outputs:
unbalanced 1/4″ jack sockets mono
Normal Operating Level = +4dBv
Maximum Gain from Input = +10dB
Monitor Output:
stereo 1/4″ jack socket
tip = left
ring = right
outer = earth
Normal Operating Level = +4dBv
Maximum Gain = +10dB
Headphones Output:
stereo 1/4″ jack socket
capable of driving stereo phones of 4ohms to 600ohms impedance
Tie Buss Svstem:
Stereo Input – stereo 1/4″ jack socket
tip = left
ring = right
outer = earth
Gain to Stereo Output = 0dB
Stereo Output – stereo 1/4″ jack socket
Normal Operating Level = +4dBv
Mono Input – mono 1/4″ jack socket
Gain to Mono Output = 0dB
Auxilliary Input – stereo 1/4″ jack socket
tip = cue
ring = echo
outer = earth
Gain to Output = 0dB
Auxilliary Output – stereo 1/4″ jack socket
Normal Operating Level = +4dBv
PFL Input – stereo 1/4″ jack socket
Gain to PFL Output = 0dB
tip = audio
ring = DC
outer = 0V
DC connect to 0V = PFL activated
floating = PFL not activated
PFL Output – stereo 1/4″ jack socket
Normal Operating Level = 4dBv
DC PFL activated = 0V
PFL not activated = -8VDC into 1MEGohm load
Metering:
illuminated VU meters 0VU = +4dBv
PFL Busy Indicator:
turns on when PFL is activated

PANEL CONTROLS

Input Channel
Place the mixer in front of you and familiarise yourself with the various functions of the panel controls. Starting at the top of an input channel the first control is:
a) GAIN
This control adjusts the sensitivity of the first amplificati stage of the microphone/line input. Use this in conjunction with the PFL function, as described in the OPERATION section, to find the correct setting.
b) MIC-LINE
Selects the sensitivity and input impedance of the XLR input socket to suit either microphone or line-level signals.
c) PAD
Reduces the sensitivity of the XLR input socket to enable very high signal levels to be accommodated. Use if the gain control has already been turned down fully and there is still too much signal.
d) HF
Boosts or cuts the high frequency or treble content of an incoming signal.
e) MID FREQ and MID
The first control selects the mid-range frequency to be cut or boosted by the MID control.
f) LF
Boosts or cuts the low frequency or bass content of a signal.
g) CUE
Used to send a proportion of the signal present in the input channel, via the cue master (see OUTPUT SECTION), to an external destination. This auxiliary send derives its signal before the channel fader and is thus unaffected by fader settings. Designed to feed on-stage foldback monitor systems for PA use or to feed headphone/loudspeaker systems in recording/broadcast situations. Can also be used as an effects send to feed external devices, providing it is remembered that the feed will still be present when the channel fader is down.
h) ECHO
Can be used in a similar way to the above control, except that it is wired post-fader and the feed to an external effects device will reduce as the channel fader is lowered. Use in conjunction with the echo master (see OUTPUT SECTION) to set correct operating level.
i) PAN
Use to position the signal anywhere between left and right in the stereo mix.
j) PFL
PFL is an abbreviation for Pre-Fader-Listen. When selected it enables an operator to check the level of a signal present in the input before the channel fader. The prefade signal level is governed by settings of the gain control (a) and equalisation controls (d, e, and f). See OPERATION SECTION.
k) PEAK
The LED (Light Emitting Diode) peak indicator flashes a warning when the signal level is too high and approaching the onset of clipping distortion.

1. FADER
   Four fader settings are marked for each input channel, these are: ‘+10’, indicating the extra 10dB boost available on the fader; ‘O’, (normal operating position) indicating unity gain between input channel and output section; ‘-10’; and ‘infinity’ or channel off.

OUTPUT SECTION

a) VU METERS
The VU meters can read any one of 4 signal sources:
i) PFL: when any PFL button is pressed on the mixer, an LED just above the right VU meter will illuminate (as a reminder), and the meters will follow the selected PFL level (in mono).
ii) The output signal level of a stereo tape recorder when TAPE is selected in the monitor section.
iii) The main stereo mix output from the mixer when STEREO is selected in the monitor section.
iv) The output from the MONO fader when MONO is selected in the monitor section.
b) CUE MASTER
The master control for all the input channel cue sends. When the associated PFL button is selected the VU meters will show the level leaving the mixer at the cue output on the rear panel.
c) ECHO MASTER
The master control for all the input channel echo sends. When the associated PFL button is selected the VU meters will show the level leaving the mixer at the echo output on the rear panel.
d) ECHO RETURNS 1 & 2
These identical sections allow 2 line-level signals (eg the outputs from 2 effects devices) to be added to the stereo mix. Each have a level control, performing the same function as an extra input channel fader, and a pan control, to position the effects signal in the mix as desired.
A cue send control is also provided in each section, so that a proportion of the effects signal can be sent to the musicians’ headphone monitoring or foldback system.
PFL select is available in each section to allow monitoring of the effects signals before committing them to the main mix.
e) LEFT AND RIGHT MASTER FADERS
Control the output level of the stereo mix. Each provided with a PFL switch to allow quality checks, on the left and right mix busses independantly, prior to the final output stages of the mixer.
f) MONO MASTER FADER
Controls the output level of the mono mix.
This mix is a mono sum of the main stereo output, is derived after the left/right master faders, and therefore depends upon their position for output level.
PFL select is available to check the signal supplying the mono output for level and quality.
g) MONITOR SECTION
This section selects the signals available for headphone monitoring via the top panel socket and/or for control room monitoring via the monitor output socket on the rear.
There are separate volume controls for headphones and output to amplifier. Selecting PFL in any section of the mixer will override the monitor outputs.
The three monitor source select buttons (Mono, Stereo, Tape) can be used in permutations described as follows:

rear panel.
Mono + Stereo| Stereo mix
Mono + Stereo + Tape| Stereo tape return
Stereo + Tape| Stereo tape return.

The VU meters, except when any PFL function is selected, will follow the selected monitor source.
We suggest, as a normal procedure, that the Mono button be left depressed. This will allow simple switching between Mono and Stereo by operation of the stereo button alone. Similarly, as can be seen from the above priority table, both Mono and Stereo buttons can be left depressed whilst operating the Tape button for A-B checks.

OPERATION

Objective: To preserve the quality of the original sound.
Assisted by:
Level controls, gain controls, mixer meters, peak indicators, tape meters and tape.
Level control – Operator control changing the signal level between sections of a console system, often a passive device (fader, pot).
Gain control – Operator control changing the shift in signal level between the input and output of an amplifier in one section of a console system.
It is said that a monkey with a typewriter would generate the works of Shakespeare given enough paper and new machines in 6 million hours. How long would it take him to mix “I am not in love’? Those of you with time to spare may like to consider whether an automation system would increase or decrease the time taken. Get to know your equipment and its capabilities and limitations. Particularly important in console systems are gain and level controls used in conjunction with level indicators. There is no way known to beat the physical constraints of thermionic noise and maximum output level within audio amplifier systems but intelligent use of gain and level controls gets acceptable, even excellent results from modest equipment. Allen and Heath consoles have more than the minimum number of controls essential between microphone and tape since this allows presentation of the original sound quality for a wider variety of sound sources having different signal amplitutudes.
Signal dynamic range: The difference in signal level between the loudest and the quietest sections of a programme.
System dynamic range: The difference in signal level between the loudest and quietest signals that can be faithfully reproduced or transmitted by an audio amplifier system.
The worst that can happen:
Start with a piece of programme to be mixed and recorded and two things may happen to reduce the quality of original sound:

1. The audio amplifier system noise is allowed to be greater than the ambient (background) noise of the original sound. Details of the quieter sections of programme are lost.
2. The audio amplifier system maximum signal level for faithful reproduction is exceeded by the original sound. Details of the louder sections of the programme are lost and the audio system adds masking distortions generated by overload of the amplifiers.

Make some simple tests:

• Set up a mic and get someone to play the piano in the studio, set the mic in close to the piano.
• Select PFL on the appropriate mic input, you are hearing only the input pre-amp and equaliser sections of the console.
• Wind up the mic gain control till you get a PFL meter indication of about 0 VU on loud notes. Set the monitor level control to reproduce the tone level of the piano as it is in the studio.
• Can you hear any noise below the programme?
• Can you hear any distortion of the notes?
• Now back off the monitor level and advance the mic gain control till the channel peak indicator flashes only on loud notes.
• Can you hear more or less noise below the programme?
• Can you hear more or less distortion of the notes?
• What is the PFL meter now reading (do not worry it will not burn out)?
• It would be OK to run all signals this hot but for one thing.
• Back off the monitor level some more and advance the mic gain control till the channel peak indicator is on all the time.
• Sounds bad doesn’t it. The gritty distortion you hear added to the programme is the sound solid state amplifiers make when overloaded and is called clipping distortion.
• The one thing referred to above is your friendly piano player hitting the next phrase a little louder – pushing the signal level in the pre-amp above amplifier overload point to produce the horrible sound we just got – once the pre-amp has clipped no amount of hard work later on will cancel it. Your mixer system has a meter calibrated to guide you where to set the mic gain control so that those extra loud phrases come out clean too ?
• The margin of loudness between the 0VU reading and the clipped signal reading is the overload margin of the system.

Going the other way:

• Reset the piano mic 20 feet back.
• Back at the console reset the controls to get 0VU again on loud phrases and reset the monitor level for a true volume in the control room. It probably sounds OK.
• Now ask the piano player to give you his favourite quiet subtle number. Reset the mic gain for 0VU on the PFL meter.
• Notice anything about the noise level behind the programme?
• You are probably losing detail in the quietest phrases and also are bugged by a continual background hiss.
• Do not worry – we are exploring the worst that can happen.
• Hard fact number two – noise added to the programme by the preamp cannot be removed by hard work later on either.
• The optimum mic gain setting for the particular programme and mic placement produces 0VU on loudsection, now you know why the console has meters and peak indicators on it and why there is a PFL switch for every section. These take the guesswork out of setting gain controls.
• The PFL meter tells you the channel signal level as determined by the input gain control. Meter indications of 0VU on loud sections are the maximum that can be used and still retain the overload margin to faithfully reproduce the unexpected changes in loudness from a performer. Run hotter at your peril. Run cooler and you will be adding noise unnecessarily at the preamp stage.
• The channel peak indicator is a permanent, eye catching warning that the overload margin in the channel amplifiers has nearly gone, the longer it stays on and the more often it comes on the closer you are to adding clipping distortion at the preamp (or equaliser) stage.
• At the end of the day your ears are the best and final judge of sound quality and you may decide to override what the meter says. Experience has proved (you may test it yourself) that certain acoustic sources are not reliably analysed for signal level by meters. If in doubt trust your ears.
• Get it right at the input preamp first.

All that has been said about the balance between added noise and added clipping distortion applies to the other parts of a console system not just the preamp. This is why the faders show a zero ‘0’ calibration point from which to work as a start for level balancing and why PFL switches are provided on the output faders as well as on input channels. In each case you can see the signal level at that point in the system and correct your gain and level control settings to preserve the original sound quality.
Tape and tape meters
The audio world rightly gets excited about new developments in tape manufacture that provide lower distortion at existing signal levels or lower noise from the tape itself and occasionally both at the same time. Noise reduction is expensive and some systems need great care in use if other signal degradations are to be avoided.
Do not assume that a signal faithfully reproduced at your console output, nicely beating the meters to 0VU, will come back off-tape so sweet. In the same way that audio amplifiers have limitations on performance so do tape and tape deck amplifiers.
Start with a must and make it a habit.
Use a 1kHz tone generator if available, music synthesiser or FM radio tuned to inter-station noise to feed the console outputs and adjust for 0VU meter readings, check that yourmonitor section is selected for output monitor (stereo), not tape monitor.
On the tape machine select input monitor and adjust the record level for 0VU reading. Record a few minutes of tone. Switch the tape machine to replay monitor and the console monitor section to tape monitor status. Replay the tape tone.
Provided your tape machine is reasonably well calibrated the meters will show the tone replay level to be 0VU. On the console check that the meters show 0VU and adjust the output level controls of the tape deck if necessary. The console inputs for tape signals are preset to a standard signal level (0VU = +4dBm). There are however three standard signal levels that different tape decks may produce:
0VU = +4dBm (1.23V RMS)
0VU = 0dBm (0.775V RMS)
0VU = -10dBV (0.3lV RMS).
Consult the page of this book “Tape deck interface” for guidance on obtaining the best from the tapedeck – console system. Your Allen and Heath console is engineered to work perfectly with all three standard level systems.
The important point is that the signal level between console and tape deck that corresponds to 0VU is the operating level of your studio system and when console and tape machine are in agreement you have a fair chance of making good recordings.
Once again your ears are the final judge of the off-tape quality and may lead you to overrule record level meters. Use the console monitor section “tape” switch to check the off-tape quality. You will usually hear added noise but more importantly listen for added distortion. Tape is operated much nearer its overload point than console system amplifiers. Your console amplifiers deliver rated performance (below audibility) at all signal levels right up to the onset of clipping 18dB above 0VU. The audio amplifiers in most modern tape decks are similarly blameless. Tape however typically generates 3% third harmonic distortion only 8 to 10dB over 0VU recording level and more bad sounding tapes are made as a result of recording too hot than through clipping console amplifiers. Not only that but tape overloads sooner at high and low frequencies than at mid-band frequencies (the 1kHz reference point) so imagine the distortion figures at 100Hz and 10kHz where bass and strings come.
The point of this is – do not expect the hardware to perform the impossible, it is already performing a miracle by comparison with full blown record house equipment of the early days of sound recording, get the console-tape deck interface right and learn the capabilities of the audio amplifiers and tape you use.

TAPE DECK INTERFACE

Your 21 series mixer is supplied with output levels and tape return levels set for 0VU on meters = 4dBm. It is a very simple operation to change operating levels to suit tape machines that require a lower operating standard. The output levels from the stereo and mono outputs and also the input level to the stereo tape input can all be independantly adjusted.
Stand the mixer on rear panel, ensuring safe and adequate support. Remove bottom panel using POZI 2 screwdriver. The illustration above shows the positions of the level-match adjustors. Simply remove the adjustor link and replace the plastic plug the opposite way round. Make sure that the plug is aligned with the pins on the circuit board as you replace it.

69 Ship Street
Brighton BNl 1AE
England
Tel, (0273) 24928
Five Connair Road
Orange) CT 0477
USA
Tel. (203) 795 3594

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