Page updated:
18 Feb 2002
Advanced Mixer Design. |
Page updated: |
Since the readership of WW covers a broad spectrum, familiarity with the recording process could not be assumed, and a concise account of it was published alongside the technical material below. This will be added to the site soon; watch for the "Multitrack Recording" department of The Institute.
MIXER DESIGN.
Competition to sell studio time becomes more cut-throat with
every passing week, and it is clear that advances in console
quality must not harm cost-effectiveness. The only way to
reconcile these demands is to innovate and to keep a very
clear view as to what is really necessary to meet a demanding
specification; in other words the way forward is to use
conventional parts in an unconventional way, rather than simply
reaching for the most expensive op-amp in the catalogue.
The technical problems that must be overcome in a
professional mixing console are many. A large number of
signals flow in a small space and they must be kept strictly
apart until the operator chooses to mix them; crosstalk must
be exceedingly low.
There will be up to 64 input channels, each with many stages,
all have the potential to add distortion and noise to the
precious signal. Even summing these signals together, while
sounding trivially easy, is in practice a major challenge. In
short, requirements are much more demanding than those for the
most expensive hi-fi equipment, because degradation introduced
at this stage can never be retrieved.
Fig l. System diagram of complete mixing console, showing
division into inputs, group monitor contributions and master
modules. Routeing matrix determines which group of inputs
shall be fed to a given track on the multi-track tape machine.
Several channels share one effects device.
The major functions of consoles are largely standardised,
although there is much scope for detailed variation. Fig 1
shows the system diagram.
Fig 2. One input channel. Gain control is 70dB and tone
control is standard Baxandall shelving type with addition of
mid-range lift and cut. Two auxiliary sends are shown.
Fig 2 shows a typical input channel for a mixing console. The
input stage provides switchable balanced mic and line inputs;
the mic input has an impedance of 1K - 2K, which provides
appropriate loading for a 200 Ohm mic capsule, while the line
input has a bridging impedance of not less than 10K. This stage
gives a wide range of gain control and is followed immediately
by a switchable high-pass filter (usually -3dB at 100Hz) to
allow removal of low-frequency disturbances.
The tone-control section (universally known in the audio
business as "EQ" or EQualisation) typically includes one or
more mid-band resonance controls as well as the usual shelving
Baxandall-type high and low controls. Channel level is
controlled by a linear fader and the panpot sets the stereo
positioning, odd group numbers being treated as left, and even
as right. The PFL (prefade-listen) switch routes the signal to
the master module independently of all other controls; a logic
bus signals the master module to switch the studio monitoring
speakers from the normal stereo mix bus to the PFL bus,
allowing any specific channel to be examined in isolation.
Fig 3. Block diagram of typical group module, showing
switching between direct output and tape replay for monitoring
purposes.
Fig 3 shows a typical group module and Fig 4 the basics of a
master section; a manual source-select switch allows quality
checking of the final stereo recording and two solid-state
switches replace the stereo monitor signal with the PFL signal
whenever a PFL switch anywhere on the console is pressed.
Fig 4. Block diagram of master module, with tape send/replay
switching and electronic PFL switching.
AUXILIARY SENDS; FOLDBACK AND EFFECTS
Sends come essentially in two kinds: prefade sends, which are
taken from before the main channel fader, and postfade sends,
which take their feed from after the fader, so that the final
level depends on the settings of both. There may be anything
from one to twelve sends available, often switchable between
pre and post. Traditionally, this means laboriously pressing a
switch on every input module, since it is most unlikely that a
mixture of pre and post sends on the same bus would be useful;
the 3200 minimises the effort by setting pre/post selection
for each bus from a master switch that controls solid-state
pre/post switching in each module.
Prefade sends are normally used for "foldback"; i.e. sending
the artist a headphone feed of what he/she is perpetrating,
which is important if electronic manipulation is part of the
creative process, and essential if the artist is adding extra
material that must be in time with that already recorded. In
the latter case, the existing tracks are played back to the
artist via the prefade sends on the monitor sections.
Postfade sends are used as effects sends; their source is
after the fader, so that the effect will be faded down at the
same rate as the untreated signal, maintaining the same ratio.
The sum of all feeds to a given bus is sent to an external
effects unit and the output of this returned to the console.
This allows many channels to share one expensive device (this
is particularly applicable to digital reverb) and is often
more appropriate than the alternative of patching a processor
into the channel insert point.
"Effect returns" may be either modules in their own right or
a small subdivision of the master section. The returned
effect, which may well now be in stereo, the output of a
digital reverb., for example, is usually added to the stereo
mix bus via level and pan controls. EQ is also sometimes
provided.
MICROPHONE INPUTS.
It is now rare to use input transformers to match the
low-impedance (150-200 Ohm) microphone to the preamplifier,
since the cost and weight penalty is serious, especially when
linearity at low frequencies and high levels is important. The
low-noise requirement rules out the direct use of op-amps,
since their design involves compromises that make them at
least 10 dB noisier than discrete transistors at low
impedance.
Fig 5. Low-noise microphone amplifier with wide gain range
and balanced line output. Transistors in first stage avoid
noise problem of op-amps.
This circuit, shown in Fig 5, therefore uses a balanced pair
of low-noise, low-Rb PNP transistors as an input stage,
working with two op-amps to provide load-driving capability
and raw open-loop gain to linearise signal handling.
Preamplifier gain is spread over two stages to give a smooth
0-70dB gain range with the rotation of a single knob. This
eliminates the switched 20dB attenuator that is normally
required to give the lower gain values, not only saving cost
and complication, but also avoiding the noise deterioration
and CMRR degradation that switched attenuators impose. The
result is an effective input stage that is not only quieter,
but also more economical than one using specialised low-noise
op-amps.
THE EQ STAGES.
Fig 6. Parametric mid-band EQ stage. EQ and centre
frequencies are independently variable, being set by the
parameters of the state-variable filters.
A signal is tapped from the
forward path, put through a state- variable band-pass filter
which allows control of centre-frequency and Q, and then added
back. To improve noise performance, the signal level at all
locations (in all conditions of frequency, Q, and boost/cut)
was assessed, and it proved possible to double the signal
level in the filter over the usual arrangement, while
maintaining full headroom. The noise generated is thus reduced
about 6dB.
Fig 7. Standard panpot circuit at (a) showing how pull-up
resistor Rlaw draws current through wiper contact resistance, which
is usually greater than the end resistance of the pot, limiting
maximum attenuation. Arrangement at (b) uses NICs to replace
pull-up to modulate law with panpot setting. Left/right
isolation increased from -65dB to -90dB.
THE PANPOT
This limitation is removed in the Soundcraft active
panpot shown in Fig 7b by replacing the pull-up with a
negative-impedance-converter that modulates the law-bending
effect in accordance with the panpot setting, making a close
approach to the sine law possible. There is no pull-up at the
lower end of the wiper travel, when it is not required, so the
left-right isolation using a good-quality pot. is improved
from approx -65dB to -90dB. This concept has also been made
the subject of patent protection.
SUMMING TECHNOLOGY.
There is, however, danger in assuming that a virtual earth is
perfect; a typical op- amp summer loses open-loop gain as
frequency increases, making the inverting input null less
effective. The 'bus residual' (i.e. the voltage measurable on
the summing bus) therefore increases with frequency and can
cause inter-bus crosstalk in the classic situation with
adjacent buses running down an IDC cable.
Increasing the number of modules feeding the mix bus
increases the noise gain; in other words the factor by which
the noise of the summing amplifier is multiplied. In a large
console, which might have 64 inputs, this can become
distinctly problematic. The Soundcraft solution is to again
exploit the low noise of discrete transistors coupled to fast
op-amps, in configurations similar to the mic preamps. These
sum amplifiers have a balanced architecture that inherently
rejects supply- rail disturbances, which can otherwise affect
LF crosstalk performance.
Fig 8. Left: Virtual-earth summer at (a) effectively eliminates
cross-talk, since there is almost no signal at the summing
point. Voltage-mode circuit at (b) allows cross- talk.
Balanced virtual-earth summing circuit at (c) requires a
separate inverter for each channel to provide the anti- phase
signal.
As a console grows larger, the mix bus system becomes more
extensive, and therefore more liable to pick up internal
capacitive crosstalk or external AC fields. The 3200 avoids
internal crosstalk by the use of a proprietary routeing matrix
construction which keeps the unwanted signal on a bus down to
a barely measurable 120dB. This is largely a matter of keeping
signal voltages away from the sensitive virtual-earth buses.
Further improvement is provided by the use of a relatively low
value of summing resistor; this also keeps the noise down,
although since it drops as the square-root of the resistor
value, at best, there is a clear limit to how far this
approach will work before drive power becomes excessive; 4.7K
is a reasonable minimum value.
External magnetic fields, which are poorly screened by the
average piece of sheet steel, are rejected by the balanced
nature of the 3200 mix buses, shown in Fig 8c. The operation
is much the same as a balanced input; each group has two
buses, which run physically as close together as possible and
the group reads the difference between the two, effectively
rejecting unwanted pickup. The two buses are fed in antiphase
from each input, effectively doubling the signal level
possible for a given supply voltage. Overall mixing noise is
reduced by 3dB, the signal level is 6dB up and the noise,
being uncorrelated for each bus, only increases by 3dB.
The obvious method of implementing this is to use two summing
amplifiers and then subtract the result. In the 3200, this
approach is simplified by using one symmetrical summing
amplifier to accept the two antiphase mix buses
simultaneously; this reduces the noise level as well as
minimising parts cost and power consumption. The
configuration is very similar to that of the balanced mic amp.,
and therefore gives low noise as well as excellent symmetry.
SOLID-STATE AUDIO SWITCHING.
Fig 9. Hard switching with JFETs in voltage mode (a) and with
analogue gates in the current mode (b), which prevents gate
elements from being driven outside their voltage
capabilities.
Secondly, there is channel muting; this not a hard switch,
since an unacceptable click would be generated unless the
signal happened to be at a zero-crossing at the instant of
switching; the odds are against you. The 3200 therefore
implements muting as a fast-fade over 10ms; this
softens transients into silence while preserving
time-precision. It is implemented by a series-shunt JFET
circuit, with carefully timed and synchronised ramp voltages applied to
the FET gates.
PERFORMANCE FACTORS.
The choice of device is also critical, for cost
considerations discourage the global use of expensive chips.
In a comprehensive console like the 3200 with many stages of
signal processing, this becomes a major concern; nonetheless,
after suitable optimisation, the right-through THD remains
below 0.004% at 20dB above the normal operating level. At
normal level it is unmeasurable.
Words= 2695
A large mixing console arguably represents the
most demanding area of audio design. The steady advance of
digital media demands that every part of the chain that takes
music from performer to consumer must be near-perfect, as the
comfortable certainty that everything will be squeezed
through the quality bottleneck of either analogue tape or vinyl
disc now looks very old-fashioned. 
The auxiliary sends of a console represent an extra mixing
system that works independently of the main groups; the number
and configuration of these sends have a large effect in
determining the overall versatility of the console. Each send
control provides a feed to an auxiliary bus; this is
centrally summed and then sent out of the console.
The microphone preamplifier is a serious design challenge. It
must provide from 0 to 70 dB of gain to amplify deafening
drum-kits or discreet dulcimers, present an accurately
balanced input to cancel noise pickup in long cables and
generate minimal internal noise. It must also be able to
withstand +48V DC suddenly applied to the inputs (for
phantom-powering the internal preamps in capacitor mics) while
handling microvolt signals. The Soundcraft approach is to use
standard parts, which are proven and cost-effective through
quantity production, in new configurations. The latest mic
preamplifier design, as used on the 3200, is new enough to be
covered by patent protection.
Since large recording consoles need sophisticated and complex
tone-control systems, unavoidably using large numbers of
op-amps, there is a danger that the number of active elements
required may degrade the noise performance. A typical mid-band
EQ that superimposes a +15 dB resonance on the flat unity-gain
characteristic is shown in Fig 6.
To give smooth stereo panning without unwanted level changes,
the panpot should theoretically have a sine/cosine
characteristic; such components exist, but they are
prohibitively expensive and so most mixing consoles use a dual
linear pot. with its law bent by a pull-up resistor, as shown
in Fig 7a. This not only gives a mediocre approximation to the
required law, but also limits the panning range, since the
pull-up signal passes through the wiper contact resistance
(usually greater than the end-of-track resistance) and limits
the attenuation the panpot can provide when set hard left or
right.
One of the biggest technical challenges in console design is the
actual mixing of signals. This is done almost (but not quite)
universally by virtual-earth techniques, as in Fig 8a. A
summing amplifier with shunt feedback is used to hold a long
mixing bus at apparent ground, generating a sort of audio
black hole; signals fed into this via mixing resistors
apparently vanish, only to reappear at the output of the
summing amplifier, as they have been summed in the form of
current. The elegance of virtual-earth mixing, as opposed to
the voltage-mode summing technique in Fig 8b, is that signals
cannot be fed back out of the bus to unwanted places, as it is
effectively grounded, and this can save massive numbers of
buffer amplifiers in the inputs.
There are two main applications for
electronic switching in console design. The first is "hard"
switching to reconfigure signal paths, essentially replacing
relays with either JFETs (Fig 9a) or 4016-type CMOS analogue
gates which, since they are limited to 18V rails and cannot
handle the full voltage swing of an op-amp audio path, must be
used in current-mode, as shown in Fig 9b. Note that when gate 1
is off, gate 2 must be on to ensure that a large voltage does
not appear on gate 1 input. Full-voltage range analogue gates
do exist but are very expensive.
Primary requirements of modern consoles are very low noise and
minimal distortion. Since a comprehensive console must pass
the audio through a large number of circuit stages (perhaps
over 100 from microphone to final mixdown) great attention to
detail is essential at each stage to prevent a build-up of
noise and distortion; often the most important trade-off is the
impedance of the circuitry surrounding the op-amp, for if this
too high Johnson noise will be increased, while if it is too
low an op-amp will show degraded linearity in struggling to
drive it.
