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The ISA features four classic, transformer-based microphone pre amps
with switchable impedance and direct Instrument Inputs are combined
with an optional eight channel 192kHz ADC. An all-new, Soft Limiter
circuit precedes the A/D, providing the perfect path into any DAW,
Digital Console or direct to Hard Disc Recorder.
The input stage provides enhanced control and creativity allowing you
to switch between four carefully selected input impedance settings. The
original ISA 110 setting is supplemented by three further impedance
settings to either perfectly match the pre-amp with any microphone and
maximise level, or to use different settings creatively to shape the
sound of the microphone being used. Should the need arise to place
extra processing between pre-amp and conversion, insert points are also
featured on every channel.
Protecting the ADC, Focusrite's Soft
Limiter is a new optical circuit design, entirely preventing any
digital overs, whilst reducing to an absolute minimum the undesirable
distortion artefacts that common limiter circuits typically generate.
This limiter requires the optional ADC to be fitted in order for it to
operate.
The ADC embodies cutting-edge conversion technology, encompassed within
Focusrite circuitry, providing eight high quality conversion channels,
at sampling frequency rates of 44.1, 48, 96 and 192kHz. Running at
96kHz allows for up to 16 (2 x 8, parallel) digital outputs. Digital
output formats are dual 9-pin D-sub (AES/EBU, SPDIF) and dual ADAT
light-pipe.
Full digital metering is provided for every channel, alongside moving
coil peak meters, designed to catch even the fastest transients.
A second ISA428 analogue unit can be connected utilising the four extra
ADC input channels on the rear of the unit containing the optional ADC.
This makes full use of the eight channels provided and expands
front-end capacity from four pre amps and eight digital outputs to
eight Pre amps and eight digital outputs.
- Four ISA-series transformer-based pre-amps
The
benefits of Focusrite's classic pre-amp topology include superb
common-mode rejection, an excellent overload margin, an extremely low
noise floor and super-wide bandwidth. - Dedicated analogue insert points
Switchable
insert points are also featured on every channel, allowing routing
(between pre-amp and output) to additional external processing. - Accurate moving coil peak meters
Clear metering designed to catch even the fastest and most elusive of transients. - Four front panel instrument inputs
Instant access on the front fascia and no separate DI box required. - Eight line inputs
Four
additional A-D converter inputs combine with the four pre-amp inputs to
provide eight line inputs, routable discretely to the eight-channel A-D
converter option. - Eight Channel optional 192kHz A/D conversion card
With
the additional four ADC inputs, two ISA428's can be used with a single
ADC, providing eight channels of pre-amplification, A-D conversion and
VU metering. - Custom Global A-D converter soft clip limiter
Focusrite's
custom optical design protects the A-D converter by preventing digital
overload. It requires that an A-D converter be installed. - Switchable Impedance
Ideal
for mic matching or extended microphone creativity. Settings include
the "Vintage ISA110" setting, providing the original Focusrite
signature sound. - Variable Focusrite high pass filter
Provided on every channel, this is the same filter as featured on the original ISA110 and the Focusrite forte console.
Specifications
Inputs
Transformer-based ISA design
Gain range: Line = +/- 18dB, Mic = 0 to 60dB (both in 6dB steps), Inst = +10 to +40 dB variable
nput impedance: Line = 10k ohm, Mic = Variable - 600, 2k4 and 6k8 ohm + original ISA 110 settings, Inst = >1M ohm
Analogue Performance
Noise: Line = -96dB, Mic = -128dB (EIN with 150 ohm input resistance at 60dB of gain)
THD: Line = 0.003% with 0dBu, 1kHz input and 20Hz to 22kHz band-pass filter, Mic = 0.003%
Analogue Input and Output connectors (balanced)
D.I inputs: 1/4 inch Jack (unbalanced)
Mic inputs: XLR (balanced)
Line: TRS Jack (balanced)
Insert Sends and Returns: TRS Jack (balanced)
Additional ADC inputs: XLR (balanced) Line level
Analogue Outputs: XLR (balanced)
Digital Output Option
AES/EBU and S/PDIF dual 9-pin D-types
Optical - ADAT Lightpipe
Bit depths 16-, 20- or 24-bit
Frequency options 44.1, 48, 88.2, 96, 176.4 and 192kHz
Clock options Standard or ?256 x?
Meter - 0 dBFS = +22dBu
Digital Performance
SNR - 120dB A-weighted
Weight
7.36kg
16.2Ibs
Dimensions
484mm (W) x 85mm (H) x 250mm (D)
19" (W) x 3.5" (H) x 9.8" (D)
The ISA Pre
The ISA microphone pre-amplifier was first introduced back in 1985.
This transformer-based pre-amplifier formed the first half of the
ISA110 mic-pre and EQ module on Focusrite's critically acclaimed Forte
console. This console, and the later Studio console were hailed by many
as the finest sounding consoles ever built.
The ISA microphone pre-amplifier went on to form the cornerstone of all
ISA and RED Focusrite products. The topology has never changed, except
for the addition of a variable impedance circuit, allowing ISA users to
either perfectly match the pre-amp with any microphone, thus maximizing
level, or to use different settings creatively to shape the sound of
the microphone being used. The original ISA110 setting was supplemented
by three further impedance settings.
The Focusrite pre amp is renowned for its transparency, but there's
also that subtle warmth that transformer core saturation contributes.
This seeming contradiction of warmth plus beautifully open,
unrestricted high frequencies, is a classic hallmark of the mid 80's
Focusrite designs.
With processors whose bandwidth is strictly limited to the audible
(20Hz to 20kHz) bandwidth alone, significant roll-off can still occur,
affecting the higher audible frequencies of the signal. Therefore, in
order to capture every subtle nuance, exactly as it occurs whilst
avoiding any roll-off, the pre-amp must operate over a far greater
bandwidth. The extremely broad frequency response of Focusrite pre-amps
- close to 200kHz at the top end - guarantees accurate phase response
across the much narrower range of human hearing, the roll-off point
being moved entirely outside of the audible hearing range.
All critical inputs are
transformer-coupled, providing a dual gain structure - an expensive
option, but the optimum route to high quality signal handling.
Focusrite use Lundahl LL1538 transformers.
Also originally employed was a 'Zobel network' to counteract the
reactance of the primary coil of the transformer. A Zobel network is a
damping circuit that is applied across the secondary of the mic input
transformer. This damping network tailors the naturally occurring high
frequency resonant point of the input transformer. The Zobel network
featured in ISA and RED pre amps was set by ear, has never changed, and
as such is a key contributor to the Focusrite sound.
Finally, all the components featured in the ISA pre-amp were always
chosen with a specific frequency response in mind. Rather than
accepting the effect of the components chosen, the designer worked
diligently with the values of every component in a relentless effort to
achieve the desired response.
FAQs
Q: What are the basic features of the ISA 428?
A: Four Focusrite mic pre's, eight line inputs, four instrument inputs, optional 8-channel 192kHz A/D conversion.
Q: Which applications is the ISA 428 suitable for?
A: The ISA 428 can be used as a multi-channel high
quality front end for Digital Audio Workstations, allowing
multi-channel recording to HD. Equally it can be used simply as the
perfect interface/ A/D converter for synths/other line level devices.
It also provides additional channels for anyone who has run out of mic
pre's on their analogue console (either live or recording,) and is
especially useful as a source of additional mic pre's for digital
consoles.
Q: Which Focusrite pre is featured on the ISA 428?
A: It's the original Focusrite transformer-balanced
mic pre that featured in the classic analogue Focusrite consoles in the
1980s. This is also the classic pre that features in the ISA 430
Producer Pack.
Q: Do the pre's have the usual phantom power and High Pass Filter controls?
A: Yes, and more, each pre has switchable mic
impedance so that you can match to your chosen mic's impedance, or
'mismatch' for creative 'input response colours'. Also there are
inserts per channel, plus sweepable HPF, phase reverse, and phantom
power.
Q: What's significant about the impedance switching for each pre?
A: Each pre can either be matched perfectly to any
microphone, (vintage or modern,) or offset to offer a variety of
'response colours' by interacting with any particular microphone. The
impedance of each pre amp is switchable (via a single switch labelled
'Impedance') between 4 settings: original ISA 110 (Zobal network
influence for the classic Vintage Focusrite sound,) Low (600 Ohms,
tends towards flat and tight sounding,) High (2.4k, relatively open,)
and Higher (6.8K, relatively lively, great for room ambience.)
Q: Are insert points featured?
A: Yes, switchable in- or out- of circuit on each of channels 1-4.
Q: What are the four extra inputs on the left hand side of the front fascia for?
A: They are unbalanced inputs which enable you to
easily connect unbalanced sources like guitars/basses to the unit
without the need for an external DI box.
Q: Why are there eight line inputs but only 4 mic pre's?
A: For two reasons. Firstly it means you can route 8
line level sources to the 8-channel 192k converter with a single unit.
Secondly, you can convert the 4 pre system to an 8-pre system with 2
units, see below.
Q: Why is it called "428"?
A: Because it's easy to convert a 4-pre-to-digital system to an 8-pre-to-digital system.
Q: Can I use 2x ISA 428s with 1x A/D card? If so, how does the routing work?
A: Yes. Mic pre's 1 to 4 on the first unit route
directly (via the limiter) to digital outputs 1, 2, 3, 4. Mic pre's 5
to 8 on the second unit then route out of their respective analogue
outputs, and loop back in to the line inputs 5, 6, 7, 8 on the first
unit. These then route directly (via the limiter) to digital outputs
5-8.
Q: But if I run an 8-channel system how can I monitor levels?
A: Easy. Each 428 includes 8 output meters, as well as 4 channel peak-reading input meters.
Q: What's the specification of the A/D option?
A: AES, (both single and dual wire specs.) SPDIF,
ADAT? formats, sample rates selectable between 44.1, 48, 88.2, 96,
176.4 and 192kHz, (ADAT is max 96kHz of course, via 2 ports,) 16, 20,
and 24 bit with adaptive dithering, internal or external word clock,
and 256X clock, S/N Ratio better than 120dBfs ?A-weighted'. Connections
are via 2 x 9-pin D-type connectors and standard lightpipes, word clock
is via BNC in and out.
Q: Does the ISA 428's A/D option feature word clock as standard?
A: Yes, word clock may be fed in via a BNC connector on each A/D to allow syncronisation to any word clock master source.
Q: Why do I need word clock anyway?
A: When using multiple pieces of digital equipment it
is necessary to make sure that their bit-streams are all in sync. In
order to do this all equipment need to be synchronised to a common word
clock system. Somewhere in this system a word clock master must be
dictating the word clock for the rest of the equipment ('word clock
slaves') to follow. Failure to sync all pieces of digital equipment to
a common word clock source will result in audible clicks and glitches
in programme material. Note that the 428 regenerates word clock at its
BNC output, further boosting word clock stability.
Q: What if my system only operates at 16 bit, 44.1kHz?
A: No problem, the ISA 428 features adaptive dithering
and supports most sample rates, (simply select the rate you need from
the ISA 428's front panel,) so you can use an ISA 428 with a 16, 20 or
24 bit system operating at 44.1, 48, 88.2, 96, 176.4 or 192kHz.
Q: How many rack spaces does ISA 428 take up?
A: The ISA 428 is a 2U device.
Q: What rear panel connections are featured?
A: The ISA 428 has 4 XLRs for microphone input, and 8
XLR line level inputs. There are 4 balanced XLR analogue outputs, plus
separate balanced quarter inch send and return insert sockets per
channel. Plus digital connections if the optional A/D is fitted of
course, see above. Finally there's a voltage-switching power socket to
connect to the internal power supply.
Q: Is the ISA 428 a Class A device, and why is that important?
A: Yes, the ISA 428 is a Class A device. Why? Class A
is a type of amplifier design in which you have a standing DC current
running through your amplifier circuits all the time. As the signal
comes along you vary what you're taking from that, rather than
switching between supplying a positive current for one half of the
waveform and a negative current for the other half. This results in the
ability to represent audio in a more linear (distortion free) manner
all the way through the circuit. Cheaper processors use IC amplifiers,
which run close to Class B and don't have the same standing DC current,
which means the transistors inside the chips switching off and on,
inevitably resulting in a less linear performance.
Q: Should balanced connectors be used with the ISA 428?
A: Yes, where possible. Alternatively, if using an
unbalanced instrument source, you can connect to the four unbalanced
1/4" inputs.
Q: What's the ISA 428's bandwidth? Does it have the same kind
of spectacular bandwidth that has given the Red and ISA range units
their reputation for 'open-ended' sound?
A: Yes. The bandwidth is the same as the classic Focusrite units of old: 10Hz-200kHz!
Q: Is there an optional digital input card?
A: No, because the ISA 428 is primarily a 'front end'
product. In other words, the only devices which are likely to be
connected to the 428's inputs are analogue sound sources such as
microphones, guitars etc.
Q: Why is the 24 bit/192kHz specification important?
A: An A/D converter works by sampling the audio
waveform at regular points in time, and then quantizing those values
into a binary number, which relates to the number of bits specified.
The quantized signal must then be passed through a D/A converter before
it becomes audible. In simple terms, the D/A essentially joins the dots
plotted by the A/D converter when the signal was first converted to
digital. The number of dots to join, combined with how little those
dots have been moved, determines how accurate the final signal will be
compared to the original. The greater the sample rate and bit rate, the
more accurate the whole digital process is. So 24 bit/192kHz
performance will ensure more accurate digital transfer of your audio
information compared to the old 16-bit/44.1kHz standards. This is
especially important if further digital signal processing is to be
applied to the signal once converted to digital, as any mathematical
operations taking place on the data, (for example as a result of a gain
change, or dynamic effect process,) may result in quantization and
rounding errors. The higher the resolution of the digital data, the
smaller the audible effect of these errors.
Q: What is dithering? Why do I need it?
A: When dropping down from e.g. 24 bit to 16 bit,
quantizing errors occur, (because 24 bit sampling involves more samples
than 16 bit, so when you reduce the bit depth the extra samples have
'nowhere to go.') At high signal levels these errors are random and not
audible, but at lower signal levels the errors correlate more closely
to the audio and become audible as distortion. Dithering effectively
?randomises' the truncation errors at lower levels, causing the 'least
significant bit' distortion to disappear.
Q: Can I retrofit a digital board to an analogue ISA 428 at a later date?
A: Yes, and you can do it yourself - it can easily be
retro-fitted at any time without any soldering etc, just a few screws
to undo, and one clip-connector to join to the main PCB.
Q: How can the ISA 428 operate with the ADAT lightpipe format
at 96kHz? I thought the maximum sample rate for this format was 48kHz?
A: Not any more. The ISA 428 supports the new 96kHz ADAT specification, using two discrete optical ports.
Q: How many digital outputs can I use at the same time?
A: It depends on which sample rate you choose.
At 48kHz or lower: 8 AES + 8 S/PDIF (or 2 x 8 AES) + 2 x 8 ADAT = max. 32 outputs simultaneously.
At 96kHz: 8 AES* + 8 S/PDIF (or 2 x 8 AES) + 8 ADAT = max. 24 outputs simultaneously.
At 192kHz: 8 AES* + 8 S/PDIF (or 2 x 8 AES,) no ADAT** = max. 16 outputs simultaneously.
- Both AES single-wire and split-wire configurations are
supported; split wire will obviously reduce the maximum number of
simultaneous outputs.
- *At 192kHz: the ADAT ports are muted.
The digital outputs can always be fed with any mix of mic, line, inst. inputs.
Q: When the A/D is used, are my analogue outputs available for use?
A: Yes. The 4 line outputs can run simultaneously with all of the ADAT, AES and S/PDIF outputs.
Q: Do I need to buy an option cable to use either of the A/D cards?
A: Yes; ADAT optical cables are available from many
sources, and Focusrite offer their own 8-channel 9-pin to 4 phono (RCA)
connectors S/PDIF cable and 9-pin to 4 XLR connectors AES-EBU cable.
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