Focusrite ISA One Analogue by danijoss
The ISA One is Focusrite’s baseline offering on the ISA series, a tabletop microphone preamp and DI. It is a remake of their classic ISA110 preamp (ISA stands for Input Signal Amplifier), albeit sans EQ, the company’s first design by none other than its original founder, Rupert Neve. In fact, the ISA One comes with a “certificate” that specifies that it uses the same Lundahl input transformer and Zobel network as the original unit.
On the front panel one finds the main gain pot (a stepped pot with 10dB increments) and a continuous trim that can provide an extra 20dB, for a total of 80dB of gain that should be more than enough for anything one can throw at it. Five switches facilitate the highpass filter, insert point, gain range, polarity reverse, and 48V phantom power. Further down one finds the source selector for microphone, line, and instrument sources.
One of the more interesting features is the variable impedance mic input: the ISA One offers four different values, offering a classic low setting of 600 Ohms, the original 110 module impedance of 1400 Ohms, and two higher-Z settings at 2400 and 6800 Ohms. The manual is very helpful in this regard, describing the theory and applications of this feature. At this point, it is worth noting that the 6800 Ohm setting is extremely useful for ribbon microphones; in fact, at this price, no other high impedance transformer-coupled mic preamp comes to mind.
The DI circuit also boasts two impedance settings, and can use a dedicated output (as well as the front-panel through out), so that it can work independently of the mic preamp. This, coupled with the unit’s portable nature, facilitated by the leather handle on the top, makes the unit surprisingly helpful as a high quality front end for singers that also play an instrument, or for DI-feed and cabinet miking live or in project studios. There is also a phones amp, that can be fed either from the unit’s output, along with an external input, to facilitate zero-latency monitoring for DAW-based setups, or use a separate stereo input to effectively work standalone. The ISA One therefore represents great value, as it packs a mic preamp, a DI, and a phones amp, all capable of operating independently, in one box priced very competitively. Additionally, a digital output option card is available, but this review is based on the analogue version.
The last front panel feature is the fairly large VU meter, along with two LED peak meters for the mic and DI channels. They can be easily calibrated without opening the unit, as well as able to monitor pre- and post-insert, and do so accurately.
The rear panel facilitates connections for all the above features, as well as power and calibration trimmers. The connections are all fully balanced, in XLR’s or 1/4” TRS jacks. The unit is about half-rack wide, but, including the handle, is somewhere between 2 and 3U high. Thus, also accounting for the slanted front panel, it is inconvenient to rack mount a pair of them with a tray, although possible. It is worth mentioning that the ISA Two, in fact a 1U rackmountable 2-channel version, fulfils this need, but sadly it has no VU meters.
The manual states a maximum headroom of 24dBu, therefore more than enough for pretty much any possible use (most audio interfaces clip at +18/+20dBu). There is no output transformer, thus the unit sounds cleaner than the rack mount ISA110 (with the EQ bypassed). While it cannot be said to be a “clean” preamp as such, and is certainly not of the transparent variety, its coloration is rather subtle. In fact, some may find it just enough to liven up certain very linear ADC inputs, without causing so-called “stacking” problems (the accumulated effect of overdubbing many sounds with the same “coloured” preamp). At least as far as the mic preamp is concerned, the ISA One certainly sounds professional and more expensive than it actually is. In fact, the ability to get the most out of any microphone using the variable impedance settings further adds to its suitability for a variety of tasks: while the “110” setting of 1400 Ohm is a fine starting point, the Low-Z point can work very well with vintage condensers in particular (some were designed with Low-Z preamps in mind), whereas the highest setting, as previously mentioned, works great with ribbons. In a nutshell, the higher the input impedance, the flatter the mic’s response becomes, as the transmission line approaches voltage transfer, and the level comes up as well. The lower it is, the more the characteristics of the mic’s output circuit and transformer will be allowed to filter the sound. The rules of bridging connections apply, but, when it is so simple to just click a button and compare the four options on offer, it is so much simpler to judge this aurally and creatively take advantage of intentional mismatches.
The DI is also very clean, although a less impressive sound overall; it can, however, be useful, when amplification is also required, in conjunction with direct injection, for example very weak instruments that would benefit from the boost.
The balanced insert point is also a great addition: it enables long cable runs (such as those when using patch bays) to external EQs and compressors, with the convenience of the front panel bypass switch that enables process in/out comparisons. Fully balanced inserts are a rarity at this price range (in fact, even in all but the more expensive mixing desks), yet extremely helpful, especially for people working without a console, and thus going direct to ADC. Even with a console, as we use it, it becomes very handy to use both insert points (say, the ISA One’s for an eq, and the desk’s for a compressor), thus having independent bypass switches for both.
We have had the unit for three years, and it has seen a lot of use. It has regularly been compared against other preamps at various price points, from budget Golden Age Project Pre73’s, to Universal Audio’s tube 610’s, to API 512c’s and the occasional Avalon, the rackmount ISA110, as well as referenced against many transformerless preamps. It more than holds its own against the more expensive devices. What it is obviously lacking is an output transformer and a “post” trim, so that the transformer can be pushed into saturation and the level brought back down to safe ADC levels (although the is not a concern if it is used with a high-headroom console).
We routinely use the ISA One with the following mics: a Blue Bottle tube mic with the B7 or B6 caps (LDC), an EV RE20 large diaphragm dynamic (both the latter with vocals), an Audix i5, and a Royer R101 ribbon (with a guitar amp).
The Blue sounds absolutely lovely with the Low and “110” impedance settings; smooth, balanced, and warm. In contrast, the 610 sounds very sibilant for some voices. The 512c yields a very interesting but completely different, more upfront and aggressive sound (unsurprisingly so). The Pre73, depending in the settings, sounds marginally less detailed but could be made warmer, although this mic certainly doesn’t need any more of that going on, especially if worked close.
The RE20 benefits from the higher impedance settings, with the best sounds on “110” and medium. However, the 512c usually outperforms the ISA with this mic, as it translates the punch and immediacy of the large dynamic much better. The 610 is interesting when pushed to distortion, and we do feel the need to brighten it up a bit using its onboard shelf eq. The Pre73 is not bad at all, but clearly far from the best choice.
The i5 on the guitar amp (overdriven sound of a MESA/Boogie rectifier through a 2x12” cab - our house rig) works very well with all preamps. The ISA again seemed to be the most neutral, with all impedance positions other than “low” being potentially useful. The Pre73 is particularly nice sounding in this example, especially if we need this mic to work on its own. Hold on to that thought…
The ISA One is by far the only preamp at our disposal that can make the R101 sing… the high-Z setting brings up this “ribbony”, extremely smooth yet fast high end and made the amp sound like itself, if only a touch too dark (a R121 would be slightly more balanced in this application). If combined with the i5, positioned as close to coincident as possible to the Royer, this changes our previous assessment: the 512c is usually a better complement to the Royer/ISA combo, filling in some fizz and plenty of percussive feel.
All of the mics sound sufficiently thinner with RME preamps from the Octamic 2, as well as with the pre's on the Focusrite/Audient 2802 that we use.
Note: every session is different, and the above remarks are averages of sorts, the sort of thing that usually happens. Sometimes, the odd part calls for something different. We use the above information in practice as a starting point, or when we don’t have time to test many combinations.
The ISA One is almost certainly a worthwhile addition for home and project studios, as it is extremely versatile, subtly coloured, but not so much to make it picky. It is also worthwhile for any studio that does not have preamps with high input impedance (again, 6800 ohms at the highest setting), as it will make most ribbon mics shine and deliver their true potential. Street price has gone down a bit since its introduction, and the mic pre alone is worth the price. If someone also happens to need the independent DI and phone amp, this is a real bargain. Although it has been presented as a primarily “character” preamp, we find it to be certainly less so than other popular units, yet with enough transformer attitude to set it apart from the “clean” models. Between the preamps we own, we’d choose the word “sweet” (as opposed to “warm”, “aggressive”, “clear”, “punchy”, “fuzzy”, etc, although we do hate do use such expressions). We use it as general purpose, as in fact it rarely fails, although statistically we find we use it more for vocal work than instruments, and always with our ribbons.
Despite its flexibility, we feel that output and line input transformers would have been nice, allowing the unit to work as a line amp, perhaps in the master buss insert point of a desk for coloration, with the added bonus of the VU meters (though that would certainly raise the price quite a bit). As it is, the line in/out path is so clean one barely notices a difference at all when used in this fashion. Perhaps a mod kit will become available at some point. Our last complaint is the handle: we concede it may be useful for people carrying it around, but otherwise it’s just in the way of things that want to sit on top of it. We have yet to open the unit and see if there exists an easy way to non-destructively remove it. And while we’re at it, the slanted face may be marginally nice if the unit sits on tabletop, but is certainly annoying for people that prefer rackmounting, yet don’t want the ISA Two (again, VU meters, or simply because they need just one channel).
Tech specs from the manual:
Mic Input Response
•Gain range = 0dB to 60dB in 10dB steps + 20 dB of variable gain
•Input Impedance, variable as follows:-
Switched Impedance setting Equivalent Input Impedance at 1kHz
Low = 600 Ohms•EIN (equivalent input noise) = -126dB measured at 60dB of gain with 150 Ohm terminating impedance and 22Hz/22kHz band-pass filter
ISA110 = 1400 Ohms
Med = 2400 Ohms
High = 6800 Ohms
•Noise at main output with gain at unity (0dB) = -97dBu measured with a 22Hz/22kHz band-pass filter
•Signal to noise ratio relative to max headroom (9dBu) = 106dB
•THD at medium gain (30dB) = 0.0009% measured with a 1kHz -20dBu input signal and with a 22Hz/22kHz band-pass filter
•Frequency response at minimum gain (0dB) = -0.5dB down at 10Hz and -3dB down at 125kHz
•Frequency response at maximum gain (60dB) = -3dB down at 16Hz and -3dB down 118kHz
•CMRR=98dB (Channel 1, 1kHz, maximum gain with +24dBu input)
•Crosstalk Channel to Channel: with 10dB@1kHz input to chA, chB output =104dBrA. With 10dB@10kHz input to chA, chB output = 84dBrA
Line Input Response
•Gain range = -20dB to +10dB in 10dB steps + 20 dB of variable gain
•Input Impedance = 10kOhms from 10Hz to 200kHz
•Noise at main output with gain at unity (0dB) = -96dBu measured with a 22Hz/22kHz band-pass filter
•Signal to noise ratio relative to max headroom (24dBu)=120dB
•Signal to noise ratio relative to 0dBFS (+22dBu) = 118dB
•THD at unity gain (0dB) = 0.001% measured with a 0dBu input signal and with a 22Hz/22kHz band-pass filter
•Frequency Response at unity gain (0dB) = -0.3dB down at 10Hz and -3dB down at 200kHz
Instrument Input Response
•Gain range = 10dB to 40dB continuously variable
High = 1MOhms•Noise at minimum gain (+10dB) = -92dBu measured with a 22Hz/22kHz band-pass filter
Low = 300kOhms
•Noise at maximum gain (+40dB) = -62dBu measured with a 22Hz/22kHz band-pass filter
•THD at minimum gain (+10dB) = 0.001% measured with a 0dBu input signal and with a 22Hz/22kHz band-pass filter
•Frequency Response at 10dB gain with -10dB input = 10Hz-100kHz +/- 0.6dB
•Frequency Response at 40dB gain with -40dB input = -2.5dB down at 10Hz and 0dB at 100kHz
High Pass Filter
•Roll off = 18dB per octave 3 pole filter
•Fixed Frequency 75Hz measured at the 3dB down point
•Moving coil (MC) meter is factory calibrated to 0VU = +4dBu with 1kHz sinewave. With the VU Cal button pressed, the meter can be adjusted on the rear panel to allow 0VU to equal +10dBu to +26dBu with the centre detent being equal to +22dBu
•Peak LED meters calibrated in the detent position for 0dBFS = +22dBu, calibration is adjustable on the rear panel to allow 0dBFS to equal +10dBu to +26dBu