The Focusrite ISA One microphone preamplifier review

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of

Monday December 17, 2012

The Focusrite ISA One certainly has an excellent pedigree. Its circuitry is derived from the Focusrite Forte mixing console, designed by the legendary Rupert Neve. Neve’s designs almost define audio electronics. So if the ISA One has a little piece of Rupert Neve inside, you can expect its sound quality to be impeccable.

The ISA One is a single channel device at a price that will only partially break the bank. It will accept a microphone line or instrument input. Clearly, however, its principal application is as a microphone preamplifier, so let’s take a look at that.

At the top left of the front panel is a gain control. This is not quite intuitive in operation. There is a switch to select a maximum of 30 decibels of gain, or a maximum of 60 decibels of gain. In the '30' position, the rotary switch can select 0, 10, 20 or 30 decibels of gain. In the '60' position, the rotary switch can select 30, 40, 50 or 60 decibels of gain. It’s interesting to note that there are two ways you can achieve 30 decibels of gain. That is the coarse gain control. Alongside that is a fine 'trim' control, which adds additional gain from 0 all the way up to 20 decibels. So we can see that the range of gain of this microphone preamplifier is from 0 dB all the way up to 80 dB, which should cover any possible requirement from the largest bass drum with the most sensitive microphone, to the ticking of a watch at twenty paces.

The other normal features of a microphone preamplifier are of course provided. There is a switch for 48 volt phantom power, a phase invert switch and high-pass filter. There is also a switchable insert point, so if the pristine sound of the ISA One isn’t quite to your liking, you can insert additional signal processing into the chain at this point. The most likely use would be for a compressor.

Focusrite ISA One

Now, as we know, a microphone preamplifier can’t just be a microphone preamplifier these days. It has to have something special or something additional to justify its existence, size and cost. In this case we have a variable input impedance which is labeled on the front panel ‘Z-In’, 'Z' is the normal abbreviation for impedance. This is a single button that can select 4 different impedances, indicated by red LEDs. The low impedance setting is 600 ohms. The setting labeled ISA 110, which is modeled on another piece of equipment in the ISA range, is 1400 ohms. Medium impedance is 2400 ohms. High impedance is 6800 ohms.

It’s worth asking the question why we need to have this switchable impedance. Isn’t there just one value for impedance that would suit all circumstances? Well, as Focusrite themselves point out in the manual, almost all professional microphones are designed to have a fairly low impedance output, somewhere around 150 – 300 ohms and you could consider about 200 ohms to be the norm. The reason why the impedance is lowish is firstly to be able to drive long cables without high frequency roll off due to cable capacitance, and secondly, it’s less susceptible to picking up noise. You could ask the question why can’t the output of the impedance of a microphone be even lower? The answer to that is in the limitations of the phantom power to supply the microphone with current. But 200 ohms in normal circumstances works just fine.

The next question is what should the input impedance of the preamplifier be? If you remember your school science, you might have learnt in the electricity lessons that you get maximum power transfer when the input impedance of an amplifier is the same as the output impedance of the device that is driving it, and this is indeed so. But, there is a problem with that. When we say that the output impedance of a microphone is 200 ohms, we are giving the value at around the 1 kilohertz point. At other frequencies, it is very likely that the output impedance will be different. This is particular so with microphones that use a transformer as the output and even more so with ribbon microphones. So, because effectively the preamplifier that has a low input impedance is imposing a different load on the microphone at different frequencies, this alters the frequency response of the microphone. It’s not going to damage the microphone in any way but it is going to extract a different kind of signal from the microphone than the designers intended. From this point of view, a low input impedance is not good.

Focusrite ISA One

Over time, preamp manufacturers have converged on an input impedance somewhere around 1200 ohms up to about 2000 ohms, which is still relatively low. This is not what you would remotely call high input impedance but it is high enough so that it is not unduly loading the microphone. The inaccuracy in frequency response caused by any kind of loading effect is minimized. So, if you feel that you would like to load the microphone even less and potentially get an even flatter frequency response, you might consider using the medium or the high input impedance settings on the ISA One. The high setting is not all that high, so you are not going to get into any trouble by doing that. It is purely what sounds best to you on the day.

As for the reasons for using the low input impedance setting, one reason for that could be if you had a microphone which had a particularly low output impedance, then it might be appropriate to use the lower input impedance setting on your preamplifier. To be honest, if a microphone has an unusually low output impedance, the designers of that microphone have gone against common custom and practice and they really need to explain in detail why, because the conventional output impedance of round about 200 ohms really has worked fine over a period of decades. The other reason why you might choose the low impedance setting is that the changes it makes in the frequency response of the microphone might just, in your situation on the day with whatever sound source you are using, sound better. So, it’s worth experimenting with this, just to see whether it sounds better to you. In truth, although it certainly is desirable to have equipment which is capable of a flat frequency response, very often it is simply what sounds better that is the correct way to go. The line input facility of this unit is useful and it works well, and other than that there really isn’t a great to say about it. So, lets move on to the instrument input.

This is intended for an instrument like an electric guitar. With a conventional electric guitar of the kind that doesn’t have an internal preamp, which is most of them, then the output impedance of the guitar pickup is pretty high. This means that it needs to be coupled to an input which has a very high input impedance, because even a medium input impedance will load the pickups too much. It simply dulls the sound and you won’t get the crispness you would hope for in that situation. So, connecting an electric guitar to a line input is not likely to produce good results. But here we have an input which is exactly right for an electric guitar. There are two impedance settings, a high setting and there is a low setting. Low is 300 kilo ohms, that’s 300,000 ohms, which is actually pretty high. High is 1 megaohm which is 1 million ohms, which is as high as you could ever possibly need, and the pickup is loaded to a minimal extent.

Focusrite ISA One

So, once again, you would choose whichever setting got the best sound from the instrument. There is a gain control on the instrument input and this ranges from 10 decibels to 40 decibels in one continuous rotary control. Changing impedance can affect level, but in reality it’s the tone that is more important here.

It’s interesting that this instrument input can be used independently of the microphone line input and you can be using the ISA One as a microphone preamplifier and at the same time be using it as a DI for an electric guitar. That is a feature that some people could find useful.

Over on the lower right of the front panel of the Focusrite ISA One, we have a monitoring section which is intended for headphone monitoring. There is a choice here - you can either use it to monitor the output of the preamplifier, or there is a button to select cue mix. With cue mix selected, then you can apply a signal from your digital audio workstation to the cue mix sockets on the back and this is the signal which will be fed to the headphones for monitoring by the performer.

It probably won’t have escaped your notice that there is a fairly large VU meter on the front panel of this piece of equipment. You could ask the question here, what is the function of a VU meter in this context? VU meters are notoriously inaccurate. They measure steady state signals very well, but for a signal which is impulsive, even in the slightest, the meter is not fast enough to respond accurately. Therefore it will under-read on peaks. So, in general, the VU meter is not a very good measuring instrument. However, what the VU meter is good at is telling you that something is going on. So, if there’s a signal going into the preamplifier and the VU is twitching, that means all is well. Other preamps may have a signal-present LED which is useful too, but my preference would be for the VU meter. I could live without it but hey, I can live with it too. To supplement the VU meter there is also a small LED bar graph with just six steps. Even so, that will tell you your peak level. So, it’s like having the best of both worlds.

The Focusrite ISA One has an optional digital output. So without the digital output this is just a conventional preamp and you will connect it to your audio interface, or analog to digital converter. But, you might think that since you’ve gone to the trouble investing in a quality Focusrite product, with Rupert Neve-influenced electronics inside, you might prefer to leave the analogue digital conversion to Focusrite. There would be strong grounds for believing that Focusrite would be able to optimize the A-D conversion to the requirements of the preamplifier. So, if you choose the digital output option, then you will be able to digitize the signal inside the ISA One to a resolution of 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz - and here are the biggies - 176.4 kHz and all the way up to 192 kHz for the ultimate in audio quality. Naturally the unit will sync to a variety of clock sources.

Now, you may have seen some of the marketing materials for this preamplifier and you will find that there are two things that the marketing plays on. One is the use of a Lars Lundahl transformer on the input of the preamp. Why is the transformer important? Well, going back in history, all microphone preamplifiers of professional quality used a transformer on the input.

Focusrite ISA One

Gradually, over a period of years, components became available that allowed transformerless preamps to be designed, and eventually they reached the same level of quality of transformer-based preamps. Some might say they potentially exceeded the ability of a transformer based preamplifier. So, what we have here is a preamp which effectively harks back to an earlier era of preamplifier design. There is every good reason for having that transformer in there, because it is a principal part of that circuit and I am quite sure that Rupert Neve wouldn’t have it any other way. Why is it a Lars Lundahl transformer? Well there a number of manufacturers of transformers and Lundahl is a prominent manufacturer. I’ve used Lundahl transformers myself and I know that they are very effective.

In a circuit that is properly designed, the effect of a transformer on sound quality should be negligible. If you can hear the transformer, then that is a sign of a bad design, or it might be a sign of a design where you are intended to hear the sound of the transformer. There is something to be said for that. In the old days of audio electronics, we couldn’t get away from transformers and we so longed to have a transformer-free signal chain, considering that in the recording process, a signal could go through 10 or 20 transformers through its lifetime. But, now we have a choice. We can choose a preamp which doesn’t have a transformer, or you can choose this preamp which does.

The other interesting feature that the marketing for the ISA One mentions is the bespoke Zobel network. Amplifiers of various kinds, large and small, are often troubled by the load that you attach to the output. So, the amplifier itself might behave perfectly well and reasonably but then, if you attach a tricky load (or no load) to the output, the amplifier could start oscillating. If you’ve ever experimented with electronic circuitry, you can easily experience this. So what we ideally want is to protect the output to the amplifier so that you can connect any load to it, however unreasonable, and the amplifier will not oscillate. This is the function of the Zobel network. So the Zobel network is actually quite a common thing. It could in theory affect the sound quality. I think we can expect here that Focusrite has taken the trouble to ensure that the Zobel network matches the intrinsic sound quality of the other components of the preamplifier.

Focusrite ISA One

Ultimately the $64,000 question is what does this preamp sound like? The principle defining feature of the sound of this preamplifier is its cleanliness and clarity. If you want the preamp to respect your sound source, whether it’s a microphone, a line input or an instrument, then this is the preamp that will do that. This is not, in my opinion, a 'personality' preamplifier. This is one that takes the signal as it is and does what it has to do with it, without adding anything and without taking away. If you want a preamplifier to have character, then you should probably be looking at a vacuum tube preamplifier because the ISA One most definitely isn’t that. But then, with a vacuum tube preamplifier, along with the supposed warmth that you get, you will get distortion. Sometimes warmth can be superficially pleasurable, but ultimately the effect of the distortion component of the signal quality can become tiring. But not with the Focusrite ISA One because it takes the signal as it is and amplifies it. That’s what it does. Also, if you have a really good vacuum tube microphone, there is a strong argument to say that you really ought to keep the signal as it is. What would be the point, for instance, of using a vintage U47 microphone and plugging it into a vacuum tube preamp? A vacuum tube preamplifier is going to change the sound of the microphone and the sound is why you chose the microphone in the first place. So, if you have a really good microphone, the chances are you will get a better quality of signal by using a preamplifier such as the Focusrite ISA One. In fact, I would go as far to say that if your microphone isn’t so good, then that is when you need the vacuum tube preamplifier.

In summary, this is an excellent product. It’s exactly what you would expect from a high quality Focusrite preamp. The price is reasonable and you will be fully satisfied with your purchase, I am sure of that.

You can hear the Focusrite ISA One in this video...


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