As I have revamped my theremin recording setup, I thought I’d describe it. It’s about as minimal as I can achieve.
curly black cable = audio from theremin, blue cable = audio to amp
First I’ll describe my “recording studio”. It’s the middle of a long living room. When I’m playing, there is a dining table and chairs to the right, under the extension roof. To the left are sofas and a TV. Behind me is my amp and subwoofer, and a bookshelf with my effects pedals et cetera. In front is a computer desk with an iMac on it. The windows are double glazed but not soundproof and the floor is wood. There are no sound-damping tiles as you would find in a professional studio. It is not acoustically well suited to recording. I can hear when it rains, when a vehicle drives by (I’m on a bus route), and when birds sing. Various things rattle when I play low notes.
Consequently I prefer to record directly from the audio signal rather than putting a microphone in front of the amplifier. I do have a microphone – a Shure SM57 – for recording things other than the theremin. It’s the model that recording studios buy by the dozen for miking acoustic instruments. For the theremin I use a DI Box, a small device that you plug in between the theremin and the amplifier that splits the signal so that one output can go to the amplifier and the other can plug directly into your recording device. The input and the amp output accept regular 1/4” mono jacks (guitar cables) and the recording output is a balanced XLR socket (for microphone cables). The advantage of using balanced cables is that they prevent noise sneaking into your audio in a long cable, so you can have your recording setup well away from the theremin without worrying about introducing hiss to the recording. As it happens, my amp and sub both have XLR outputs, but the amp does add a bit of hiss, so I prefer to use a DI box before the signal gets to the amp. The one I use (on Peter Pringle's recommendation) is marked "Chord Passive DI Box with Noise Filter 173.294UK" – it is a rebadged Leem FDR60. It is also sold as the QTX Sound Passive Filter 173.294. Which one you can get locally probably depends on where in the world you live.
From the DI Box (or microphone) the sound goes into an iRig Pro. This is where magic happens and the analogue audio signal is transformed into a digital signal. The plus points of the iRig Pro are that the digitiser is good and produces a high quality digital output, and it is very small and light and can output to either a USB cable to plug into my computer or a lightning cable to plug into my iPad for recording away from the studio – at gigs or whatever. It is a single function device, so recommended if portability and simplicity feature high on your preferences rather than wanting a larger, more complicated, all-singing, all-dancing, multifunctional device. The only downside I have found is that it requires a 9 volt battery to work, so there is a danger of it running out of power at an inconvenient time (i.e. when you are using it.)
My preferred software on the computer is Amadeus Pro (Mac only) although the cheaper Amadeus Lite would probably be sufficient to meet my requirements. It’s very straight forward to use, (as is the iRig Pro), which is an important consideration for me – I can do the complicated technical computer stuff, but it requires a completely different mindset to playing the theremin and jumping between the two makes my head spin. Also it has very effective noise reduction tools. I really don’t like hiss. If I want to do a lot of messing around with the audio I will clean it up in Amadeus Pro and then open the file in Garageband which is fun to use and comes with a lot of interesting effects, but doesn't have the noise reduction tools that Amadeus does.
If I’m using the iPad I use the iRig Recording app. With the in-app purchases purchased it’s quite a powerful little tool with effects and so on, including noise reduction – not as powerful as Amadeus but I have created tracks solely using the iPad and a selection of audio apps and been very happy with the results.
One final word about recording and reverb. The theremin sound can be very dry without a touch of reverb added. Mostly I use the reverb built into my amp for a subtle effect, or a reverb pedal if I want to use it as a strong effect rather than an enhancement. But I don’t record those reverbs – I put the DI Box in the audio chain before the reverbs. This is because reverb is best in stereo and my rig is strictly mono. The reverb built into my Mac (AU Matrix Reverb) suits me fine, but plenty others are available as plug-ins for your audio editing software.