Mono instrument tracks are easy to deal with but they can sound a little “lifeless” in a mix. We all know that there are countless plug-ins and hardware units that can create a massive stereo image from mono signals but this is often at the cost of mono compatibility. I was recently switched on to a technique using Inductor based EQ that can create an interesting stereo image with no fancy plug-ins or M/S trickery. Let me show you how it works.
What Is An Inductor?
Without diving headfirst into a rabbit hole of complex electronics, an Inductor, which might also be called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. Typically an inductor is made up of an insulated wire wound into a coil around a ferrite core.
Inductor Based EQ
Once again I really don’t want this article to be an electronics lecture (I’ve sat through enough of those in my time) so let’s just say that several well known British brands from the golden age of pro audio started developing hardware EQs and mic pres that used Inductors in place of the more commonly used capacitors. These EQ’s had a very pleasing sound and have been imitated, tweaked and updated ever since.
One of the main characteristics of Inductor EQ is the way it treats frequencies with respect to time. High frequency signals pass through an Inductor quite quickly however, lower frequencies pass through much slower meaning that audio that is treated with Inductor based EQ exhibits interesting and pleasing kind of phase smearing.
Creating A Stereo Image
The process is very simple. I have recorded an acoustic guitar with a single microphone plugged into a Warm Audio WA273-EQ which I know to feature an Inductor based EQ circuit. I’m pushing a little extra 12KHz, dipping the mids a little at 700Hz and boosting the lows at 110Hz to make the most of the EQ on the way in. You can hear this in the audio file below.
In Pro Tools (or any DAW) I am duplicating the track to give me two mono channels then panning them hard left and right. Using the channel inserts on my console, or if you choose you can use hardware inserts in Pro Tools, I am routing the audio back through the Warm Audio WA273-EQ using it now, not as a mic pre but as a line level processor.
The “trick” here is to use the High Pass filter to alter the tone of each of the channels very very slightly. The left side has the High Pass set to 50Hz and the right side set to 80Hz. By using different filter settings the inductors in the filter are every so slightly changing the timings of the upper more audible frequencies and so we get a slight phase difference between the two channels and a very pleasing stereo image is created.
Just to spice it up a little and hey, because I can, I am taking this one stage further. The Warm Audio WA273-EQ not only features a great sounding EQ but it also has a beautiful British sounding (oh we all know what I mean) mic pre, which from experience we know sounds great when you hit it just a little harder. This in turn means you hit the EQ circuit a little harder and creates and ever more enhanced stereo image that is harmonically richer.
You can see from the images below that this time I boosted the signal into the WA273-EQ by 20dB. This was not enough to cause the incoming acoustic guitar signal to break up but you do get a more harmonically interesting tone without what you might call distortion. I then used the built in output trim pots to ease the output back so not to over cook the insert returns.
This audio example is the same acoustic guitar part just with the WA237-EQ being hit a little harder at the preamp stage.
The Plug-In Alternatives
The nice thing about this technique is that it doesn’t just work on hardware but on well coded recreations in plug-in form. Below you can see the Universal Audio Neve 1073 plug-in on both channels in my session. Left set to 50Hz and right set to 80Hz high pass filter.
It’s a very subtle effect but it’s works really well where you don’t want to start diving for the big heavy processing guns. Try this with your own EQ’s, hardware or software, and let us know how you get on in the comments section below.