HMX - The Benefits of Harmonic Distortion
OK so we all know that plenty of engineers love the sound of tape, tubes and transformers.
All of these devices can exhibit plenty of non-linearity (although many would say that back in the day people tried to not push these types of devices into saturation if possible)…
However each adds a particularly characteristic type of distortion defined by their transfer functions, substrate materials and circuit location.
Our HMX circuit provides useful tonal options that are created by unique distortion and harmonic generation topologies with a nod to the saturation artifacts of yesterday’s designs.
Classic Sources of Distortion:
Tubes (Valves for us Brits!)
Tubes often distort asymmetrically and therefore produce significant amounts of even order distortion. If any of you have ever studied signal generation, you should be aware of the fact that symmetrical shapes (like square waves) contain odd only harmonics and asymmetrical shapes (like sawtooth waves) contain a mixture of the two.
Though there are methods and circuit arrangements that can cancel out even order distortion to some degree but for now lets assume that ‘typical’ tube tone comes from rich 2nd order harmonic distortion (effectively a musical octave) and therefore asymmtrical waveshapping can create a rich, larger than life sound.
Typically when a tube reaches clipping (running into the power rails) - a somewhat soft shape is created (softer than solid-state electronics) which provides a ‘musical’ entrance into the saturation transfer curve.
Transformers are very popular at the moment and are enjoying a retro revival. They work by inducing current in coils (primaries and secondaries) that are wound around a core of ferrous material. Popular metals used in transformer laminations are silicon-steel, nickel and cobalt.
Effectively a transformer passes signal from one winding to the other through induced magnetic fields. This magnetic field is known as the flux field. Flux is produced within the transformer core and the amount of magnetic field that can be produced within the core is directly related to its size. The bigger the core, the more flux it can withstand. Due to the nature that magnetic flux changes polarity like an alternating cycle (a.c) - saturation (the point at which the transformer cannot withstand anymore flux generation) is largely symmetrical.
Therefore transformers provide odd-harmonic content as they reach saturation. Interestingly, transformers also provide low level distortion during initial magnetism of the core (coercive force) and there are also several parasitic influences that effect the response of a transformer.
Transformers provide most of their saturation below 500Hz (core size and material have a huge impact on this) and therefore lots of 3rd and 5th harmonic are added to the lower mid-range and low end making instruments seem punchier and often easier to be heard on smaller speakers. For musical people, 3rd harmonic distortion basically adds a tone that is an octave and fifth higher than the fundamental (think power chord!).
Tape is a magnetic recording medium and therefore exhibits all typical magnetic flux field saturation issues just a transformer. Tape is symmetrical in saturation and also suffers from a magnetic problem known as hysteresis. Effectively hysteresis presents itself as a lag in output signal compared to changes in input signal. In simple terms, magnetic devices remember what signal just passed, they hold on to it and therefore smear transient material as they magnetically discharge.
Tape provides similar punchy low frequency odd-harmonic distortion when pushed into saturation, it smears transients and also suffers from coercive force problems (low level distortion). However tape bias is used to stimulate the magnetic particles into action to reduce this low level distortion at the trade-off of high frequency response.
On top of all magnetic saturation issues, tape speed and head gap size / width have large effects on the frequency response of the tape machine - ultimately (even with correct alignment procedures) tape exhibits a slight bump in the low frequency range (50-100Hz) and often a slightly lumpy lower midrange with some extreme HF roll-off.
HMX - The Audient Dark Art Colour Control
OK so HMX has been a feature on our Black Series and MiCO. HMX is a clever little drive circuit that uses discrete class-a MOSFET electronics to produce plenty of smooth musical distortion. It is an asymmetrical saturation stage reminiscent of valve saturation but it has also been cleverly voiced to produce some heavy low frequency wave-shaping and subtle frequency response shifts that look very similar to tape head bump and therefore HMX offers a blend of yesterdays colour in a modern adjustable form.
This is the distortion profile of a MiCO with HMX turned off. The unit is being fed 0dBu into the XLR input and is padded to use the mic amp gain (as a line input) with 0dBu at the output stage back into the dScope…therefore unity gain from in to out.
As you can see, there is no harmonic distortion to the 1kHz fundamental down to -80dB (very low indeed considering this is going into a mic amp!).
Engaging HMX set to zero produces an increase in 2nd harmonic distortion - remember things are a little warmer and richer with this present, albeit 50 dB below signal level.
Adjusting the control up to halfway produces some 3rd harmonic also at a very low level - adding just a bit of push to the sound, notice that the sine wave is still very sine-like!
Cranking the control to maximum produces more noticeable thickening with 2nd, 3rd and 4th harmonics being added - with even order still dominating.
So that is the cleaner side to HMX - a subtle harmonic thickener that acts in the low order to produce some colour and warmth.
HMX also produces a subtle frequency shift that enhances low end much like tape could. This is especially useful on sources that needs a little size to them! Notice how this adds some weight to the low end and opens up the midrange.
HMX - The Dark Side
Righto, lets bang into it and crush things to see how much waveshaping can be had!
1kHz, preamp drive creating more HMX saturation, notice the asymmetrical waveshape - much like many tube stages… plenty of harmonic content being created now.
Heavier drive shows lots of asymmetrical clipping but also indicates that positive going transients may not be completely destroyed in the saturation process!
As can be seen HMX is capable of downright filthy overdrive and therefore is a fun tool to use on electric guitars and bass!
If we investigate HMX at lower frequencies it exhibits waveshapes that aren’t too dissimilar from that of transformer saturation, albeit asymmetrical versions… I’m not suggesting that HMX sounds like a transformer - in fact it wasn’t designed to in the slightest, but it is a very useful colour control that produces many similar attributes to classic distortion mechanisms.
Up at 5kHz waveshaping is slightly less brutal, producing a smoother top-end with some semblance of fidelity! Again asymmetry can be seen here.
Overall, this is a nice feature to have on a clean microphone preamplifier such as the MiCO.
Please note that the above graphs are very ‘rough and ready’ to highlight the range of tone available and are by no means a perfect scientific study of HMX.