SPL MikeMan

There's more to building a serious mic front end than putting a standard circuit in a flashy box. Fortunately, SPL's circuit design is even more impressive than their custom anodised front panels, as Paul White discovers.

Most people know of SPL through their Vitalizer, which is still one of the most coveted enhancement devices on the market today, but in this latest product, technical guru and SPL founder Wolfgang Neumann has focused his creative energies on the more down‑to‑earth subject of mic preamp design. Most mic preamps now offer reasonable performance, but very few attain the lowest noise and distortion figures made possible by modern component design. All too often, state‑of‑the‑art components turn in an indifferent performance because of the way they are used; maybe the circuit grounds aren't quite right, perhaps the power supplies aren't as clean as they should be, and so on — but Neumann has put a lot of hard work into this design to make sure that the SSM 2017 Analog Devices amplifier chips work to the very best of their capabilities.

Essentially, the MikeMan comprises two independent mic amps, each with a Gain control providing up to 72dB of gain, a Phase Invert switch, a 35dB pad, and switchable phantom power, all on illuminated switches. A 10‑step bargraph PPM meter shows the output signal level at all times, and a specially‑designed output stage provides a balanced XLR feed and an unbalanced jack feed which may be used simultaneously. A Ground Lift switch is fitted for use in ground loop‑sensitive situations.

The maximum gain of 72dB is 12dB more than is normally available from a conventional mic preamp, and has been achieved by using the amplifying devices in an instrumentation amplifier configuration — the same circuit topography as is used for ultra low noise, very low distortion measurement and calibration test gear. A common mode rejection ratio of over 90dB is claimed, which basically means that the interference‑cancelling effect of the balanced input stage works rather better than most.

The normally vulnerable input stage has an exceptionally high overload margin and is protected from DC offsets or damage from excessive signal level, while the phantom power circuitry uses precision components so as not to compromise the efficacity of the balanced input stage. Additional smoothing circuits are also employed to limit the rise time of the phantom power voltage when it is switched on, so as to avoid disturbing thumps and bangs.

Hardware Design

The MikeMan comes in a 1U package which has been newly designed to conform to the impending legislation on RFI (Radio Frequency Interference) susceptibility. A novel cosmetic touch is the use of a special anodising technique on the front panel, which produces a random deep blue swirl pattern. No two front panels are the same, so perhaps SPL should consider providing a photocopied 'fingerprint' of the panel as an aid to identifying stolen equipment. The mains power supply is integral and uses a toroidal transformer, while the audio circuitry conforms to a carefully‑designed grounding system and is built using very high‑tolerance 1% and 0.1% metal oxide film resistors.

Paper Values

OK, so it's obvious that a great deal of care has gone into this mic amp, but does it perform any better than the one in your mixer? After all, the printed EIN noise figure is just 126.6dBu, which is more or less exactly the same as that boasted by most mixer mic amps. What this figure doesn't tell you is how well the mic amp performs at intermediate gain settings; cheaper designs often produce the best 'paper' figures when used flat out, but in most practical situations you'll be using the Gain control somewhere in the middle. Furthermore, you'll find that many spec sheets quote an A‑weighted figure, which makes the noise performance look better than the more honest, unweighted one. Yet another variable is the frequency range over which the mic amp works. Most cheaper models roll off soon above 20kHz, whereas this unit is only 0.1dB down at 40kHz, and half a dB down at 100kHz. It is widely recognised that audio equipment needs to have a bandwidth far in excess of the human hearing range to sound good, especially if several devices are to be connected together; the cumulative frequency response gets worse the more devices you cascade.

SPL have bravely published both noise and distortion figures for all gain settings in 10dB steps, where it is evident that the best noise performance is achieved with gain settings of 30 or 40dB — the most commonly‑used range. That's not to say that the other settings don't turn in good performances too — the distortion figures are creditably low throughout. Again, this sounds great on paper, but what are the practical benefits?

Performance

All a good mic amp should do is take the signal from the microphone and make it bigger without changing it in any other way. By this definition, the SPL MikeMan behaves very much like the ideal mic amp, and in practical terms, it is noticeably quieter and cleaner‑sounding than the mic amps found in most mid‑priced mixing consoles. The wide bandwidth and very low distortion means that the sound is quite uncoloured, and because of the care that's gone into the design, there are no problems with low‑level hums, buzzes, or interference from Radio One!

If there is a criticism, it is that no output level is provided, which means that for direct‑to‑tape recording you have to use the mic Gain control to set the recording level. The XLR output works at a nominal +6dBu level, which means it will match professional balanced gear nicely, but the unbalanced jack output runs at a nominal 0dBu, meaning that the mic amp gain will need to be set below optimum in order to match semi‑pro, ‑10dBV machines. This shouldn't create significant problems in real life, but an output Gain control, or at the very least a switchable ‑10dBV pad on the unbalanced output, might have been more elegant. Knowing Wolfgang Neumann's approach to audio design, it's my guess that he decided to leave them off in order to keep the signal path as clean and short as possible. As a side benefit, because the SPL MikeMan has such a high input level capability, it may also be used in conjunction with the Pad switch to control the level of line signals being fed to tape.

Although the SPL MikeMan is rather more costly than a standard mic preamp, it manages to deliver esoteric levels of performance at a price far below what you'd expect to pay for a premium, guru‑designed mic front end. For anyone interested in recording critical sounds or for getting their mic signals directly to tape without going via a mixer, I'd say the MikeMan is something of a bargain — it'll take some beating at any price.