Electronaut M63 Tube Microphone Pre-Amp

By Barry Rudolph

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The Bomb The Electronaut M63 is a hand-made, dual-channel microphone and instrument pre-amp. This all-tube three-rack space unit is made in Chicago by owner/designer Rob Roy M. Campbell, a self-described musician, recordist, engineer and experimenter.

Campbell created the M63 to be a recording instrument rather than strictly a microphone preamplifier and he did not shy away from incorporating some nonlinearities into the design, in direct contrast to the "straight wire with gain" approach. This design aesthetic, coupled with superb craftsmanship and attention to detail, pervades the unit making comparisons to other microphone pre-amps difficult--the M63 is in its own class.

Unique Design, Construction And Package

The M63 comes in an optional foam-lined, rugged travel case ready for the studio. The first production run (40 including my review unit) has a hand-fabricated aluminum chassis made by Campbell in his metalworking shop. Two large, backlit Sifam VU meters dominate a front panel, which was milled from thick aluminum plate and then anodized black and engraved.

Inside there are three printed circuit boards: one for each channel plus the power supply. These boards are 1/8-inch thick--twice standard PCB thickness and with copper traces three times thicker. These details eliminate the disadvantages of using printed circuit boards but make field replacement easier.

The boards connect to the front panel controls and switches using beefy Tyco MetriMate connectors fitted with gold-plated pins. All pots are by Clarostat and the switches are made by Carling except the locking NKK phantom power on/off toggles.

Each channel's pre-amp signal path begins with a Lundahl LL1636 amorphous-core input transformer feeding a 12AY7/6072A mic pre-amp stage fixed at 50dB of gain. The fact that the input stage's gain is fixed and not variable like a modern pre-amp is the core reason behind the unit's sound and operation.

The Trim control pot following the input stage acts like a "volume control" to set the level to a 12AU7/ECC82 line driver stage that's followed by another volume control called Attenuate. This output stage uses a Lundahl LL1676 output transformer and provides up to 13dB additional gain when driving a 600-ohm load.

The M63's larger heavy-duty toggle switches do not pass audio. Used for the tactile feel, they operate relays that feature gold-plated contacts. In addition to the M63's mains AC power on/off, there are four toggles per channel: -20dB pad, +48-volt phantom, (phase) polarity reversal and the unit's two operational modes: microphone or instrument input--a DI path that uses a front-panel locking Neutrik -inch jack. All four of the front panel controls use impressive-looking Daka-Ware knobs cast in Bakelite. They look like the knobs on an old Ampex 350/351 analog tape deck.

The power supply uses a custom-designed shielded toroidal transformer mounted on stilts above its PCB and a 12X4 tube rectifier instead of diodes. Campbell says this feature promotes longer tube life via the slow ramp-up of filament voltage.

Solid-state regulators are used for +48-volt phantom power, 12-volts DC for the tube's filaments, jeweled power indicator and the VU's light bulbs. All tubes are mounted in ceramic sockets but there are no retainer or hold-down brackets. Electronaut sources tubes from Electro-Harmonix and pre-tests them for each unit ordered.

The Bomb

A New Color In The Studio

My first test was at my Tones 4 $ Studios for a voiceover recording for a promotional piece about Alan Parsons' Art & Science of Sound Recording DVD Series. I compared the M63 to my solid-state RTZ Professional Audio pre-amp. I used my transformer mic splitter box (Jensen JT-MB-E transformer inside) to route a MXL Revelation large diaphragm tube condenser mic to both pres at the same time and recorded their outputs to separate tracks into Pro Tools HD at 24-bit/96kHz.

I went with the "cleanest" set up with the Trim control at about 11am and the Attenuate control full CW while the RTZ was set to 50dB of gain. No pad was used on either pre-amp. I had two vastly different sounding signal chain paths running here and the differences were not unexpected.

The M63 added a thick analog layer to the MXL's sound--a mic that uses a pentode (EF86) instead of a triode and is very 'colorful' on its own. On the other hand, the RTZ pre-amp portrayed the MXL as it actually is. For speech recording, the M63 (and this mic) sounded well saturated like an old recording but without the noise, analog tape print-through, modulation noise, and distortion.

I used no processing; by virtue of its design, the M63 imparts a noticeable tube compression even when set up as cleanly as possible. I used a Pete's Place Blast Filter for insurance against plosives and found an "s" though the M63 was sanded off--smoother and more pleasant-sounding. The M63 consistently produces mostly second harmonic distortion products in the range of 0.2 to 0.4% and as rich and luxurious as this sounds, I never heard it fuzz out into unusable audio.

Next test was a simple drum kit recording at another studio. I tried the M63 in three tests: stereo overheads (wide-spaced RØDE NT5 cardioids), kick (Shure Beta 52), and snare (SM57).

For the overheads, I used no pad on the M63, Trim barely cracked off the furthest CCW (off) position, and the Attenuate control straight up 12-o'clock. With the super hot mic levels coming in and the Trim control set to nearly off, it was very touchy to set a proper level. Instead of off at full CCW, perhaps a minimum Trim level should be offered with the remainder of available level spread out over the rest of the control's range.

I next wanted to try a different setting to see if I could alter the drum sound without changing mic positions, the drums or the drummer. I turned on the M63's -20dB pads and raised the Trim to about 8 o'clock and turned the Attenuate knob full CW. What a huge change!

Without the -20dB attenuator, the first drum sound was live sounding with the sound of the room, kick and snare well heard in the overheads--as if I used a compressor. In fact, zooming in on the recorded audio's waveforms in Pro Tools showed that some of the peaks were flattened although I never peaked (red) on the HD192's or PT's meters.

The second drum sound was closed-down and dryer sounding with no peak flattening. I liked this sound but it was a much tamer animal. Clearly, the M63 could use a -10dB attenuator position for yet another drum sound somewhere in between those two extremes.

Using the two channels of the M63 for kick and snare drums produced a fat, clear sound without doing anything other than putting the mics in the right places! The Beta 52 is a bright dynamic that can give you too much click on certain drums but in this case it was dialed in like I spent hours tweaking it. Ditto for the snare drum--through the M63, the lowly SM57 sounded much thicker and ready for final EQ.

At a third studio, I recorded a Collings acoustic guitar equipped with an LR Baggs piezo-electric pickup system. I recorded a Royer R-121 ribbon placed six inches away from the 12th fret and the guitar's piezo output into the M63's Instrument input at the same time. These lower level sources makes the M63 with its high gain a perfect choice.

I found the mic channel (Trim at 2' o'clock Attenuate full CW) to sound warm, thick and present--this is NOT the typically over-bright acoustic sound! The DI channel (Trim at 11 o'clock) was one of the best direct acoustic sounds I've heard and was totally useable in the final mix.

At a fourth studio, I recorded a stock Fender Jazz Bass (passive pickups) using the Instrument input. But before plugging in and after selecting the Instrument mode, I heard a buzz that varied with the Trim control setting. A self-shorting input jack would solve that issue.

The buzz went away as soon as my bass player plugged in and he immediately remarked that playing through the M63 reminded him of a bass guitar amp. We did find the M63 distorted on certain peak moments especially when playing hard on the E string. Backing down the instrument's volume control seemed to help this problem but it also diminished the bass guitar's sound and punch.

More Than A Mic Pre

Maybe not technically and specification-perfect, the M63 is a mash-up of old and new electronic design philosophies and more of a vintage guitar amp than recording studio kit. It's easy to love because of the mojo this winner brings to the party.

Barry Rudolph is an L.A.-based recording engineer. Visit his website at: WWW.BARRYRUDOLPH.COM.

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