Bogen MXM Modifications

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Considerations for the Direct Output Buffers

How the modified MXM will be used. The availability of post-fader direct channel outputs on the modified Bogen MXM is the stand-out feature of this rehabilitated equipment. This allows independent access to five high-quality tube microphone preamps for recording up to five mics on separate tracks. Before "retro-fitting" direct channel outputs into this dedicated 5-to-1 mixer, I first had to measure gains and maximum audio levels in the MXM's various stages. Although I wanted to preserve the MXM's original mixing function, I recognized that the master (mixed) output would seldom (if ever) be used concurrently with the direct outputs. These two functions are not really compatible. (This could change if channel mute switches were added; see my section on suggested future modifications.) In other words, the MXM's channel faders would henceforth be used mostly for setting individual direct recording track levels, and not for setting a mix. I kept this in mind while designing the direct output buffers.

Gain characterization of the Bogen MXM. By injecting sine wave tones and observing output levels, I characterized the gain of all stages in the rebuilt MXM. Augmenting a simplified block diagram, this data is presented in Figure 9. Channel 1's overall gain is 40 dB for its line input or 61.5 dB for its mic input. (Voltage step-up by its microphone input transformer thus accounts for about 21.5 dB of gain; this was similar in the pentode channels.) Overall gain in each of the pentode-resident mic input channels (2 through 5) is 67 dB. In the summing amplifier/tone control stages of the master channel (a 7025 twin triode), amplification is a further 11 dB. I measured gain in these combined stages with the bass and treble controls in their tonally neutral positions (as evaluated by square wave optimization rather than knob position; I marked the neutral knob settings on the front panel). Gain at the final voltage amplification stage of the master channel (the first triode of the 6CG7) is 21.5 dB. Thereafter, the second 6CG7 triode, which is used as a cathode follower, has no voltage gain (0 dB). In coupling this cathode follower to the balanced master output jack, the MXM's output transformer steps the signal down by 15 dB. The balanced output was terminated with a 600-ohm resistive dummy load for all measurements.

Comparison with solid-state mixers. Importantly, preamp (input channel) gain is fixed (i.e., not variable) in the Bogen MXM. I therefore call its channel volume knobs "faders," not "gain controls." This differs from a typical modern solid-state mixing board, in which each channel has both a gain control (usually a knob near the top of the channel strip) and a fader (nearly always a slider at the bottom). In the MXM, the preamps work at maximum gain all the time. With this much gain, the channel faders have to be kept on the low side (i.e., lots of attenuation) for most mics most of the time. This is why the MXM manual tells us to start with the master knob fully clockwise and then to bring up the channel knobs.

A fixed preamp gain of over 60 dB in a solid-state channel would lead to severe distortion most of the time. Imagine the gain knob at the top of a Mackie mixer channel stuck fully clockwise. Fortunately, the MXM's tube circuit design gives its preamps much more overhead than is possible with solid-state preamps. Moreover, with tube preamps, the increase in distortion as levels ascend toward that overhead is gradual and results mostly in music-enhancing even-order harmonics, perceived by humans as warmth (this is why tube audio is good). In contrast, as the solid-state preamp vainly tries to deliver levels impossible by design, it simply clips the waveform, introducing high levels of harsh-sounding odd-order harmonics. So the louder the input signal to an MXM channel, the greater the amount of tube distortion, no matter where that channel's fader is set. Ultimately, however, overloud signals and/or some of today's "hot" mics may engender too much distortion (even a tube preamp has its limit--albeit usually considered a "soft limit"). I therefore suggest switchable input pads as a suggested future modification.

Absolute levels and overhead. The specifications in the Bogen MXM manual gives a (presumably) nominal balanced master output figure of "+18 dBm @ less than 1 % distortion (into 600 ohms)." This equals 6.16 V (rms) or +15.8 dBV. Given the gain of the master channel (see above), this corresponds to a mix bus level of -1.7 dBV when the master fader is fully clockwise. My tests showed that the summing amplifier has the most restrictive overhead of all stages; severe distortion begins abruptly as mix bus levels rise above +2.3 dBV. Of course, the input channels (mic preamps) have considerable overhead beyond this level, otherwise their faders would be of little use. Although qualitative (I lacked a distortion analyzer for this project), my observations using the oscilloscope bore this out: preamp output waveforms remain below (soft-)clipping thresholds to beyond 25 Vp-p or about +19 dBV. An ADAT recorder line-level input is nominally -10 dBV and has an absolute (digitally determined) maximum at +5 dBV. Given these facts, I decided to use unity-gain buffers at the modified MXM's direct channel outputs, without additional pads in most cases (I say "additional" because the channel faders serve as variable input pads to the buffers). ADAT input levels are aligned with the modified MXM's gain diagram in Figure 11; I will discuss this further below.

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