Bogen MXM Modifications
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No project of this magnitude is ever really finished. Fertile ground for continuing modifications to the MXM include:
1. Pre- and/or post-transformer input pads. As mentioned earlier, certain mics and/or recording situations may be too hot for the MXM's fixed-gain tube preamps to handle, resulting in excessive distortion and very low channel fader settings. In this case, switchable input pads would be advisable. Some study would be required here, but possibly the best place for such pads would be on the secondary side of the input transformers. This might enhance any aesthetic benefit of the magnetic coupling ("hair") while keeping the tube distortion ("warmth") in check. In any case, a good location for mounting toggle switches controlling input pads would be on the chassis plane directly adjacent to the input transformer sockets. This strategy would keep the signal paths short. Additionally, these locations lie along one side of the instrument in the case of Channels 1 through 4, so convenient access holes could be cut in one of the perforated steel covers, and the chassis-mounted switches operated with a pencil or other stick.
2. Input polarity reversal switches. These might be useful on the primary side of the input transformers, particularly for Channel 1, whose polarity is reversed in the master (mix) output as I noted earlier. On other channels, such switches might be useful for exotic stereo miking techniques (among other applications), perhaps involving the modified MXM's own mixing capability. I would locate the necessary DPDT toggle switches on the back panel next to their respective XLR input jacks to minimize signal path elongation.
3. Channel mute switches. Presently all channels with non-zero fader settings pass their signals on to the master (mix) channel. The modified MXM would be more useful overall if channels could be selectively disengaged from the mix bus while still feeding their respective post-fader direct outputs. That way a mix of certain mics could be recorded while directly recording others. The most logical place for the necessary switches would be right on the front panel next to the channel faders, although this could create crowding and/or visual aesthetic concerns. If Channel's 2 through 5's "speech filter" slide switches turn out to be less useful than mute switches would be, the former could in fact be converted to or replaced by the latter.
4. Switchable phantom power. The MXM supports only dynamic microphones at this time. This situation was acceptable in the early 1960's, however today's studio uses plenty of condenser mics which require a +48 V bias on XLR pins 2 and 3 (referenced to pin 1). This is a challenging situation because it would require additional power supply circuitry in the already crowded (although neat) chassis. Perhaps a connector could be added for an external "wall-wart" style transformer/rectifier and then a regulator, filter capacitors, and +48 V bus could then be added within the MXM. I would install any phantom power switches next to the XLR inputs on the rear panel if matched 68-K-ohm resistors were used to couple bias to pins 2 and 3. As a possible alternative, the bias could be applied at the center tap of the input transformers. This appears to be pin 6 on the Bogen T-155 (from Bogen Manual), which is presently unused. However, depending on how this transformer is designed, the effect of DC on magnetic permeability is important to consider. Moreover, certain fault conditions could potentially result in highly undesirable transformer burn-out.
5. Channel-selectable V.U. meter. Currently the big V.U. meter looks impressive but has questionable practical value. Wedded to the master channel and "calibrated" only by an obscure "gain diagram" (Figure 11), it is all but meaningless for the direct outputs. For a luxury modification, a 6-position rotary switch could be added so that the metering circuit could monitor any of the five input channels or the master channel. Of course, appropriate actual calibration would also be desirable. I'm uncertain where I would mount such a rotary switch (below the meter?).
6. Reverse channel 1's buffer protection blunder. See the above paragraph entitled "One Ill-Conceived Feature," and short out the appropriate 1.0 M-ohm resistor.
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