Bogen MXM Modifications
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The Bogen MXM is an all-tube five-channel mic preamp/mixer from circa 1963. A fortunate friend rescued a dusty old MXM as it was being thrown away, and then asked me to rehabilitate it into "something useful" for his ADAT-based project studio. Since this equipment embodies some of the best audio technology from the harmonically rich vacuum tube era, I gladly accepted the task. This article presents my approach to its restoration and modification, including the addition of independent buffered direct outputs for each channel. The MXM was accompanied by a 30-watt tube power amp (a Bogen MO30; link comming soon).
In the manufacturer's operating instructions, the MXM is described as a "broadcast quality" unit, "designed for theatres, radio stations, recording studios and public address installations." The MXM mixes five channels to a single master channel (see the block diagram in Figure 1, top). Each input channel has a large rotary fader on the front panel. All channels use shielded 9-pin plug-in transformers for microphone coupling. Channel one employs a 7025 (=12AX7) dual triode; a rotary switch on the front panel sets this channel's input to mic, phono, or aux. The remaining input channels (2 through 5) are mic-only, and each uses an EF86 pentode. These channels feature front-panel-mounted "speech filter" slide switches for low end-roll-off. The pentode channels are also associated with a rear-panel jack for a "remote volume control".
The five MXM channels are mixed in a summing amplifier consisting of one triode of a 7025 tube. Corresponding with two large knobs on the front panel, master "bass" and "treble" controls are in a network between this summing amp and the next 7025 triode. A 6CG7 dual triode is the final tube in the master channel path; its first triode precedes the master volume fader (the eighth large knob on the front panel). In cathode-follower mode, the remaining 6CG7 triode drives the MXM's "monitor" and "hi impedance" ground-referenced outputs, and via a plug-in transformer, the balanced "low impedance" output. A 6BF6 tube is associated with the large V.U. meter on the right side of the front panel. Rounding out the MXM's 9-tube complement is a 6X4 as high-voltage full-wave rectifier. Each tube is fitted with a shield. The heaters of all tubes in high-gain stages (the four pentodes and two 7025's) are supplied with DC to minimize hum.
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