Bogen MXM Modifications
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Overview. As shown by the block diagrams in Figure 1, overall signal flow in this five-input mixer was minimally changed upon modification, but I added solid-state buffers that drive independent direct outputs for the five preamp channels. For each channel, the audio signal for the respective op amp-based buffer is taken from the channel fader's wiper. Thus, channel faders are used to set direct output levels concurrently with relative mix levels in the modified MXM. I designed the channel output buffers to have high impedance inputs (1 M-ohm), which minimally load the preceding tube preamp stages (see below and Figure 10). Other changes to the signal path visible by comparing the block diagrams (Figure 1) are: (1) simplification of Channel 1's input options, and (2) elimination of the original "high-impedance" unbalanced mix output, with use of a solid-state output buffer instead. I present these modifications in greater detail below. As an overall upgrade to the MXM's signal path, I replaced all of the original coupling capacitors. In most cases I chose Sprague "Orange Drop" polypropylene film units (this is most apparent in Figure 3, bottom), which are famous for their musicality.
Channel 1. Originally, Channel 1 had a three-position screwdriver-slotted rotary switch on the front panel to select between microphone, phonograph and auxiliary inputs. As seen in Figure 5, this three-pole (3P3T) switch not only selected the input coupling, but also swapped filters within a feedback network (between 7025 triode stages) to add equalization (EQ) for the phono setting. Passive filter networks at the "magnetic" and "crystal" phono inputs contributed additional EQ for the different 1960s-era phono cartridges. This EQ is obsolete by modern standards, as are monaural phonographs. I therefore eliminated all EQ in this channel rather than attempting to rebuild it. A third setting of Channel 1's input selector switch completely bypassed the preamp, coupling the auxiliary input jack directly to the top of the channel fader. Not anticipating much use for this, I opted to eliminate this luxury for simplicity's sake.
Figure 7 (top) shows a schematic of Channel 1 after my modifications. The 3P3T switch and phono EQ circuitry is replaced by a SPDT toggle that simply selects between a transformer-coupled mic input and a 1/4-inch line input. The feedback network between 7025 triode stages is identical to that of the original design when the microphone option was selected. It results in a flat audio frequency response as the sole option in the modified channel. A Neutrik 1/4-inch jack provides a "line" input when selected by the toggle switch, and is isolated with a series 33 K-ohm resistor for stability. The 1-M-ohm grid resistor at pin 7 of the 7025 tube insures a ground path for any grid current, and also sets the input impedance of the line input to 1 M-ohm. Accounting for this added resistor, I changed the microphone transformer's load resistor from 100 to 111 K-ohm, so the equivalent mic input circuits of Channel 1's original and modified designs are identical. After modification, I measured this channel's microphone input impedance as 320 ohms.
Channels 2 through 5. Aside from upgrading their coupling capacitors, I left the design of these pentode-based mic preamp channels the same as in the original MXM (compare the post-modification schematic in Figure 7 [bottom] with the original schematic in Figure 5). However, I eliminated the 9-pin "remote control" jack and its wiring to the pentodes' second grids. This obsolete feature could only be a noise antenna now. On testing the completed project, input impedances for these channels measured exactly 150 ohms.
Summing amplifier and master channel. Figure 8 shows a schematic of the MXM's remaining tube audio circuits after modification. As detailed below, I substituted an op-amp-driven line output for the original "high impedance" master channel output. Otherwise, I upgraded capacitors without altering this section's basic design. This included the only coupling capacitor upgrade that did not employ a Sprague "Orange Drop:" the original coupling between the 6CG7 cathode follower and the output transformer, line outputs, and V.U. meter driver was a 5-microfarad electrolytic capacitor; I replaced this with a new, nonpolarized, 6.8-microfarad polypropylene film/foil capacitor from Solen.
V.U. Meter Circuit. My modifications to the MXM did not alter its own particular V.U. meter circuit. However, comparison of this circuit (Figure 5 and Figure 8) with Bogen's published schematic showed that someone else had made modifications here. The two main differences were: (1) The voltage divider at the grid of the 6BF6 uses resistors of differing values in the two schematics. Moreover, the resistance nearest ground in our MXM consists of two resistors in parallel--evidence that someone calibrated the V.U. meter to their own specifications using parts on hand. (2) The meter is driven by the 6BF6's cathode circuit in Bogen's schematic, while it is coupled to this tube's vacuum rectifier plates in our particular MXM. I rebuilt it the way the last technician modified it--parallel resistors included (Figure 8). After all, V.U. meter readings are arbitrary without calibration. (As a form of calibration, I align the V.U. meter's tested response to real units in Figure 11; see also my discussion on gain structure below.)
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