Changes to the MXMs Audio Signal Path

Posted on Feb 6, 2014

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. Thu

Changes to the MXMs Audio Signal Path
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s, 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. 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 This EQ is obsolete by modern standards, as are monaural phonographs.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...

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