A Rebuilt and Modified Altec 1567A Mic Preamp-Mixer HeadHere is a modified Altec-Lansing 1567A microphone preamp-mixer head that includes a channel breakout modification. Besides a near-total rebuild of the vintage unit, highlights of the modifications include:
photo of the modified unit's front panels is shown below. If you are
considering a standard breakout modification, do not let this photo
scare you. In this particular case, my client had some specialized
requirements. In addition to getting five independent tube preamps, he
wanted the ability to cascade channels in various ways to experiment
with distortion effects, including driving input transformers into
saturation. All input and output jacks are thus accessible from the
front, on an "auxilliary panel" which is permanently attached to
(below) the vintage unit. This panel also hosts toggle
switches that control relays near the input transformers in the vintage
unit, and the "pre-triode attenuator" knobs and pads (in CH1 and CH2, a
lot of attenuation is needed when experimenting with transformer
distortion). It's possible to get a standard channel breakout mod of an
Alltec 1567A without using up additional rack space.
- Independent, buffered differential outputs added for the mic pre channels (CH1-CH4), with clip warning indicator LEDs.
- Relay-mediated switching of input pad, polarity, and impedance on balanced inputs of CH1-CH4.
- Provision of a high-impedance unbalanced input option for CH1 and CH2.
of attenuator pot and pad in high-impedance link between input
transformer (or unbalanced input) and triode for CH1 and CH2.
function eliminated; original master channel made a fifth independent
channel (CH5) with variable attenuator- and pad-equipped high impedance unbalanced input.
- Negative feedback (gain) in CH5 made variable.
- Soft startup feature for DC-heated triodes (three 12AX7s), via standby switch.
a complete description of this equipment, including theory of operation
and its performance, you can read the following liberally-illustrated
reports. They are Adobe Acrobat (PDF) documents. The "Quick Start Guide" (1.09 MB),
available when I shipped the gear, includes unpacking and power-up
instructions, a summary of the modifications, schematics of
recommended patch cables, and instructions for rack-mounting. The "Technical Report" (5.72 MB) followed a few months later, and has complete schematics, circuit
descriptions, and performance test results. The block diagrams crowded
into your screen below are presented at greater resolution in each of
the report PDFs. Compare these block diagrams to the generic one shown
on my breakout modification hub page.
Notice that channel mute switches are not needed for this Altec 1567A
modification because the mix bus is removed (it can be reconstituted
externally if desired).
the idea of a channel breakout modification resonates with you, bear in
mind that it need not be this complex. Feel free to email your questions and comments to me at the
address shown on my home page (clarkhuckaby.com). Thank you for your interest.
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