Sunday, February 06, 2011

This past week has been one obstacle after another on the electronics front. Old vehicles, learning software and a mass of other trifling happenings has pushed me off the map for a bit. Though, on a somewhat superstitious mumbo-jumbo front an illustration has been drawn regarding the delay of gratification cultivating a proportionally larger boon for the field which had so recently been repressed.

I went hunting for a home stereo preamp yesterday, armed with a budget figure based on a locally advertised unit carrying more modern features than I need but at roughly 10% of the mid 1990s MSRP. Then I happened on a couple of broadcast mixing consoles, an Autogram AC-8 and a Wheatstone A-32, which more or less derailed my hunt for a preamp, having consumed, to the dollar my established budget.

I'm running through the AC-8 in this installation as I am somewhat familiar with this beast, already having it's smaller sibling the AC-6. Typical broadcast console interface, a mixer refined to the barest of features, routing switches and volume controls.

One of my favorite things about old radio mixers is the ease of access they afford the technician. These were often designed with 100% up time and field repairs in mind, the Autogram/Rockwell stuff exemplifies this by confining the various stages into cans that can plugged and unplugged in seconds while another channel is active.

From the upper left of the picture on the base chassis: input terminal blocks; output stages left and right program, mono and left and right audition; amplifier and power supply board and output terminal blocks. The front panel modules are two MPA-1 microphone preamps, one for each input channel which is set to mono via the configuration switch next to the empty sockets, followed by five stereo channels of MT-1 socketed interstage audio transformers and a pair of IA-1 variable gain op-amp modules.

With the exception of the microphone preamp and op-amp modules, all the routing and level controls are passive. Two pair of balanced input wires terminate to an input selection switch which feeds the module, a stepped attenuator and an output routing switch.

Large, low loss connectors to impede the 600 ohm line as minimally as possible.

Here's a gutshot of the MPA-1 microphone preamp module from the AC-6 I took 6 years ago. Input transformer at bottom feeding the active circuitry.

Here's the innards of an MT-1 module, a Triad A-67J.

More fixing stuff and less showcasing on the horizon, once I climb through that Wheatstone unit..


pearshapedhuman said...

Wow! Nice score!

Will look spiffy in the future Rochambeauphonic Workshop.

crochambeau said...

Thanks! I'm stoked to have landed it, I'll tell you about what happened when I run into you next.

My plan is to make this one 100% transformer coupled stereo channels, and the AC-6 wired to support direct output of the microphone preamps.

The Wheatstone has sliders, and looks a bit higher class (it's a modular), but my gut is telling me the Autogram will be superior in terms of sound quality. P