HomeChannel Breakout Modification of Vintage Mic Preamp-Mixer Heads

In the 1950s through the early 1970s, vacuum tube-powered multi-input "mixer-preamplifiers" were common in many broadcast and recording studios, and in public address applications. They were dedicated mixers with four or five transformer-balanced low-impedance inputs (often using plug-in transformers) and tube gain stages. Most had a monaural output; frequently they had tone controls, a volume control, and a VU meter in the master (mix) channel. They usually supplied connections for linking multiple heads to increase the number of channels feeding the mix. Examples of vintage preamp-mixer heads are the Altec-Lansing 1567A, Bogen MXM and MXM-A, Ampex MX-10 and MX-35 (which had two mix busses and outputs, but no pan pots),  RCA OP-7 (which used a passive, transformer-based mixer), and Stromberg-Carlson AU-57 (which had a built-in power amp).

With their transformer-coupled tube mic preamps, these old mixer heads have a great warm vintage sound. Well-maintained or restored units are still useful in today's recording studio, except frequently the "extra" input channels go unused--audio engineers appreciate their sound as mic preamps, but don't need them as mixers very often. That's why I developed the "channel breakout modification" for vintage mixer heads. This gives each mic preamp channel its own independent, buffered (low-impedance) output. It liberates all of the tube preamp channels for simultaneous use in today's multi-track recording environment. The following simplified block diagram illustrates this concept:

Simplified block diagrams of mic preamp-mixer head before and after channel breakout modification.

In this block diagram, a generic four-channel vintage mixer head on the left is modified with additional circuitry (in red) on the right. At the wiper of each channel's rotary fader pot, a solid-state (IC-based) balanced output driver stage is added. As shown, the original mixing function can be retained; in that case it's best to add a mute switch to each channel. Thus, channels not being mixed or processed through the master channel can still be used independently. For example, say you have two mics on a snare drum using channels 1 and 2; you can record their mix to one track via the master channel while muting channels 3 and 4 and using those for tom-toms assigned to their own separate tracks. Or, say you have one mic on a guitar amp going into channel 4; you can mute channels 1 through 3 and use them for other instruments, while recording two separate tracks for the guitar--one from the channel breakout and one through the master channel--perhaps with a funky tone control or gain setting on the latter, which may be useful later in the mixdown.

I realize that tube audio "purists" may criticize this channel breakout approach, because using solid-state output drivers violates the all-tube "purity" of the vintage gear. But few modern studios have an all-tube (or even all-analog) signal path from microphone to final product. Today, tube gear is used for effect (warmth). Adding transparent solid-state stages to the vintage gear can increase connectivity and versatility, while retaining the desirable tube character. I would offer to tube purists who balk at modifying their vintage mixer heads the following irony: breaking out the channels with solid-state drivers can actually add more tube tone to your overall recording project than keeping your head "pure," because in the latter case your "extra" tube channels go unused.

Here are links to two breakout modifications I have done: The first was a Bogen MXM, which has been online here since 2005. It uses ground-referenced (unbalanced) output drivers. The second is an Altec 1567A, which was an unusually extensive modification that included disabling the mixer function and turning  the master stages into an independent channel. The four mic preamp channels were given differential (balanced) outputs using THAT1646 chips from That Corporation; these are excellent and versatile output drivers which I now prefer. In each of these modifications, the vintage circuitry was completely rebuilt, including upgrading the tube sockets, wiring, and coupling and filter capacitors. While this may not be necessary in all cases, it is recommended for service-neglected gear of this age, such as many vintage pieces from online auctions, for example. Rebuilding assures their reliability for years to come.

My present goals for breakout modifications is to lower their cost and minimize the bench time they require. A client's total cost per channel should compare favorably to the cost of new tube preamps of similar quality. To this end, I'm planning the modification of a second Bogen MXM, which is currently stipped down and awaiting rebuild. It will serve as a prototype for developing standardized printed circuit boards for the solid-state output drivers and their power supply. These boards will be compact and versatile enough for installation in a range of vintage models. I will report my progress on that project here. Meanwhile, if you are interested in breaking out the channels of your own vintage mic preamp-mixer, or have questions and comments, please email me at the address given on my home page at clarkhuckaby.com.

First Bogen MXM Breakout Modification (2005)

Altec 1567A Extensive Modification Including Channel Breakout

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