CUSTOM 32x24 RECORDING DESK
Designed/Built by Brian A. Roth 1977-78

1. View from left (note theSpectrasonic and UA limiters!)

2. Another view.

3. Other snapshots.

This desk, built in 1977/1978, was a very ambitious project for a 23 year-old "kid", but I had a passion for excellent performance and construction quality.  The desk was delivered in early 1978 to Maranatha Studios in Santa Ana, California, and remained in use for about a dozen years.  I understand that some/all of the desk has been reinstalled in another SoCal studio.

Overall design concepts.

The signal flow was a mixture of MCI "inline" design with ideas borrowed from API, Neve, and others.  There were only eight program mixing busses (not counting stereo mix, echo sends, etc.), BUT each I/O module had a "D" (Direct) switch that connected the I/O channel signal directly to the corresponding multitrack input.  If the "D" switch was not activated, then the output of a submaster was routed to the multitrack input.  Since most multitrack recording consists of one mic source on one tape track, this "D" method eliminated a lot of extra circuitry in the signal path.

My mic preamp design was an interesting beast.  In the "non-padded" mode, the microphone fed into a Jensen 115-KE transformer.  When the "-20 dB" switch was flipped, the Jensen was replaced by a unity gain differential amplifier.

The EQ section had a sweepable 12 dB/octave low-cut Bessel filter, adjustable from 25 Hz to 350 Hz.  The low and high frequency EQ sections were switchable between shelving and peaking.   Continuous sweep range for the LF section was 45 to 350 Hz, and the HF was 1.7 to 15 kHz.  The mid band was a fully parametric section, tunable from 100 Hz to 1.6 kHz or 1000 Hz to 16kHz (selected via a "F x 10" switch), with bandwidth adjustable from 2 octaves to 1/3 octave.  The low cut and EQ sections were totally "hard wire bypassed" when not needed.  Thus, it was possible for only two opamps and their associated components to be in the signal path when no EQ was required, and with the "D" switch activated instead of using a submix buss.

In order to minimize phase shift in the audible passband, my design goal was for a -1 dB point at 75kHz on the top end (exception: when the Jensen tranny was in circuit, the -1 point moved down to 40 kHz), and approximately 10Hz for the bottom -1 dB point.

Each module had an overload peak detector that sampled all gain stages, so that (hopefully!) no combination of screwball level or EQ settings could cause a clipped signal without the red LED on the channel strip lighting up.  The solo sends were taken after the pan pots, so non- destructive stereo "solo-in-place" was available.  Every solo switch had an associated lamp or LED, so it was easy to find what paths were soloed.

To show what era this console was from, it had only had four echo/effects send busses.  A dual concentric send pot fed two sets of four echo buss selection switches.  Two cue sends were also available, and could be used as additional post-fader effects sends during mixdown.

Components and mechanicals.

The faders were expensive 100-mm-throw P&G's (with the multi-pin connectors on the rear) and rotary pots were custom-ordered Bourns conductive-plastic 5/8" square devices.  It may not be immediately apparent from the blow-up of a 5x7" photo, but I used a LOT of concentric pots in order to make the panel space more compact.  Speaking of which, the knobs were from Rogan; these all had dual hex key setscrews to attach them to the pot shaft, something you don't see much of anymore.

The square white (channel mute) and yellow (solo) illuminated pushbuttons above the faders were Swiss-made EAO, a brand that has shown up in some Otari and MCI recorders.  The remainder of the pushbuttons were gold contact Schadow switches, and the mini toggle switches were gold contact C&K's.

The meters were the same model Dixson meters that Ampex used on the AG-440 "A" and B series recorders, but with a twist.  I added a LED "peak winkie" onto the dial face of each meter, borrowing the concept from Tascam's mixers of that era.  The threshold of the LED peak detector was adjustable, and I calibrated it to correspond to the midrange saturation level of the tape on the studio's Stephens 24 track recorder.

Now, onward to the internals.  I used the then-brand-new NE5534 opamps throughout.  All resistors in the signal path were metal film.  Capacitors for filters, EQ, etc. were tight-tolerance polypropylene or polystyrene models.  Several sealed reed relays were in each module to perform various signal switching operations.

A steel "tray" attached to the front panel acted as support for the four PC boards that made up each I/O module.   I borrowed an idea from Neve consoles from that era(although I reversed the genders) and used a gold plated male connector from Amphenol on the module in lieu of the more common "edge connector", and it mated with a female edge connector in the mainframe.

The patchbay seen in the "Another view" link above was custom fabricated.  A single slab of black phenolic was machined to accept the Switchcraft long-frame jacks.

The wood trim was solid Teak wood, and yes, that is "gen-u-wine" leather covering the wrist rest.  The Ampex ATR-100 remote control was built into a panel on the left end of the console.

I put a lot of thought into the design of the power supplies.  On one hand, I needed to accomodate low AC Mains conditions, yet I wanted to avoid excessive heat dissipation if using high "raw" DC rails into the regulators.  After some agonizing,  I finally decided to use three Acme Electric ferro-resonant bulk DC supplies to provide fairly well-regulated 24 volts DC sources, regardless of AC mains variations.  These fed electronic regulators mounted under the right-hand "producer's desk" seen in the photos, so the final voltage regulation was performed inside the actual console frame.

Well, that's all I can think of regarding the console. It was one helluva construction project!  I hope you enjoyed the tour.

Postscript.

I lost track of the desk as it went through various owners, but I received an email from the current owner who sent me this picture in 2004.  Various refurbs were done, and it continues in operation to this day.

M77 console in its new home, 2004.

My shop is set up to do all manner of from-scratch audio projects, from design to installation. I've done everything from this console to building a multitude of studios and multi-media presentation theaters, as well as modifying vintage gear into "rack-up" boxes for an outboard rack. By the way, I still have all my plans for this desk, and I can make custom channel strips or other "rack-ups" based upon my modules. Contact me and we can discuss your specific needs. Plus, I'm always available to assist with any professional audio "issues" you might have in your studio.

Contact info:

Cell/voicemail/pager:
405-630-7509

Return to Roth Technical Services homepage