Study Hall

Supported By

The Days Of Rosewood & Iron: Restoring And Enhancing An Early ’80s Mixer

Inside a process of reverse engineering an unknown past engineer’s brilliant but outmoded work and re-purposing it for a second life.
The surface of the Yamaha M1516.

Maintaining The Character

In recent years, shifting trends have popularized “out of the box” summing of DAW stems, a technique utilizing analog circuit paths and summing buses to add character to mixes previously created completely with computer software. I wanted to maximize signal quality, minimize possible noise, but still maintain the unique character of Discrete OpAmps and iron core transformers to make DAW summing.

Age degrades the quality of electrolytic capacitors but modern improvements in the manufacture of these components have left us with superior replacements. I replaced all electrolytic capacitors that could affect the signal path for tracking and mixing, including the power supply.

Real LCR filters for the EQ.

I started by generating component numbers for the Yamaha schematics so I could keep track of values, since these were drawn by hand before the common usage of component numbering (C1-C?? for the capacitors, R1-R?? for the resistors, etc.). All electrolytic capacitors in all input and output channels were replaced with Nichicon capacitors of the highest quality: low ESR (FG & VZ) for DC power, BiPolar (ES) for audio coupling and Esoteric (KZ) for the mic preamp. I also recapped the power supply with high-temp Nichicons (VZ), and tested the Yamaha NE80200 DOAs and reflowed their solder joints.

The most important component in the M1516A is the PGM summing opamp, a Toshiba TA7322P, designed in the mid 1970s. The one modern replacement for this upgrade is the OPA604 by Texas Instruments. I designed a circuit board to facilitate swapping them out. Lower noise and distortion measurements combined with higher slew rates (how fast the opamp can change output level) compound in the signal chain, resulting in more detail and dynamics.

This is most evident in the PGM summing bus. The OPA604 makes for a more detailed mix, benefiting from the higher slew rate and lower distortion.

Input channel full of new parts.

Calculated Decisions

Yamaha chose to use Gyrator filters in the EQ for the M1516. A Gyrator circuit uses an opamp to act as a simulated inductor in an LCR filter circuit. But the Gyrator circuit reduces cost at the expense of character. LCR filters with real inductors have pleasing non-linearities.

With a little online searching, I was very lucky to find inductors that fit the required values and size. I replaced all EQ filter circuits with passive LCR filters made with 1 percent Dale resistors, 5 percent Kemet film capacitors and 1 percent Bournes/MCE inductors. The results are very similar to the API553 but with more frequencies available to choose from.

A mountain of opamps and capacitors.

I also widened the bandwidth from 0.5 to 1.3 octaves and spread the frequency centers a bit, designing a circuit board for tight layout, which fit neatly into the section previously occupied by the Gyrators. This EQ filter upgrade makes a noticeable difference. The wider bandwidth is more musical and less surgical, and the frequency centers are more evenly distributed across the spectrum. The only external evidence of alteration is the ELCO direct out connector, but internally it’s a repurposed “plug-in” killer.

The tactile satisfaction of affirmative knob clicks and feathery fader slides, unique to analog gear, is an experience many of us miss and some of us never even had. Perhaps it’s time to revisit this pleasant past, when creativity and ingenuity pushed the limits of analog gear.

One more look at the control surface of the restored Yamaha M1516.
Read More
A Twist Of The Knob: Techniques To Stay Focused On The Matter At Hand

Supported By

Celebrating over 50 years of audio excellence worldwide, Audio-Technica is a leading innovator in transducer technology, renowned for the design and manufacture of microphones, wireless microphones, headphones, mixers, and electronics for the audio industry.

Church Audio Tech Training Available Through Church Sound University. Find Out More!