I had the good fortune of recently acquiring a plate reverb, courtesy of my friend James Cunningham. Jim ran Studio Technologies in the 1970s, which manufactured the Ecoplate plate reverb unit. Jim took the original EMT 140 design and improved it by using a plate material with a brighter sound, adding modern electronics and higher gain pickups, and providing a substantial discount on the inflated price that plagued the German EMT units.

After lugging it down the stairs, we located it under the stairwell outside our recording space. We decided to forego the 3/4" plywood box the unit is meant to go inside, for space and weight considerations. The acoustical isolation afforded by the box is nice, but because it is located outside our recording space it is well isolated as-is. Not to mention, it tips the scales at around 350 pounds when it's cased.

Interestingly, this unit is a prototype Jim built before going to market professionally. Unit #1B is for all practical purposes identical to a production unit. However, years of pillaging and disuse meant that it needed some work to put it back in service. This site attempts to document my efforts in bringing this echo chamber back to life for use in my studio. This is an ongoing effort, and is not yet completed.

The Ecoplate needs a power amplifier for the driver mounted in the middle of the plate, and two preamps to get the pickups' output up to line level. When the unit got to the studio, I first wired on some appropriate connectors to the bare wires from the speaker and preamps. I then jury-rigged a power amp using a much too powerful Hafler P3000 150W per channel amplifier. I turned the gain almost completely down on the amp, and it appeared to power the driver at an appropriate level (using a pre-recorded vocal track described below).

I then took one pickup's output and sent it to my Avalon U5 direct box. I had to crank the DI output, but after trying it through two different microphone preamps to no avail (Sytek MPX-4Aii and Ampex MX-10), it was the best of what I had. Not to mention, the only unit with a Hi-Z input as required. I only have one of these DIs, so I settled on operating the plate with only one pickup - thus half its intended output.

I picked a vocal track that had been previously recorded through the Ampex MX-10 using an Electro-Voice V-2A ribbon microphone. I applied an equalizer on the track to mimic the frequency response curve the driver amp is meant to employ. I sent this out through the amp to the plate (with no damping plate at this point - see below) and recorded the output. I then patched the DI into my MOTU to record the resulting reverb track. The result was... not that great.

This clip contains 3 sections. The first is the dry, pre-recorded vocal. The second is the Ecoplate track. The third is a mix of the two. Notice the hum in the reverb track - an astonishing noise floor. Subsequently, I boosted the track gain in Cakewalk by 12dB to get it to what you hear in the 2nd and 3rd sections. The third section is a mix intended to show what it might sound like on its own someday. I put a gate on the reverb track to cut out some of the hum for this section.

These tests demonstrate there is potential for improvement. The Ecoplate is supposed to shell out clean audio, not sound so muted and noisy. Time to hit the workbench.

Mechanics

Damping Plate

The Ecoplate uses a damping plate mechanism to adjust the decay time of the reverb. The damping plate can be adjusted to provide 2-5 seconds of decay. Decay rate is adjusted by moving the damping plate closer to or farther away from the echo plate. The damping plate is mounted in parallel to the echo plate (made of stainless steel). Unfortunately, the included damping plate (figure a below) is of an earlier, flawed variety. It uses fiberglass insulation mounted to a 3/4" plywood sheet, which does not properly absorb sound.

Using plans provided by Jim, I built the new damping plate as seen in the second figure below. It uses lightweight steel channel as a frame, housing "pin perf" acoustical ceiling tiles (back of tiles shown). The channel ends were mitred to a 45, and riveted together to form the frame. The ceiling tiles are cut to fit the custom size, and held firm to the frame using flexible silicone glue.

This plate was built without a mechanical reverb time adjustment mechanism, and instead has 4 screw shafts protruding from the top of the plate frame. The damping plate rides and rests on these shafts, so you simply move the plate to the distance you want. Jim provides plans to build a mechanically-adjustable damping plate, and remotely-controllable servo motor/cam mechansisms can be built if so desired. However, the scale of my studio doesn't necessitate the extra cost or labor. I built eye hooks into my damping plate to use the existing screw shaft adjustment mechanism. This damping plate is effective and lightweight, yet sturdy enough for all intended purposes.

Tuning the plate

Coming soon!

Pickups

The pickups on my unit were from an EMT 140. These units were screw-mounted to the echo plate (causing potential vibration), have a high signal to noise ratio, and lower overall gain. Jim manufactures and sells a set of modern, high gain piezoelectric transducers that surface mount to the echo plate. These provide 20dB in additional gain over the EMT pickups, have a higher signal-to-noise ratio, and have the added benefit of mounting to the plate using a conductive adhesive. Figure a below shows the old EMT pickups after removal. Figure b shows the new pickups mounted to the plate, their leads connecting inside the junction box.

The reverb had been sitting dormant in a basement for years, and its electronics had been pilfered for use in another unit whose electronics had gone bad. It needs a power amplifier to drive the modified speaker mounted in the middle, and a set of preamps to get the pickup's output up to line level.

Chassis (02-28-06)

Using schematics provided by Jim (power amp schematic, power supply and preamp schematic), I sourced and purchased the parts necessary to build two pickup preamplifiers, a 20 watt power amplifier, and their power supply. I decided to build these plate electronics into a 2U rackmount box. Having just finished a project using a lot of chassis-mounted connectors and tall components in a 1U rack chassis, I spared myself the hassle and went with the extra 1U of rack height.

I used a gray Par-Metal case with a clear brushed aluminum front panel. The front will have only a power switch and indicator light. The rear of the unit is shown in the photos below. A mono input signal enters via an XLR, is amplified and sent to the speaker using a Speak-On connector for durability. The two pickups enter the preamplifiers from the plate by two locking 1/4" jacks. The output of the pickups is amplified to line level and sent to the two XLR output jacks.

Power Supply (03-08-06)

The figures below show the completed power supply. This is a pretty simple 15V bipolar supply. I got my sea legs again working with circuit board point-to-point. As a result, I think I'll learn how to build PCBs after this project is over - this jumper wire business is kind of a PITA. I bench tested the supply for a few hours and it is stable with no hot regulators or other parts. I'll call that a winner!

Off-board connections are made with Molex connectors for ease of use, cleanliness, and good lead isolation. The Molex connector at the top in the photo is for the 120V AC supply. The three other Molex connectors route DC +/- power and ground to the preamp and power amp circuit boards, as well as the LED power indicator (just gets positive voltage, with a dropping resistor in series taking it to +12V).