Artificial Reverberation in the Analog World, Part Three

Artificial Reverberation in the Analog World, Part Three

Written by J.I. Agnew

What I set out to make was essentially a character piece, something that would define recordings of suitable styles of music that were done within my own facility.

It was intended to set itself apart from the EMT plates or the various examples of spring reverb units, as described in Issue 182. (Part One of this series appeared in Issue 181.) After all, if you are looking for the classic EMT 140 sound, the only way to authentically obtain it is by using an EMT 140.

I wanted my character piece to have low noise; be bandwidth-limited by the electromechanical transducer system and not by the electronics; have low-order harmonic distortion only; and have the ability to be overdriven with a wide range of creatively-usable sounds rather than just breaking up into gross distortion. I also wanted it to have balanced inputs and outputs, a full-bodied reverb sound with a slow, gradual decay, and the ability to also work gracefully with low-frequency content.

I decided to use a spring system with special high-impedance transducers. The power supply would be kept in a separate housing, to assist with maintaining the lowest-attainable noise floor (these electromechanical transducers do tend to pick up any stray EMI/RFI, and huge power transformers in close proximity are best avoided).

For convenience it was decided to house the reverb units and power supply in 19-inch rack-mountable boxes. Since I was not bound by any size limitations that would negatively impact the future marketability of these units, the boxes could be made as large as needed. A multi-spring system of the largest size that would physically still fit within the format envisaged was used and since we had decided to let go of reason and practicality right from the onset, we would use vacuum-tube electronics.

No, not another 12AX7-based design, capacitor-coupled to the next half with a clumsy electrolytic cap bypassing the cathode bias resistor! Please no! Enough of these 12AX7 tubes; they are everywhere by now. Also, it is not like we have a shortage of other tubes, in fact, we do have MUCH better tubes than the 12AX7 available for audio applications. Some are even still being manufactured today. But I won’t be using any of these either; that would be too easy.


Several vacuum tube reverb units and power supply units at the assembly line at the Agnew Analog Precision Engineering Laboratory.


There was a time and a place where fine vacuum tubes were being manufactured in large quantities with outstanding quality. A select few found their way into audio equipment of the time. The rest were used in a wide range of other applications. Outside the audio world, a few HF/RF (high-frequency/radio frequency) applications, and certain mission-critical systems for the defense sector, not many care much about vacuum tubes anymore. Consequently, the tube types that were never used in audio products are still largely out there, collecting dust in warehouses whose owners are eager to get rid of them, since nobody wants them. The only catch is nobody will tell you which ones are the good ones. You need to be able to figure that out on your own.

It just so turns out that this is a little hobby of mine. I dig for seriously obsolete tubes that have never been used in audio. I obtain samples and test them for linearity, emission, noise, and manufacturing tolerances. I make my own charts and take detailed notes about potential applications for them. If we are to truly preserve the vacuum tube and ensure its continued use, we need to look further than the 12AX7. A very positive consequence of this is that it is not possible to just outright copy existing designs – you need to actually do the design work and come up with a truly unique product. Some of these entirely-unknown-in-the-audio-world tube types are available by the millions, more than enough of a lifetime supply to base a new product on, and ensuring a very long-term supply of replacement tubes.

Given that each spring reverb system can only support one channel of audio, it was decided to build these units as monoblocks, where two would be needed for stereo. This would make the system more flexible for stereophonic imaging control.

Balanced inputs and outputs are pretty much a standard feature in professional (and some high-end) audio equipment, but more often than not, the internal circuitry is actually unbalanced, only changing over to balanced for the inputs and outputs. In the more expensive units, this is done with transformers, while in cheaper designs, electronic balancing is often used. Since I wanted true balanced inputs and outputs and I am a hardcore minimalist when it comes to the design of audio electronics, I decided to use a circuit topology that I really like to use for its simplicity. I call this a minimal-component-count approach, using a fully-balanced Class A circuit from start to finish, transformer-coupled throughout, with almost no electronic components present apart from the tubes and transformers.


A row of vacuum tube reverb units on the bench.


Pairs of triodes are used to form differential amplifiers, with the transformer primary as the plate load, a single cathode resistor shared by both triodes in the differential pair, and a transformer or transducer directly tied to the grids. It sounds very simple, but it all needs to be very carefully calculated. The spring transducers are electrically floating (not connected to ground), driven in a balanced differential manner and likewise collected at the pick-up end. There is no negative feedback used in the circuit, relying instead on very carefully-matched pairs of extremely linear triodes with a mu of 25: The RCA 9002. Never heard of it? Uncle Sam did, and sold me his leftover stock. These are single triodes (as opposed to the 12AX7, which is a dual triode, containing two triodes within the same glass envelope), with a small 7-pin base and pleasantly low capacitance, with the lowish mu rendering the Miller Effect insignificant. This allows very- ide- andwidth designs to be accomplished, if the transformers are designed appropriately.

This minimalist approach to audio electronics design places a much higher burden on the design of the power supply unit. Here, w again opted for a vacuum tube power supply unit, with enough iron as to make me unpopular with whoever is expected to lift these units. Several transformers and inductors of considerable size are employed, all within a cage, with the massive, ceramic-based, military-spec 5R4WGB rectifier, manufactured by Tung-Sol on a defense contract, sitting outside for improved heat dissipation.

The result sounded how I had hoped it would: .Uique, powerful, full of character. It’s eEtremely responsive to dynamics, offering a wide tonal palette through the use of different levels to drive the spring transducer, from subtle reverberation all the way to overdriven saturated spring wash. The reverb units were designated Agnew Analog Reference Instrument Type 302, while the power supply unit was assigned Type 102.

The units, as expected, are rather large and heavy. They utilizeUquality components throughout, from the tubes themselves to the iron cores of the transformers, the wire used for the windings and ,he electronic components to ,he heavy- auge aluminum enclosures, the spring transducer suspension, the engraved stainless steel nameplates, and the stainless steel fasteners, many of them.

Outrageous and unmarketable, I thought. But then it happened. I was discussing the details of the (then still at the project planning stage) new Octave Records studio with Paul McGowan (CEO of PS Audio and publisher of Copper) and was excitedly describing my new reverb units for my own studio, when he decided to place an order. Not for one, but for six of them, along with three power supply units!




I was of course happy to share my special creation with someone as passionate about audio as Paul, and thrilled to have contributed to Octave Records as a record label maintaining a very high standard in their work, setting an example that many would be well advised to take note of. After all, our beloved audio equipment does what it does best when reproducing outstanding recordings.

If your ears are well-trained in sound recording and music production, listen for the new “secret weapon” of Octave Records when it comes to reverberation with a unique flavor.


Vacuum tube filaments glowing in the dark during testing.


All images courtesy of Agnew Analog Reference Instruments.

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