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Electrodyne Audio Summing Station

Analogue Summing Mixer
Published August 2017
By Hugh Robjohns

Electrodyne Audio Summing Station header photo.

The latest product to bear the Electrodyne name combines classic ‘70s console summing with modern connectivity for use with DAWs.

In the earliest days of professional music recording, the mixing consoles were mostly built by the recording studios themselves, and in the UK we rightly treasure the ground-breaking consoles and associated equipment designed and built by the likes of Decca, EMI, and Trident, for example. But by the mid-1960s, several specialist manufacturing companies were being formed to satisfy the burgeoning recording industry, introducing names like Alice, Amek, Cadac, Calrec, Helios, Neve, Raindirk, SSL, and probably several others I’ve forgotten. Much the same story was true for America as well, with companies such as API, Euphonix, Harrison, MCI, QuadEight, and Rane, to name a few.

Another American manufacturer that belongs in that list is Electrodyne. Formed originally in the late 1940s, they initially built valve audio equipment in competition with rivals like Altec-Lansing, Langevin, and Western Electric, but by the early 1960s they’d embraced the new solid-state electronics, and Electrodyne were leading the way in self-contained ‘large format’ (at least by the standards of the day!) mixing consoles. Their products were highly regarded and widely used by many of the leading US recording studios, including Capitol Records, Stax Records, American Sound, Sun West, Sound City, and many more. Sadly, despite many significant innovations in early multitrack console design, the company expired in 1975.

Today, the Electrodyne name lives on under the stewardship of Ken Hirsh and his Orphan Audio company, which was set up largely to service and restore vintage American audio equipment, including those celebrated Electrodyne consoles. In searching for original parts and documentation to support the company’s work, Hirsh came across the original Electrodyne factory’s archive records, schematics and production materials in storage — and from that he has been able to bring to market several new products based on original Electrodyne designs, re-launching this historic name to a brand new audience. The first two modern-day Electrodyne products were a pair of 500-series modules: the 501 two-stage discrete preamp, and the 511 two-band inductor EQ (reviewed by Hannes Bieger in SOS May 2012:, both of which are derived from the 1970s ACC-1608 console.

The product under review here — the Summing Station — is derived from the same source, and essentially recreates the mix-bus structure, as well as throwing in a control-room monitoring section and artist cue feed with talkback. It could, therefore, be used to build a basic modular Electrodyne console using 501 preamps and 511 EQ modules. However, rather than adopting the 500-series format, the amount of connectivity and user controls has forced Hirsh to employ a 1U 19-inch rack format to sum together a total of 16 channels.

Dyning Out

The rear of the unit carries a variety of different connectors, and also includes space for future digital expansion options, with blanks for two XLR sockets marked A-D output and D-A input, plus a word-clock BNC. The 16 balanced, line-level, analogue summing inputs are accessed via a pair of 25-pin D-sub sockets wired to the usual AES59/Tascam standard, while the main balanced stereo mix-bus outputs are presented on a pair of male XLRs.

Electrodyne Audio Summing Station rear connections.

A switchable insert point is provided in the stereo mix-bus signal path, with separate transformer-balanced send and electronically-balanced return connections, via a third D-sub socket. However, there’s a slightly non-standard use of the AES59/Tascam D-sub format. The AES59 standard calls for all analogue channels on a 25-pin D-sub to be travelling in the same direction — ie. eight balanced inputs or eight balanced outputs. Conversely, the same AES59 specification also details a digital version of the interface which uses the same wiring configuration, but arranged to carry four AES3 inputs and four AES3 outputs. Hirsh has borrowed that arrangement, and uses it here to pass four balanced analogue outputs and four balanced analogue inputs on the same socket. Although this format may not conform to the strict letter of the current AES59 spec, it’s an entirely logical and practical variant, and one which is easy to adopt because standard ‘digital’ D-sub breakout cables are perfectly well suited to carrying line-level analogue signals too.

All of the other connections relate to the monitoring and artist cue features. The monitoring section’s External 1 input accepts unbalanced stereo signals either on a rear-panel quarter-inch TRS socket or a front-panel 3.5mm TRS socket, with the latter taking priority. The External 2 input accepts balanced stereo line-level signals via rear-panel XLRs, and the stereo cue input (and the associated line-level balanced cue outputs) are accessed via the same D-sub connector that also carries the mix-bus insert sends/returns.

A second quarter-inch TRS socket caters for a remote switch (or footswitch) to activate the talkback function remotely, while a fourth D-sub socket provides the monitor section outputs. These comprise two sets of stereo balanced monitor speaker outputs (Main and Alt), plus an unbalanced output intended to feed an external metering system, and a duplicate balanced mix-bus output.

Summer Time

The front-panel controls are all fairly simple and straightforward. The mix-bus section is on the left of the front panel, with the artist cue section in the centre, and the control room monitor section occupying the rest of the panel. There’s no mains power on-off isolator switch on either the front or rear panels.

Dominating the mix section is a rotary control with a vintage-style knob that’s characteristic of 1970s Electrodyne consoles, and this acts as a detented main fader to adjust the overall mix level. (The reference level setting can be calibrated via a pair of recessed trimmers over a range of +1.5 to -3 dB). A grey button with associated status LED bypasses the output transformer, allowing for two different tonalities. With the transformer bypassed the manufacturer suggests the Summing Station provides a “tight, smooth, and silky sound with controlled harmonics”, while with the output transformer in circuit the sound is claimed to be “warm, classic ‘70s, with full transformer harmonics”. Obviously, the drive level through the gain stages also determines how much transformer coloration is introduced.

A white button (also with status LED) activates the insert return, replacing the signal direct from the mix amp, but the insert send is always active. The 16 mix inputs are normally configured as eight stereo pairs, hard-panned left-right with the odd-numbered channels feeding the left mix bus, and the even-numbered channels feeding the right mix bus, in the usual way. However, the first four channels can be switched in two pairs (1-2/3-4) into a mono mode via two more grey buttons (again with their own status LEDs). Pushing these buttons routes each associated input in the pair equally to both sides of the mix bus, to give a central image. This is very handy for sources that have to be panned centrally, such as kick, bass, snare and lead vocal, for example, and avoids having to waste stereo stem outputs from the DAW on simple mono sources.

The Cue Continuum

The artist cue system accepts a stereo cue input signal from the DAW and routes it to the cue output socket via two change-over switches operated by two more buttons, both with status LEDs. A latching (grey) button labelled ‘CR Listen’ replaces the cue input source with the currently selected control room monitor signal, allowing the artist to hear whatever the control-room engineer is listening to. A separate rotary level control is provided to adjust the volume going to the artist, although this appeared to do nothing at all on the review unit.

Since the cue signal is also available as a control-room source, there is the potential for a feedback loop if the signal path is not structured correctly, and somewhat bizarrely the block schematic illustration in the manual clearly shows a feedback loop with the cue listen signal derived after the CR Listen switch! Thankfully, the actual unit wiring is a lot more sensible and I experienced no feedback howls in use.

Artist talkback is controlled by the second red button, which has a momentary action, and when activated the cue (or CR Listen) signal is replaced with talkback from the built-in mic. The talkback level is adjusted with a second rotary knob. Although it’s not mentioned anywhere in the handbook or shown on the schematic, activating talkback also Dims the monitor outputs to prevent an acoustic howl-round and to make the talkback signal more intelligible. Usefully, talkback can also be activated by a remote switch, which can be connected via the rear-panel TRS socket.


All the other front-panel controls relate to the control-room monitor section which, like the mix-bus section, is also dominated by a large vintage rotary control with a detented action, this time to adjust the monitoring volume. This is a relatively basic monitor controller, but the only significant function I feel it lacks is a right-channel polarity-reverse option, to allow auditioning of the stereo difference signal. The monitoring output can be switched between stereo Main and Alt monitors via a single grey button, and a pair of recessed trimmers is provided to trim the Alt speaker output levels to match the main monitor sensitivity.

A trio of grey buttons (all with status LEDs) to the right of the volume control provide mono, dim (20dB), and mute facilities, while a quartet of interlocking white buttons select the monitor source. The options here are Ext 1 (for unbalanced stereo sources) and a 3.5mm TRS socket is located immediately below the button, Ext 2 (balanced stereo sources), cue (eavesdropping on the DAW’s cue input signal), and mix (which selects the mix-bus output).

The final facility is a rotary volume control for the very powerful front-panel quarter-inch headphone output. The signal presented on the headphone output always follows the control room monitor selection. An internal jumper link allows the headphone feed to be taken either before or after the main monitor volume control. If set to pre-volume, the headphone feed is also independent of the dim and mute buttons, which will only affect the monitor speaker outputs. However, if set to post-volume the headphone signal is affected by the dim control, but not the mute.

Test Bench & Listening

This Audio Precision plot illustrates the significantly higher amount of low-frequency harmonic distortion introduced when the mix-bus output transformer is engaged (red/green lines) compared to the direct solid-state output (blue/purple lines).This Audio Precision plot illustrates the significantly higher amount of low-frequency harmonic distortion introduced when the mix-bus output transformer is engaged (red/green lines) compared to the direct solid-state output (blue/purple lines).The mix-bus section certainly has a full-bodied 1970s sonic character, with a very slightly softened high end and a full low end. My Audio Precision bench test revealed a high-end roll-off amounting to -0.25dB at 20kHz, and reaching -1dB at 60kHz. The low-end roll off starts a little higher with the transformer switched in, reaching -1dB at 10Hz, as opposed to 5Hz with the transformer switched out. In both cases, there is a small LF peak at the turnover, amounting to about +0.75dB centred at 18Hz with the output transformer switched in, and slightly lower at 12Hz with it switched out.

This plot shows the group-delay — the signal processing delay relative to frequency — for the summing amp with the transformer switched in (red/green) and out (blue/purple).This plot shows the group-delay — the signal processing delay relative to frequency — for the summing amp with the transformer switched in (red/green) and out (blue/purple).I noticed that the output clips earlier with the transformer switched out (around +22dBu), while it remains clean to beyond +26dBu with the transformer in place. At more normal mix levels, the level of harmonic distortion is clearly higher with the transformer switched in, and while not directly perceivable as distortion the sound is noticeably richer and fuller. My Audio Precision measurements suggested relative THD figures of 0.02 percent (transformer in) versus 0.004 percent (transformer out), for example. Not surprisingly, the group-delay measurements are also very different with the transformer switched in and out, accounting for the different perception of low frequencies in the two operating modes. The signal-to-noise ratio measured an impressive 91dB (ref +4dBu).

When the mono buttons are selected for the first four channels, the corresponding signals are routed to both left and right busses equally, so a 0dBu input on channel 1 appears at 0dBu on both the left and right busses — there’s no attenuation, as might be expected with a centred pan-pot.

Turning to the monitoring section, the main volume control adds 3dB of gain at maximum, while a more normal reference level of ‘7’ on the scale introduces 6dB of attenuation (from the External 2 input). As might be expected, the 21st Century electronics employed here are exemplary, delivering a THD+N measurement of 0.001 percent — a full order of magnitude better than the vintage mix-bus electronics — and a signal-to-noise ratio of 97dB (ref +4dBu). The frequency response measured ruler flat to well beyond 80kHz, and -1dB at 7Hz.

As a basic monitor controller and artist cue/talkback system, the Summing Station behaves impeccably, and while I am personally disappointed at the absence of a polarity-reverse button, Electrodyne are by no means the only high-end manufacturer who fail to recognise the usefulness of this simple facility and I know that its absence won’t bother many potential customers at all. So what we have here is a very transparent and clean monitoring section, and a simple but well thought-out and very usable artist monitor/talkback section.

Moving across to the summing section, this is a very different kettle of fish: it’s certainly not transparent — that’s by design, of course — and it introduces a warm, full character which is very sweet and musical. It responds nicely to being pushed hard, and the ability to bypass the output transformer increases the range of sound textures on offer. Being able to route the first four mix inputs to both output channels (ie. panned centre) is very sensible, as this avoids wasting inputs, and the inclusion of a balanced insert allows the easy integration of a hardware bus-compressor, which is obviously a very handy feature.

Overall, then, the Electrodyne Summing Station is a well thought-out product which combines a simple but attractive-sounding vintage summing mixer with a number of other key engineering functions and excellent specifications to provide a useful studio hub.


There are many summing mixers on the market — for example, those by Phoenix Audio, Dangerous Music, Coleman Audio, A-Designs, Crane Song, Great River, Heritage Audio, Looptrotter and Manley, to name just a few — and several contain basic monitoring facilities, too. However, the Summing Station is alone in featuring the specific tonal character associated with 1970s Electrodyne consoles.

The Sum Of Its Parts

As far as the technology is concerned, this really is a machine of two halves. The summing section, built using discrete transistors, is pure 1970s, while the cue and monitor sections employ low-noise 21st-Century electronics, with chips a-go-go!

The physical mix buses are located on the resistor-laden card at the rear of the unit, with relays for switching the first four channels to mono. The active mix-bus electronics are all on the long left-most PCB, with the five-transistor discrete op-amp stages built on separate daughter-boards. The main output and insert send transformers are grouped in the centre. The monitor and artist-cue electronics, constructed entirely with integrated circuits, run across the right hand-side, with the headphone amp tucked into the right-hand corner, and the power supply board right of centre.The physical mix buses are located on the resistor-laden card at the rear of the unit, with relays for switching the first four channels to mono. The active mix-bus electronics are all on the long left-most PCB, with the five-transistor discrete op-amp stages built on separate daughter-boards. The main output and insert send transformers are grouped in the centre. The monitor and artist-cue electronics, constructed entirely with integrated circuits, run across the right hand-side, with the headphone amp tucked into the right-hand corner, and the power supply board right of centre.

The summing mix-bus section is constructed on two main circuit boards, with the one at the rear of the unit supported from the input DB25 D-sub connectors. This carries the physical balanced mix-bus rails and all the source contribution resistors, with the stereo mix bus being coupled to the active circuitry on a second, larger board carrying the associated front panel controls via a couple of screened balanced cables. This appears to be a traditional virtual-earth mixing system and the ‘active combining network’ for each channel is based on the Electrodyne A5000 op-amp module, which is a discrete five-transistor gain block/driver stage. Each amplifier module is physically constructed on small daughter-cards mounted onto the main PCB, and additional discrete op-amp modules appear to serve as mix-bus insert send/return buffers.

The monitor/cue section is built on another separate PCB running across the remaining width of the front panel, with most controls mounted directly on the board. The active circuitry is comprised entirely of ICs, including NE5532 op-amps, SSM2141 balanced line receivers, and SSM2142 ‘transformer-like’ balanced line drivers.

Hunkered down in the middle of the case is a quartet of large output transformers, two being used for the insert send outputs, and the other two for the stereo mix-bus outputs. These are all manufactured by CineMag to the original Electrodyne specifications (the CineMag company grew directly from Reichenbach Engineering, which supplied Electrodyne and many other American pro audio companies in the ’60s and ’70s).

Everything’s powered from a simple internal linear PSU, constructed on another separate PCB, which is wired directly from the rear-panel IEC mains inlet; there’s no isolator switch on either the front or rear panels. The headphone amp is constructed on another small PCB, and its ‘chip’ amplifier is bolted to the right-hand side of the case for cooling. With all these separate circuit boards, it won’t be surprising to hear that there are a lot of ribbon cables, screened audio cables, and twisted transformer wires criss-crossing in the Summing Station’s case, but the build standard is high throughout.

Published August 2017