Can Chandler’s resurrection of one of the most revered preamps of all time help you get that classic Abbey Road sound?
Chandler Limited’s close relationship with Abbey Road Studios has generated many delicious hardware products over the last 14 years or so. Mostly derived from EMI’s mid-period transistorised equipment, including mic preamps, equalisers, compressors and more, these Chandler products have been recreated with commendable precision and accuracy in the unique electronic circuitry that defined one of the UK’s most significant music recording eras. However, Chandler have recently been busy recreating some of EMI’s earlier valve-based equipment, starting with the REDD.47 ‘gain stage’.
This was EMI’s first ‘generic’ valve amplifier module designed exclusively for use in REDD.51 recording consoles (see boxes). In this new REDD.47 Mic Amplifier incarnation, the original EMI design has been cleverly adapted to serve as a stand-alone single-channel mic preamp that meets modern workflow expectations. It is replete with a direct instrument input as well as incorporating facilities that were originally part of the console, rather than the amp module — although the revised design has been fully approved by Abbey Road Studios.
It’s worth noting that, unlike modern console designs, which employ fully integrated active circuitry, the technology in the late 1950s forced EMI to take a rather different approach when designing the REDD consoles. The console itself was essentially a passive device with stud-faders, passive EQ units, resistive mixing and transformer-based Mid-Sides converters. Each section was buffered by identical but independent fixed-gain modules. In the earliest REDD.1 console, these amplifier modules were provided by Siemens and built into a separate six-foot-tall rack, but in later designs they were relocated into base units at each side of the console.
Consequently, these generic amplifier modules were employed universally as mic preamplifiers, line-receiving input amplifiers, EQ section drivers, fader gain-stages, output line drivers, talkback amps, and so on. Moreover, every module incorporated input and output balancing transformers with a relatively simple two-valve circuit, the combination naturally contributing significantly to the overall sound character.
Although conceptually similar to the Siemens V72S amplifier modules used in earlier REDD consoles, EMI’s own REDD.47 — designed as a lower-cost alternative to the Siemens modules — delivered a recognisably different sound character, generally described as more punchy and aggressive compared with the ‘laid-back’ timbre of the V72S. The REDD.47 timbre can be heard on pretty much everything recorded in Studio Two at Abbey Road between January 1964 and the summer of 1968, including virtually all the Beatles tracks from this period.
Overview
There’s no mistaking the REDD.47 Mic Amplifier for having its origins in anything other than a vintage unit! It occupies a large and heavy 2U rackmounting chassis, painted in the traditional industrial battleship grey. The two sizes of black chicken-head knobs have apparently been copied from the original REDD consoles, while the various toggle switches and instrument input socket protrude through milled cut-outs in the front panel. And talking of toggle switches, the mains power switch (with its old-fashioned indicator bulb — not an LED — behind a red jewelled lens) is arranged in the American ‘up-for-on’ fashion, while all the others follow the UK standard of ‘down-for-on’.
The front-panel controls are equally simple, starting with an almost comically large chicken-headed rotary switch to set the amplifier’s coarse voltage gain. Whereas the original design offered three gain settings (34, 40 or 46 dB), Chandler have expanded the options to a more practical seven, with 6dB increments between 16 and 52 dB. The second over-sized rotary switch provides a ±5dB fine trim control, which is also a greatly expanded range compared with the original design’s alignment trimmer. However, this is actually far more than just a fine gain trim; the lower end of the range alters the amplifier’s operating conditions to give a cleaner tonal character, while higher settings introduce more harmonic distortion (up to about two percent), with a thicker and dirtier timbre, as well as making the amplifier overdrive more easily. With the fine gain Set control at +5dB, the overall maximum preamp gain is 57dB.
Four toggle switches arrayed across the panel select the rear XLR input (mic) or front-panel instrument input (DI), a 20dB input pad, output polarity reversal, and 48V phantom power. None of these options were incorporated in the original REDD.47 module, and they are, in my view, essential additions. There’s no warning indicator for phantom power, I presume to retain more of a vintage look, and the instrument input is routed straight into the first-stage EF86 valve in a similar configuration to a similar-period guitar amp like a 1957-vintage Selmer.
Beatling About
The styling of the Chandler Limited REDD.47 is classic British retro, borrowing from the simple, but effective, early EMI console layouts. Setting the preamp up for any specific task is trivially simple, while the addition of modern facilities makes this a very versatile design.
With a maximum gain of 57dB (or 52dB if you don’t want the increased colour afforded by the fine gain trim control), this is not really the ideal preamp for low-output mics, distant placement techniques, or spoken-word applications! However, it is well suited to typical music-studio roles with conventional mics and loud sources, and it’s actually pretty quiet and clean-sounding for a valve-based vintage design.
That last point is important as the REDD.47’s pleasing distortion effects come mostly from overdriving the input stage, rather than increasing the internal voltage gain, and this point was made most obvious to me when trying to dirty up an electric guitar connected via the instrument input. No matter how I adjusted the controls I just couldn’t get anything like the effect I was expecting, and certainly nowhere close to the classic Beatles’ ‘Revolution’ guitar sound which is so strongly associated with the REDD.51 console.
Bench Testing
From a construction point of view, the REDD.47 is very tidy indeed, with two large PCBs separated by a steel screening plate, and mounted on supporting rails across the base of the unit. The linear power supply occupies the right-hand half of the chassis, with the mains transformer mounted on the right-hand wall. There is no internal switch to change the operating voltage.
The audio board occupies the left side, with neat wiring looms running to the front-panel switches as well as the rear-panel connectors and the input and output transformer cans. The EF86 and E88CC valves are mounted horizontally, both in metal screening cans, with all other circuit components mounted conventionally via through-holes on the PCB.
Measuring the gain through the unit, I found the results to match the panel markings within 0.25dB at all of the different coarse-gain switch positions, although the fine gain switch markings proved a little optimistic. The maximum achievable gain was 54.1dB, rather than the specified 57dB. The pad switch introduced 20dB of attenuation, and the output control offered up to 90dB of attenuation.
The minimum microphone input level for a +4dBu reference output was -50dBu, delivering 0.05 percent THD, rising to 0.15 percent THD with a -34dBu input delivering +20dBu at the output. The maximum input level (with the pad and minimum gain) measured +12dBu for 0.25 percent distortion, rising to three percent THD at +15dBu, delivering +4 and +6 dBu at the output, respectively. This restricted output level seems to be caused by the front end of the amplifier clipping well before the output stage.
The maximum instrument input level is about +6dBu before things get really crunchy, while +10dBu gives around two percent THD. The input pad has no effect on the front-panel instrument input. The frequency response extends between 7Hz and 50kHz at the -3dB points.
REDD Letter Day?
Chandler’s REDD.47 is an impressive update on a revered ‘classic’. It’s almost unique, as there have been precious few hardware incarnations of EMI’s first modular amplifier until now. It has a very different character from that of Chandler’s other classic EMI preamp, the silicon-transistor TG2 preamp from the 1970’s-era TG console.
Alternatives
I’m unaware of any other commercially available REDD.47 preamps, or DIY kits for that matter, and although Drip Electronics offered a PCB for DIY construction, it’s no longer available. I think the Chandler Limited REDD.47 rates as the second-most expensive single-channel, no-frills preamp currently available (Retro Instruments’ OP-6 wears the ‘most expensive’ crown). Everything else at this price level that I can think of offers at least two channels, or a single channel with full channel-strip or EQ facilities. Clearly, a lot of value is placed upon the EMI heritage and the rarity of the design, but it does boast an excellent build quality and a seductively attractive sound character.
Siemens V72S Versus REDD
Siemens’ design for the V72 amplifier module was much simpler than the V76 used in the original Burkowitz/CLG consoles, which allowed a much smaller front-panel width. It was transformer-coupled in and out, but employed just two EF804S pentode valves to provide a fixed 34dB of gain (the V76 had three EF804s plus an E83F).
However, EMI’s REDD consoles required matched 200Ω impedances between each passive signal path element (such as the stud-faders and EQ circuitry), but the standard V72 amplifier had a 32Ω output impedance. Although REDD’s engineers fixed the impedance mismatch by installing 168Ω resistors in series with the output of each amplifier module in the console’s wiring, this inevitably also reduced the effective stage gain by about 6dB. So EMI commissioned Siemens to build special 40dB gain modules, designated as the V72S (S for studio — the regular model’s 34dB gain was standardised throughout the German broadcast industry). Around three-hundred V72S modules were manufactured in total.
Although V72S modules were used in the REDD.17 and REDD.37 consoles, they had become prohibitively expensive, especially because so many were required in the four-track REDD.37 consoles. Not surprisingly, the REDD engineers designed their own equivalent gain module, which was introduced as the REDD.47 amplifier, and the REDD.37 console was essentially re-engineered as the REDD.51
This new amplifier was housed in a similarly sized case to the V72, and the circuit design still employed balancing transformers on the input and output (this time with the correct 200Ω output impedance). However, the gain was provided by an EF86 pentode valve followed by an E88CC dual triode, and a front-panel switch (with a separate screwdriver-adjusted fine gain trimmer) allowed 34, 40, or 46 dB of overall gain.
Whereas the Siemens V72 modules each had their own internal mains transformer and linear power-supply section, making them useful as stand-alone gain stages, the REDD.47 design made significant costsavings by using a shared 380VDC high tension power rail derived from the console’s master power supply (with a low-voltage AC heater supply). However, although this arrangement reduced manufacturing cost dramatically, and brought a worthwhile improvement in the console’s hum and noise performance, it also made it necessary to power the console down whenever a module had to be replaced. (V72 modules could be ‘hot-swapped’ without powering down the console.)
This might not sound like a big deal, but EMI’s engineers apparently underestimated the amount of heat generated by the arrays of REDD.47 modules housed in the console’s side units, which gave reliability and performance problems. Consequently, changing faulty modules was a far more frequent occurrence in the REDD.51 desks, stalling a lot of sessions because the process was much more time-consuming than in the earlier (and more reliable) V72-equipped REDD.37 consoles.
While the technical aspects of the REDD.47, such as headroom and signal-to-noise performance, were easily on a par with the V72S, the EMI design had a slightly different sound character which is generally described as being punchier and more assertive or aggressive, compared to the V72’s smoother and more rounded — almost ‘laid-back’ — timbre. Most of EMI’s engineers preferred the more exciting tonality of the REDD.47, and it certainly set the standard through the 1960s.
Seeing REDD
REDD is an acronym for ‘Recording Engineering Development Department’, EMI’s original technical design and manufacturing division formed in 1955 and based in Hayes, near Heathrow Airport, London. EMI had started making stereo recordings the year before but really lacked the appropriate electronic equipment, so REDD’s first project was a ‘Stereosonic’ studio mixing console called, logically enough, the REDD.1. This was in the form of a six-foot bay full of valve amplifiers and associated circuitry, controlled from a very basic six-input passive desk called a REDD.8. The latter was built as a stop-gap to make the REDD.1 system usable while a more operationally advanced design was being developed.
However, while planning this improved console the REDD team, led by Abbey Road’s Technical Engineer Len Page, learned of the work of Peter Burkowitz at the Carl Lindstroem Gesellschaft (CLG) in Germany. CLG was the technical wing of Electrola studios in Cologne — EMI’s German affiliate — and Burkowitz had developed a far more advanced stereo console for the Electrola studios, using Siemens V76 fixed-gain amplifier modules with passive faders and basic equaliser facilities. Consequently, EMI contracted Burkowitz to design a similar console for Abbey Road studios, the first of which was introduced in early 1958 as the REDD.17, replacing the previous REDD.1 desks.
Recording technology developed very quickly through the 1950s, and the REDD.17 enjoyed a relatively short life as a studio console. It actually played a more significant and lengthy role as a portable stereo location-recording console, largely because it was self-contained and easily broken down for transport.
It was the new generation of four-track tape recorders that drove the requirement for a new type of mixing console just a year after the REDD.17 entered service. Once again, the basis for this new REDD.37 console came from an innovative Burkowitz/CLG design in August 1958. Besides its ability to work with four-track tape recorders, the updated design also featured more versatile ‘plug-in’ EQ modules identified either as ‘classic’ or ‘pop’ with different preset bass and treble turnover frequencies. CLG apparently built one REDD.37 console for EMI’s Pathé Marconi studio in France, while the team in Hayes built three more. The original prototype went into London’s Kingsway Hall (with permanent ‘classic’ EQ settings), while the subsequent two (with the replaceable EQ boxes) went into Abbey Road studios One and Two. It was these consoles that were employed for all of the Beatles’ recordings up until January 1964.
The REDD.37 console provided individual passive LF/HF tone facilities on each channel, with eight passive Painton quadrant stud-faders, panning and routing controls, level attenuators and various other facilities including two dedicated echo send/return paths. Although very simple by modern standards, this was a very powerful and high-quality console in its day, and the roots of all subsequent British mixing consoles really stem from its pioneering design. The necessary gain-stages around the passive signal paths were provided by (slightly modified) Siemens V72 valve amplifier modules, each with a nominal 34dB of fixed gain. Siemens V72 modules had been used previously in the REDD.17 console, and were ideal for the REDD.37 too as they were half the width of the older V76, and thus easier to package in a console that required 31 of them!
The virtually contemporaneous successor to the REDD.37 was the REDD.51 — an almost identical desk designed at the end of 1958, even before the former consoles were being installed. Strangely though, the first REDD.51 console, while built in 1959, wasn’t actually installed until mid-1963 (in Abbey Road Studio Three), while the second desk went into Studio Two in January 1964 and became famous for its use on almost all the subsequent Beatles recordings. A third desk went to EMI’s studios in Italy.
To the casual eye, it’s quite hard to tell the REDD.37 and REDD.51 consoles apart, although the latter looks noticeably less industrial, with rounded cabinet casings and generally more pleasing aesthetics. However, although the two designs are essentially identical in terms of facilities, controls, and signal paths, the REDD.51 was far more cost-effective to produce because it employed EMI’s newly designed REDD.47 preamp modules in place of the hugely expensive German Siemens V72S units employed in all previous REDD consoles. Not surprisingly, the new amplifier design also brought a new, more assertive sound character, too, which has become indelibly associated with recorded music of that period.
The REDD.51 consoles remained in service for the rest of the decade, but today the only known surviving console resides at Mark Knopfler’s British Grove Studios in London — the others appear to have been scrapped. In the mid-60s a cost-saving restructuring within EMI resulted in the short-sighted closure of the REDD division, only to be replaced a few years later by the Central Research Laboratories (CRL). The introduction of eight- and then 16-track tape machines towards the end of the 1960s demanded much more expansive and flexible consoles, and CRL started work designing the TG Series of transistorised consoles in 1967. By 1969, all of the REDD.51 consoles had been replaced with TG-series desks, ending EMI’s valve-console era.
Pros
- Thoughtful enhancements to an historic vintage preamp design.
- Surprisingly versatile sound palette.
- Useful ‘rumble’ filter.
- Attractive retro styling.
Cons
- The price tag!
- Hot line inputs can be difficult to handle.
- Instrument input is perhaps not quite what you’d expect.
Summary
The REDD.47 amplifier module was directly responsible for the sonic character of so much British music recorded throughout the 1960s. Chandler’s attractive homage cleverly builds on the original design to deliver a versatile microphone preamp with a sublime sound quality.
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