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Chandler Limited REDD.47

Valve Microphone Preamplifier By Hugh Robjohns
Published December 2015

Chandler Limited REDD.47

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.


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 obligatory internal shot. Output transformer on back panel, input transformer on left-hand side, mains transformer on right-hand side. EF86 valve on left and E88CC on right. The row of red capacitors at the front is for the rumble filter.The obligatory internal shot. Output transformer on back panel, input transformer on left-hand side, mains transformer on right-hand side. EF86 valve on left and E88CC on right. The row of red capacitors at the front is for the rumble filter.The rear panel is somewhat spartan, with just two XLRs for the transformer-balanced input and output, plus an IEC mains inlet and fuse holder. The internal PSU is configured at the factory for a single fixed mains voltage (120 or 220 V AC, as marked on the rear panel). Conforming to the original REDD.47 module’s design parameters, and thanks to the use of custom-wound transformers matching EMI’s original specifications, the input and output impedances are both 200Ω. This was necessary in the 1950s because of the matched-impedance interface technology that was prevalent at the time, and doesn’t sit comfortably in the modern matched-voltage interfaces used today. Specifically, a 200Ω input impedance is rather low by modern standards, and a little high on the output side. In practice, neither is likely to cause any problems, but the low input impedance will affect the tonality of dynamic microphones, and I wouldn’t advise using the REDD.47 in the studio driving a long output cable back to the control room!

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.

The rear panel may be “somewhat spartan”, but it gets the job done.The rear panel may be “somewhat spartan”, but it gets the job done.Two more conventionally sized chicken-head knobs are employed to adjust the tuning of a ‘Rumble’ filter, and to attenuate the output level. The latter is a conventional potentiometer, scaled from 0 to -10, but actually attenuating the signal all the way down to silence at the minimum position. Again, neither of these features was provided as an operational control on the original module, but a simpler high-pass filter option was available via a circuit board link. In the Chandler recreation, this intriguing filter option has been made accessible and expanded considerably to provide selectable turnover frequencies of 30, 45, 60, 70, 90, 110, 130, 180 Hz, or Out. This filter uses the input transformer as an inductor to form the high-pass circuit and has a relatively gentle slope. The filter ‘Out’ option is provided because the filter circuitry increases distortion slightly.

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.

Audio Precision frequency response plots showing the full range of high-pass filter settings. Note the expanded low-frequency range stopping at 1kHz.Audio Precision frequency response plots showing the full range of high-pass filter settings. Note the expanded low-frequency range stopping at 1kHz.With high-output mics in front of loud sources a broad range of controllable coloration effects can be achieved through the microphone input, if required, but the REDD.47 also excels when configured as a very clean and detailed preamp, too, with a full low end, a powerful mid-range, and a slightly grainy (almost edgy) high end. Given the revered musical coloration which overdriving this preamp can introduce, many will also want to insert it as a line-level processor across a mix, and this is easily done thanks to the inclusion of a 20dB pad switch and a minimum gain mode of 16dB. The provision of a variable output attenuator ensures sensible output levels even when deliberately overloading the input.

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.

This AP chart shows how the frequency response varies with increasing gain settings, working on a  fixed -31dBu input signal level. The mild resonant peaks at 12 and 25 kHz are due to the transformers, and tend to flatten out slightly with higher gain settings and heavier output loading impedances.This AP chart shows how the frequency response varies with increasing gain settings, working on a fixed -31dBu input signal level. The mild resonant peaks at 12 and 25 kHz are due to the transformers, and tend to flatten out slightly with higher gain settings and heavier output loading impedances.However, I quickly discovered that playing a guitar track recording at line level through the preamp delivered exactly what I was expecting, with a great deal of controllability over exactly how dirty (or clean) the sound should be through adjusting the coarse and fine gain controls. After researching the REDD.47 history (from the outstanding and highly recommended Recording The Beatles tome by Kevin Ryan and Brian Kehew:, this made perfect sense: the REDD.47 was designed as a modular gain stage and the inventive Abbey Road engineers apparently used to daisy-chain modules, using one to raise the signal level sufficiently to overdrive the input of another.

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.

This chart shows how the overall THD figure builds with increasing level through a  very linear region at moderate levels before rising very steeply.This chart shows how the overall THD figure builds with increasing level through a very linear region at moderate levels before rising very steeply.The frequency response remained consistent with different gain settings, the -3dB points being at 7Hz and 40kHz. The LF roll-off seemed to move up slightly to about 15Hz at the highest gain setting. Minor transformer-related peaks were evident at 12Hz (+1dB) and 25kHz (+3dB) when the output was loaded with 600Ω. With a high-impedance destination, the output level was raised by about 1.5dB overall, and the HF peak increased to +4dB. These peaks both tended to flatten out with increasing gain.

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.

This chart shows how the output level saturates and finally limits, and should be compared with the previous one to see how the level relates to distortion.This chart shows how the output level saturates and finally limits, and should be compared with the previous one to see how the level relates to distortion.My impression from memory, is that the REDD.47 sounds larger but noticeably softer through the bottom end, whereas the TG2 is much more solid and ‘contained’. Both share a dynamic, punchy mid-range character, though, and the REDD adds a subtle but attractive grainy character at the top end. Overall, its vintage ‘valve and transformer’ sound character is quite evident, and very musically pleasing, yet this is no one-trick pony and its technical performance is perfectly adequate for modern recordings nearly 60 years on: it has the versatility to deliver fine, clean and delicate, or strong and colourful, character equally well. Perhaps this shouldn’t come as a surprise: it was, after all, originally designed to be used in consoles employed in Abbey Road’s classical Studio One as well as pop sessions in Studio Two! Designer Wade Goeke’s updates and additions are very sympathetic to the original design, but also enhance it, to make this preamp a relevant tool in modern production situations. So what’s not to like? Well, there’s the price: quality like this doesn’t come cheap. A two-channel stereo unit would have been nice, especially for line-level mix-bus ‘coloration’ duties, although on such a device a stronger input pad or a tweaked front-end design would be beneficial to cope better with typical high converter or interface output levels.


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.


  • Thoughtful enhancements to an historic vintage preamp design.
  • Surprisingly versatile sound palette.
  • Useful ‘rumble’ filter.
  • Attractive retro styling.


  • The price tag!
  • Hot line inputs can be difficult to handle.
  • Instrument input is perhaps not quite what you’d expect.


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.


£1999 including VAT.

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Chandler Limited +1 319 885 4200