660 and RS.124 Compressors

Agustín Saravia
agusaravia@hotmail.com

A puzzling moment the recordist faces when recreating the techniques of the Beatles is that their compressors and limiters lack modern controls to fine tailor this processing. If we know the theory this is far from being a limitation, rather it helps to avoid producing disastrous results. When faced with this scenario the two most important controls on which we shall rely are release time and the gain reduction meter.

Provided we aren't going for a creative result, like the distorted drums in "Revolution" or the gurgling sound of harmony vocals in "Octopus's Garden", compressors are such devices that are used with subtlety for taming peaks and overall loudness. Therefore, in what seems like a contradiction, adjusting compressors by ear may not be the best option. This is due to the main processing being on the attack stage of the soundwave, which goes from the order of hundreds of microseconds to tens of milliseconds. This small time frame is veiled to human ears for identification of loudness levels and time shifts. Our ears start to identify distinct attacks in the order of 1/20th of a second, or 50 milliseconds, for music and a few less milliseconds for speech.

Release controls of compressors usually go from 50 milliseconds to various seconds, hence, their effect is perceived by ear. However, most of the other characteristics of vintage compressors usually have to be relied on the gain reduction meter, which isn't as fast as the soundwave attack but help us to monitor visually what our ears can't.

A rule of thumb for long release times and high compression levels is that they out average each other. High gain reduction levels get away with short release times and long release times get away with low gain reductionn levels. Therefore if we are processing a mix or submix of instruments, each with different attack times, a long release time and low gain reduction of -1dB helps to avoid the compressor ducking on the attack of every instrument and then being obvious. On the contrary, if we are trying to tame an instrument with inconsistent levels a greater gain reduction of -3dB to -10dB may be of help but using a shorter release time.
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The same could be said of attack times, and that's the reason at EMI Studios the slow attack of the RS.124 was preferable for mixes and rhythm instruments while the fast attack of the 660 was preferable for lead instruments.

In the next examples the compression meter is as important as the release time setting. The other controls are of less interest.

1 Mixer Bus and Audio Programme

During the Beatles 4-track years, i.e. 1964-1968, the REDD.51 mixer usually featured a matched pair of RS.124 and a pair of 660 inserted before the faders of busses I, II, III, and IV respectively. Busses I and II featured RS.124 while busses III and IV featured 660. Due to the gain structure of the REDD.51, the input of the RS.124 and 660 were hit by a signal of -14dBm. This gain would be equivalent to -32dBFS in the digital domain when 0dBu is aligned with the EBU standard of -18dBFS.

Table 1. Bus, Compressor Type, and Audio Programme

Bus	Device	Programme
I	RS.124	Stereo mix left channel, mono mix, drums and bass (1964-1965), bass (1966-1968), rhythm section (1966-1968).
II	RS.124	Stereo mix right channel, rhythm guitars (1964-1965).
III	660	Percussion, lead guitar, vocal.
IV	660	Drums (1966-1968), vocal.

Fig. 1a. Ardour emulating a REDD.51 mixer. The center section shows a pair of RS.124 and a pair of 660 inserted before faders (Blue) of busses I, II, III, and IV.

Fig. 1b. A pair of RS.124 and a pair of 660 in Ardour.

2 RS.124

The 436B is a variable µ compressor designed by Altec in 1958 for radio broadcasting applications. It features an input gain knob as single control, an attack of 50ms, a release of 1s, a threshold of 0dBm, and a compression ratio of 2.5:1 at maximum compression. Led by Len Page, engineers at EMI Hayes modified the 436B by adding a stepped bridged-T output attenuator, a six-position recovery switch, and trying different values of capacitor C4, until finally settling on 0.5µF in 1963, shortening attack to 25ms and release to 500ms. Three RS.124 units survive to this day at Abbey Road Studios, bearing serial numbers 60050A, 60070B, and 61010B. Attack times of these RS.124 units are 50ms, 25ms, and 16ms for serial numbers 61010B, 60050A, and 60070B respectively.

Table 2. 436B and RS.124 Specifications

Model	Attack (ms)	Release (ms)	Ratio	Threshold (dBm)
436B	50	1000	2.5:1	0
RS.124	25	500
		840
		1340
		2340
		3990
		6790

Fig. 2a. 436B serial number 1061, Feb. 27, 2012.

Fig. 2b. RS.124 serial number 60050A and 61010B at Abbey Road Studios, Nov. 2012, photograph by Hannes Bieger.

Fig. 2c. Schematic of the 436B. Attack time is determined by the product of C4 by R12, release time is determined by the product of C4 by R9, C4 is 1µF.

Fig. 2d. Schematic of the RS.124. Attack time is determined by the product of C4 by R12, C4 is 0.5µF.

2.1 Compression Ratio

Being the RS.124 a variable µ compressor, its ratio settles at 2.5:1 with a very soft knee.

Fig. 2.1. The RS.124 at maximum compression features a ratio of 2.5:1 and a threshold of 0dBm.

2.2 Control Settings

In every photograph of the Beatles in the studio control room the RS.124 features consistenty the same settings: Recovery at 1, Input Control at 3, and Output Attenuator (dB) at 0. This results in -1dB of gain reduction when a line-up tone of -14dBm is fed to the input. Ken Scott recalls that for mixes and most of the time the gain reduction needle almost barely moved, it just tickled in the signal peaks. Emerick wrote down -10dB of gain reduction for dilruba in "Within You Without You". Hence, balance technicians at EMI Studios aimed for greater gain reduction for individual instruments, like guitar or bass, than for an ensemble or rhythm section.

Table 2.2. Audio Programme and Gain Reduction

Programme	Recovery	Input Control	dB Compression	Output Attenuator (dB)
Mix	1	3	-1	0
Rhythm Section	1	3	-1	0

Fig. 2.2a. Settings for rhythm section and mixes.

Fig. 2.2b. Session for "Yesterday", EMI Studios, Jun. 15, 1965, photograph by Koh Hasebe.

Fig. 2.2c. Session for "Why Don't We Do It in the Road", EMI Studios, Oct. 10, 1968, photograph by Linda Eastman.

Fig. 2.2d. Let It Be sessions, Apple Studios, Jan. 22, 1969, photograph by Ethan Russell.

3 660

The 660 is a variable µ compressor designed by Fairchild in 1959 for disc cutting. It features two attack settings of 0.2ms and 0.4ms, six release time settings from 300ms to 5000ms, two threshold points, a DC or limiter threshold preset from factory to +2dBm, and an adjustable AC or compression threshold, and a maximum compression ratio of 20:1. As mentioned above, the DC threshold was factory preset to +2dBm bringing compression ratio to 4:1. Engineers at EMI Hayes modified the 660 by replacing the input gain knob with a stepped -55dB attenuator to precisely recall input gain settings, a feature useful for classical music sessions spanning through several weeks. As with the RS.124, the 660 were modified and calibrated in matched pairs to funtion in stereo when side chained. This feature allowed some interesting effects, like the gurgling backing vocals by Harrison and McCartney during the guitar solo in "Octopus's Garden", which are compressed when triggered by bubbles blowed by Ringo through a straw in a jar glass.

Table 3. 660 Specifications

Model	Attack (ms)	Release (ms)	Ratio	Threshold (dBm)
660	0.2	300	4:1	+2
	0.2	800
	0.4	2000
		5000
		10000
	0.2	10000

Fig. 3a. EMI modified 660 unit 519 at Abbey Road Studios, Nov. 2012, photograph by Hannes Bieger.

Fig. 3b. Schematic of the 660.

3.1 Compression Ratio

Being the 660 a variable µ compressor, compression ratio depends on the setting of AC and DC thresholds. Since DC threshold was factory preset to +3dBm, which means that compression ratio was 4:1 when AC threshold was set to maximum. The DC threshold can be adjusted by turning a screw marked DC THRESHOLD mounted on the chassis inside the housing, while the AC threshold can be adjusted by turning the control marked simply THRESHOLD on the front panel.

Fig. 3.1a. The compression ratio and threshold of the 660 are preset from factory to 4:1 and +3dBm respectively, as can be seen on curve 3 of the graph above.

3.2 Control Settings

In every photograph of the Beatles in the control room the 660 features the same settings: Input Gain at -14dB, Threshold at 7, Time Constant 2. Instruments with long sustain benefit from -3dB to -5dB of gain reduction. Drums need subtle amounts of only -1dB to enhance the sustain of cymbals.

Table 3.2. Audio Programme and Gain Reduction

Programme	VU	Input Gain	Threshold	Time Constant
Vocal	-3	-14	7	2
Drums	-1	-14	3	2

Fig. 3.2a. Settings for vocal.

Fig. 3.2b. Settings for drums.

Fig. 3.2c. Let It Be sessions, Apple Studios, Jan. 22, 1969, photograph by Ethan Russell.

Fig. 3.2d. Session for "Maggie Mae", Apple Studios, Jan. 24, 1969, photograph by Ethan Russell.

References

[1]  436B Amplifier, Altec Lansing, Anaheim, EE.UU., 1960.
[2]  Model 670 Stereo Limiter, Fairchild Recording Equipment, Long Island, EE.UU., 1959.
[3]  B. Kehew y K. Ryan, Recording the Beatles, 2ª ed. Houston, EE.UU.: Curvebender, 2010.
[4]  M. Lewisohn, The Beatles Recording Sessions. Nueva York, EE.UU.: Harmony, 1988.
[5]  A. Nisbett, The Sound Studio, 7ª ed. Waltham, EE.UU.: Focal Press, 2003.
[6]  L. Page, RS.124 Compressor Amplifier (Altec), E.M.I., Hayes, Inglaterra, Dibujo No. RS.124/D1/2, 2 de abril, 1965.
[7]  K. Scott y R. Owsinski, Abbey Road to Ziggy Stardust. Los Ángeles, EE.UU.: Alfred Music, 2012.

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