Chandler Limited RS124 Mastering Compressors/Matched Pair
The Chandler Limited RS124 Compressor is a recreation of one of the most coveted audio valve compressors of all time, the legendary EMI/Abbey Road RS124, reimagined for use in the modern studio environment.
First designed in 1960 by Abbey Road Head of Technical Bill Livy, Deputy Head Len Page and seasoned EMI Audio Designer Mike Bachelor, the Abbey Road RS124 compressor is one of the most desired and mysterious pieces of recording gear ever developed.
Born out of the insufficient Altec 436Bs acquired by EMI in the late 1950s for Abbey Road, the RS124 compressor is most closely associated with recordings made by The Beatles at Abbey Road Studios. Present on virtually every Beatles session, the RS124 left its remarkable signature across most of the catalog.
The RS124’s undeniably smooth character and ability to tame transients without muffling the sound at steep compression levels, quickly became popular with Abbey Road Studios engineers and widely used throughout the now historic studio, for tracking, buss compression, mixing, and mastering. They were so integral to the recording and mixing process that a pair can be seen in almost every control room photo in Abbey Road Studios during the 60s period.
Through the years, from incomplete information and supposition, some have tried to recreate the holy grail of tube compressors, though never fully capturing the elusive Abbey Road RS124 in all its glory.
Schematics themselves only told part of the story, and access to Abbey Road Studios’ historic units and various hand written modification notes would prove paramount towards understanding the true nature of the Abbey Road RS124 compressor and variances between units.
Many revisions were carried out since its inception, and as a result, the Abbey Road RS124 valve compressor has remained a mystery for decades, until now.
Available now, for the first time ever, the Chandler Limited RS124 Compressor harnesses the sound and personalities of Abbey Road Studios’ favorite historic RS124 Compressors (Ser. No’s 60070B, 60050A, and 61010B) still in use at the famed studio to this day, all in a single unit.
Along with features traditionally found on the original counterparts, Input Control, Output attenuation, ‘Recovery’ (release) control and the unique ‘Hold’ setting, the Chandler Limited RS124 Compressor includes modern facilities such as adjustable attack, selectable output impedance, and a special twist, dubbed the ‘SuperFuse’.
Bring home the sound of a legend reimagined for the modern era, today.
The Chandler Limited RS124 Compressor features an internal power supply. Connection is made using the supplied IEC power cable. Power is preset internally to operate at either 120V or 220V.
On the rear panel of the RS124 compressor exists two XLR jacks (wired pin 2 hot), one for input, and the other for output. Both input and output connections are +4, transformer balanced. Output impedance is 600/200ohms switchable, both offer tonal and gain differences. The 600ohms setting is more amenable to the modern studio environment. The 200ohms impedance setting was Abbey Road Studios’ standard.
Stereo Link Chandler Limited RS124 Compressors are stereo linkable via a standard 1/4″ cable, using the female 1/4″ connector available on the rear of the unit. While “Stereo-Linked”, each compressors control-set would still be managed individually.
Cycling power to the RS124 is achieved by turning the On/Off power switch (located far right of the front panel) to the desired position. When the RS124 is powered on, the jewel light on the front panel will illuminate.
Stop: The balance feature is preset at the factory and should not need to be adjusted. In any “push/pull” amplifier, one side pushes waveform, while the other side pulls it in an equal fashion. Over time, the “push/pull” balance may become irregular, and as a result, unwanted artifacts will occur in the audio. This is common to most tube based compressors, including the RS124.
When the “push/pull” circuit becomes unbalanced, it may exhibit subtle rumbling or distortion and modulation of the source signal; this is more noticeable at quiet sections of music, and low frequencies.
Located at the far left of the front panel is the “BAL.” control-set, a circuit rebalancing utility. If the RS124 is suspected of having become unbalanced, simply depress the silver colored momentary switch. This action will induce a ticking sound heard in the monitors. Use a small flathead screw driver and slowly adjust the trim pot (located above the “BAL.” label) back and forth across the range, until the ticking sound is barely audible; the ticking sound will not fully disappear. Once the ticking sound is at its minimum audible level, rebalancing of the RS124 has been achieved.
Historic Note: Early versions of the EMI / Abbey Road Studios RS124 did not have front panel “BAL.” functionality. As a result when the circuit became unbalanced, the compressor would have to be pulled during the session and rebalanced by Abbey Road technical staff. This was scenario was cumbersome, and the resultant session interruption not ideal.
Abbey Road engineer Len Page had a proverbial ‘Light-Bulb’ moment, coming up with an ingenious modification.
A small balancing circuit “BAL.” including a neon bulb, momentary switch, and trim pot was added to the RS124. When the momentary switch was activated, a ticking sound was sent to both amplifiers, with one being phase reversed. If the circuit in a balanced state, the ticking would be canceled out. Otherwise, the engineer would adjust the trim pot until the audible ticking was virtually removed. This allowed for convenient in-session rebalancing of the compressor by the recording engineer in a matter of seconds.
The “Input Control” is variable*, and allows adjustment of input signals applied at the XLR input connector on the rear of the compressor. The “Input Control” is labeled 0 to10 from left to right.
The RS124 is a variable transconductance compression circuit. Clockwise adjustment of the “Input Control” simultaneously raises incoming signal level, while lowering the compressor’s thresHold and increasing the amount of compression applied to the source material.
Counterclockwise rotation of the “Input Control” simultaneously reduces incoming input level, while raising the thresHold and lowering the amount of compression applied to the source material.
Output Attenuator (db)
The ‘Output Attenuator (db)’ control is variable*, use this control to lower the overall output level of the RS124. Rotation in a counterclockwise motion from right to left, beginning at 0 and continuing to -10 will attenuate the output signal of the RS124 to the desired level.
Chandler Limited’s RS124 Compressor provides adjustable attack time via the stepped switch, labeled “ATTACK”, a feature not available on historic units. There are nine attack time settings available, the fastest setting is achieved at the far left position labeled “1”, each subsequent position is slower.
The Chandler Limited RS124 includes three favorite historic RS124 compressors still in use at Abbey Road Studios today, in a single unit for the first time ever.
Included amongst the nine attack options are three positions marked in red. These red colored settings reflect the serial numbers (60070B, 60050A, and 61010B) of historic RS124s. To dial up one of Abbey Road’s favorite historic RS124s, simply set the “ATTACK” switch to the desired serial number.
Historic Note: Each historic RS124 was unique, exhibiting its own character and personality, variances between units were enough so that Abbey Road engineers would request a specific serial numbered unit depending upon the source material it was to be applied to. Furthermore, historic EMI/ Abbey Road Studios RS124s featured a fixed attack time, i.e. the attack timing was hardwired internally to a set time constant. Overall, not all RS124s shared the same values.
“Recovery” is an eleven position stepped switch which controls the compression release time. The “Recovery” switch provides six Recovery or release time options, starting left to right and from slowest (6) to fastest (1), including five identical “Hold” positions denoted by a red colored dot.
Adjusting the “Recovery” switch to any of the “Hold” positions, will keep the source signal compressed to the most recently processed peak level.
Historic Note: The “Hold” feature was an ingenious way to prime or preset the compression level of the incoming signal as a way to overcome potential processing issues.
During mastering, to prevent abrupt compression onset at the beginning of a track, Abbey Road engineers would roll tape forward to the highest peak level and set the RS124 to the nearest available “Hold” position from the desired “Recovery” setting. Now with the compression level held or preset, the engineer would rewind the tape and begin from the top of the track, and any artifact of initial compression onset was removed.
Another use of the “Hold” facility was to prevent level in-rush when there was a precipitous drop in level during quiet passages of a track below the compression thresHold. Here again, engineers would switch the “Recovery” control to the nearest “Hold” position until the quiet passage had passed, before setting it back to the previous release value.
The Chandler Limited RS124 features compression level monitoring facilities via the large ‘db Compression’ meter located at center of the front panel. The ‘db Compression’ meter illustrates the amount of compression applied to the incoming signal, and is labeled right to left, 0 to 30db. When compression is induced, the ‘db Compression’ meter’s needle will move from right to left, indicating the amount of gain reduction applied to the source signal. NOTE: The meter will move faster or slower depending upon attack and Recovery time settings. However, the meter is analog and ultimately slow in nature, as a result the amount of compression applied to the source signal may be greater than visibly observed.
Note: The vintage RS124 and new version RS124 as well as the Altec 436, have an unregulated power supply. Because of this design there can be some variation in the resting place of the meter’s needle, this is normal. For example, if your AC voltage is a bit high, the meter may rise slightly above the meter’s thick black zero bar.
Available on the far right of the RS124 front panel, exists what appears to be a fuse cap, it is not, it’s a switch! This switch disguised as a fuse cap, is a feature dubbed the ‘Superfuse’. Turning the switch to the left will activate ‘SuperFuse’ mode and the RS124 power jewel light becomes brighter indicating the feature has been engaged. What is ‘Superfuse’ mode? While we will not divulge the inner-workings of this special feature, it does change the personality of RS124 to become faster and more aggressive overall. When activated, the source material may become more lively and appearing to jump out of the monitors. WARNING- “Superfuse” is addicting!
From time to time, replacing vacuum tubes may be necessary. The RS124 compressor requires three vacuum tubes: 6BC8, 6CG7, and 6AL5. WARNING: HIGH VOLTAGES are present! Before replacing any tubes, please power off the unit, disconnect the power cable and wait 5 minutes for the voltage to discharge.
On the rear of the unit exists a circuit protection fuse. If fuse replacement is required, cycle the power switch to ‘OFF’, and use a 250mA Slow-blow type.