The Liverpool is a hot-rodded version of the EMI RS124 compressor, which was used in early Beatles albums recorded at Abbey Road Studios. Its lineage predates the Fab Four’s seminal albums, however, as the RS124 was itself a modified Altec 436C compressor, fashioned by EMI engineers. Incorporating an all-tube signal path and solid-state power supply, the single-channel, feedback-style Liverpool compressor uses a dual-triode 6BC8 remote cutoff tube as its gain control element—a variable-mu circuit design.
Hit It Hard
The Liverpool’s operation manual states that the 6BC8 is re-biased by a “6AL5 vacuum tube rectified side-chain control voltage which causes the 6BC8 to smoothly change its mutual conductance.” This design reportedly allows the Liverpool to handle a wide range of input signals quickly without introducing harmonic distortion.
Five rotary controls grace the Liverpool’s front panel and adjust input gain, threshold, attack time, recovery time and output gain. The Input and Threshold controls each have more than three-dozen detents, facilitating precise, repeatable setups. The Input (gain) control is located directly after the input transformer in the signal path and before the tubes; raising it drives the gain-reduction tube harder and increases compression depth (up to 30 dB max). The Liverpool’s Threshold control also increases compression depth by adjusting the compression ratio from 2:1 to 4:1 as you rotate it clockwise (which simultaneously lowers the threshold). A second gain stage amplifies the signal. The Liverpool’s Output (gain) control is a stepped attenuator with a range of 0 to -30 dB (in 5dB steps, presenting constant impedance).
Neither the Altec 436C nor the EMI RS124 offered adjustable attack times. The Liverpool’s Attack control is a stepped affair offering six attack times ranging from 3 to 138 milliseconds. The fastest setting is faster than either the 436C or RS124 provided, while the three slowest settings provide longer attack times than those models yielded; the remaining two attack-time selections are close to what the vintage units provided, respectively. In between each attack-time selection is a setting (labeled ‘C’) that bypasses compression and allows the unit to be used as a line amplifier, with power output of +20 dBm.
The stepped Recovery control selects one of six release times ranging from 127 ms to 6 seconds. In between each release-time setting is a setting (labeled ‘H’) that turns on the compressor’s Hold function (one of the mods introduced in the EMI RS124). The Hold function maintains compression depth by preventing its release phase from occurring.
The trim control on the Liverpool’s front panel, below the Attack control, adjusts the provided VU-style gain-reduction meter to show 0 dB of gain reduction with no signal input and all controls turned down or off. (The smaller trim immediately below the backlit meter is not intended for use.) A power switch and associated status LED finish off the front panel.
On the Liverpool’s rear, you’ll find balanced XLR I/O connectors, a ground terminal, an IEC receptacle for the 6-foot detachable AC cable, a ¼-inch tip-sleeve jack for linking two units for stereo operation, and a voltage selector. Liverpool accommodates 115 or 230 VAC at 50 to 60 Hz. Fifteen to 30 minutes of warm-up time is recommended to achieve optimal performance.
The Liverpool’s frequency response is stated to be 10 to 40,000 Hz ±1.5 dB—more extended than the vintage compressors on which it’s based (the 436C had a 30 to 15,000Hz response for the same tolerances). THD is cited as 0.08-percent at 1 kHz with the output attenuator set to 0 dB. The noise level is said to be 82 dBu below the rated output. Maximum output level is about +34 dBm (hot!) with the compressor bypassed. The Liverpool is covered by a 90-day warranty for labor and materials, 360 days if you register the unit within 30 days of purchase.
Working with the Liverpool in recording sessions, I found it a little difficult to find the sweet spots for the Threshold and Input controls. The Threshold control had a much greater effect on compression depth than it did on the actual threshold for signal processing, for which it seemed to have a quite limited range of adjustment. Because of that, I found it best to set the Threshold control strictly for the ratio I wanted and then adjust the Input control to determine how much of the signal would exceed the threshold. Even with the Threshold control set for the lowest possible ratio and highest threshold, I often found myself setting the Input control very low—between ‘2’ and ‘3’ to avoid compressing tracks too much; raising the control from 2 to 3 typically increased gain reduction a whopping 20 dB.
Once I learned the ropes and put on my kid gloves, I got great results using the Liverpool to compress lead vocals. There was no mistaking this was a tube compressor; the sound was rich and full-bodied. I could increase flattering distortion by raising the Input control—thereby driving the input tube harder—and lowering the output gain, although this often increased compression more than I wanted. Using the same technique with the compressor bypassed (using the Liverpool as a line amplifier) sounded terrific, dishing out velvety tones.
Heavy compression produced the euphonically dense sound variable-mu compressors are renowned for. (This phenomenon is due to the gain-control tube saturating.) On electric bass guitar—recorded via DI using an upstream Millennia TD-1 Music Recording System in FET mode—the Liverpool delivered a supersize track brimming with lush overtones and exhibiting beautifully leveled dynamics. I was shooting for a non-percussive, compact sound. Very fast attack and release times and more than 15 dB of gain reduction produced a rounded attack and wonderful sustain, without any hint of pumping or unpleasant distortion (like fast time constants can sometimes cause on bass tracks). I also got great electric guitar tones—dense and creamy—using a fast attack and slow recovery.
The Liverpool’s multiple compressor bypasses (which, remember, are situated between attack-time settings on the Attack control) are meant to help ease A/B comparisons of wet and dry signals, but they didn’t always act as advertised. If I switched the Attack control to a bypass position before compression was released and level restored, the gain reduction would persist in the supposed bypass mode.
Regardless, I’d rather the compressor featured a single, isolated switch for bypass; that would allow me to adjust attack times on the fly without potentially whipsawing dynamics as the compressor gets repeatedly turned on and off. I could put the Liverpool into Hold mode to prevent gain recovery while stepping through attack-time settings, but such a workaround ideally shouldn’t be necessary. My only other disappointment was that, considering the relatively high price tag, I’d like to see internal sidechain filters or an external sidechain input.
My criticisms concerning the Liverpool’s operational quirks don’t take away from the fact that it’s a terrific-sounding compressor. If you’re looking for a compressor that delivers rich texture, girth and density, be sure to give the Liverpool a test drive.
Michael Cooper is a recording, mix, mastering and post-production engineer and a contributing editor for Mix magazine.
Compressors can sometimes produce an accentuated thump when processing the first note of a percussive track. To preclude this, play the track back and, the instant gain reduction is achieved, place the Liverpool into Hold mode. Then rewind the track and compress it from the beginning.
COMPANY: Grove Hill Audio
PRODUCT: Liverpool Tube Compressor
PROS: Sounds terrific. Can be used as a line amplifier.
CONS: Finicky operation. No internal sidechain filters or external sidechain input.