Grove Hill Audio: Liverpool Tube Compressor

When I plug in a piece of gear (or use a plug-in) I want to be inspired. Sometimes all I need is a little mojo to pull me out of a stall or tailspin. Phase-linear EQ and transparent gain are just fine — and of course play a role in music recording — but at the end of the day, I just don’t want my hard-earned cash to be burned on boring. The compressor about to be reviewed here is not boring, and it’s funny how what was at one time created out of a need for utility is now one of many desired colors in today’s "sonics paintbox."

By today’s standards, the "Fi" of Beatles recordings wasn’t the most "Hi," but man, listening to those recordings now, they certainly have soul, and they express human spirit and innovation. Part of the puzzle, from the band’s first album on, was the gear. This review is about a second-cousin-once-removed of an integral piece of Beatles recording history.

The Liverpool from Grove Hill Audio is a hand-assembled, all- tube compressor with a feedback-based topology. It’s a take on the Altec 436 compressor, several of which were modified and put to use on many a classic album recorded at EMI / Abbey Road Studios. The original 436 design went through three revisions, and EMI purchased a number of B models and implemented a series of modifications on them, naming their versions the RS124. Joe Meek was also an early adopter of the 436, and he also modified them, giving them much faster attack and release times, leading to his signature heavy-pumping mix sounds. There is a history lesson on the Altec 436 and RS124 in the Recording The Beatles book [Tape Op #56]. (If you are interested in the history of recording gear, this beautiful resource is a must have.)

Like the Altec 436, the Liverpool employs a variable- transconductance compression circuit utilizing a medium mu, semi-remote cutoff, twin-triode tube as the gain-reduction element. This tube is re-biased by a 6AL5 tube–rectified side- chain control voltage which causes the mu tube to smoothly change its mutual conductance, providing the change in gain. There is no ratio control, but with circuits of this design, the ratio is increased as input level goes up. Other well-known compressors of this type are the Fairchild 660/670, the Manley Variable Mu, the Pendulum Audio ES-8, and the classic Collins 26U-1.

The Liverpool is not a clone of the Altec 436 or EMI RS124, but something of a hybrid that respectfully brings the spirit of the Altec together with some modern design enhancements. Most notable is the ability to push the unit harder on the front end for some interesting tone-shaping options — more on this later.

I asked Steve Palermo at Grove Hill Audio about the ideas and impetus behind the Liverpool. Steve had an Altec 436C of his own that was typically used for "effect" but not typical compressor duties. After some mods and component upgrades, the unit had new life and greater usefulness, which inspired him to dig deeper into the design that would ultimately become the Liverpool. As he explained to me: "The criterion for the Liverpool design was that ‘the sound’ had to be ‘right.’ It had to stand on its own, while complementing vintage and modern recording gear alike. The quality had to meet professional standards, with low noise and wide bandwidth, and it had to be suitable for modern recording techniques. The final version became a unique iteration of the original Altec 436 and the Abbey Road compressor. Of equal importance, the street price had to be within reach of the larger recording-arts community."

The front panel, layout, and design of the Liverpool is logical and practical. Controls from left to right are Input, Threshold, Attack, Recovery, and Output Attenuator. In the middle sits a nicely sized and easy-to-read meter that displays gain-reduction. Inside, the transformers are custom made, and the tubes are 6BC8/6BZ8, 6CG7, and 6AL5. The back of the unit sports straightforward XLR I/O and a 1/4" stereo-link jack.

All controls are stepped for precise recall and have a nice feel when turned. On the Attack knob, every other setting is "Comp Off" — which allows the user to A/B between the desired attack setting and the compressor bypassed, with one click of the knob and back. This control is similar (in how you use it) to the "Hold" feature introduced by EMI in the RS124, which also makes an appearance on the Liverpool. Every other setting of the Recovery knob is for Hold, which is effectively an infinite release time. Hold was intended to prevent the RS124’s output from recovering while compressing a mix. This was primarily used to keep the quiet ending of a song (or the noise following the last note) from swelling up in volume. As the song ends, the engineer would click the Recovery knob one position left or right to engage Hold. Hold was also useful for the beginning of a song in a two-pass approach, especially if the song had a strong initial transient. In the first pass, after the song kicked in, the engineer would engage Hold and then stop the tape machine — in a way, "priming" the RS124. On the second pass, the engineer would start in Hold mode, then switch to the desired Recovery time after the song’s first notes came through. This technique was employed to prevent an audible "thunk" on the RS124’s initial clampdown of the signal, as the unit had a relatively slow attack time. However, as is the case with much gear, you can use the Hold function in unintended ways with fun results. Examples of this use can be heard on guitar tracks on The Beatles Let It Be (and I am sure on countless other recordings made at Abbey Road Studios). It just locks the audio in place and holds it there with no pumping. It’s a cool feature both for effect and functional use in leveling as mentioned above.

I used the Liverpool on several sources and found it to be a nice tone-shaper, as well as a useful compressor, and I tended to want to run guitars through it more than anything else. For example, you can really cream an acoustic guitar and still have it sound just fantastic with the Liverpool. Even at 20–30 dB of reduction, the result was still musical. I set the attack and release times to about medium, and played with the input and threshold to achieve the right amount of tonal-shaping and squeeze. The Liverpool imparted a nice, warm, roundness to the midrange, and it smoothed out any pokiness, helping the processed track sink right into the mix. Clamping down on a fuzzed-out White Album–sounding electric guitar track with the Hold function was a treat. It was "that" sound. In more gentle use, the Liverpool did its job and added color and character to guitars if desired, and less so if backed off a bit. If you have recorded a guitar sound that is similar to those that came out of Abbey Road in the ‘60s, the Liverpool will take you that extra mile to get you to the finish line.

I also liked the Liverpool on bass. The tube-y vibe was big, fat, and warm, while definition was still maintained. The ability to overdrive the input gain to achieve some harmonically pleasing dirt was a nice bonus. Even at really high gain-reduction (with the meter being completely pegged), the Liverpool sounded great. The Hold function was also cool used in this application for complete level lock. Note that this was a specific and intentional choice for a bass tone, and one that was "vintage" in effect. For a more pop or contemporary aesthetic, the Liverpool can achieve a nice, fat, sit-in-the- mix bass sound with plenty of character as well.

Does it do the Beatles drum thing? Yes — depending on what you think the Beatles drum thing is. You can get reasonably squishy-squashy with the Liverpool and dial in some sounds reminiscent of what the Altec and Fairchild compressors will do when set to achieve such sounds. And remember, when you are struggling to get that Ringo or John Bonham sound, it may simply be that Ringo or Bonham didn’t play drums on your song. Non-judgmentally, the Liverpool is far from the fastest compressor out there, and even at extreme attack settings, the transient of a snare hit comes through, in my opinion to nice effect. Situationally of course, I am confident you can find a use for the Liverpool on almost every mix.

The Liverpool does a nice smooth thing to a vocal, and dialing in useable settings was a snap. It was very difficult to make this compressor sound bad. I also liked it paired in a double-compression scenario with an Empirical Labs Distressor [Tape Op #32] for a real in-your-face intimacy. Again, the ability to shape and add grit with the input stage to some degree was a great option, even when the Liverpool wasn’t employed for dynamic control.

I only had a single unit, so I couldn’t use it on mix bus, or on a stereo drum bus, but I found it flavorful on a mono drum overhead, especially when blended in with the rest of the drums. I also played around with sending various elements of the kit to the Liverpool in a parallel-compression scheme, and found that to be useful and fun when situationally appropriate.

Did I mention I liked the ability to drive the input into tube overdrive for character? Only about a dozen times! Like many tube compressors, the Liverpool can be "colorful," and that’s a good thing. If you need transparent utility, there are plenty of compressors that will do the trick and do it well. The Liverpool delivers control over both dynamic-range and tone, with the ability to drive your signal into fuzz land. So in many ways, you are getting yourself two boxes in one — a compressor and a color creation tool. I really liked adding some grit to a DI-recorded guitar with this unit, and without obviously audible compression.

If you want to hear this compressor really working, you can certainly get there. When used tastefully, it is smooth and a nice tonal addition to a mix. It has an ability to sweetly draw notes and tones out of an instrument, especially in an open mix with just a few elements, so its effect can really be heard. In summary, the Liverpool is a high-quality compressor that achieves the familiar compression characteristics of its family lineage. It is easy to use and has a variety of applications — not to mention that, compared to a modified original Altec, it is very affordable. You know the sound. You love the sound. Now it’s possible you can afford the sound.