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Thermionic Culture Phoenix HG15

Mono Valve Compressor By Neil Rogers
Published March 2014

Thermionic Culture Phoenix HG15

Vic Keary’s latest design is a compressor that’s also a mic preamp and an EQ — but not, apparently, a channel strip!

With the emphasis on vacuum tubes or ‘thermionic valves’ always being at the heart of the signal path, British manufacturers Thermionic Culture have built a steadily growing reputation for the high quality of their professional outboard gear. The company’s latest offering has been given the lengthy name of ‘Phoenix HG15 High Gain Valve Compressor’, and it seems a clear attempt to make the high-end quality their design philosophies produce available at a more accessible price.

The HG15 can be described very broadly as a single-channel version of Thermionic Culture’s well-respected stereo Phoenix compressor, but a closer look at the unit’s features shows that there’s rather more to the HG15 than just compression: it also includes a mic/line preamp of sorts and some basic EQ. So is it a channel strip? Well, sort of, but not quite. Despite sporting those additional features, the supplied literature doesn’t make any claims that the unit is a channel strip: it’s described as a mono-compressor with the gain boosted enough to act as a preamp on certain sources, namely vocal recording with a condenser microphone.

Thermionic Culture are known for their slightly different way of doing things, so we shouldn’t be surprised, or alarmed, that it’s not a straightforward product. Indeed, fans of their equipment appreciate how their sometimes ‘quirky’ design choices are aimed squarely at helping you mould your audio in a musical and well-thought-out way.


On first examination, everything gives the impression that this is a high-quality product. All the controls are well laid out and pleasing to the touch: the potentiometer caps (‘knobs’ to you and me) may be plastic, but the feel of quality is very much there, as you’d both hope and expect at this price. Gain reduction is indicated via a large VU meter, which will be familiar to users of other Thermionic Culture products and, set as it is against the brown-red casing of the unit, looks very appealing indeed. One slightly frustrating characteristic of Thermionic Culture’s compressors for me is that the user has no option to switch the meter to monitor input or output levels, but given the ability of valve processors to change their sound according to the degree to which you ‘drive’ them, perhaps using your ears is no bad thing.

In the input section, you’re offered the choice of line or mic input, a 14dB pad and a switch offering 48V phantom power. Switched to mic input, a maximum of 50dB of gain is available, which should be more than enough for recording vocals with a condenser mic, or fairly loud sound sources with a moving coil mic. Realistically, it’s not going to be enough for a passive ribbon on any source, or a moving coil mic on quieter sources, so an additional preamp would be required. I liked the fact that you have to pull the phantom power toggle switch out slightly to engage it: it’s a nice safeguard to help you avoid accidentally switching it on when using the device in line mode.


The compressor section comes next in the signal chain, and its array of controls is where things start to get interesting. As well as being able to govern the amount of compression applied by using your input and output controls, you have five fixed threshold settings to help you decide at what point the compression kicks in. When engaged it ranges from setting 1 to setting 5, with 5 being the maximum. There are no dedicated attack and release times available; instead you have the choice of five ‘time constants’, each offering different attack and release characteristics, all of which are explained in the manual; position 1 is fast attack/medium release, position 2 is fast attack/ slow release, and so on. This type of settings-based approach is not new by any means, but it’s not as limiting as you might think: indeed, the manual mentions the legendary Fairchild 670 as an example of a device that employs it.

There are no ratio controls either. Instead, the HG15 uses a variable system that automatically increases the ratio depending on how hard you are hitting the compressor. The last control in this section is a ‘side-chain bass cut’, which enables you to make the compressor less sensitive to the high-energy low frequencies. The idea is to make it easier to avoid over compressing the signal whenever very low parts of the audio spectrum pass through the detection circuit (whether that be a kick or bass note, or someone inadvertently knocking a mic stand!). A basic stereo-link facility allows you to sum the side-chain signal for use on two HG15s simultaneously.


Moving on to the basic EQ section, there’s a bass cut control, with a choice of two turnover frequencies: 50Hz or 100Hz. This provides a fairly steep shelving type EQ and is aimed squarely at compensating for microphone proximity effect in typical recording situations. Also included is a ‘presence’ control, with six switched settings governing the gain of a broad ‘band lift’ EQ, which peaks at 3kHz. Thermionic’s Vic Keary tells me that this is inspired by the mid-lift function of the company’s Earlybird preamp, and that the six different settings offer not only different gain, but different bandwidth characteristics. Lastly, there’s an ‘Air’ control, which is used to boost the high-end frequencies in a shelving fashion at any one of 10,13 or 18 kHz. Again, there are six different ‘Air’ settings for these frequencies and, as with all the EQ controls, these are explained very clearly in the manual.

The ‘Presence’ and ‘Air’ controls of the EQ section can be switched in and out with an active EQ switch, which is very useful for checking what you’re doing, or for switching between sources in a recording or mixing session.

In Use

I received the review model during a very busy period of recording at Half-Ton studios, and after a glance at the manual I felt confident enough to throw it straight in at the deep end. For its first task, I tried it as a preamp for a condenser mic on an acoustic guitar recording session. With no compression engaged at this point, I was struck immediately by how good it sounded in this application, with a slightly sweet and flattering sound, but seemingly neutral at the same time. A good preamp should give you the feeling it’s getting the very best out of your microphone and this was very much the case in this instance — and, for that matter, with any large-diaphragm condenser mic I used it with during several busy sessions.

Though the potential 50dB gain is far from an earth-shattering statistic, the amount of gain available was more than enough for recording vocals with a typical condenser mic on the six or so vocalists I used it on over the review period, and I imagine this is what a lot of people will want to use it for. As I’d suspected, it was lacking as a preamp when paired with a rather less sensitive Shure SM7 dynamic mic for similar vocal sessions. Nonetheless, I expect it would be up to the task when paired with some moving coil microphones that require less gain, or when close miking louder sources such as guitar amps, for example.

As I got a feel for the unit over this busy tracking period, I began to use the compressor and EQ more and more. Applying 3-4dB of gain reduction at the medium attack/fast release setting quickly became a habit when recording vocals, and yielded very pleasing results with several different voices.

I also tried the HG15 on bass guitar, which it seemed to cope with well and I found flicking though the time-constant options an easy way to find suitable attack and release times. A touch of the presence EQ worked really well on one particular bass recording, also. I was less keen on the results when trying the HG15 on heavier electric guitar parts: it just didn’t seem to be its thing. However, it excelled on a clean electric guitar, the presence EQ doing a lovely job of gently brightening a rhythm part.

In the relative calm of some subsequent mixing sessions, I had a chance to really get stuck into the compression that’s the raison d’être of this unit, and to get to know the EQ sections better. I tried hitting the compressor a bit harder, just to see what it did with a few different style vocals, but whilst it worked well on one male vocal in particular, I found myself backing it off pretty sharpish once I started getting beyond 6-8dB of gain reduction. Things became too sibilant and harsh for my liking at those settings, which surprised me slightly. More surprising was the fact that when I took it past about 10dB of gain reduction, the compression character seemed to smooth out again. Curious indeed! Experimenting with the side-chain bass cut seemed to help with this quirk, and some careful auditioning of the time-constant options produced more pleasing and natural-sounding results on a track whereThe rear panel hosts a socket for stereo-linking two Phoenix HG15s.The rear panel hosts a socket for stereo-linking two Phoenix HG15s. I wanted a more heavily compressed vocal.

I found it quite difficult to fine-tune the amount of compression at times, as switching from setting 1 to 2 on the threshold control made it jump 3-4dB in some cases, and I often found myself adjusting the input gain and output trim to get it where I wanted.

I had a lot success running some guitars back out through the unit in a mix session, applying just a touch of compression and some presence boost. I also started to notice the coloration that you can get from the HG15 if you pull back the output trim and drive the input a bit harder. The slight thickening effect of the harmonic distortion is subtle but appealing and very effective — and as the lack of input/output metering meant I couldn’t see how hard I was pushing things, I found it strangely liberating: I stopped worrying about it and just listened!

The EQ controls in general seem to have been designed by someone who understands the needs of real-world recording and mixing situations, and the selectable options made sense to me and were very usable indeed. I found myself using the Air EQ the least, although it did a lovely job of adding some sparkle to a particular female vocal I was mixing. I can imagine these controls proving more and more effective as you become more intimately acquainted with the HG15, and more confident in how things are translating in your mixes.


This may not be labelled, or packaged as one, but I find it difficult to think of the HG15 as anything other than a channel strip. With the right mic, it has the potential on so many sources to offer a complete recording chain of preamp, compressor and EQ. Would I recommend it to someone as their one and only piece of hardware for recording and mixing? With the right mic, yes — although with some mics, you’d need another preamp or Cloudlifter-type device to provide enough gain. The preamp stage and the general sound of the unit is fantastic; it’s a great unit for general tracking, and for use with vocals and acoustic instruments in particular. Running things back out through the unit when mixing, I found it made some excellent subtle ‘shaping’ options available to me, and for light-to-medium compression duties I was very impressed indeed.

When thinking about how to sum up my opinion of the Phoenix HG15, I found myself browsing some of the other products that Thermionic offer and contemplating the HG15’s price. With a matched stereo pair of these coming in at just a shade over the price of their standard stereo Phoenix compressor (which lacks the HG15’s EQ facilities), they must have made some interesting design choices to meet the price point. But there’s no indication of corners having been cut, and the slightly unusual features that are very much in keeping with the company’s style are to be applauded.

If I’m being picky, although it excels on some sources, and across the board when used sparingly, I’m not 100 percent convinced about the style of variable-ratio compression when using it heavily, without also being able to fine tune the attack and release times. I’m not saying it isn’t effective, I just think it presents rather a different learning curve than a typical compressor. The quality of the preamp and the usefulness of the EQ section, however, more than compensate for this, and make the Phoenix HG15 a seriously tempting proposition!


Suggesting alternatives is difficult, because it depends really on whether you’re looking for a compressor ‘with benefits’ — in which case there are very few, if any, directly comparable devices — or a full channel strip, which brings a huge number of devices into consideration. As always with Thermionic Culture, the quality is high, and you won’t find much that is this good for less money.


  • Great sounding preamp.
  • For subtle and medium compression duties this is excellent: particularly on vocals and guitars.
  • Basic but well thought-out EQ options.
  • A more affordable way into the Thermionic sound.
  • The whole unit encourages a particular way of working: using your ears.
  • Excellent manual.


  • It’s so nearly a full channel strip!
  • Switched threshold control can feel a bit crude.
  • The compression style takes a little getting used to, particularly if used aggressively.


Though described as a compressor, this device actually offers sufficient gain to act as a mic preamp for many recording duties, and also has some basic, but useful, EQ incorporated. It’s a relatively affordable way into the Thermionic Culture family of hardware, with its impeccable-sounding signal path and useful, musical processing options.


£1872 each or £3600 for a matched pair. Prices include VAT.

Thermionic Culture +44 (0)1279 414669

$2799 each or $5598 for a matched pair (matching units requires a seven-day lead time).

Independent Audio +1 207 773 2424.