The BG2 is a handmade, all-tube, single-channel compressor that uses a twin-triode 6BC8 as a variable-mu gain-changing element followed by a push-pull pair of 6V6 beam-power pentodes for its output stage. The gain-reduction stage and 6AL5 tube circuit are similar to the Abbey Road RS124 (which was adapted from the vintage Altec 436C), while the output stage is like old RCA BA6-A or Gates Sta-Level units.
Under the hood, the unit reveals excellent point-to-point hand wiring and large Altran Corp. output and input transformers. The main circuit board attaches to the rear inside wall of the unit, with ceramic tube sockets that accept the horizontally positioned tubes on the rear panel. I like that the hot tubes are distanced from the circuit board—users have to be careful when inserting the BG2 in a rack, as there is no cover or screen protecting the tubes. (This works out well for ventilation; tubes do generate heat.) There are also smaller circuit boards for the output attenuator resistor network and the power supply rectifiers, with a 3-terminal regulator for the tube’s DC filament voltage.
The electronics are housed in a rugged, two-rackspace cabinet, and the dark gray front panel has a customized U.S.-made Simpson gain reduction meter and large knobs for Input and Output gain pots, to go with a six-position release time rotary switch. The six release times (1-6) are 270, 400 and 700 ms, then slower at 1, 1.7 and 2.8 seconds. Without a separate threshold control, adjusting the Input gain control sets the compression amount, and the Output gain sets final makeup gain.
Technology Explainer: Understanding Compressors and Compression, by Barry Rudolph, Mix, Jan. 1, 1999
Other front panel controls include old-school on/off AC power switch, pilot light and mains fuse; a Link on/off switch for stereo linking a second BG2; and a two-position attack time switch, with Fast at 30 ms and Slow at 110 ms. There is a kind of gain range switch called Input Impedance to use either the 150 or 600-ohm impedance primaries of the input transformer. There is also a Compression On/Off switch—it’s not a bypass but just disables the sidechain so you can use the BG2 as a line amp only. (The Altec 436C worked as a line amp, but you had to pull out the 6AL5 tube.)
I thought highly of the Feedback switch to choose between “American” and “British” feedback styles in the amplifier circuit. American mode is like the Altec circuit and is more aggressive with more harmonics, color, gain and compression. The British mode has less gain and a cleaner sound. Switching between them while auditioning audio will produce a noticeable click because of the inherent circuit changes taking place inside.
In the Studio
One of my first tests was on a female lead vocal track that was dynamically wild, requiring special treatment to blend well into the mix. In Pro Tools HDX 12.8.2, I placed the BG2 as a hardware insert after an EQ plug-in on the vocal track.
I started conservatively: Brit mode, slow attack, slowest release time #6, and 600 ohm input impedance. I set the Input knob to get about a 3dB max of gain reduction (indicated) on the loudest peak. I was looking for clean and smooth leveling.
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While the EQ plug-in got the spectral balance dialed in, the BG2 reined in the volume level differences and added a smooth, thick, “chesty” tone. The vocal sat in the track better with only minimal automation moves here and there. Far from a noticeable “squashed” sound, I could now better hear the singer’s vibrato, glottal stops and other nuances, even with a pounding rhythm track surrounding her.
Next, I tried a more aggressive treatment on the same vocal track, going to 12 dB or more of gain reduction. I changed to 150 ohm impedance to push more level into the input stage and the detector circuit. I also switched to American mode, but kept the slow attack and used either release time #2 or #3.
This was where I would have liked a slightly different collection of release times, as I could have used a #2.5. Based on the singer’s phrasal spacing, I was looking for a release time slow enough that the BG2 did not re-attack in between short phrases. There should be more release-time choices at the faster end of the six positions and fewer slow times. (Highland Dynamics’ Bryce Gonzales has offered to modify the release times and the two attack time choices on request.)
I should also note that the subtle nuances brought up in the first setup are more pronounced here—most noticeably breath inhalations as the BG2 attacked at the start of each new phrase after the release time expired. If the inhalations became a nuisance, I would go in and automate or clip-gain those moments down as part of the mixing process. With this artist, the finished sound was worth this extra tedium—it’s part of the mix!
Drum and Bass
It’s a definite sonic statement to use the BG2 heavily on individual drum kit tracks or overheads. I used the maximum Input gain control setting (10), 600 ohm, American mode, slow attack and release #1 for a healthy “snare drum smackdown.” Using British mode offers a close, drier sound with less kit and room leakage brought up in between snare drum hits. This is all fun, and using the BG2 is very natural, with lots of variability as each control interacts with the others.
Like most vintage-style compressors without threshold and ratio control knobs, there is no hardware bypass on the BG2. My workaround is to duplicate the track in Pro Tools—one with the BG2 inserted and the other without. I could switch between them as I did for processing a bass guitar track. I was sold immediately after the first A/B comparison. The BG2 is great for bass guitar; you can go clean or dial in enough sonic grit and color for edge, and always get good leveling with increased sustain.
I also tried the BG2 simply for color as a line amp with the Compressor Off mode. I was able to get a grainy tone on a direct, five-string bass recording, anywhere from an overdriven guitar amp to a fuzz-tone bass to a fairly clean tube hi-fi sound. Start with 150 ohm and British mode—Input was at 4 and Output was at 3 to match the duplicated bass track in my Pro Tools session. American mode bumps level up and adds more dirt in the bottom octaves, especially when an open B string was played. You can (unintentionally) thin out the resultant bass sound if you overdo this processing effect.
The BG2 marries the sound and operation of a couple of classic tube compressor circuits in a reliable and modern unit. Its versatility and simple operation combined with a first-class quality build make a solid marriage. As an investment in great sound, I can highly recommend this hot-rodded compressor for vintage tone shaping and processing. n
Barry Rudolph is a Los Angeles-based audio engineer and educator. You can visit him at www.barryrudolph.com.
Company: Highland Dynamics
Product: Highland Dynamics BG2 Compressor
Pros: Super-versatile and useful tube compressor; lots of sounds possible.
Cons: Pricey but worth it!
The BG2 has enough gain to amplify a connected microphone plugged directly into its input XLR. For direct guitar, I used the 150 ohm mode with a Jensen transformer DI (passive) box connected. My Fender Strat sounded super-clean with the BG2’s Input knob “dimed out” (10) and barely registered any gain reduction. An all-in-one tube direct box/compressor is an asset in any studio. Loved it!