AVALON DESIGN
VT-747SP
Stereo Opto-Compressor / Program Equalizer
By Barry Rudolph
A clear case of more than the sum of its parts, the VT-747SP from Avalon Design combines a tube-based, stereo opto-compressor, side chain processor and discrete transistor program equalizer in a two-rack-space package. This stereo line-level processor is designed for stereo bus compression/EQ, signal conditioning of stereo keyboards, input or output processing for DAWs and analog stereo mastering. Its one of a few new pieces of equipment that exemplifies a current trend: clever signal routing and interconnections coupled with advanced control features making for new and more powerful analog signal manipulation.
The 747 looks similar to Avalons VT-737spmic/compressor/EQ preamp with its backlit push button switches, oval-shaped gainreduction VU meter and overbuilt, mil-spec appearance. Pushing each switchreveals that all in/out switching and routing of each section is by any one of22 sealed silver contact by-pass relays. For example, if you have the compressorswitch out, the signal does not flow through that stage. I like the feelof working this unit with its knurled metal knobs and positive detents on theinput. Output gain controls, 0dB center-position EQ faders and center detents onthe Side-Chain Threshold controls.
Operation of the 747 is intuitive for anyengineer who knows compressors and equalizers. There is a single stereo trackinginput level control accepting maximum levels of up to +36dB. This is not aconcentric control, so it is not possible to offset input levels for the leftand right channels separately; Youll have to do that in front of the 747.There is a +10dB push button that jumps the level up if you are working with10dB levels from synths or semipro recording equipment or want to change theinternal gain structure by overloading the front end with super-hotlevels. Both the +10dB button and input control affect compression as you wouldexpect.
The Threshold control adjusts the level whencompression begins from 20dB to +20dB, and the Compression control setsratios from 1:1 to 20:1. A passive optical attenuator is used for gainreduction, introducing very little additional noise or artifacts to the sound. Ilike the small blue LED that flashes with small gain reductions that may gounnoticed on the meter. Attack times are adjustable from 1ms to 200ms while theRelease control adjusts release times from 50ms to seven seconds. There areslightly faster values than the compressor section of the Vt-737SP unit.
The large VU meter reads only gain reductionwith a 20dB range and offers good resolution of the important first 5dB of gainreduction. As this unit is stereo, the meter indicates a summed value of GR forleft and right channels with no way to monitor the channels individually. Thisreminded me again that you cannot separately process two mono signals with the747, as both channels functions are inexorably linked. The Make-Up gaincontrol adjusts the output of a discrete transistor variable gain Class Aamplifier from 0dB to+10dB. When TSP (Twin Signal Path) mode is used, anadditional single triode buffer stage is inserted directly after the compressorbefore this make up amp.
Furthermore, in TSP mode the discretetransistor Class A output amplifier after the main equalizer is swapped for adual-triode tube line amp. Final output level is controlled by a single stereotracking Output control with a range of 20dB to +6dB. Again, not a concentriccontrol that you could individually set left and right output levels. I foundfinal output levels closely matched switching between TSP and the default solidstate path. The sonic difference between these two paths is subtle unless theunit is operated at maximum throughput levels, where near-distortioncharacteristics of these two classic paths become noticeable. I did notice alittle more grunge when compression very hard and using the TSP mode. TSP is thesmoother, rounder, warmer, glowing path... all those tube words. Finally a pairof high-resolution LED meters with 60dB of range measure the output level.
Avalon refers to the Vt-747spsfrequency-conscious side-chain section as Spectral Opto-Compression. This partof the unit is an improvement over the simpler method used in the Vt-737sp,where 2-band section of the 4-band parametric EQ are re-routed to the side chainpath. In the 747, there are dedicated high-frequency and low frequencyparametric controls that adjust frequency-dependent compression. They are not part of the main equalizer. The high-frequency control coversfrom 600 to 10kHz with a Q of 1.0. and the low-frequency control overlaps from60 to 1kHz with a Q of 0.7. Each ofthese controls has separate boost and cut ranging from 15dB to +15dB.
When you use Side-Chain, these boost/cutcontrol act as compressor threshold adjustments for the frequency selected onthe HF and LF controls. By selecting 100Hz and cut, the compressor will clampmore at frequencies at and around 100Hz. By making the compressor more or lesssensitive at certain frequencies, you can create unusual tone shapes andtreatments, or fix real-world problems like excessive sibilance (de-essing) orresonant peaks encountered in live sound work. The Side-Chain Listen buttonmonitors the sound of the side chain signal and lets you precisely tune inthe frequencies you want to dynamically change. In use, I found this anothercompelling reason to own this unit. As it was easy to stress certain bands bydiminishing their level in the side-chain and reducing the need for overallcompression.
This feature worked well on program sourceswhen I wanted a compressed bottom end on an open and transparent high end. Ithink this unit would be an instant favorite with live sound mixers for use onthe stereo front-of-house mix. The Spectral Compression was useful with a synthpatch where the internal chorus rolling through resonance caused a giant leveljump. I couldnt fix the chorus itself (I was mixing a clients tape that Ididnt record), so by finding the center frequency of the resonance peak whilein Side-Chain Listen, I was able to squash just enough to level the peaks only.
The final section of the 747 is the 6-bandstereo graphic program equalizer with a hard-wired EQ bypass switch there is aswitch that places the passive equalizer either before or after the compressorsection, which I liked. As it is a graphical equalizer, you can see from acrossthe control room which bands are being equalized another nice touch. The sixfrequencies and performance of the equalizer are +/-24dB of 15Hz shelving;+/-8dB at 125Hz peaking type; +/-4dB at 500Hz peaking; +/-4dB at 2kHzpeaking;+/-10dB at 5kHz shelving; and +/-20dB at 32kHz shelving. The midrangeequalizers have a Q of 0.1 while the high and low frequency shelves have a Q of0.2 both very broad program EQs.
I liked the equalizer a lot for masteringwork and in use across the stereo output of a low-cost digital mixer I have beenusing lately. The 32kHz band is a popular holdover frequency from the AvalonVT-737SP. Boosting 32kHz and 15Hz provided a wonderful sound for studio playbacklistening hype and live sound work. Boosting 32kHz definitely affects all theway down to pedestrian frequencies around 10Hz; likewise changing 15Hz affectsmost of the low frequencies too. As much as I liked it, I could use two otherin-band frequencies (20 to 20kHz) for most studio and mastering work. (How abouta 16/32kHz and a 15/30Hz octave switch?) Strangely, none of the frequencies orthe boost/cut values are silk-screened on the front panel. A good way to ensurejob security for the engineer who is familiar with the 747, but maybe notendearing to the casual or first time user. Again, singe the 747 is a strictlystereo unit, there is no way to have different EQ curves for left and rightchannels.
The VT-747SP, priced at $2,495 is afinishing tool that updates the process of stereo mix compression withadvanced control and superb sound. Its precise control over the interaction ofdynamics and equalization provides endless sonic sculpting and polishingpossibilities for the creative engineer.
Barry Rudolph is an L.A.-based recording engineer. Visit his Website at www.barryrudolph.com.
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