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Avalon VT-747SP Stereo Compressor & EQ

The 2-U space Avalon VT-747SP offers a stereo tube discrete Class A opto-compressor with a six band equalizer. It does not offer separate controls for the left and right channel, which may not offer as much flexibility as some other units. The good news is you don’t have to worry about matching the two channels, and paying for costly Grayhill detented knobs to do so. Though it does not offer the option for processing two discrete signals, you are able to use it when tracking a single source. Avalon Designs advised that the purpose of the unit is for stereo signals when recording such as keyboards, stereo buss compression and mastering applications. As with any unit with tubes and an internal power supply, it is recommended to provide enough ventilation for the unit.

Overview: In looking at the front faceplate from the top left, there is an input knob that has a 0 gain in the middle, with -20 db - +8 db control. Underneath them there is a +10 db gain button, TSP button which activates the tubes into the signal chain, and an EQ pre or post setting to the compressor. Avalon was smart enough to have colored lights which confirm when all the buttons are activated even from a distance. There is an attack and release knob below them. Even though there are 3 levels of controls, the display is not crowded and everything is easy to access. Next is the Side Chain Threshold of up to – or + 15 db for the LP and HP. The LF offers 60, 70, 80, 100, 160, 200, 250, 300, 400, 600, 1K, while the HP offers 600, 700, 800, 1k, 2k, 2k5, 3k, 4k, 6k, 10 k. Having overlapping freq.’s offers additional flexibility. There is a side Chain active button and SC listen button. The ratio goes from 1 -20, and a make up gain which goes up to 10 db.

The 6 band EQ (to the right of the VU Meter) offers a shelf EQ at 15Hz, with a boost-cut of -24 dB to 24 dB. A 125 Hz bell EQ of -8dB to +8dB, a 500Kz bell of -4dB to +4dB, 2kHz bell -4dB - +4dB, 5kHz shelf of -10 db to + 10dB, and a 32kHz shelf of -20dB - +20dB. All EQ sliders have a detent at 0dB. Though the top of the sliders indicate what type of EQ it is (shelf or bell), the sliders do not indicate what freq.’s they represent. I was rather disappointed that Avalon decided not to include this information. At the far right on the unit there are two 60 dB range LED meters which provide fast L-R output information, an output control of -20dB - +6dB, as well as a hard-wire bypass EQ switch.

Opto-Compressor: The compressor uses a minimum signal path that uses sealed silver relays for the routing, and bypass functions. The attenuator is a passive controller that has a Class A variable gain make-up amplifier. It offers soft to hard knee limiting, and provides a large VU meter to advise on how much gain reduction is being applied to the signal which sits in the center of the unit. There is also a side-chain path that offers both low and high end filtering. This allows particular frequency’s to pass through the compressor without being compressed. This can be helpful when trying to control a mix that has dominate low frequencies such as a strong kick drum or bass guitar. By selecting the right frequency, you could prevent the compressor from grabbing the low end too much to allow the top end to be compress more smoothly. Many compressors offer a HP filter, but most do not offer both a low and high pass filter. This makes the Avalon 747SP more flexibility. There is a blue light that advises the speed and activation of the compressor. There is even a listen mode for the side-chain for monitoring.

Six Band Equalizer: The 747SP offers a discrete Class A six band EQ in a passive design. Unlike a parametric EQ, this graphic EQ does not offer a Q factor so it is more limited for broad strokes. However Avalon Design was smart in offering a low shelf band, three bell mid freq.’s, and two higher shelf bands. For a 2 bus mix, or at the mastering stage, I found the selected frequencies, and their character offer what is needed to control low end, and give a pleasant clean sweetening character to the music. Specifically, the mid freq.’s can help reduce clutter in a mix very effectively. While being a clean EQ it does not sound sterile, and has a touch of smoothness to it. You also have the option to put the EQ before or after the compressor offering additional flexibility.

TSP –Twin Signal Path: With a push of a button, you have a choice of running the signal through a Class A discrete solid state components, or through three high voltage dual triode tubes that are in the discrete amplifiers in the output transformer. Though there is only a subtle difference when engage, there is a noticeable pleasant rounding of the signal when activated.

In use: I ran several different styles of music through the 747SP, as well as tracking instruments with it. The compressor was very effective in helping mixes gel while adding a wider spacial character with the optic compressor. It is able to be very transparent or offer a pleasant clean smoothness to mixes. The EQ is not a surgical type that can fix things, but it can add sparkle, reduce clutter in the mid’s, and control the low end in a mix quite well. I recorded an acoustic guitar in stereo through Rode NT5’s going through my Neve Portico pre’s. The signal was then routed through the Avalon 747SP. In activating the TSP with a ratio of about 3.5, a fairly fast attack and medium release, I reduced the signal up to -3dB. With this setup, the compressor was very clean while controlling the transients. In addition, it added a little bit of fairy dust by gently adding smoothness to the guitar. Regarding EQ, I always believe in using proper mic placement which is the most effective EQ in my book. However when I boosted the 32Khz slider, it added a bit of air to the already enhance sound that the compressor and TSP offered. The result obtained was one of the best sounds I ever tracked with an acoustic guitar.

By using the TSP button, one can add subtle smoothness to the signal. Depending on the source material will dictate if the 2 bus mix sounds better with or without the TPS button. If you (sadly) receive tracks where the engineer squashed the dynamics too much to make the recordings loud, the TSP button can help reduce some harshness one might hear in a mix. It might also sound a little better with the TSP button activated with heavy rock music. When you have mixes that were done well, and you just want to give some transparent control to the mix, I preferred a cleaner signal chain without the TSP activated.

The compressor has the ability to be very transparent in controlling transients just as well as many other optic compressors out there. In addition, by working with the +10 button, make up gain, side chain, and output control, you can obtain a little bit of character to the signal as well. Even so, it is still cleaner than the character of compressors like the Manley Mu or Thermionic Culture Phoenix. It also does not offer the punch of an API 2500 compressor. This is the reason why studios need more than one type of compressor to meet the many needs of their studio. In working with many high end optic compressors, I can say with confidence that the 747SP’s compressor is on par with the best of them. All of them of course have their own personally and strengths which will dictate which one will meet the needs of your studio. However there are two distinct aspects that make the 747SP even more attractive. Besides having a good compressor, it includes a great sounding EQ, and is priced $1,000 - $2,000 less than some of the higher end compressors. With it’s clean control, 3 dimensional sound, and lower cost to many of its rivals, it left me no choice but to add this unit to my rack.

Pros: The compressor can provide a very transparent sound when you want to just control the transients without altering the sound of a mix. In addition, it can add some character and smoothness if so desired. The 6 band EQ is very natural sounding which allows broad strokes to enhance your tracks. The price of the unit is cheaper than many of it’s competition.

Cons: The EQ sliders don’t have labeling for the frequency they represents. Time to get my label maker out. Some may not like the lack of separate controls for the left and right channels.

VT-747SP SPECIFICATIONS
Circuit topology Three (3) dual triode vacuum tubes (Sovtek 6922), high-voltage discrete Class A mode.
Input gain range Balanced, Class A, 20k ohms, -20dB to +16dB with high gain switch
Maximum input level +36dB balanced XLR pin 2 hot
Maximum output level +30dB balanced 600 ohms, DC coupled, discrete Class A
Output type XLR connector, output trim gain -45dB to +6dB
Noise 20kHz unweighted -92dB
Distortion THD, IMD 0.5%
Frequency response -3dB 1Hz to 200kHz
Gain reduction meter High quality, illuminated gain reduction meter 0dB to -20dB range
Output meter Twin 20 segment LED VU meters with -27dB to +30dB range
Compressor type Optical passive attenuator
Make-up gain Variable 0dB to +10dB stereo tracking
Threshold - Ratio Threshold variable -30dB to +20dB, ratio-compression variable 1:1 to 20:1
Attack - Release Attack variable 2mS to 200mS, release variable 100mS to 5 seconds
Side-chain spectral contour Variable threshold -15dB to +15dB, frequency 70Hz to 9kHz twin ranges
Equalizer type Discrete Class A, passive design optimized for full range program material
Frequency bands (6) Variable amplitude LF +/-24dB 15Hz shelf response
Variable amplitude MF1 +/-8dB 125Hz selected Q response
Variable amplitude MF2 +/-4dB 500Hz selected Q response
Variable amplitude MF3 +/-4dB 2kHz selected Q response
Variable amplitude MF4 +/-10dB 5kHz shelf response
Variable amplitude HF +/-20dB 32kHz shelf response
AC power Internal toroidal 100v to 240v, 50-60Hz selectable, 70 watts maximum
Dimensions 19 x 12 x 3.5 in (482 x 305 x 89mm)
Weight 22lbs (10kg)
Dimensions-shipping carton 21 x 18 x 8 in (533 x 457 x 203mm)
Weight-packed 26lbs (11.8kg)

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