The latest addition to Avalon Design's high-quality line of preamps, equalizers and compressors is the VT-737 Vacuum Tube Direct Signal Path, a complete single-channel recording pathway for processing any audio source, regardless of signal level or source impedance.When I think of all the individual pieces of equipment I might have to patch together to simply record avocal, the VT-737 seems like a godsend; it combines all the functions of a mic tube preamp, tube optocompressor and a discrete Class A transistor 4-band equalizer with sidechain routing ability, all followed by a tube line level-balanced output stage.

I reviewed the new SP (Special Performance) version that up- grades the original VT-737's purple knobs and faceplate. In addition to the standard Avalon metal knobs and a more detailed faceplate, the SP has a new, higher-level mic input transformer.

The Avalon VT-737 is housed in a two-rackspace, aluminum and steel cabinet and is styled like the rest of the Avalon line, with backlit push-button switches and an oval VU meter. At first glance, the front panel may seem busy, but all four processes are grouped in an ordered, logical way


A front panel switch selects three possible input sources: a rear XLR- balanced line input to the equalizer and/or compressor sections; a front %-inch unbalanced high-impedance (1-megohm) mic input for directly recording instruments; and a rear transformer-balanced XLR input for studio mics. With three input paths, you can immediately switch from recording a direct guitar to recording a vocal to processing an already recorded track with the EQ and compressor. There are also switches for 48 VDC phantom power and (output) phase flip. Another switch selects a bass cut filter operating between the preamp tubes; it's a -6dB/octave passive design with a variable comer frequency of 30 to 140 Hz.

One continuously variable gain control trims the level for all three inputs via a ganged-pot that sets input level and gain of both dual triode stages together in the cascaded tube mic amplifier. The VT-737 uses four Sovtek 6922 tubes: two for the mic preamp, one for the compressor and the fourth for the line output amp. For more mic gain, a front panel switch called High Gain zeroes out the negative feedback in the mic preamp circuit, resulting in an open loop gain increase of 18 dB (for a max total of 58 dB). In operation, I used the unit at the upper end of the gain range, where I found a more "linear" feel when setting level. I also used High Gain for a hotter mic sound but found the adjustment of an exact, working level touchy. In any case, getting a starting level is easy with the smooth, noiseless controls.

In/out switching for each section is via sealed silver contact bypass relays, so if the compressor is switched out, the signal bypasses that stage. Using the mic preamp section only (without EQ or Compressor switched in), I compared the VT-737 to some of the other mic preamps I use. I found the sound very clear, open and flattering on everything I recorded. With a tube U47, the VT-737 was a tinge brighter than some of the other mic pre's. (Or, I could say, the others were a tinge duller...whatever). One small change I would like is a dedicated mic preamp output, so that the mic pre and the EQ/compressor sections could be used on different signals simultaneously.


The opto-compressor features two Class A triode tube sections. The optical attenuator acts as a passive level controller. The Threshold control sets threshold level from -30 to +20 dB with a center detent position of 0 dB. Preamp input gain and threshold settings are totally interactive; shaping the nature of the compressed sound becomes something of a juggling act between the controls.

The Compression control adjusts both the ratio and the "knee" of the slope; a continuously variable pot selects compression ratios from 1:1 to 20:1. The Attack control adjusts attack time from 2 ms to 200 ms. Release time is adjustable from 100 ms to five seconds. There is also a compressor in/out switch, a meter switch that toggles the VU between output and gain reduction and a very cool EQ to Comp Pre button that inserts the equalizer section either pre- or post-compressor. The compressor is very smooth, and I found it nearly impossible to make anything sound bad through it. The compressor is comparable to Avalon's AD2044 Opto-Compressor in operation and sound.


The VT-737 uses an all-discrete, Class A transistor circuit in the 4-band equalizer. The equalizer is divided into two separate equalizers: a Treble/Bass EQ and a dual mid-band circuit. The Treble/Bass equalizer is a passive, shelf-response design with +24 dI3 of gain. The Bass frequency selection points are 15/30/60/150 Hz; Treble Frequency selection points are 10/15/20/32 kHz. This part of the equalizer was very good, broad and smooth. Even boosting at 32 kHz affected the high frequencies in a subtle and positive way.But you can really carve in the equalizer's dual mid-band section. The variable low-mid control selects the 35 to 450Hz range while the high-mid section affects from 220 to 2.8k Hz (t16 dB). Both sections have a "Hi-Q" switch that narrows the Gaussian curve width from a Q of 0.2 to 0.8. A "x10" switch multiplies the indicated frequency ten times. I'd like to see these sections fully parametric with a little higher Q, but this may not be possible within this price/performance envelope. The equalization was smooth and perfect for vocals but not quite as cranky as I sometimes want when recording instruments such as percussion or drums.

An "EQ to SC" switch routes the two mid sections to the sidechain input of the compressor. (In this mode, the Treble and Bass sections remain in the final signal path, and the ability to switch them in/out and pre/post is unaffected.) Using the two overlapping EQ sections of the dual mid-band equalizer, it is a' simple matter to zero in on problem frequency peaks and make the compressor clamp more. Note that you must find new settings for Threshold, Attack and Release to effect the desired sidechain modulation at the same time as proper main-chain compressor operation. In practice, the sidechain feature worked well on certain synth patches having sizable level jumps when a filter rolled through resonance or a chorus effect caused giant, in-phase level build-ups. I also used this feature on a difficult bass guitar; the instrument had uneven output level from one string to the next, and the G string was really loud when an open G was played.

My results on de-essing were mixed, as I found it hard to arrive at optimum settings that squashed sibilance sufficiently yet didn't overcompress the signal. If you have a singer with sibilance problems, you may need to rely on an external de-esser.

The output tube stage where you "make up" or "take away" level to achieve your final recording level to tape has a trim range of -40 to +10 dB. The tube amplifier is followed by a transformerless, high-voltage, discrete, transistor-balanced output amp circuit.

I got big, fat and impressive results on acoustic guitars, vocals and direct bass. I recommend the VT-737 for any recording task requiring a big, open and natural sound.


The internal toroidal power supply can be powered by any AC source in the 100 to 240-volts range. An integrated "soft start," 40-second power-up routine is said to enhance tube life, and Avalon recommends following this with 30 minutes of warm-up time for optimum performance. Maximum gain is 58 dB using the balanced mic input; maximum input level is +10 dB at 25 Hz from the balanced mic input; maximum output level is +30 dB into 600 ohms; EIN noise 20kHz unweighted is -92 dB; THD is 0.5%.

Current retail price is $2,495 for the VT-737SP, while the original purple-knobbed VT-737 is $2,195. Barry Rudolph is an L.A.-based recording engineer. Visit his Web Site at http://home.att. net/~brudolph/.

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