Avalon Design, who introduced the world to the AD2055 and the AD2077 equalisers, have turned their efforts towards the vacuum tube pre-amplifier market. JAYSON CHASE reviews the newly released VT-737.



Pure Genius

For those of you who may still be unfamiliar with Avalon products, the best way to describe them is 'pure genius.' Avalon are renowned for their discrete class A circuit designs and components, and phenomenal sound quality. This is a tool for people who enjoy their work.

From The Top

Removing the VT-737 from its box is a feat in itself - at l0kg it's no lightweight, and everything about the VT-737 is solid, from the chunky cross head rotary controls on the front panel to the rather large heat sink on the rear. Looking through the vents on

the top of the unit reveals the positioning of the vacuum tubes which illuminate behind the pre-amplifier input section, and a nifty little LCD display which Avalon have named a 'tube life monitor,' which displays the number of hours and part-hours that the tubes have been operating. Avalon recommend that the vacuum tubes are replaced after 5000 hours of use. Space is limited within the confines of the chassis, but it's clear to see the precision that was involved in developing the circuit layout; the main board is broken down into various sections, and there are several daughter boards which are mounted above the main surface.

From The Side

The front panel is equally well laid out, made from solid aluminum with an additional slab to mount the single VU in the middle of the facia. The use of large purple synthetic knobs is an interesting choice, aside from attracting the attention of anybody who is remotely interested in what you have in your racks, the cross head design does actually make them easy to use without your fingers moving the other knobs around it. The VT-737 is supplied with the flexibility that is required from a pre-amplifier and them some! The inputs, compressor and equaliser feature those three important words 'discrete Class A.'


The VT-737 comes standard with three input connectors, you are able to choose from unbalanced quarter-inch mono TRS sockets mounted on the front panel and XLRs for the transformer-balanced microphone input and discrete high-level class A input amplifier for the line input. A phase reverse relay can be placed into any of the input signal paths, and a high gain switch is also at hand should the overall gain of the pre-amplifier need to be increased - 8dB for line level, and an additional 18dB for TRS jack and microphone inputs. The variable range of pre-amplifier gain itself should be more than sufficient for most applications; line inputs are -20dB to 30dB, quarter-inch TRS -30dB to 30dB and, for microphones, it's zero to +40dB. Phantom power (+48V) and a passive 6dB per octave high-pass filter, with a range that can be varied from 30Hz to 140Hz, is also supplied.


Alongside the pre-amplifier is the opto-compressor which features twin class A vacuum tube triodes that are used for gain matching. The name opto is an abbreviation for the optical attenuator, which is employed to act as a passive level controller. The compressor itself is worthy of review in these pages, the amount of control for the dynamics is enough to put most compression boxes to shame and, with variable ratios from flat to hard knee limiting at 20:1, and release times that can be adjusted from 100ms to 5s, variable attack times from 2ms to 200ms, and adjustable threshold, this compressor is designed with creativity in mind. The large VU meter in the middle of the unit acts as a visual indicator of the amount of gain reduction taking place, as well as the overall output of the VT-737. The option to switch the equaliser into the size chain for de-essing and the ability to move the compressor pre or post EQ are the simple features that are so often missed by manufacturers and so useful to engineers.


The four-band EQ is not parametric, nor is it the standard sweep variety, it's more of a cross between the two. The labels 'Bass' and 'Treble' look very out of place on the equaliser, but don't let this fool you into thinking there may be a dip in standards between the equaliser and the rest of the components in the VT-737. Whichever section of the signal path you care to follow, a 100 percent discrete class A component is never far away.

At the bottom end of the frequency spectrum is the 'Bass' knob. Control for this area comes in the form of a shelving EQ that has a centre frequency selector with four positions, ranging from an impressive 15Hz to 150Hz, and a mighty boost-cut control which can add 24dB in either direction. The same type of EQ is employed for the high end of the spectrum, the 'Treble' knob has four pre-selected positions from 10kHz to 32kHz. The boost-cut range is slightly lower at *20dB. The high and low shelving EQs cannot be totally removed from the signal path if you decide not to use them, although setting the boost-cut knob to zero is sufficient for signals to pass through untreated.

The two mid-range EQs are probably the most dynamic equalisers you are likely find on a pre-amplifier. Both are supplied with a pre-selected adjustable Q. With the high Q push button switch in, they form a narrow bell shape and, with the push-button out, they form a wide bell shape. Some would say that a pre-amplifier such as the VT-737, that is built around fully discrete class A technology, should have fully parametric equalisers as part of its arsenal after all, they are essential aren't they? Not in the case of the VT-737 V's sonic performance.

The mid-range EQs perfectly complement the system, the low-mid can swing the centre frequency from 35Hz to 450Hz. Push the freq. xl0 button, and the range will switch to 350Hz to 4.5kHz. The same goes for the high-mid EQ; with the freq. xl0 button out, it will provide a range from 220Hz to 2.8kHz and, with the button in, it will achieve a centre frequency between 2.2kHz and 28kHz - the term mid-range has a new meaning. The size of the overlap can be tamed by using the freq. x10 button. Treating the same frequencies with separate equalisers is not always advisable, although it can create some interesting effects. What it does allow for is scope for the user to move up and down the spectrum, and it offers several choices of how to treat the signal.

But, as ever, the proof is in the pudding, and using the VT-737 EQs adds a new dimension to the term transparent. There are many ways to gauge how good an equaliser really is, the most simple of which involves switching all the signal processors out of the signal path, leaving just the input and output stages in place, and listening for the difference between what is going in and what is coming out.

Many equalisers will sound good when they are processing signals, but fail the test when they are set to flat, by colouring the signal that passes through them, leaving you with unwanted artifacts. Not so with the VT-737; signals fed to any of the three inputs will appear at the output stage untainted when the signal processors are by-passed. The integrity of the signal path through a system such as the VT-737 is something that we should be able to take for granted and, if you were using an Avalon VT-737, you would be right. But the same cannot be said for all units in the higher echelons of pre-amplifier mythology. We use signal processors for the purpose of treating the audio that passes along the signal path, and not to compensate for the inadequacies of components and circuit designs.

The use of such a high performance compressor and equaliser within the VT-737 is enough to encourage anyone to buy two of them, link them together and drop them across the inserts on the stereo bus for the final mix.

At Work

The VT-737 is simple to use, and there is no need to look through the two-page leaflet that accompanies the box. The controls are familiar and well laid out. Thankfully, there aren't any flashing LEDs, the thought behind the design seems to focus on supplying what is needed rather than superfluous gimmicks. There are two sides to the VT-737; the serious signal processors, which I am sure share the same technology used in the AD2055 pure class A parametric equaliser, and the AD2044 pure class A opto-compressor.

These processors are not for the heavy handed, the precision of the equaliser, coupled with the control that can be achieved using the opto-compressor, is the most agile de-essing combination that I have had the pleasure of using. The range of the equaliser section would hold its own against units built specifically for equalisation, and the compressor is just awesome. After all is said and done, there is a catch. To have only one Avalon VT-737 in the rack is not enough. The diversity of the system far exceeds the limitations of most mono applications and, after the initial introductory period, it won't be long before the idea of stereo processing becomes almost impossible to resist.


Circuit topology


Four dual triode vacuum tubes, high-voltage discrete class A Noise 20kHz un-weighted: -92dB

Noise Microphone EIN


-116dB, 22Hz to 22kHz un-weighted.

Distortion THD, IMD


O.S percent.

Frequency response 0.5dB


10Hz to 120kHz input filter includes Frequency response -3dB: 1Hz to 200kHz line in-out.

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