REVIEWS:From AUDIO MEDIA September, 1999



Twin Signal Path Compressor-Equaliser

By Benedick Grant

For a relatively young company, Avalon Design has attained a formidable reputation as a manufacturer of premium-quality analogue signal processors. The Vt-747SP is a stereo signal processor, combining an opto-compressor with a six-band programme equaliser. The unit features Avalon's TSP (Twin Signal Path) architecture, which allows the user to choose between a tube-based output amplifier, and a Class-A discrete transistor amplifier. The compressor has an additional two-band equaliser on the side chain signal path. This feature allows frequency-dependent compression to be carried out without the need to used an external equaliser.

A first glance at this processor inspires confidence. Externally the appearance is of an impeccably designed well-engineered product. Lifting the unit confirms this. It weighs a hefty 22lb (10kg) - significantly more than many power amplifiers!

The case is of steel construction, with a 6mm anodised aluminum front panel. A separate meter bezel is 12mm thick! Internal construction is of a similar standard. The torroidal transformer and other power supply components are contained in a separate steel sub -enclosure, to shield the audio circuitry from electromagnetic interference.

The circuitry occupies over 20 separate printed circuit boards, the main ones being securely attached to the chassis by means of hex-head screws. There are three valves: Russian-made Sovtek 6922s. The compressor is configured around a passive optical attenuator.

Ventilation slots are cut into the upper and lower panels of the case, and the rear panel has a substantial heat sink. The unit can draw up to 55 watts of power, and must therefore be mounted in a well-ventilated enclosure.

On the rear of the panel is an IEC mains socket, and voltage selector, allowing an operation voltage of 100V 110V or 220V to 240V, at 50Hz or 60Hzto be chosen. Mains and signal earths are brought out to an external terminal block, allowing them to be linked, or the signal ground "lifted".

Audio inputs and outputs and outputs are electronically balanced and on XLR connectors, wired pin two hot. The maximum input level is +36dB; the maximum output is +30dB. The outputs are DC coupled. All signal routing and bypass switching is accomplished by means of sealed silver relays. A protection circuit mutes the outputs for 40 seconds when the processor is first switched on.

No external access to the compressor side chain is provided, although the provision of a dedicated side chain equaliser largely removed the need for such a feature.

The manufacturers specifications are: Frequency response 1Hz to 200kHz (-3dB), distortion THD, IMD 0.5 percent, noise 20kHz unweighted -92dB. The Vt-747SP is enclosed in a 2U, 19-inch rack mounting case., which is 12 inches deep (482mm x 457mm x 203mm).


This unit is designed for stereo operation rather than processing two discrete signals, so there is only one set of controls, operation on both components of the stereo signal.

Controls for the compressor are located on the left-hand section of the control panel. There is a dual range input gain control. This normally operates through the range -20dB to +8dB with a detent a 0dB. However pressing an adjacent switch selects the range -10dB to +18dB. The threshold above which compression takes place can be varied from -30dB to +20dB. Attack and release times are variable from 2ms to 200ms, and from 100ms to 5s respectively. On the front panel these controls are calibrated simply from slow to fast. There is no programme/dependent, auto release setting.

The two-band spectral equaliser, which operates on the side/chain signal, can be switched in or out of circuit. It comprises two discrete sections: a shelving response low-frequency filter, sweepable through the range of 70Hz to 100kHz, and a bell response high-frequency filter with t a frequency range from 500Hz to 9kHz. Up to 15dB of boost or cut is available on each filter. A side-chain listen facility allows the user to switch the output to monitor the side chain signal in order that the effect of the equaliser can be auditioned to aid "homing in" on the part of the signal to be compressed. The compression ratio can be set within the range 1:1 to 20:1. A blue LED illuminates progressively to indicate compressor speed and activation. The large back-lit, mechanical VU meter monitors gain reduction. The compressor can be switched in or out of the signal path.

On the right-hand side of the front panel are the programme equaliser and output controls. This six-band equaliser is normally located after the compressor in the signal path but can be switched so that it precedes the compressor. Its controls comprise six sliders. The equaliser is a simple set of passive tone controls. The low- and high-frequency bands are shelving response filters, operating at 10Hz and 32kHz respectively. Up to 16dB of boost or cut is available on each of these bands. The four mid-frequency bands are bell-shaped response filters, operating on 100Hz and 500Hz, and 2kHz and 5kHz. The parameters for each are carefully selected to produce a musical-sounding equaliser, which has a minimal impact on the phase response of the signal. These filters are designed for making subtle alterations: 6dB, 4dB, 4dB, and 8dB of boost or cut respectivly are available on the four mid-bands. The equaliser can be switched in and out of the signal path.

A gain control allows the output level to be adjusted through the range -20dB to +6dB. A pair of 20-segment LED bargraph meters monitor the output level.

Pressing the TSP (Tube Signal Path) switch selects the valve-based output amplifier instead of the transistorized amplifier.

This unit was a delight to use. Whereas many manufacturers feel the need to cram their controls (to satisfy the demands of their marketing departments), and then cram the circuitry into the smallest possible enclosure, and use the cheapest possible components (in order to satisfy accountants), it is apparent that Avalon design their processor from a specification drawn up to meet the needs of the user. Controls are carefully chosen - none is superfluous, yet the unit provides effective and comprehensive control of the signal. Equally pleasing is the layout of the control panel. All controls are clearly labeled in black, which contrasts well with the silver of the panel, facilitating the reading of the legend in poor light conditions.

The chunky aluminum knobs have knurled sides, making them pleasant to grip, and there is plenty of resistance in the pots, making them reassuringly firm to turn. Similarly, the faders of the equaliser have a positive feel, and are easy to adjust.

I was particularly glad to see separate gain reduction and output level meters because on many units, one has to switch the same meter between the two functions. It would have been even better if the output level meters could also have been switched to monitor the input signal level.

As on would expect from its appearance and the standard of engineering, the sonic performance of this unit is quite impressive. As well as trying the device out with a variety of stereo programme material, I experimented with vocals, both de-essing and carrying out ordinary compression, as well as bass and a variety of per cussive sources. Even with quite heavy compression the unit sounds clean and uncoloured - rather difference from most old "classic" optical compressors. This is a unit which I would happily use for just about any stereo mix compression task. It is equally effective processing individual instruments whether subtle level control, or effect-type "pumping" compression is required.

De-essing and frequency-conscious compression is theoretically best carried out using a multi-band compressor, or one using a subtractive circuit, which removes only the unwanted part of the signal spectrum, rather than reduction the level of the entire signal once the threshold is exceeded. However the system on the side chain is generally faster and easier to set up than a multi-band compressor, as well as being very effective. This compression functions as a very competent de-esser, do the side chain filters are a very worthwhile addition.

The equaliser is effective and simple to use, and is ideally suited to making spectral adjustments during the mastering process. The high and low filters, despite the fact that their operating points are outside the main audio spectrum, are useful for contouring the overall sound of a mix. I found also that the two upper-mid bands were very well suited to making adjustments to vocals, and were particularly effective at lifting vocals to the fore in a mix.

The Twin Signal Path feature adds to the versatility of this device: there are two valve stages, the first uses a single triode, being a suffering stage between the compressor and equaliser, and the second a twin-triode output line amplifier. There is a subtle but distinct difference in the sound of the class-A and valve amplifiers. Both are quiet, but the latter provides that ever-popular valve warmth.

Avalon supply minimal documentation - in fact just four A4 sheets - with the Vt-747SP. This information includes a copy of the sales brochure, a circuit block diagram, a "quick set-up operating guide", which explains the functions of the controls, and useful photocopiable patch sheets, which can be used as a convenient way of recording settings. No circuit diagrams were supplied.

Although the valve circuitry is operational within about a minute of the unit being powered up, the valves actually require longer than this to reach stable operating temperature. During this period the sound of the processor changes significantly so, like an valve-based audio processor, this unit should be switched on at least 30 minutes before use to allow it to warm up completely.


This powerful and beautifully engineered processor would be equally at home in a production studio, for use as a main mix compressor and equaliser,, or in a dedicated mastering facility. Although the compressor will cope admirably with most programme material, the equaliser is primarily a creative device, and is not suitable for making extreme corrective adjustments.

by Benedick Grant

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