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TL Audio VP1

Valve Recording Channel
By Hugh Robjohns

TL Audio's heavyweight valve recording channel not only provides you with gating, de-essing, compression, equalisation and limiting, but also lets you experiment with the sounds of different types of circuitry.

TL Audio are one of the UK's leading proponents of hybrid solid-state/valve audio equipment, and the VP1 mono valve processor is their new high-end recording channel. Within the 3U rackmounting case the VP1 contains a mic preamp stage with selectable valve or Class-A solid-state electronics; a parametric four-band valve equaliser; a compressor with selectable solid-state or optical elements and a valve gain make-up stage; an optical output limiter; a VCA-based expander/gate; a de-esser; and a rotary output fader. An optional 24-bit/96kHz digital board provides built-in A-D conversion, with selectable sample rates and bit depths, and a full complement of AES-EBU, S/PDIF and Toslink output formats.

TL AUDIO VP1The design philosophy of the VP1 is to furnish each signal processing stage with the most appropriate technology — valve, solid-state, or hybrid. There are no fewer than seven separate valve stages, all running with a 250V stabilised HT (anode) supply. The valve path of the mic preamp employs a Siemens EF86 pentode at the front end, followed by a Sovtek ECC83 (12AX7A) — in much the same configuration as the company's highly regarded dual-channel mic preamp, the PA1. Another ECC83 is used as the gain-controlling element in the compressor, and four more are used in the equaliser.

Nuts & Bolts

The VP1 weighs a substantial 8kg, measuring 132mm front to back. Although there are seven valves in the box, it doesn't seem to get much more than comfortably warm even though the only cooling vents are at the top of the front panel. Power comes via an IEC mains inlet with separate fuse holder and voltage selector, and the VP1 consumes 50VA.

The audio connections on the rear panel comprise a pair of electronically balanced XLRs for mic input and +4dBu line input. A quarter-inch jack socket is also provided for a -10dBV unbalanced line input. The Instrument input is mounted on the front panel, using another tip-sleeve quarter-inch jack socket. The two analogue outputs are provided via XLR (balanced +4dBu) and quarter-inch jack socket (unbalanced -10dBV). A pair of TRS quarter-inch sockets provide two unbalanced inserts, one in the side-chain of the compressor and the other in the main audio path following the mic preamp. A third TRS socket can be used to link the dynamics side-chains of a pair of VP1s, allowing accurate gain tracking in stereo applications.

A metal blanking plate identifies the position where the optional DO1 A-D converter card can be installed. However, this facility was not fitted to the review unit so I am unable to comment on its performance. The module is controlled through dedicated switches on the VP1's front panel, allowing the resolution to be selected from 16, 20 or 24 bits, and the sampling frequency from 44.1, 48, 88.2 or 96kHz. There is a dedicated LED bar-graph output meter here to check the headroom of the digital signal. A Lock LED illuminates when the DO1 card is properly synchronised to its internal crystal or an external word clock (a switch on the module itself selects internal or external clocks). As mentioned previously, AES-EBU, S/PDIF and Toslink optical outputs are provided simultaneously.

User Interface & Controls

In general, the front panel has plenty of space around the control knobs and switches, with clear, white legends on the dark-blue paintwork. Aside from one green and one blue knob, most are in shades of grey — and the difference between them can be hard to discern in low light. However, the panel layout is logical and I found I quickly became familiar with where everything was.

The input section at the left of the panel has two rotary switches, one to set coarse gain and the other to select the mic input, line inputs (both mixed together), or instrument input. The Gain control covers a 40dB range in 10dB steps, providing a maximum of 50dB for the mic input and 20dB for the instrument source. A continuous Trim control adds another ±15dB and there is a variable 12dB/octave high-pass filter (25Hz to 1kHz). Four buttons engage the high-pass filter, polarity reversal, 30dB pad, and phantom power. A pair of LEDs indicate Peak (nominally 5dB below clipping) and Drive, the varying brightness of which shows how hard the valve stage is being driven.

Next along are the two VCA sections: the Expander/Gate and De-esser. Both comprise three rotary controls, an On button (with status LED), and four-bar LED gain-reduction meters. The gate (for it is more a gate than an expander) is provided with variable Threshold (0 to -40dB), Attack (0.5 to 10mS) and Release (30 to 300mS), and the gain-reduction meter is calibrated at 10, 20, 30 and 40dB attenuation. There are no ratio or range controls, the gate always applying its full gain reduction of around 80dB when the signal falls below the threshold. The de-esser is equipped with Depth (0 to 20dB), Frequency (700Hz to 7kHz) and Bandwidth controls, along with an On button and gain-reduction metering, although this time calibrated at one, three, six and 14dB attenuation.

The compressor is provided with all the usual facilities, including its own On switch. The Threshold spans ±20dB and is wired so that the threshold reduces as it is turned clockwise. A switch selects a hard knee instead of the default soft-knee slope, and the ratio can be adjusted between 1.5:1 and 30:1.

The Attack and Release are adjustable manually, although there is also a degree of programme-related control helping to ensure, for example, that fast high-level transients don't punch holes in the track when a slow release time is selected. The Attack time is variable from 0.5 to 50mS and the Release from 40mS to 4S. A Gain Makeup control allows up to 20dB of level to be clawed back after compression — the amplifier circuit employing a valve.

One of the nice aspects of this compressor is the ability to change its gain-control element between a solid-state transconductance amplifier (similar to that in TL Audio's C1 compressor), and an LED/LDR (light dependent resistor) opto-circuit. The latter provides the characteristically slow, but highly musical style of compression which can be so attractive with voices in particular. Another unusual aspect of the compressor section is the inclusion of a Hold control, which delays the start of the release after the signal has fallen back below the threshold. It has a fixed delay of 10mS which has been chosen to allow the compressor to be used with a fast recovery time, even when processing low-frequency signals. (Generally, compressors with very fast release times try to track the waveform of a low frequency signal, causing unpleasant amplitude distortion.)

The Link switch allows two VP1s to work together for stereo processing, assuming the appropriate connections have been made. However, the controls of both units remain fully independent, and so must be set to identical values to maintain an equal dynamic response. Having said that, bending this rule allows one VP1 to control a second in a voice-over configuration. In this case, the Link switch is pressed only on the 'slave' VP1 carrying the music to be dipped, so that it reacts to the control signal derived from the 'master' VP1 carrying the controlling voice. The amount of voice-over control is determined by the threshold and ratio set on the 'master' unit, and these will still function even if its compressor section is bypassed, allowing significant gain reduction to be applied to the slave unit without having to compress the life out of the controlling voice signal — a handy facility.

Equaliser, Limiter & Output

The equaliser in the VP1 is a four-band semiparametric design, each band having its own valve to provide the necessary gain or attenuation. There is an overall bypass button to enable a 'reality check' between original and equalised sounds, as well as the ability to switch the EQ pre-compressor instead of its normal post-compressor position. The sound may be radically different between the two operational modes since the compressor reacts to the loudest signal irrespective of frequency — placing the EQ ahead allows a degree of frequency-selective compression.

The top and bottom EQ sections are relatively steep 12dB/octave shelf types with ±15dB gain swing and switchable turnover frequencies of 60, 120, 250, and 500Hz for the LF band, and 2.2, 5, 8, and 12kHz for the HF band. The two middle bands provide variable frequency and bandwidth control, the lower spanning 30Hz to 3kHz, while the upper ranges between 1kHz and 18kHz, there being a useful overlap between the bands. The bandwidth reaches between a precise and narrow Q value of seven, down to a very broad and gentle 0.7 (about 1.5 octaves).

The final two stages of the signal path are a peak limiter and output level control section. The output control is a simple rotary fader, allowing the signal to be faded down to silence or have its level boosted by up to 15dB. The idea is to allow the VP1 to feed multitrack tape or digital recorders directly, bypassing a recording console, but retaining full operational control of the signal level. With 15dB of gain in hand, the VP1 can also drive digital recorders calibrated to receive +18 or +20dBu (for peak 0dBFS) with relative ease.

The limiter is another optical design, but one with a fast response and very high ratio to control high-level transients. Its side-chain control signal is conveyed independently from that of the compressor, via the Link socket, to share control signals between the limiter in a second VP1. This output limiter stage is equipped with a variable Threshold control (0dBu to +20dBu), an On switch, and an LED to show when signals have exceeded the threshold. I found this indicator distracting, as it remained functional even with the Limiter out of circuit. Furthermore, the LED remained bright for about four seconds after the signal had fallen back below the threshold and the gain had long since been returned to normal! This time-stretching arrangement makes it tricky to set the limiter to catch just the occasional brief transient, as you have no idea how hard (or often) the limiter is working.

TL AUDIO VP1 front panel controls.

Also, as the limiter acts after the output control, different programme dynamics will be obtained if the output control is adjusted. While this offers some protection against unintentional overloads if the output level is increased, it also means the dynamic control of the material will vary correspondingly. Equally, carefully setting the limiter to provide a modicum of dynamic control may be blown to pieces if the output control is subsequently backed off. However, provided the user is aware of the implications of this arrangement it can be used to advantage, but it may well catch out those less familiar with this unusual way of working.

An illuminated VU meter can be switched to monitor one of three signals via a four-way rotary switch. The options are input level (post-preamp but pre-compressor and pre-EQ), gain reduction of the compressor (not the limiter), or output level. In the gain-reduction mode the meter continues to show the gain reduction that would be applied, even if the compressor is switched out of circuit, which, again, I found disconcerting.

The output level meter mode has two options: normal and 'O/P +10'. Normal means the 0VU point equates with +4dBu via the balanced output (and -10dBV from the unbalanced output). However, if the VP1 is being used to drive a professional digital recorder, the typically much higher input levels required would mean the meter's needle would be wrapped around the end stop the whole time. Consequently the 'O/P+10' position attenuates the display by 10dB, such that 0VU equates with +14dBu (or 0dBV).

The VP1 In Action

This is, by and large, a very nice machine — both in sound quality and ergonomics — and overall, I found it to be a very creative tool, largely because of its flexibility and versatility. The options to select valve or solid-state preamps, transconductance or optical compression, and move the EQ pre- or post-compression all afforded the opportunity to experiment quickly and easily to see what worked best on a given signal source. Thus experimental ideas could be explored without the time-consuming business of replugging and configuring separate units.

There are a few frustrating quirks — the top three on my list being the 'time-stretched' limiter LED, the gain-reduction meter working even when the compressor is switched out, and the output level control being post-limiter. However, I doubt they would prove a real hindrance once I was familiar with the machine.

But let's face it, you only buy a valve unit for one thing — its sonic character — and the VP1 provides that by the shed load, with real controllability too. Beware, however, that the sound quality changes appreciably as the unit warms up, so allow 20 minutes or more before trying to record anything critical. The noise floor is reasonable for a valve preamp, although with high gain settings the review model suffered some rattling microphony when the front panel was tapped. Of rather more concern was the mains-related hum which remained at a constant level regardless of the input gain switch setting. However, both of these problems can often be attributed to a less-than-perfect valve and, given my experience of other TL Audio products, I doubt very much that this is a consistent weakness of the VP1 per se. Nevertheless, if you intend to purchase a VP1 you should have a careful listen to your particular unit before walking out of the shop with it! The performance of individual valves can vary greatly and they can be damaged by physical shocks during transportation.

Not surprisingly, neither hum nor microphony troubled the Class-A transistor input stage (or the line or instrument inputs for that matter), and the overall noise floor seemed lower and slightly smoother than the valve stage. The quoted EIN is -124dBu with a 150Ω source, which is a little short of the best preamps, but perfectly acceptable for most applications, particularly when you remember that the VP1 would typically be used with high-output condenser mics capturing close vocals.

The Instrument input provided a suitable impedance and sensitivity for electric guitar pickups (1MΩ) and the wide-ranging gain controls made it easy to match levels and enable the appropriate degree of drive to be obtained from the valves.

The gate proved useful and easy to adjust, and the de-esser was a versatile facility with many more uses than its name implies — key rattle and honky reeds on wind instruments could usefully be tamed, for example, as well as acoustic guitar finger noises. The equaliser is lovely, with just the right ranges on the controls to allow smooth creative control, as well as reasonably surgical tweaking if necessary. It also exhibits that characteristic warmth and bloom you would expect from a valve-based equaliser, changing slightly depending on how hard you drive it.

If working with bass instruments, or a bass-heavy mix, the compressor's release setting was surprisingly critical, although using the Hold control helped a lot. I found the optical mode suited bass instruments rather well, while the transconductance mode seemed faster and cleaner.

The peak limiter felt a little heavy-handed for my tastes, with a recovery time which seemed way too slow. I also wasn't convinced of its ability to catch brief transients — the sort that often cause grief with A-D converters — and am not therefore overly confident of its declared role of providing protection from clipping when working with digital recorders. This is almost a case of the treatment being worse than the disease, as I found I could achieve far better results by leaving the limiter switched off, relying on careful level setting and perhaps a more subtle degree of dynamic control from the compressor section.

Taking On The Competition

The only product I can think of which compares with the VP1 is the Millennia Media STT1, which I reviewed back in SOS April 2001. This unit offers a similar degree of signal processing flexibility and switchable topologies, including both valve and solid-state signal paths. Although a side-by-side comparison was not possible for this review, my recollection is that the technical performance of the STT1 was slightly better overall. However, the VP1 is priced significantly below the STT1 in the UK and still provides superb flexibility with professional performance. It would make an ideal front end for a range of analogue or digital systems, is easy to use, and looks good in the rack.

Published March 2002