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TL Audio Indigo VP2051 & PA2001

Valve Voice Processor & Valve Preamp By Paul White
Published November 1996

The TL Audio Indigo range keeps expanding, with cost‑effective hybrid valve/solid‑state processors emerging for every studio purpose. Paul White discovers that Indigo can be a very warm colour...

The success of TL Audio's valve signal processor range so far is a testimony to the fact that the vintage valve sound is in great demand: as the rest of the studio continues to go digital, for many people it becomes more important to find some way of injecting character back into their sound. There's nothing wrong with good digital equipment, any more than there's anything wrong with a plate of steamed vegetables, but both are improved by the addition of a few subtle spices. And in audio terms, subtle spicing is exactly what valve equipment provides.

The two newcomers to the TLA Indigo series are the VP2051 Valve Voice Processor and the PA2001 Valve Preamp (we reviewed TLA's Indigo EQs in August '96 and their Tube Compressor in September). I'll start with the PA2001.

PA2001 Valve Preamp

Presented in a 1U, mains‑powered package, and featuring switchable low‑pass filter, phase invert and phantom power, the PA2001 is a straightforward four‑channel mic amp. All its channels are identical, with XLR mic inputs and both balanced XLR and unbalanced jack outputs at +4dBu and ‑10dBv respectively.

Like most modern valve designs, the design of the PA2001 is hybrid, meaning that solid‑state circuitry is used in some areas and valve circuitry in others. To maintain a very low noise floor, the electronically‑balanced XLR input feeds a solid‑state gain stage. This is followed by a switchable high‑pass filter operating at 90Hz (12dB/octave), and only then does the valve circuitry take over, with a dual triode being used in the second preamp stage.

The PA2001 would make a worthwhile purchase for any studio owner wanting to capture the valve mic sound from an existing mic collection.

The amount of valve coloration added to the input material depends on how hard the valve is driven: adjusting the input gain control varies the drive and hence the degree of 'character'. A simple LED system, the only metering available on this unit, monitors the output of the valve stage. Available headroom is indicated by the LED starting to illuminate when the signal exceeds the normal line level by 6dB. When only 10dB of headroom remains, the LED will be fully on. With most sound sources, the 'valve sound' starts to happen at around the point when the LED takes an interest in the proceedings.

All that's left to mention about this simple unit is its output level control, which comes at the end of the preamp signal chain, directly before the output driver stages, and provides 60dB of mic gain.

VP2051 Valve Voice Processor

All‑in one vocal processors, or 'voice channels', have suddenly become very popular amongst manufacturers (the LA Audio Classic Channel and the Focusrite Green Voicebox are just two examples, both reviewed in this very issue), and it will be interesting to see if they become equally popular with the project studio owner. The idea is sensible enough: combine a mic preamp, EQ, and a compressor in a single unit, giving the user the option of a high‑quality signal path to tape, bypassing the mixing console. Even when you're using a mixer, a unit of this type may be used as a conventional 'insert point' signal processor.

As far as I can tell, the mic amp section of the VP2051 is based on the same circuit as the PA2001. It features both mic and line‑level inputs on separate, balanced XLRs, plus a further unbalanced line input on a rear‑panel jack, and an unbalanced instrument input on a front‑panel jack. A single gain control covers the range 0dB (line inputs) to +60dB (mic input), and the instrument‑level jack also has a dual‑position sensitivity switch, allowing it to make use of either the high mic‑amp gain or the more modest line‑amp gain. This is useful, as it allows such diverse instruments as active or passive electric guitars and basses, keyboards, or even high‑impedance mics, to be accommodated. The sensitivity switch also functions as a mic/line selector for the rear‑panel mic and line inputs. In line mode, the line input jack and line XLR are mixed, allowing simultaneous use.

The EQ section, which uses all‑valve circuitry, is identical to that used in the TLA 2011 equaliser, and is configured rather like a console EQ, with high and low shelving sections, plus two mids with fixed Qs and switchable frequencies. Both the high and low shelving filters have switchable shelving frequencies (12kHz/8kHz and 120Hz/80Hz respectively), while each of the mid‑range frequency switches has four positions. The lower mid can operate at 250Hz, 500Hz, 1kHz or 2.2kHz, while the upper mid overlaps this slightly by offering 1.5kHz, 2.2kHz, 3.6kHz or 5kHz. Cut or boost of up to 12dB is available in each section, and a bypass button takes the EQ out of circuit when not required. A pre/post switch means that the EQ can come either before or after the compressor in the signal path.

The Voice Processor provides a powerful, sweet‑sounding audio toolbox that can be used to hone a single signal to perfection.

The compressor itself is a fairly simple affair, but no less flexible for that. It has an insert point, allowing an equaliser to be used in the side‑chain for de‑essing, switchable fast/slow attack and release times, and just three knobs: Threshold, Ratio and Gain Makeup. Like other TLA designs, the compressor has a soft‑knee characteristic, with a ratio variable from a gentle 1.5:1 up to 30:1, which is as close to true limiting as makes no difference. The attack time is switchable between 0.5ms and 20ms, and release can be set at either 40ms or around two seconds. Some interaction is provided between the time constants (a fast release time will be lengthened if a slow attack setting is used), so the system isn't quite as rigid as it might appear.

Up to 20dB of makeup gain is available to compensate for level losses incurred in the compression process, and the compressor can be bypassed when not in use. A rear‑panel Link socket allows the compressor sections of two VP2051s to be linked, for true stereo operation using the Stereo Link button on the slave unit. It is also possible to use two units to create ducking effects, where the level of the signal passing through one channel controls the level of another signal passing through the other channel.

To the far right of the unit is the output stage, comprising an output Level control, the compressor's Stereo Link button, and an 8‑section bargraph meter that can be switched to read either gain reduction or output level. Both balanced XLR and unbalanced jack outputs are provided, operating at +4dBu and ‑10dBv respectively.


As expected, both units' mic amps have identical specifications, showing a ‑127dBu Equivalent Input Noise when terminated with a 150Ω source, with gain set to maximum. In practice, the noise is comparable with what you'd expect from the input stage of any competently‑designed, solid‑state console. Both units also have a very wide frequency response, extending from 10Hz to 40kHz, and flat within 1dB.

Specs aside, what really counts with a valve unit is the subjective result, and in this area, both boxes stand up very well indeed. The mic preamp has a clean, transparent sound at normal gain settings, but as soon as you crank on enough gain to get the LED flashing, you can hear the change in timbre as the tube coloration starts to thicken the sound, making it more throaty and smoother round the edges — almost compressed. To my ears, both mic preamps perform identically.

The VP2051's compressor has the effect of bringing the sound closer to the listener without making it stifled or dull, as some compressors do. Even with fast attack and release settings, there's little sign of excessive pumping, even at high gain‑reduction settings. The result, rather, is a full, produced sound with plenty of detail and intimacy. This is a very difficult compressor to abuse.

The EQ section of this unit is also very forgiving, in that you can be quite heavy‑handed with it and still produce a musical result. Though not as flexible as a parametric EQ, it sounds noticeably nicer than most mid‑price console EQs, filling out the bottom end without boominess, and adding 'air' at the top end without harshness. The switchable frequencies seem to work particularly well on vocals. Thin‑sounding vocals can be underpinned in the lower mid‑range without loss of clarity, while indistinct vocals can be given cut without introducing fatiguing aggression. Plug in a good capacitor mic, and you get very close to the sound you'd get by putting a valve mic though a separate valve compressor and valve EQ — very nice indeed.

With electric guitar, the compressor and EQ work very nicely to create a wide range of clean sounds, but it's surprising just how much difference placing the EQ before or after the compressor really makes. Because guitar sounds generally require a fair bit of upper‑mid boost, putting the compressor first results in a bright, twangy country‑style sound; putting the compressor after the EQ produces a much warmer sound, with plenty of sustain.


Both these units are excellent performers, with the facility to control the amount of valve coloration in the mic preamp stages adding greatly to their tonal flexibility. The PA2001 mic amp would make a worthwhile purchase for any studio owner wanting to capture the valve mic sound from an existing mic collection; the only compromises I feel have been made are the lack of more comprehensive level metering, and the absence of status LEDs for the switches, particularly the phantom power switch. The VP2051 Voice Processor provides a powerful, sweet‑sounding audio toolbox that can be used to hone a single signal to perfection. Given that TLA valve gear is actually very sensibly priced, both these boxes offer a lot of quality for the money, and they're likely to remain useful for many years, no matter how far the digital revolution encroaches on the rest of the studio.


  • Convincing vintage tube sound.
  • Good range of features.
  • Well engineered and stylishly presented.
  • All the modules in the Voice Processor stand up well as processors in their own right.


  • PA2001 would benefit from status LEDs on all buttons.


TL Audio have created the right balance of sound, features and price. The VP2051, in particular, is a wonderful all‑rounder that can be used to sweeten just about any mono signal, not just a microphone.