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TL Audio VI-1

Valve Interface By Paul White
Published March 1996

Paul White checks out a signal processor with no knobs, no buttons and no display, designed to add valve warmth to 8‑track recording.

It seems that the more sophisticated digital technology becomes, the more people seem to want to patch a valve somewhere in the system. It would be easy to write off this trend as misguided nostalgia if it it wasn't for the fact that valve circuits do 'something nice' to the sound, usually by adding some kind of benign distortion.

Why we should prefer the sound of slightly distorted music has been the subject of much debate, but I have my own theory. The vast majority of us have been brought up on a diet of recorded music, and most of what we've heard over the past three decades has been recorded and reproduced using analogue equipment. Older material may have been made using valve equipment, and valve mics remain popular even for the recording of contemporary music. The net result is that over the years, we have developed expectations as to what good recorded music should sound like, based on our experience of analogue technology.

Now that it's possible to do most things digitally, the 'analogue sound' has been pushed out of the picture, and to put it simply — we miss it, because we were comfortable with it. This is quite understandable: if somebody takes off your rose‑tinted glasses, it's only natural to want to put them back on, to stop the world looking unnaturally blue. What TLA have done in the case of the VI‑1 is develop an 8‑channel, rackmounted, 'rose‑tinted' box for use with digital 8‑track recorders.

Technically speaking, the VI‑1 could be used in all sorts of applications, from 'valvalizing' your console insert points to warming up several digital synths or samplers. To make it as flexible as possible, internal jumpers can be switched, allowing the input and ouput levels to be set independently at either ‑10dBV or +4dBu. In addition to ensuring the VI‑1 can match any type of pro or semi‑pro recorder, the independent nature of level switching also means the box can be used as a level converter.

Inside the 2U case are eight channels of circuitry, which interface with the outside world via balanced, quarter‑inch jacks. Low noise, solid‑state circuitry is used to handle the balanced input and output stages, as well as the level shifting, while a single triode valve stage is used to add the necessary 'flavouring'. Each stage is based around half of a 12AX7 (ECC83) valve, bringing the total valve complement up to four. The audio frequency response extends from 10Hz to 40kHz, flat within half a dB, and the unweighted noise floor is 90dB, a figure roughly comparable with the noise performance of a typical 16‑bit digital recorder. The inputs have an input impedance of 10Kohm (per leg of the balanced connection), while the outputs are a commendably low 47 ohms. The maximum level is +18dBu.

The front panel sports a solitary mains switch and an almost apologetically‑small LED. There's no bypass switch, so if your plans include switching in and out of circuit without patching, you'll need the optional VI‑S switcher unit, which will demand another 2U of your rack space. Providing the input and output levels are set to the same value, the circuitry has unity gain.

Next to the VI‑1, the VI‑S looks positively busy, but it's actually a very simple box comprising entirely passive switching. Rear‑panel, balanced jack connectors are provided for both the mixer inputs and outputs, and for the multitrack inputs and outputs. Eight sets of front panel switches then determine whether the VI‑1 is in the record path or replay path, and each channel also has its own bypass switch. Because the unit is passive, there are no status LEDs. Of course, if you want to be able to hijack the VI‑1 for use with keyboards or for warming up a channel insert point, you'll still have to use a patchbay or put up with unplugging leads from the back panel, but for dedicated multitrack use, the VI‑S provides for all possible options.

In Use

It's quite difficult to describe the effect of the VI‑1, because what it does is so subjective and also quite subtle. Because the effect is related to how hard the valves are driven, it becomes more pronounced with louder or more powerful material, while with low‑level signals, very little change seems to take place at all. In an A/B, unity‑gain test, the VI‑1 definitely enhanced the tone of bass instruments such as bass guitars, bass synths and kick drums, but without appearing to change their level in the mix or their basic tone. It's as though the sound has been subtly changed to appear louder, thicker and smoother; yet when you try to analyse exactly what's changed, you can't really put your finger on it.

The impression is that all the sounds in a mix are better integrated and somehow not so thin, but all this is achieved without any perceptible change in the underlying tonality. I could detect very little change in the way high frequency signals were treated. There's certainly no added brightness or harshness as you sometimes get with an exciter, but even though the overall effect is more a case of adding warmth than brightness, the top end doesn't actually seem to suffer in any way, or become overwhelmed by the slight additional bass‑end presence.


Assuming that you like what the VI‑1 does to the sound, and I think most people will, the box is incredibly good value and simplicity itself to use. On the other hand, the VI‑S costs three quarters as much as the VI‑1, which I feel is a bit on the steep side for a passive switch box. For professional users who need to work quickly, the VI‑S makes sense, but I think I'd stick to the VI‑1 plus a conventional patchbay and put up with the slight inconvenience. One thing is for sure though — until the DSP boys figure out exactly what it is that makes analogue circuitry, and valves in particular sound so 'comfortable', their future is assured.


  • Subtly flattering sound.
  • VI‑1 very good value.
  • Quiet and transparent in operation.
  • Level‑shifting facility is a useful bonus.


  • VI‑S switching unit is a little expensive.


A simple and cost‑effective way of reintroducing some of the warmth and humanity of tone that digital recording sometimes seems to filter out.