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TL Audio 2031

Indigo Valve Overdrive By Paul White
Published December 1997

TL Audio 02031

TL Audio are well established as producers of down‑to‑earth valve‑hybrid and all‑solid‑state studio equipment that delivers a high level of performance at an affordable price. Most of their signal processors have obvious counterparts from other manufacturers — compressors, equalisers, mic amps and so on — but the latest addition to their Indigo range is slightly unusual. Whereas other hybrid products incorporate valves as a means to another end, the Valve Overdrive unit is designed solely to provide valve coloration to signals, though additional equalisation and filtering is also included.

The Indigo Valve Overdrive is a mains‑powered 2‑channel processor equipped with balanced line inputs on XLRs (+4dBu), unbalanced line inputs on jacks (‑10dBV), and instrument inputs on unbalanced jacks. The instrument inputs have integral speaker simulators, though how well these work, coming before the tube overdrive rather than after it, remains to be seen. Dual gain settings are available on the instrument input to accommodate low‑level sources such as electric guitars, or higher level sources such as electronic keyboards. Both XLR and jack outputs are fitted, the XLRs balanced at +4dBu and the jacks unbalanced at ‑10dBV. Of course, it's what goes on between the input and output jacks that's really interesting.

All three inputs are active all the time, so it is possible to mix three sources if required, though there's no way to balance the three levels unless the sources themselves have gain controls. Each channel has a variable‑gain valve preamp (up to 20dB) permanently in circuit, followed by a second valve stage that may be switched in or out as needed. This second stage adds a variable amount of gain boost, up to a maximum of 20dB, and is used to create more obvious overdrive effects. In all, there are four dual‑triode valves in the unit. In addition to the front panel Boost button, there's also a rear‑panel footswitch jack for each channel, to bring in the boost for live performance.

After leaving the Boost stage, the signal passes through a variable‑frequency, 12dB/octave high‑cut filter that can be swept from 500Hz to 10kHz. The filter may be bypassed when not required, but it's included to enable the user to 'trim' off any harsh higher harmonics created by high levels of overdrive. It has a similar response to the low‑pass side‑chain filter of a Drawmer gate, and is useful in attenuating unwanted high end without seriously affecting frequencies below its cutoff point.

Further equalisation is available via a 3‑band stereo equaliser positioned in the centre of the front panel. This comprises an 80Hz shelving high‑pass filter, a fairly gentle 800Hz mid peaking filter with a Q of 1, and a 5kHz shelving low‑pass filter. All three sections have a +/‑10dB range, but because the controls are stereo, both channels have to share the same settings. However, it is possible to bypass the EQ independently on the two channels.

Finally comes an output gain control accompanied by a green 'signal present' LED (‑22dB), a yellow peak LED (+10dB), and a red clip LED (+16dB).


In some ways, you can view the Valve Overdrive as a musical instrument equivalent of a voice channel, as it contains a means of adjusting gain, equalisation and coloration. However, it may also be used via channel insert points to add colour to a vocal sound, or via group or master insert points to warm up a whole mix or subgroup within a mix. By adding a second boost stage, the designers have given the Valve Overdrive plenty of scope for both subtle warming and heavy overdrive, so applications range from stereo mastering to adding real dirt to a guitar or organ sound. It's also possible to use the overdrive and EQ to radically transform synthesizer sounds — something that the hat‑on‑backwards white lab‑coat brigade will probably go for in a big way.

In some ways, you can view the Valve Overdrive as a musical instrument equivalent of a voice channel.

My first test was to DI an electric guitar via the unit to see if the speaker simulator, augmented by the filtering and EQ, could really produce a plausible rock guitar sound. Without the EQ or filter switched in, boosting the sound past the point where the clip LED came in produced an obviously clipped and rather unpleasant sound, as you might reasonably expect. Using the variable low‑pass filter, it's possible to filter the rasp out of the sound, but by the time the result is smooth enough to be usable, the filter frequency is somewhere around 700Hz, which produces a very mellow tone indeed, with no bite at all to speak of. Though usable in certain applications, this isn't a solution to creating ready‑to‑record rock guitar sounds, and even frantic use of the 3‑band EQ failed to save the day. However, I produced some nice analogue‑style fat tones from digital synth sources by using the overdrive to roughen them up, and then EQ'ing the roughness out of the end result.

Producing clean guitar sounds is a different matter entirely; as long as you only drive the signal hard enough to light the yellow LED, it's possible to use the EQ to create a convincing guitar voicing that records nicely. The same technique can also be used to record bass to good effect, though I'd usually add a compressor to the signal path when recording bass. Similarly, low to medium overdrive settings work fine on complete mixes, though I have to confess that there's a very fine line between having no apparent effect and going into audible clipping. The SPL 2‑channel Charisma I've had for review (for publication in a future SOS) seems far more controllable in this application, and the conceptually simpler Ridge Farm Gas Cooker (reviewed back in SOS June '95) behaves more predictably.


While the Valve Overdrive succeeds in providing gentle valve coloration combined with EQ, a variety of input options and low background noise, I feel it is rather less successful at producing true overdrive sounds, mainly because of the odd decision to place the speaker simulator before the overdrive stages. Speaker simulators normally come at the end of the chain to filter out the harsher distortion products, so putting it before the distortion stage makes little sense to me.

The most successful applications of this little box are stereo mix sweetening with modest amounts of coloration, keyboard sound‑shaping and the production of clean guitar tones with just a hint of warmth. Even so, there are less costly ways of doing both jobs rather better, so, much as I love the rest of the TL Audio range, I have to say that I think this one has rather lost the plot.


  • Nicely engineered, with clear controls.
  • Sweet‑sounding EQ with useful variable low‑pass filter.
  • Inputs to handle all levels, from guitar to +4dBu line.


  • Heavy overdrive sounds are unpleasant and are not helped by the speaker simulator being on the instrument input stage.
  • Modest overdrive sounds are very subtle, but increasing the gain soon brings on audible clipping.


Though useful for general mix warming and keyboard sound treatment, I feel the poor speaker simulator performance, combined with the rather sharp transition from very little coloration to audible clipping, limits the usefulness of this unit.