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TL Audio 2011 & 2012 Indigo Range

Valve Equalisers By Paul White
Published August 1996

Paul White casts a critical ear over TL Audio's new budget dual‑channel valve equalisers to see if high quality really is possible at such a relatively low price.

TL Audio should need little by way of introduction for our regular readers — this small UK company have made a big impact building quality valve‑based audio processors at a fraction of the cost you might expect to pay for an esoteric, American equivalent. Even so, they have their sights set on an even wider market, which is why they've launched the new Indigo series. By trimming away unnecessary features and scaling up for mass production, TL Audio have managed to make a further drastic price reduction while still offering the same basic quality of circuitry and performance. Indeed, much of the Indigo circuitry is identical to that in TL Audio's premium models.

The two models on review are both 2‑channel equalisers, though the range also includes a 4‑channel mic preamp, a 2‑channel compressor and a 2‑band valve overdrive unit.

The Equalisers

Before exploring the features of the individual equalisers, it's helpful to look at what they have in common. Like the rest of the Indigo range, the EQs run on mains power, not external adaptors, and are fitted with both unbalanced jacks and balanced XLRs to handle the audio ins and outs. A further pair of front panel jacks accept high‑impedance instrument sources with switchable sensitivity, and up to +/‑ 20dB of front panel gain control is available for level matching. All three input types may be used simultaneously, with the obvious proviso that there's no independent level.

The 1U cases are surprisingly heavy, and have ventilation slots in the side and top covers to help dissipate the heat from the valves. To help keep costs down, the same 4‑socket valve circuit board is used in each model, which is why the 2‑valve units are shipped with two empty sockets. Although the front panels are finished in indigo (hence the name of the range) with white and blue legend, the rest of the box is simply plated — presumably another cost‑saving measure.

The EQ2011 is a dual, 4‑band equaliser providing shelving high and low controls combined with two sweep mid‑range controls, rather like the EQ you might find on a good mixing console. Directly before the input Gain control (which has a range of +/‑ 20dB) comes the Instrument input, an unbalanced jack with switchable Hi or Lo gain settings, the Hi setting being suitable for electric guitars and basses fitted with passive pickups. These will also accept high‑impedance mics, but they aren't designed to replace a proper mic preamp. A Peak LED warns of excessive input gain.

A button switches the low‑frequency shelving control from 80Hz to 120Hz and up to 12dB of cut or boost is available. A similar range of cut and boost is offered by the two mid‑band controls where the low mid can be switched to operate at 250Hz, 500Hz, 1kHz or 2.2kHz, and the high mid can be set to 1.5kHz, 2.2kHz, 3.6kHz or 5kHz. The filter bandwidths of the mid controls are preset, and the high‑frequency shelving control, which also has a +/‑12dB range, operates at either 8kHz or 12kHz. Each channel has its own Bypass button with status LED.

The 2012 parametric equaliser follows the same kind of styling, and has the same input/output configuration, with the added bonus that both channels may be cascaded for use as a single‑channel, 4‑band parametric EQ (rather than as a dual channel, 2‑band parametric EQ) if more control is required. In single‑channel mode, the input feeds channel A and the output is taken from channel B, although output A remains active.

All four equaliser bands have the same +/‑15dB cut/boost range, but the frequencies are continuously variable rather than simply switchable. However, bands 2 and 3 have switchable ranges (/10 and x10 respectively), to give them sufficient coverage when used in both 2‑ and 4‑band modes. The Q of each section is variable from 0.5 to 5 and all the cut/boost controls have centre detents. A central button switches between 2‑band stereo and 4‑band mono operation, and separate Bypass buttons with green status LEDs are provided for both channels.

In Use

I tested both units with mixes, separate instruments, voice and electric guitar, and in all cases, the tonal quality was well‑focused but without becoming abrasive. I found the parametric EQ the most flexible, even when I was working with only two bands per channel — but then I've never been a big fan of complex EQ anyway. The 2012 parametric virtually sings in the high registers, and handles bass sounds without letting them become too 'loose' or uncontrolled, even when significant amounts of boost are applied. For those who demand finer control, the 4‑band mono mode delivers it with style.

Subtle use of this parametric EQ can add the kind of clarity and air to a mix that you normally have to turn to enhancers to achieve, and even when driven so hard that the clipping LED is on almost all the time, the sound remains clean and tight. Indeed, it's only when you crank the input gain way past danger level that you can coax any audible distortion from this unit. Tests with electric guitar also revealed sufficient tonal range to create a wide variety of clean guitar sounds, from mellow jazz to a sparkling edge.

Moving over to the 2011, I found this to behave much like a really good desk EQ, but I have to admit that I found the switchable frequency ranges just a little frustrating. I like to 'spin the dial' to tune into the right frequency spot, but here, you have to jump in steps of almost an octave at a time. Most of the time you can get what you want, and the choice of high/low shelving frequencies also helps enormously, but given the choice, I'd plump for the parametric every time. Overall, the sound is tighter and more detailed than you get from most mid‑priced desk EQs, and the sound does have a warm, well‑integrated classic feel to it.


At just over £700 each, the Indigos are not much more expensive than most decent solid‑state equivalents, yet TL Audio haven't compromised on sound quality in bringing this budget range to the marketplace. It's only when you compare a really nice EQ with an average one that you realise how much clarity you can lose. An indifferent equaliser leaves the bass end feeling slack and ill‑defined while the top end becomes harsh or nasal. That doesn't happen with these units; the overall tone is reassuringly positive and very well behaved.

At the asking price, TL Audio have come up with a supremely attractive product line, and though the all‑valve brigade might snipe at the hybrid design philosophy (see the 'High‑bred Hybrids' box), it makes a huge amount of sense from a technical and financial point of view. If you've never used anything but the EQ on your console, you should give one of these units a try — I think you'll be surprised at the difference.

High‑Bred Hybrids: The Indigos' Innards

Though we talk about these TL Audio units as valve or tube products, they are in fact hybrid designs which combine solid‑state circuitry with valve circuitry. Done properly, this provides the benefits of valve coloration as well as the low noise performance and inherent reliability of solid‑state circuitry. It also makes manufacturing cheaper, which is why TL Audio's products are so competitively priced.


  • Attractively packaged.
  • Attractively priced.
  • Excellent sound quality.


  • Switched frequency settings on the 2011 are a little limiting if you're trying to target something specific.


These valve hybrid audio processors offer excellent value without compromising on performance.