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TLA Dual

Mic Preamp/DI Box By Paul White
Published August 1995

Paul White tries out the new TLA Preamp/DI box, which combines leading edge solid‑state technology with vintage tubes in an attempt to offer the best of both worlds.

TL Audio have already established a reputation for building quality valve outboard gear, and their new two‑channel preamp/DI is designed to handle guitar, instrument and mic level signals. Presented in a sturdy, utilitarian steel case, the preamp/DI is straightforward and functional, though I wouldn't go as far as to say it was pretty!

Quick Tour

Rackmount ears are provided with the unit, and power comes in via an IEC mains lead. A dedicated low‑noise SSM chip is used as the mic preamp stage (complete with switchable phantom power for use with capacitor microphones), while a valve input stage — which utilises half of a 12AX7 or ECC83 double triode — handles the Instrument/Guitar input signal. This comes into the unit via an unbalanced jack, and when set to Guitar, the impedance of this input is 100kΩ. When switched to Instrument, however, it is 10kΩ. Guitar amps usually have a higher input impedance — more on what effect this has on performance in a moment.

A push‑button switch selects either Guitar or Instrument sensitivity, and a further button switches between the Mic and Instrument/Guitar inputs. A single gain control affects whichever input is active, and the second channel also has a phase invert button to aid in the correction of phase errors.

The selected input passes through the second stage of the valve before encountering the solid‑state balancing circuit feeding the XLR output. A peak LED is also provided, which monitors the output LED and comes on progressively to warn of clipping. This combination of solid‑state and tube technology is theoretically a good idea, because it enables a very low‑noise mic input to be followed by a stage of tube buffering and coloration. Because the Guitar/Instrument input passes through two valve stages, you might expect that this would be more coloured than the mic input, but in practice, there was no perceived difference.

Warmer By Tube?

To evaluate the effect of the Tube Preamp/DI on the output of a microphone, I plugged in a Beyer MC740, purely because I know the sound of this mic pretty well. After arranging a system where I could quickly switch between the mixer input and the Tube Preamp/DI, tests with speech showed a noticeable difference in sound quality. The Beyer mic turns in good results under almost any circumstances, and I used no EQ. Nevertheless, with the TLA in circuit, the sound seemed a little clearer, and the bottom end just a hint warmer, but with no suggestion of cloudiness or boominess. Indeed, if anything, the bass end was tightened up slightly, resulting in a positive and confident tonality that was at the same time very natural. This impression of increased clarity is a known side‑effect of adding small amounts of second‑harmonic distortion (indeed, that's how exciters work), and it's long been known that valve circuits add just this kind of coloration.

Used with an electric guitar, the sound was smooth and very warm, but I felt it was just slightly lacking in brightness compared with, say, a high‑impedance, active DI box using solid‑state circuitry. That's not to say that the TLA's tone isn't good, because it is — but I feel the 100kΩ input impedance may be loading the guitar pickups slightly.


On the whole, the Tube Preamp/DI is a well‑designed unit from a manufacturer with a good pedigree, and though the effect of valve coloration is relatively subtle, it's still very nice to listen to. Having both mic and instrument levels adds to the versatility of the device, and I can think of numerous applications, from use in the making of quality live and studio recordings, to treating an entire stereo mix with a touch of colour. The only thing that's missing is a high‑pass filter switch for bass roll‑off, but one thing's for sure — the TLA is a lot cheaper than buying a valve mic, and the results are much the same.

Brief Specification

  • Frequency Response 20Hz to 40kHz within 1dB at 20dB gain
  • Noise Mic EIN ‑127dB (22Hz to 22kHz, maximum gain, input terminated in 150 Ω)
  • Distortion 0.05% at 1kHz, mic input with 20dB gain
  • Output Balanced XLR, impedance less than 10Ω, maximum level +26dBu
  • Input Impedance Mic 10kΩ, Instrument 10kΩ, Guitar 100kΩ (Max mic gain 60dB)
  • Phantom Power 48V switchable


  • Easy to use.
  • Quiet.
  • Sweet tonal quality.
  • Mic and instrument inputs.
  • Built‑in phantom power.


  • No high‑pass filter.
  • Guitar input impedance on the low side.


Although better suited to use with mics and non‑guitar sound sources than with electric guitars, this remains a versatile unit, ideal for adding 'the valve sound' to your recordings.