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TL Audio C1

Valve Compressor By Hugh Robjohns
Published March 1999

TL Audio C1

TL Audio have gained an enviable reputation for high‑quality processors. Hugh Robjohns takes a look at the latest addition to their professional Classic range.

Tony Larking joined the small, select band of valve equipment manufacturers back in 1993 with ex‑Neve designer David Kempson. Since then his range of products has grown to its present comprehensive catalogue, and has received much critical acclaim. The product line‑up is currently spread across two ranks, providing broadly similar tools in alternative price brackets. The all‑singing and dancing Classic range is intended for the professional user, with the Ivory series providing a more affordable alternative for the project studio, while retaining the valve circuit topologies.

I have used TL Audio's Indigo processors (now replaced by the Ivory series) widely in the past and have always found them to be of very high quality. Their valve amplifier stages undoubtedly add a certain character to the sound which was rarely anything other than fabulous! With this positive experience of the more modestly priced TL Audio processors, I was keen to find out what advantages the Classic range had to offer. The C1 dual mono/stereo compressor under review here loosely corresponds to the Ivory 5021 compressor, the successor to the discontinued Indigo 2021.


The C1's well‑stocked back panel.The C1's well‑stocked back panel.

The Classic range, like the Ivory series and the Indigo before it, employs a coalition of both solid‑state and valve components in carefully designed hybrid circuits, the aim being to combine the best of both worlds. TL Audio claim that this approach enables their signal processors to deliver superb sound quality, high technical specifications, and rugged real‑world interconnectivity — a combination which eludes many vintage valve units!

The majority of the Classic and Ivory ranges use the ever‑popular 12AX7A/ECC83 twin‑triode valve for voltage amplification stages, positioned strategically in the signal path where the benefits of valve circuitry are most apparent. However, the microphone and line inputs, line outputs, and various other sections of the circuitry are fabricated from conventional solid‑state components. Apparently this design approach helps to achieve the kind of technical specifications demanded of a modern piece of studio equipment — operating noise figures better than ‑80dBu, distortion better than 0.05%, and frequency response flat within 1dB from 10Hz to 40kHz.

In place of traditional signal transformers linking the valve amplifiers with the rest of the circuitry, transistorised buffer stages allow better noise figures, lower distortion, faster slew rate and a lower shipping weight! The Classic range also uses relatively high anode plate (HT) voltages of around 250V to further lower the noise floor and extract the best possible performance from the selected valves (the Ivory series uses a lower 150V HT rail).

The standard of construction is hard to fault. High‑quality fibreglass printed circuit boards with over‑specified components are used throughout, and the case is made of strong folded and welded steel with a tough paint finish. Although the control pots and connectors are all mounted directly on the main or daughter PCBs, they are mechanically secured either to a sub‑chassis or the external case, so it's unlikely that any mechanical strain would be transferred to the circuit boards.

The comments above apply to all the Classic range devices. In the case of the C1 dual compressor, the balanced mic and line inputs are accommodated by the familiar SSM2017 audio preamplifier chip, and NE5532 dual op‑amps handle the guitar/keyboard inputs on the front panel, as well as driving the balanced line‑level outputs. One of the ECC83 valves provides a voltage amplification stage prior to the compressor, and the remaining circuitry is built around TL074 quad op‑amps. The core of the compressor — the VCA — is based on an LM13600 transconductance amplifier with the second ECC83 valve as an integral part of its signal path. In the review model, the valves were selected American GE types.

Knobs And Twiddly Bits

The C1 is housed in a black 2U steel case of a very modest 205mm depth. The unit follows the distinctive Classic series house style, with a drilled metal grille above the controls. This allows a glimpse of the glowing valve heaters and provides the only ventilation — the lid of the unit becomes quite warm after a relatively short period, but is never too hot to touch.

The C1 is easy to interface, each channel being equipped with separate balanced XLR connectors for mic and line levels on the rear panel, as well as unbalanced line‑level inputs on quarter‑inch jack sockets. The mic input can be provided with 48V phantom power via the input selector on the front panel, and both the balanced and unbalanced line inputs have sufficient gain range to accommodate nominal +4dBu or ‑10dBV signals.

Outputs are provided simultaneously on a pair of quarter‑inch jack sockets and balanced XLRs, and a push‑button on the rear panel determines the nominal output level as either +4dBu or ‑10dBu (sic). A further pair of TRS jack sockets provides insert points to the two compressor side‑chains, the send signal on the tip connection and return on the ring. The nominal operating level at this point is ‑2dBu, and connecting an equaliser here would allow frequency‑conscious compression, for de‑essing perhaps. The remaining rear‑panel facilities are a mains voltage selector, fuse, and IEC inlet socket.

The ventilation grille mentioned above extends across two thirds of the front panel, with the majority of controls grouped below it; to the right are a pair of VU meters and the auxiliary inputs. The front panel is very neat and tidy, with the seven control knobs associated with each channel arranged in a zig‑zag fashion to maximise the finger space around them. The white text legends are very readable against the black background, which is fortunate, as most of the compressor controls share anonymous grey caps — finding your way around initially requires some careful reading.

I dare say many users would be quite happy to have this machine across the desk output at all times — especially in an otherwise all‑digital signal path.

Each channel has its own set of controls, although in stereo mode the threshold, ratio, attack and release of both channels is derived from channel one's controls. Starting at the left, the first rotary switch selects microphone with phantom power, microphone without phantom, line, or auxiliary input (ie. the front‑panel keyboard/guitar input). A red LED illuminates when the phantom‑powered mic input is selected. An easily missed black button below the input selector introduces a 90Hz high‑pass filter — a useful facility, but one which I believe really should have had an LED associated with it.

A blue‑capped knob sets the input gain for the selected input across a range from +60 to ‑16dB (with a centre detent at zero), and four grey‑capped controls adjust threshold (+20 to ‑20dBu), ratio (1.5:1 to 30:1), attack and release. The last two controls are calibrated simply between slow and fast, although the time constants also have a degree of automatic programme‑dependent adjustment. For the record, though, the attack control ranges between 0.5 to 50mS and release from 40mS to 4 seconds.

A light‑coloured knob provides gain make‑up from 0 to +20dB, and another invisible black push‑button allows the compressor channel to be bypassed (a green LED illuminates when the channel is in circuit). Between the two sets of controls, yet another push‑button enables stereo linking, this time with a yellow LED indicator. In stereo mode the individual channel controls for input selection, input gain and make‑up gain remain active, but the compressor settings are derived only from channel one's controls (with commoned side‑chains) to ensure stereo imaging remains stable.

To the immediate right of the On button for channel two, another black button with an associated yellow LED switches the two back‑lit VU meters to show either the output levels (0VU equating to +4dBu) or applied gain reduction. Under these meters, a pair of quarter‑inch jack sockets enables front‑panel connection of a guitar or keyboard. Channel 1's connector is normalled to channel 2 so that a single input could, for instance, be processed with light compression on one channel and heavy on the other. A push‑button labelled Guitar/Keyboard alters the input sensitivity, introducing around 20dB more gain in guitar mode, and a green control knob trims the input level all the way down to silence. The last control is a silver toggle mains power switch for that token retro look!


From the moment you plug the C1 in, it is obvious that there are valves in the signal path somewhere — it has that hard‑to‑define quality of warmth and fullness that seems to elude the majority of solid‑state equipment. By choosing to drive the machine a little harder or softer than 'normal', it is also possible to control its 'valve character' to some extent, which makes it a very creative tool rather than a purely protective one.

The input gain control allows the compressor to be interfaced accurately with other studio equipment, as well as determining the amount of drive to the first valve gain stage. The compressor's parameters are easy to optimise, all four covering sensible ranges, and the semi‑automatic time constants allow good control of fast transients without undesirable pumping effects. Similarly, the 20dB of available gain make‑up allows gentle compression slopes to be used from a very low threshold for imperceptible 'squashing'

I found the machine worked extremely well with a wide range of material, introducing overall characters from bright and punchy to subtle and smooth, but all with that warmth and fullness associated with good‑quality valve amplification. Complex details seemed to be very well preserved through the C1 too, even under heavy compression and in normal use the C1 didn't seem to restrict the brightness of the programme material in any way, nor add any perceptible noise or unwanted distortion. I wasn't able to make a direct comparison with the Ivory series compressor (with its lower HT voltage), but from memory of the old Indigo equivalent I think the Classic C1 is probably slightly quieter and definitely a touch cleaner or sharper — more polished and elegant perhaps.

When used to process solo instruments — electric and acoustic guitars, harp, piano, MIDI keyboards, trumpet and voices — the C1 proved itself well able to handle everything I threw at it, and invariably produced a great sound when optimised. I wouldn't say it was transparent exactly, but it seemed to make 'nice' changes to the sound that I always ended up keeping! I was a little less enamoured with it across an entire mix, not because it didn't perform well, but rather I felt its character tended to override some of the more subtle aspects in the mixes I was playing around with while reviewing the machine. Of course, there are times when this kind of global effect is highly desirable — I can think of occasions in the past when it would have been absolutely ideal (with retro rock 'n' roll numbers, for example) — and the C1 was certainly quite capable of beefing up a complete track mix whilst still controlling short transients well. I dare say many users would be quite happy to have this machine across the desk output at all times — especially in an otherwise all‑digital signal path. I certainly found the C1 had the very useful ability to round off the sharp edges of fierce digital tracks, making them a little more palatable.

Overall, I'd say the C1 lives up to expectations, offering impressive sound quality in a very flexible and capable package. It is a relatively expensive machine, costing around three times as much as the Ivory‑series equivalent, but its sound quality is sublime and the build quality well up to professional standards. This is a truly creative tool which can easily justify its price.


  • Valve amplification lends character.
  • Hybrid circuitry ensures respectable technical performance.
  • Front‑panel interface for guitar and keyboards.


  • Push‑buttons for HPF without LED indication.
  • Needs colour coded identification of compressor controls.


The C1 champions TL Audio's Classic range of signal processors. With two valve amplification stages, one of which can be overdriven to a degree, combined with solid‑state circuitry, this machine offers the best of both worlds — character and technical quality.