Frequently Asked Questions

Here are answers to some of the more frequently asked questions about TL Audio products. If you have a query that is not addressed here, then please e-mail us using the Tech Support page of this site!

What type of valves (tubes) do TL Audio use?
A: We use only two types of valve - one is the ECC83 dual triode valve (also known as the 12AX7) and the other is the EF86 pentode valve (also known as the 6267). The EF86 is used only in the preamp stage of the PA-1 and VP-1 models, while every other model uses the ECC83/12AX7 only. We currently use the following brands, and recommend these as replacements:

Sovtek 12AX7WA
Sovtek EF86

You can find more info at

You can in theory use any brand of EF86 or 12AX7WA, but you may find subtle difference in audio quality (and price) between various brands. We have found the Sovtek brand to offer a good balance of quality and affordability.

What is the life expectancy of the valves in my TL Audio unit, and how do I know when the valves need replacing?
A: We would normally estimate around 4 years, but much will depend on the way the units are used.
Leaving units powered up permanently will certainly reduce the valve life - we would recommend that the unit is switched off at the end of a session (although our use of dc heater supplies reduces the fatigue on the valve heaters). Traditionally, as a valve starts to deteriorate with age, its HF performance will be affected, so that there is a perceived loss of brightness, and the sound tends to become "woolly". TL Audio products make extensive use of localised internal feedback circuitry to ensure that if a valve's characteristics change with time then the performance of the unit is not compromised - thus extending the useful life of the valve.

When replacing valves, is any calibration necessary?
A: Small gain changes may occur when replacing valves, and ideally you should have a technician re-calibrate the unit – this involves checking signal levels at various test points and adjusting if necessary. However, these gain changes are often small - so many users simply change valves and carry on as before. It is advisable to replace all the valves in a unit at the same time, particularly on stereo units where valves of different brands and ages may cause a difference in tone between channels.

My Fatman FAT-1 unit has one valve stage per channel, yet there is only one valve in the unit in total. Why is this?
A: This is due to the fact that the valve concerned is an ECC83/12AX7 dual triode valve. A dual triode valve has two completely independent valve stages within one glass housing, enabling each channel to use one stage each. This applies to most TL Audio products – generally the number of valve stages within a unit will be double the number of valves found inside.

I have been told that the value of the HT voltage supply to the valves has a significant effect on audio performance. Is this true?
A: Yes, the HT voltage supply to the valves will dictate the maximum size of audio signal that can be handled by the valves before clipping takes place - when the signal swing becomes equal in size to the HT voltage, then no more headroom is left and clipping begins. Using a high HT voltage means signal swings in the valve stages can be larger - which keeps the audio signal well above the noise floor (thus ensuring the best possible noise performance) and permits the maximum headroom. TL Audio Classic range products use an HT voltage of between 250v and 325v DC depending on the model, Ivory units employ a 150v DC HT supply, and Fatman models use a 100v DC HT supply. Unlike some other manufacturers, we have resolutely avoided using low voltage "wall-wart" supplies for any of our valve products.

What exactly is compression?
A: Compression is an essential but often misunderstood process in modern recording. Put simply, compression reduces the difference between the loudest and the quietest levels of an audio signal. This is known as reducing the “dynamic range” of that signal and is a powerful tool for an engineer helping to avoid overloading, distortion problems as well as raising the level of the quieter parts of the audio signal. Before the introduction of compressors the only way this could be achieved was by “gain riding”, whereby an engineer would control the fader manually in order to try and anticipate very large levels (which might distort the signal) or very low levels (which may get lost in noise). The introduction of compression devices meant that this process could be controlled automatically, allowing the engineer to get on with more productive jobs!

Many instruments and voices have a very wide dynamic range that need to be controlled. A singer, for instance, may be singing quietly one moment and very loudly the next, and unless compression is applied the vocal won’t “sit” correctly in the mix, in addition to the problems of distortion on loud passages and noise on quiet ones. Compressors effectively turn down the loud bits and turn up the quiet bits, to achieve a more even and controllable level.

Compressors are often judged by their ability to control the dynamics without creating noticeable audible side effects. Heavy compression can cause the signal to pump or breathe with the onset and release of the compression. Some compressor designs can dull the signal and lose the top end of the signal. TL Audio compressors use a technology based around a transconductance amplifier rather than a VCA design. This transconductance amplifier design is known for being able to retain the full frequency range and natural character of the audio signal, even when compressing the signal quite heavily. Certain TL Audio models are also capable of more severe compression based around the optional Hard Knee mode if this is desired.

There are other benefits of compression as well as just controlling the peaks and raising the quiet parts, applied properly, it can add punch and excitement to music, as well as fattening up sounds and creating a more professional sounding recording. With TL Audio products you also have the added benefit of valve stages in the signal path, which create a warmth and presence just not obtainable with solid state or digital products.

What are the benefits of valve compression?
A: Valve compression yields a particularly special sound which has become very sought after, particularly with the widespread use of digital products. The reason valve equipment sounds special is due to two things: harmonic distortion and natural compression. When the signal through a valve is increased, it tends to generate a particular type of subtle and desirable distortion, called "second harmonic" distortion. This has the effect of thickening and warming the sound, and the more the level you feed to the valve stages, the more of this harmonic distortion will be produced. You should be able to hear this effect as you increase the input gain on TL Audio units.

Secondly, valves will tend to naturally compress an audio signal, again particularly as the signal level is increased. This itself also contributes to the warmth produced by TL Audio products.

When I connect the output of my TL Audio product to the input of my mixer, I get lots of noise and distortion. Why is this?
A: The most common mistake when connecting up our products is to feed the balanced XLR line output of the TL Audio unit into the XLR microphone input of a mixer. Although both mic and line connections often use XLR sockets, they have different impedances and signal levels. A line level signal fed into a mic input will cause it to distort very easily, since a line output provides a much higher signal level than a microphone does. Also, mic inputs are of low impedance whereas a line output is generally high impedance: this mismatch often results in poor frequency response and increased noise levels. So always ensure that the line output of any TL Audio product is connected to the line input of any mixer or recorder.

If I connect the mix buss insert points of my mixer into the unbalanced ins/outs of my TL Audio unit, I find that the signal level into the TL Audio unit is very high, and I have to turn down the input gain to compensate. Is this normal?
A: Many console mix buss insert points operate at a high +4dB level, even though they use 1/4" TRS jack connectors (which are often associated with low -10dB levels). Our unbalanced jack inputs and outputs are designed to operate at -10dB (in line with much domestic and semi-pro equipment) so feeding a +4dB signal into the jack input is going to drive it very hard. The solution is to use the +4dB XLR inputs and outputs of the TL Audio unit instead. Since these are balanced connectors, and a console insert point is usually unbalanced, you need to ensure that the insert cable is correctly wired - make sure that the insert "send" is connected to pin 2 of the TL Audio input XLR, and connect the screen of the "send" cable to pins 1 and 3 of the XLR (this is normally done by connecting the screen to pin 1, then placing a short wire link between pins 1 and 3 inside the XLR). Similarly, connect the insert "return" to pin 2 of the TL Audio output XLR, again making sure that the screen is connected to pins 1 and 3. Full connection details appear in the user manual. Failure to follow these instructions may result in a loss of signal level and increased noise in the audio signal path.

Is the sidechain insert point on my TL Audio compressor the same as the insert point on a console?
A: No, the sidechain insert point is specifically designed to allow frequency conscious compression such as de-essing. This is achieved by patching an equaliser (normally a graphic or parametric type such as the Ivory 5013) into the sidechain insert point (the tip or "send" of the insert point goes to the equaliser input, while the ring or "return" is connected to the equaliser output). Boosting any frequency on the equaliser then effectively lowers the compression threshold for that particular frequency of signal, so that it gets compressed to a greater degree than other frequencies. Thus boosting the frequency that corresponds to the sibilance in a vocal performance will allow de-essing to take place.

When using a TL Audio compressor, I sometimes experience some LF distortion when using a fast release time, which disappears as I make the release time longer. Why is this?
A: This effect tends to be experienced on signals with significant LF content, when some gain reduction is taking place. The reason it occurs is a result of the way compression works: As a simple example, imagine you feed a low frequency source signal such as a 20Hz sine wave into the C-1 Compressor, whose release time is set at its fastest position (40mS). The time it takes for a 20Hz signal to complete one whole cycle is 50mS. Since the fastest release time on the C-1 is 40mS, this means that once the source signal has fallen back below the compression threshold, its gain will return to normal in 40mS, which is a shorter time than the cycle time of the source signal. As a result, the envelope of the compressed signal is distorted as it is forced in and out of compression within a single cycle. Increasing the release time to a figure greater than 50mS prevents this problem, which is why adjusting to a slower release setting will cause the distortion to disappear. For the same reason, source signals with little or no LF content will not be subject to this effect.

On some units – such as those in the Ivory range – a ‘Hold’ circuit is designed into the compressor stage. This delays the onset of the release by around 10mS, and as a result the above effect is minimized.

Can you tell me what the differences are between the original Ivory series and the current Ivory 2 series?
A: A list of the extra features found on Ivory 2 models is summarised here click for details

I have found that the peak LED on my Crimson or Indigo unit seems to illuminate quite regularly, even with modest input levels. Why is this?
A: The "Peak" LED on these two ranges of products has a different function to those normally found on solid state products, where the intention is to warn that signal clipping is about to occur. These products use the "Peak" LED to inform the user the degree to which the valve stages within the unit are being driven - the LED illuminates at the point when the valves are starting to compress the signal and generate extra harmonics. Effectively this means that the valve character of the unit is really starting to contribute to the sound at this point - increasing the input gain further will cause the LED to glow brighter as the valves are pushed harder, and even at maximum brightness there should be around 10dB of headroom left. So don't be afraid to get that LED glowing - you should notice a subtle change in the character of the source sound as the LED glows more brightly and the valves really start to work. Ultimately though, let your ears be the judge! The "Peak" LED on the Crimson range products has a more orthodox role - it starts to illuminate 6dB before clipping, getting brighter as it approaches the clip point.

On later models – such as the Ivory series – we introduced a 2 LED arrangement, whereby one LED is labelled ‘Drive’ and the other is labelled ‘Peak’. In this instance the Drive LED indicates the level of drive to the valve stages, and Peak LED is a traditional clip indicator that illuminates when approximately 5dB of headroom is left.

Why does my Indigo unit have some unused valve sockets inside?
A: The number of valves fitted in an Indigo unit depends on the model - two valves are present in the 2012 Equaliser and 2021 Compressor, three in the 2051 Voice Processor, and four in the 2001 Mic Pre-amp and 2011 Equaliser. But - to make manufacturing easier - all the Indigo units are fitted with the same valve pcb, which carries the maximum four valve sockets, one or two of which may not be used, depending on the model.

Can I take TL Audio products out on the road?
A: Yes, more and more live systems are now employing studio-quality products, since the sound of live rigs has improved so much over the years and people can now hear the difference that quality outboard makes in a live situation. TL Audio products are very solidly constructed using an all-steel chassis, and the valves are gripped securely by high retention gold plated ceramic valve bases, for maximum security. Live acts including the Rolling Stones, Tori Amos, Lynyrd Skynyrd, Wyclef Jean, Prefab Sprout, Toploader, AC/DC, Morcheeba and Travis have all benefitted from using TL Audio products on major tours in recent years.