You are here

SPL Charisma Dual-channel

Valve Signal Processor By Paul White
Published May 1998

SPL Dual-channel Tube Processor.

Though this Charisma has only two channels instead of eight, it's lost none of its original charm.

SPL's original Charisma (reviewed in SOS November 1996) was an 8‑channel device designed for use with 8‑track recorders, and its aim was to add valve coloration and saturation effects in a very controllable way. Now the company has released a 2‑channel version, so that people who don't need to process all their tracks at once can use the Charisma process on either individual mono tracks, a stereo subgroup, or a whole stereo mix.

The Charisma's two channels have identical sets of controls. Its internal circuitry features a pair of dual‑triode ECC83 valves running at 230V (though the input and output circuitry is solid state and uses SSM 2141 and 2142 balancing drivers, which permit unbalanced use without level loss). Elaborate power‑supply design is used to keep hum and noise from the valve stages to an absolute minimum, and a rear‑panel ground‑lift switch is fitted to isolate the circuit ground from the chassis ground. Both balanced jack and XLR inputs and outputs are fitted on the rear panel, and power comes in via the usual EC socket and lead.

What Does It Do?

Developed by SPL's own designers, in conjunction with Manfred Reckmeyer, the creator of the original Charisma, this unit is designed to generate the beneficial side‑effects associated with the best vintage valve equipment, notably that elusive quality of warmth and dynamic compression. The effect can sound like tape saturation (unsurprisingly, since the non‑linearities of valves and analogue tape under high‑level conditions are quite similar).

The valve's behaviour is governed by the Drive and Charisma controls: the Drive control adjusts the level of the signal fed into the valve stage by up to +24dB, and the Charisma control varies the valve's saturation characteristics. A Gain control adjusts the output level of the processed signal, and each channel has its own bypass button — a sort of Charisma bypass (I used to work with somebody fitting that description!). As Drive is increased, the valve starts to operate in its non‑linear region, but because valves saturate softly, rather than clipping in the way that most transistor circuits do, the effect is to add subjectively musical harmonics, while at the same time restricting the dynamic range by compressing individual waveform peaks.

With most valve devices, drive level is the only factor the user can vary, but SPL have added the Charisma control so that you can alter the way the valve sounds when it saturates. Varying the Charisma setting changes the saturation effect from hard to soft, and from what I can tell this determines how quickly the valve becomes non‑linear once a certain signal level has been reached. Though the effect is different to compression, there is an analogy to be drawn between the soft Charisma setting and soft‑knee compression, where the effect increases progressively with level. The Hard setting, on the other hand, features a rapid transition from clean to coloured when the signal exceeds a specific level. The manual describes the Soft side of the control as providing warm, smooth sounds, while the Hard side creates punchy, crisp sounds.

SPL have added the Charisma control so that you can alter the way the valve sounds when it saturates.

The 2‑channel Charisma has no on‑board metering but does feature two LEDs designated Max and Process. The Process LED lights when the signal level is high enough to cause the tubes to start behaving in a non‑linear fashion, and the Max LED warns that any further level will cause clipping. However, the manual wisely points out that the Process LED can only be a guide, because some signals, such as drums, can stand very heavy processing before much change is evident, whereas some types of musical signal can sound over‑processed at such low levels that the Process LED may not even have come on.

In Use

When I first switched on the Charisma, only one channel was working, so I removed the cover and found that one of the ECC83s had worked loose in its socket. I guess the unit must have received quite a jolt in transit, as the valves are securely seated in good quality sockets, but this did make me think that perhaps sockets fitted with spring retaining clips might be a good idea. Once the valve was pushed back home, the unit worked perfectly.

I tried the Charisma with single voices, solo instruments and full mixes, and found that it delivered those characteristics normally associated with 'the valve sound' very authentically and controllably. It also confirmed what the manual warns of; namely, valve distortion doesn't suit every sound source. Some musical sounds suffer from rather unpleasant intermodulation distortion when valves are deliberately pushed into saturation, and in such cases you might find that running the valves below the level at which the process LED comes on gives a subtle but tangible effect that does remain musical. The Charisma's effect on vocals also tends to vary from singer to singer, but I prefer to err on the side of minimum saturation to keep the sound natural and clear. A soft setting of the Charisma control, with the Process LED coming on only occasionally, seems a good bet. Heavier processing can be used to beef up synth or percussive sounds, especially those with spiky waveforms, and because of the way in which tubes saturate you can gain a significant increase in perceived level without increasing the actual peak level at all. This can be useful when recording to a digital medium, as it helps protect against peaks.

The more assertive Hard setting of the Charisma control seems best suited to percussive sounds, where a little crunch is perceived as better attack and more power rather than as obvious distortion, though it's also possible to use this setting as a type of limiter, just to catch peaks. There's also enough Drive available to move out of coloration and into obvious distortion, and this setting may be useful to treat sampled organ patches or to further process electric guitar tracks. The real trick is to listen, and not be tempted to overdo the processing — quite often, if you can hear a definite effect you've gone too far. Original valve mics were designed to be as clean as possible, not to deliberately distort the sound, so with most vintage gear you don't get significant amounts of distortion until signal levels are very high.


There are numerous boxes on the market that claim to let you add the valve sound to your music as if it were some kind of sonic stock cube that you could sprinkle on, and some do the job more successfully than others. As usual, SPL have taken a somewhat different approach to their competitors, by making the valve overdrive characteristics user adjustable, so that it's possible to get any amount of coloration at any signal level, and to choose between a soft, progressive coloration and fairly tough saturation. If you're into the sound of valves working hard, this is certainly the most controllable and flexible box I've yet tried for achieving that sound.


  • Very easy to set up.
  • Very controllable.
  • Authentic valve saturation sound.
  • High standard of mechanical design and construction.


  • The Process LED gives only an approximate indication of how the unit will sound, so you really have to count on your ears when making adjustments.


A very controllable way to add either subtle valve coloration or more obvious valve distortion to any audio signal.


£469 including VAT.