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SPL Charisma

8-channel Valve Signal Processor By Hugh Robjohns
Published November 1996

More and more people are choosing digital recording systems, which is creating a spin‑off market for valve processors designed to lend a more comfortable warmth to the resulting recordings. Hugh Robjohns looks at an SPL unit being touted as the ideal partner for eight tracks of digital.

The Charisma is the latest product from innovative German company SPL (whose Tube Vitalizer enhancer we looked at in our last issue). Essentially, it's an 8‑channel signal processor which uses valves to add a controllable 'warmth' or 'fatness' — in other words, it should add a little 'charisma' to your sound! It can also provide a degree of clipping protection for digital systems through the inherent soft saturation characteristics of valves — not dissimilar to analogue tape saturation.

Intended for use with digital multitrack systems such as the DA88 or the ADAT, or even DAWs (Digital Audio Workstations) like Digidesign's Pro Tools or Session 8, the Charisma operates with balanced pro‑level inputs and outputs and has eight completely independent channels.

Racking It Up

The Charisma is packaged in a standard 2U‑high, 19‑inch rackmount case with a substantial front plate. The latter is well up to the job of supporting the unit, although I feel some rear support would be a good idea in a mobile flightcase. The finish is a distinctive brushed gold colour, with simple black control knobs and markings.

Obviously, valves get pretty hot, and convection cooling is encouraged through slots on the sides and lid. The well‑written manual recommends keeping the unit away from power amplifiers or any device which itself gets hot or generates strong magnetic fields. It also suggests keeping the Charisma separated from digital equipment, to avoid interference from clock signals or MIDI data. However, the unit is well screened and CE marked, and I experienced no trouble with interference at all.

Mains power is supplied through a normal IEC socket with a voltage‑selector switch and an earth‑lift facility separating chassis and signal grounds (to break hum loops). There are no externally‑accessible fuses, but seven internal ones (none of which are labelled as to their function or value on the PCB!).

Audio connections are on rear‑panel Neutrik stereo jacks, wired for balanced operation (tip hot). A nice touch is that the rear‑panel graphics label the socketry twice — allowing it to be read when leaning over the unit from the front as well as directly from the rear! Input and output balancing is implemented through the excellent SSM 2141/2142 chips, providing superb performance regardless of whether your system is wired for balanced or unbalanced operation. Nominal input and output levels are +6dBu.

Setting It Up

The front panel of the Charisma has a power switch over on the right‑hand side, and eight columns of neatly‑arranged controls. Each channel has just three rotary controls, plus a push button and a couple of LED indicators.

The top knob is labelled Drive, and it determines the signal level feeding the valve processing stage. Calibrated from minus infinity to +24dBu, its unity gain position is marked on the dial, with a gentle detented action as the knob is rotated. Increasing the control above the zero mark introduces the characteristic 'valve sound', whilst also increasing the output level, more or less in proportion.

Adjacent to the Drive control are a pair of LEDs labelled Max and Process. The Process lamp blinks when the signal reaches a point where the valve is actively changing the sound quality. The Max indicator illuminates when the valve has reached saturation, and any further advance of the Drive control introduces overdrive distortion.

The bottom control knob is labelled Output, and again has a detented action and is calibrated from minus infinity to zero. This knob is provided to compensate for the level increase caused by the action of the Drive control. Next to the Output knob is a button which provides a complete channel bypass.

The 'fun‑knob' is in the middle, and is intriguingly labelled Charisma. Calibrated from soft to hard, this control effectively alters the biasing point of the valve governing the sound quality in a very audible way. At the soft end of the range, the effect is not dissimilar to gentle compression, but with an added richness caused by the inherent harmonic distortion. This can appear to either dull the sound slightly or lend it a lovely warmth, very much depending on the nature of the sound source itself. The hard end of the range is more like the effect of a limiter, and tends to lend a more punchy, dynamic characteristic to the sound.

Lapping It Up

Used with a certain amount of discretion, the Charisma enhances almost anything you care to feed through it — I suppose the best description of the effect is that it adds 'class'. All you have to do is juggle the Drive and Charisma controls for the desired effect, adjusting the Output merely to set the send level to the recorder or the rest of the signal chain.

Soft Charisma settings tend to be quite subtle, and affect the signal over a wide dynamic range. However, this is certainly not the case at the hard end, where there is a more obvious processing threshold. In both cases, though, the strength of the Charisma is that, through altering the dynamic and harmonic content of the audio signal, it tends to make things sound louder, fuller and richer, even if the input and output peak levels are carefully matched.

I found the Charisma to be particularly effective on drums (real, synthesized, or sampled) when used at the hard end of the scale, where it added a very analogue tape‑like quality which required far less EQ than usual to make 'right' within a mix. On acoustic guitar (especially with a pick) and DI'd bass, it can add a very pleasant thickening or warmth, and I found this helped weak and thin‑sounding instruments to edge their way to the front of the mix, without resorting to pushing the faders up. I also found that I needed much less reverb on instruments after they had been 'Charisma'd', which also helped to tighten up mixes.

The same effects were evident with vocals — particularly male vocals — and with careful adjustment, the Charisma can make cheap electret microphones sound really quite acceptable! With extreme settings, it can even turn weedy synth organ presets into ballsy pseudo‑Hammonds (but you still can't beat a real Tonewheel and Leslie combo...).

To sum up, the Charisma is well built, ideally suited to adding a classy analogue quality to your ADAT or Session 8 recordings, excellent at enhancing a wide variety of sound sources, and very easy to use. I liked it a lot, but don't take my word for it: check it out for yourself — you won't regret it!

Hugh Robjohns is a lecturer at the Centre for Broadcast Skills Training at BBC Wood Norton. The views expressed in this article are the author's own and are not necessarily those of his employer.

Opening It Up

Gaining access to the insides of the Charisma involves removing 12 short self‑tapping screws from the steel lid. A single mother board carries the encapsulated mains transformer and power supply circuitry on the right‑hand side, and in a neat row across the centre of this high‑quality PCB are eight E83CC valves. Immediately in front of these are eight vertically‑mounted daughter boards carrying the front panel controls and most of the electronics for each channel. The rear‑panel connectors all mount directly to the mother board.

The E83CC is effectively an upgraded version of the better‑known ECC83 double‑triode valve, with higher gain, improved high‑frequency capabilities and more effective internal screening.

Specifications are pretty good, with a flat frequency response from 20Hz to 50kHz, and excellent common mode rejection figures (courtesy of those SSM devices). A‑weighted signal‑to‑noise is quoted at better than ‑80dBu and distortion at 0.4%. This last figure is pretty poor in comparison with decent modern amplifiers, but remember: it is this very distortion that creates the sound quality you would be buying this unit for!


  • Classy sound.
  • Very easy to use.


  • Input rather insensitive and can be very difficult to use with some low‑output equipment.
  • Very difficult to match channels for stereo tracks.


The Charisma provides a good way of enhancing the quality of a wide range of source material, often making EQ redundant and helping things to shine through a mix. An ideal partner for 8‑track tape and disk‑based systems.