Sound on Stage (August 1997)
By Simon Bateson
Demeter Amplification's Tube Bass Preamp offers you consistent contro from gig to gig, whatever your rig. Simon Bateson checks it out.
A conservative and quality design, for the non knob-twiddling bass player.
The Demeter Tube Bass Preamp VTBP-201DBL is a black standard 19-inch rackmount design 1.5 units high, not 1U or 2U, but 1.5U. In this way, it starts to stake its claim as a "slightly special" sort of unit. It aims to provide a high quality front end for use while recording or, in conjunction with a suitable power amplifier and speaker, a performance rig of a size to suit the layer and venue while retaining a consistent set of controls. In this it succeeds, and Demeter Amplification have an impressive list of well-known clients to their credit. I should say straightaway that this is not a multi-function unit with crossovers and limiters, but a very straightforward valve unit aimed at the valve aficionado who perhaps doesn't want to carry a massive amplifier with them wherever they go. I tried it out in a number of contexts, both performing and recording, and found it reliable and easy to use, with a clean but warm signal path.
This preamp will hold no fear or confusion for the user. The front panel follows classic lines, is clearly printed in white on brushed black, and has familiar controls. Starting off on the left, two inputs on mono jacks are labelled HI and LOW. These are combined in much the same way as all the old valve amps, with a pair of resistors and switched jack sockets. In the HI socket, the signal passes through to the input valve more or less unattenuated and with a high input impedance of 1M. In the LOW socket, the signal ends up being attenuated by a factor of two; the input impedance is reduced, at around 124k. The precise meaning of HI and LOW on sockets has always been a matter of inconsistency between manufacturers: some mean level, in which case the LOW socket plays louder; some mean gain, in which case the HI socket plays louder! In this case, the labelling refers to gain. Personally I would have preferred the labels HI-GAIN and LOW-GAIN.
Following on, we have a single volume control and three standard tone controls: treble, middle, and bass. The common practice of adding a "bright" switch below the volume control has been followed. Electronically, this connects a low-value bypass capacitor between the top end of the volume pot and the wiper. Consequently, an amount of treble boost is added which depends on the setting of the volume pot; the greatest boost occurs at the electrical halfway stage of the pot, which in mechanical terms is about three-quarters of the way up. At very low or high settings, the bright switch makes no difference. If this seems a strange setup, you're right. It comes from an age when electronic components were very expensive and you had to get as much function as you could from the fewest possible parts! It will be familiar behaviour to most amplifier users because it has become a standard.
The tone controls follow the original Fender passive ladder network and are very predictable in operation as a result. They have a fair amount of interaction and a quite limited range of cut and boost, if you are using lots of bass and treble, there isn't much mid adjustment. With passive tone controls, there is always a trade-off between signal loss in the network and available tonal range. This set of controls is obviously a low-loss design, with specified adjustment ranges of just plus or minus 6dB (treble, middle) and plus or minus 9dB (bass) and, as I have implied, each of these ranges will apply to just one control at a time with the others set flat. That said, the type of instrument and speaker this preamp is likely to be partnered with are unlikely to need very drastic tonal correction. Usefully, a switch is provided to alter the turnover frequency of the bass tone control. At 120Hz, the bass control is wide ranging and powerful, while at 60Hz (my preference) you can trim the lowest frequencies.
An additional Presence control is fitted after the tone controls, with an associated selector switch for operation at 2kHz or 4kHz. This cannot completely imitate the presence control found in old valve power amplifiers. Those controls were connected around the power amplifier feedback network and had some quite complex effects such as altering the amplifier damping factor, the extent to which the amplifier controlled the speaker, in a frequency-dependent fashion. On the Demeter, the control follows the style of most modern amplifiers in being a further tone control, permitting the addition of extra high-frequency boost for brilliance. It does this perfectly well, emphasising the metallic tone of picked or pulled strings.
And that about wraps it up for the front panel controls. The only remainders are an on/off rocker switch with an associated red LED.
Around the back, we find a number of sockets and switches. On the right (looking from the front), a standard IEC connector and a finger-release fuseholder. I'm not quite sure whether this fuseholder would meet European standards for inaccessibility since the outer contact was quite easy to touch with the fuseholder cap off. It was wired the right way round, live to the rear inaccessible contact, but you can never guarantee mains leads to be wired correctly, and there is a potential path to the live mains through the transformer primary. Perhaps it would be better to make this a screwdriver release type.
Further along, we have the main signal outputs on paralleled jacks; these could be taken away to a couple of power amps. There is no crossover facility, but an effects loop is provided with the aid of two switched jack sockets. As usual, the send socket can be used as a signal take-off point as it does not break the signal path like the return socket does. The effects loop is connected post-tone and pre-volume/presence. I would like to have seen a second send socket for a tuner; it really is inconvenient to unplug and fiddle about with leads. If you are not using any effects, the send socket is the place to feed a tuner from.
Finally, we have the balanced line output facility. It comes courtesy of a Jensen signal transformer and you can choose to take the signal pre or post-EQ. The all-important ground-lift switch is present and correct. The circuit is not terribly complex, the transformer primary is fed from the main output sockets or from the first stage valve via the selector switch. The transformer is an option included in this "studio" VTBP-201DBL version, but you will need it for PA feeds too, so I would say it is a necessary option! The VTBP-201L version has no Jensen transformer and no XLR outputs.
Signal Path Design
One of the major selling points of this unit will undoubtedly be the exclusive use of valves for signal conditioning (along with the exclusive name and individual hand-built construction). Two Chinese 12AX7/ECC83 double triodes are applied, giving four amplifying stages. Without tracing the circuit in detail, I would guess that the first stage is the guitar preamp, feeding the tone control network, while the second would be a cathode follower which has no voltage gain but provides a low(ish) output impedance to feed the effects loop output. This will allow the connection of moderately long leads, but only a few metres, it's not like a solid-state unit. If you use very long cables, the sound will become dull.
On returning from the effects loop, a second amplifying stage incorporates the volume and presence control, which you can adjust to suit your power amplifier. Most power amps have volume controls though, so I think this is unlikely to be used. It's an indication of the Demeter approach that, while some manufacturers offer signal level adjustment at three or four different stages in the circuit, this preamp gives you the opportunity to buy a plain slave amp and reduce the volume control count to one!
After the signal has been fully adjusted, the fourth stage, which is also a cathode follower, drives the main outputs. Although these therefore also have a low impedance, connection to distant PA or recording desks should of course always be made from the balanced socket.
In keeping with the traditional feel of the controls, ordinary potentiometers and rocker switches have been used throughout. Thankfully, I found no trace of unidirectional cable or oxygen-free copper, and all the connections are to standard quarter-inch jack sockets.
The most striking feature of the electronics is not the major signal path design, which is very conventional. It is the care taken to maintain signal quality.
Valve circuitry of this design vintage is very much "single-ended", there is only one power supply rail and the circuitry is very susceptible to noise or variation on this rail. Valve amplifiers therefore have copious filtering or "decoupling" arrangements in the power supply, using large capacitors to act as local supplies to each stage and (hopefully) preventing disturbances, such as the heavy currents taken by the power amplifier stage, from finding their way back to the preamp stages. Unfortunately, large electrolytic capacitors are among the least satisfactory of electronic components and they used to be a lot worse than they are now! Hence the frequent unreliability of old amplifiers, with dried-up capacitors causing instability (motorboating), soggy sound, and strange discordant hums due to sagging and unsmoothed rails. Incidentally, I suggest that a lot of old amplifier problems are caused by capacitors (cheap) rather than by valves (expensive), approach your repairs accordingly!
Anyway, back to this unit. The designers have decided to adopt the benefits of modern components, there are some, you know, and have included a fully electronically regulated high voltage rail. It is a proper high-voltage rail as well. Some valve pedals and effects units of previous years have tried to run the valve anodes at 12V or less, and although they work, a valve will actually work at a lower voltage than a transistor, they do not have the same gain and overload characteristics as those run at a high voltage. Here, the high voltage rail is given some resistor-capacitor smoothing to make an unregulated and fairly ripple-free 340V supply. It is then regulated through an MJE340 high-voltage transistor to make a nice solid feed for the circuitry.
Interestingly, Demeter have also opted for a DC valve heater supply. It is unregulated and as a result has about 0.5V p-p ripple on it. The heater current flows back to the supply through the same cable as the HT supply, but the signal earthing arrangements seem OK, because there is no hum on the output. In fact, it is quite noise free, not as noiseless as it is possible to be with well-designed transistor circuitry, but a lot quieter than most of the rather careless designs actually used by equipment manufacturers. It is a fallacy that valve circuitry is noisy; valves are fine. The culprits were almost always the horrible carbon composition resistors and leaky electrolytics used in old valve amps.
In this unit, we find high quality metal film resistors and polypropylene capacitors mounted on a single small PCB, affixed to the base of the cabinet with metal brackets. The valves are horizontal in PCB-mounted ceramic holders. These have no retainers, but preamp valves are quite light so I don't expect them to drop out straightaway. It might be worth checking occasionally if the preamp is sitting on a great vibrating rig, however.
I found the method of manufacture quite interesting. With a number of specialist American and British designs, the choice of components seems to be orientated strongly towards standard parts from the popular supplier catalogues, so that there is nothing non-standard in it. This does mean that everything is easily serviced and replaced, which must be good. You could get the vast majority of parts in Maplin, in fact. On the other hand though, it does mean that the insides of products such as this look rather like the creation of a well-resourced hobbyist. For instance the controls are hard-wired to each other and to the board, rather than being board-mounted. This has been done very neatly, with sleeving and silicone used to secure long cables, but more PCB design work would have eased bulk manufacture. The intended market is small though, and the expense and inefficiency of a "hand-crafted" product enhances the image Demeter wish to portray.
The front, back, and sides are of black anodised aluminium alloy, while the casework panels are of black powder-coated steel with very neat paint removal at bolt sites to ensure good earth contact. The front panel is flanged backwards for rigidity. The top and bottom panels fit inside these flanges, but there is no internal support so they do bend inwards rather easily if pressed. There is nothing dangerous they could touch, but it would create a better impression if some sort of internal brace were fitted.
How's It Sound?
Valves and transistors are different and I don't mean that in any mystical sense. They have different measurable characteristics and normally find themselves in different circuit configurations. If you made a high-gain, high-feedback stage with valves, it would be just as linear and distortionless as the corresponding multi-transistor stage; and it would clip just as disgustingly when overloaded.
However, because valves have always been expensive and their electrical characteristics make them quite stable and predictable, they were mostly used in single-stage circuits and often without any feedback in order to obtain the maximum gain. As a result, many valve preamps, including the Demeter, add a little harmonic distortion which is pleasing at moderate levels. When wound up, they overload quite gently. The sound gets thicker with a bit more "grunt" but without nasty high frequencies coming in; in fact, the treble recedes as the overload increases. The trick with guitar amplifier overload is actually to prevent too many low frequencies getting into the overloaded stage, since the harmonics of thumps and rumbles mess up the main sound. This preamp doesn't use any clever filtering but does remove low frequencies in the capacitors connecting each stage, so it doesn't seem to get too muddy either.
The Demeter VTBP-201 is a straightforward valve preamplifer, which is actually the front end of a good traditional valve amp, but is lightweight and has a carefully designed noise-free signal path. Due to its conservative design, there isn't a great deal of gain or EQ available so you won't get any extreme effects out of it, but for the bass, that's often a good thing. It's clearly aimed at and priced for the quality end of the market and, in conjunction with equipment of a similar ilk, it can deliver a solid sound without fuss.