Paul White checks out a couple of all‑valve front ends to see whether eliminating solid‑state circuitry really makes a difference.
These two Rolls units are extremely simple in concept, but where they differ from most of their rivals is that the audio signal path is entirely valve‑based. Most valve mic preamps still use a solid‑state input stage, but the MP220 mic preamp (badged under the name of the Rolls subsdiary Bellari) uses a transformer input followed by a two‑stage valve (one dual triode) amplifier circuit. An instrument input and line setting allows the preamp to double as a DI box.
Taking the ADB3 two‑channel DI box first, this has an unbalanced jack input, and an XLR‑balanced output. Duplicate input jacks are fitted to both front and rear panels, and the controls are limited to two pad/gain switches and a ground lift. Power comes from an external 12V power supply, sadly not supplied, which implies that the valves are run in low voltage mode.
The two pad/gain switches may be used in combination to provide gains of ‑20dB, unity or +20dB, and using the ‑20dB setting, it's possible to feed in speaker level signals. There's no metering and no clip LED — you just select the best gain option, plug in and play. Because of the high 4MΩ input impedance, this unit is ideal for DI'ing electric guitars and basses, though it could also be used to add warmth to keyboards.
The two‑channel MP220 is a much more substantial, mains‑powered affair occupying 2U of rack space, with a choice of XLR mic/line input or a high impedance (1MΩ) jack instrument input. Both balanced XLR and unbalanced jack rear panel outputs are fitted; there are in fact two parallel output jacks per channel, allowing the signal to be split. Each channel has an input gain control and an output volume control, as well as switches for phantom powering, and a 20dB mic/line selector which affects only the rear panel XLR input; not the Instrument input jack.
A five‑section LED output level meter is fitted, and rear panel switches allow the XLR outputs to be switched from ‑10dBV to +4dBu for connection to mic or line level console inputs. The frequency response of both units is specified as being from 40Hz to 40kHz within +/‑3dB (from 5Hz to 40kHz on the instrument input of the mic preamp), and the signal‑to‑noise ratio is quoted as 107dB.
Both units are simple in concept as well as operation: the DI box accepts line, instrument or speaker level signals, while the mic preamp accepts low‑impedance microphones, line levels or instruments. At low gain settings, neither unit colours the sound significantly, but as the input level is increased, there is a very obvious tonal thickening which makes any decent capacitor mic sound obviously 'valvey'.
Though the tonal change isn't drastic, it does add a certain throaty intimacy to vocals, while rounding out the bottom end quite nicely. However, because neither box has an input gain metering system, you have to set up the amount of 'drive' by ear. Even if you can't use the DI box with microphones directly, you can patch it in via console insert points to colour what you already have.
Used with electric guitars, the result was less dramatic than I might have expected, but again, the sounds were delivered with enhanced warmth and a hint of compression. The only negative point I discovered was that the DI box hums alarmingly if the gain is set to maximum and no input is plugged in.
These two units actually perform very well, but given that they are priced to compete with the likes of the TLA Range, or the excellent Gas Cooker all‑tube DI from Ridge Farm (see the reviews of the TLA mic preamp/DI and Gas Cooker in SOS August and June '95 respectively), I feel that more attention should have been lavished on the presentation. Also, I'm not convinced that the all‑tube circuitry sounds significantly different to circuitry that's part‑tube, part‑solid‑state, but when it comes to valves, so much is subjective. I don't feel the ADB3 DI box is really tough enough to spend its life on the floor, which is where DI boxes generally end up, and the external power supply isn't even supplied as part of the package.
These are both interesting products, which unquestionably do the job asked of them, but they either need to be cheaper or more lavishly packaged to lure the potential purchaser away from more established alternatives.
- Simple to use.
- Both units deliver the 'valve' warmth promised.
- The units are versatile in that the DI box can also accept speaker level signals, whereas the mic preamp will take mic, line and instrument levels.
- Some form of pre‑output gain metering would really help setting up.
- Cosmetically uninspiring.
Though both flexible and genuinely useful valve processors, the strong competition will limit their sales at the current asking prices.