This new front end has a clean, warm sound and is surprisingly easy to use.
Bellari have combined elements of their analogue processors and mic preamps to produce the RP503 Tube Channel, an attractively-priced 1U rack processor that combines a mic/line preamp with a simple-to-use compressor and a three-band equaliser with a swept mid-band. Although little circuit-design detail is provided, it seems that a tube gain stage follows the transformerless solid-state mic preamp, after which the signal passes through an optical compressor before being fed to the EQ stage and output level control. My preference is for the EQ to follow the compressor as it does here, as that prevents the compressor from fighting whatever EQ changes you have made. However, there are occasions when having these processes the other way around can be of benefit, so it wouldn't have hurt for the designers to make the order switchable. Nevertheless, this is a low-cost unit, and I'd rather lose features than sound quality. Power comes in via a captive mains lead, and both the input and output are available on balanced XLRs and unbalanced quarter-inch jacks.
The input jack has a 50kΩ input impedance, which is compatible with most line-level sources but rather too low for use with electric guitars and basses. A further TRS jack provides an insert point into the compressor's side-chain, allowing an external filter to be patched in if frequency-conscious compression is needed. However, there's no straightforward insert point between the preamp and compressor, which can be useful when adding other devices to the signal chain. A switchable 30dB pad in the preamp control section (which you press for maximum gain rather than for attenuation) means that pretty much any signal level can be accommodated, including the outputs from electro-acoustic guitars and keyboards, though there's no 'instrument' option setting for guitars and basses. The mic preamp has switchable 48V phantom power with status LED, phase reverse, and a gain trim control with a 30dB range — the maximum gain is a healthy 65dB. A clip LED provides the only front-end metering.
As this unit is intended mainly for vocal use, the compressor seems to have been optimised for vocals and stripped of all unnecessary controls, leaving the user to adjust only the threshold and ratio. The ratio can be varied from 2:1 to 8:1, which should more than cover normal vocal requirements. The time constants are preset, and the amount of gain reduction shows up on a very small, circular moving-coil meter which lends a vintage feel to the unit. A chunky button switches the compressor in or out of circuit.
That leaves the EQ section, which is arranged much like a small console's EQ, with shelving high and low filters augmented by a swept mid-band. All three stages have a ±15dB gain range, and the mid-band can be swept from 300Hz to 5kHz with a fixed Q value of one. The low and high filters have shelving frequencies of 150Hz and 10kHz respectively. There's no EQ bypass button, and the EQ controls are not centre detented. An output level control offers up to 20dB of additional gain following the EQ stage.
The output offers a commendably low 50Ω impedance, and can deliver up to +22dBu of level, while the maximum line input level is +18dBu. The noise floor is quoted as 75dB, which though not exceptional is perfectly fine for a valve unit of this type. However, the more compression you apply to any signal, the more the noise content of the signal is brought forward during pauses.
On powering up the unit, I was mildly disappointed to find that the meter didn't light up, and I also detected a slight mechanical transformer buzz emanating from the case. This was pretty low in level, but careful design could have avoided it altogether. Because of the streamlined control system, using the RP503 is fairly straightforward, but there are a couple of little quirks to get used to, not least being the unusual reversed action of the pad switch. There's also the issue that most of the mic gain comes in over the last 20 percent of the gain pot's travel, a problem that shows up in many budget mic preamps. Those factors aside, the compressor is a doddle to use, as you simply set the ratio you want, then adjust the threshold until the meter shows a sensible amount of gain reduction — your ears will tell you the rest. Similarly, the EQ section works exactly like that on a typical budget console, though it sounds rather better than some. I would have liked more sophisticated level metering on the preamp, but provided that you adjust the Gain control so that the red LED flashes on peaks, then back the level off a fraction, it seems to work fine.
Tonally the unit manages both to be slightly warm and to maintain clarity, which is what you generally want from a tube design. The harder you push the preamp gain, the harder the tube is driven and so the more 'warmth' is injected into the sound. Unless you really go over the top with the gain control, though, this is commendably subtle. I like the simplicity of the compressor, which exhibits the usual easy-going character of an opto-compressor, and levels out vocals with seemingly little effort while adding a welcome density to the sound. This affirms my feeling that if a compressor is designed properly it doesn't need lots of controls to make it sound good. Even the EQ sounds pretty sweet — while it doesn't have a lot of flexibility, it works well on vocals, adding air at the top, increasing weight at the bottom, and allowing undesirable mid-range artefacts to be pulled back. As always with budget equalisers, this one sounds best when used sparingly, but then if you have a decent mic in front of a decent singer, you shouldn't need much EQ to fine-tune the sound.
Given its attractive UK price, this little unit performs extremely well, though it is not without competition in this sector of the market. If you definitely want a tube-based preamp and an opto-compressor, then the RP503 has to come out close to the top of the pile in its price range. On the other hand, if you don't insist on tube circuitry, it may also be worth auditioning the Focusrite Trak Master, as that's in the same price bracket and also turns in a very polished performance.
I like the mildly flattering sound of the RP503, and unlike some of the earlier Bellari units this one looks fairly stylish too. Criticisms are minor in nature, and though the technical spec of the unit is nothing out of the ordinary, it does have a friendly, musical sound. I feel that too much processing is often added during recording, so I welcome the simplicity of this little unit — it has exactly the tools you need to do the job with no unnecessary frills. Furthermore, as it is available via mail order from Smart Sound Direct, who offer a trial period with a money-back guarantee, there's no risk attached to buying one.