This hybrid mic preamp can go from solid‑state clean to valve warmth — and for a surprisingly low price, too. Is there a catch?
Some people like to use solid‑state mic preamps, while others prefer valve devices, and companies such as Universal Audio have responded by creating products that offer both options, or a blend of the two, in a single box. But not everyone can afford that luxury... or so I thought.
Further down the hardware food chain is Studio Projects' VTB1, a compact mic and line preamp that can also function as a high‑impedance instrument DI box, and it comes in at a fraction of the price of a UA Twinfinity. It's hard to believe that this preamp has been on sale since 2002 without SOS having reviewed it, but it's still on sale, and a couple of readers requested our opinion — so we decided it was time we put the VTB1 to the test.
The whole package is housed in a folded‑steel case, and it has rubber feet to keep it stable on the desktop. It is equipped with switchable 48V phantom power, a switchable 18dB/octave low‑cut filter at 70Hz, and a polarity‑invert button. In addition to the usual input gain and output‑level controls, the VTB1 also has a front‑panel Blend control that can be used to mix in some valve 'warmth' by balancing its solid‑state and valve signal paths. A five‑LED meter can be switched between the input and output.
The microphone front end is a solid‑state Class AB circuit, which feeds a TRS jack insert point, allowing additional processing to be inserted into the signal path. According to the circuit description, a balanced, current‑source‑fed parallel transistor arrangement feeds into a bipolar op‑amp. The transistors provide up to 45dB of gain and the op‑amp a further 15dB. Both the mic input and line output are on balanced XLRs. The line switch is on the front panel next to the instrument/line input jack. A FET is used in the line input circuit, to present a high input impedance when the device is switched to instrument mode. (In this mode, the input impedance is 1.5MΩ.) Unusually for a preamp in this price range, the input also offers variable input impedance, via a rear‑panel switch, to suit both 50Ω and 200Ω microphones. The actual input impedance of the VTB1 is 2kΩ in the 200Ω position and 300Ω in the 50Ω position, as it is normal to feed mics into an impedance an order of magnitude greater than their own impedance. Whatever theory suggests, your best bet is to try both positions for each application and see which sounds best to you; usually you'll hear a subtle difference at the low end.
One major cost saving is that the VTB1 runs from an external 12V AC power adaptor (which is included), and its valve circuitry is run at a relatively low voltage. While such 'starved tube' circuits can sound good, they do tend to sound rather different from a full‑voltage circuit. A 12AX7 dual‑triode valve with a DC heater supply (to avoid hum) is used in the valve drive circuit, where the first triode stage provides the gain and the second works as a cathode‑follower impedance converter. Output buffering and balancing is via a high output‑current op‑amp that also adds 6dB of gain for both the hot and cold balanced output legs.
There's no shortage of mic gain, with up to 60dB of preamp gain and a further 12dB available via the output gain control. An equivalent mic input noise figure of ‑128dBu (150Ω source, 60dB gain) is quoted, and the frequency response is flat from 20Hz to 20kHz within 0.5dB. Output levels of up to 21dB are available before clipping occurs.
A recorded vocal test using an Audio‑Technica AT2020 capacitor microphone revealed a reasonably lively sound with the blend knob in the clean position, though further tests comparing the sound with my (much more expensive) Universal Audio Solo 110 showed it up as being a hint less detailed.
The blend control is supposed to hold the level constant, so that the user isn't distracted by level changes while making adjustments, but it isn't entirely successful in this respect: I found that the level still increased significantly (by around 6dB) when the knob was turned towards the all‑valve position, and this made judging the subjective effect a little harder than perhaps it should have been.
I was pleased to discover that the valve coloration wasn't excessive, and that it didn't add too much mud to the proceedings either. The nicest blend to my ears was roughly half-way, with the highs being softened slightly, and the lower mids filled out. But there was nothing too dramatic going on, which is how it should be. For most pop work, I'd stick at or close to the clean end of the blend scale, but for rock, where you need a bit more punch and attitude, higher settings that bring in more of the tube character could work well.
When compared with my Golden Age Pre 73 solid‑state preamp, which uses transformers to help create a 'vintage' sound (and which is about twice the price of the VTB1), I found the VTB1 to be a little less smooth at the high end. With the valve warmth dialled all the way up, the mid-range took on a slight 'barking' quality that neither my UA110 or Pre 73 did. Nevertheless, you do have to bear in mind that the VTB1 is a very low‑cost mic amp, and within its market sector it actually acquits itself pretty well.
Repeating the tests with a dynamic mic yielded similar results — and served as a reminder than in some vocal applications, there's less difference between a capacitor mic and a dynamic mic than you might imagine!
Maybe the VTB1 isn't exactly 'esoteric', but it is quiet — and unlike the preamps on budget audio interfaces that are aimed at the same cost‑conscious sector of the project‑studio market, its blend control makes it tonally flexible.
For low‑cost valve mic preamps based on low‑voltage valve technology, look at Behringer and ART. More money will buy you something like the SPL Gold Mike, or you could consider looking for 'warmth' from a solid‑state preamp such as the Golden Age Pre 73, though this again is more expensive.