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Digitech VTP1

Valve Mic Preamp By Paul White
Published August 1996

Digitech's VTP1 seeks to outpace the competition in the crowded valve mic preamp market by offering built‑in 4‑band EQ and a digital output. Paul White sees how it runs...

Digitech's VTP1 is just one of a growing number of hybrid valve/solid‑state products aimed at the serious end of the project studio market as well as at the professional user. Essentially, the VTP1 is a mic/line preamp, and as such, can be used for feeding high‑quality signals directly to the inputs of a recorder, bypassing the ostensibly less sophisticated mic amps found in typical mid‑price mixing consoles. A digital output also makes it possible to transfer the signal to a DAT machine or hard disk recorder without leaving the digital domain.

In Detail

Packaged within the deliberately retro‑style 2U rack case are two mains‑powered mic/line channels, each with 4‑band, semi‑parametric EQ, and an 18‑bit digital output stage available in both S/PDIF and AES/EBU formats. The analogue inputs and outputs are furnished on balanced XLRs, with 48V phantom power on the microphone inputs and post‑EQ, balanced insert points for patching in external processors such as compressors. The line input also has an alternative balanced jack input, and inserting a jack disconnects the XLR.

The low‑noise, solid‑state mic preamp is transformerless, and phantom power, phase invert and a 20dB pad are all available on small toggle switches. A separate preamp is used to handle the line input, and a selector switch determines whether the mic or line input is to be fed to the valve gain stage via the Pre Gain control. This control sets how hard the valve is driven, while a Post Trim control attenuates the gain after the valve stage; the output stage level is monitored by a charming, circular moving‑coil meter. Clip monitor LEDs are fed from four separate points in the signal path to warn if any one stage is being overdriven excessively. A low‑cut shelving filter is also available, with an 12dB/octave response at 75Hz.

As a mic preamp, the unit is very flexible.

The two valves (one class A 12AX7 parallel triode gain stage per channel), are run at a stabilised 235V; lots of other valve equipment works by running the valves at a much lower voltage, but purists may argue, quite correctly, that the resulting sound isn't exactly the same as you get from a valve running at proper life‑threatening voltages.

Switching in the EQ brings a 4‑band equaliser into circuit, which the manual describes as 'semi‑parametric'. Obviously the Americans have now changed our mother language to such an extent that 'semi' now means 'not', because what we actually have is the familiar 4‑band EQ with two sweep mids and shelving high and low sections. All four bands provide up to 15dB of cut or boost, and the high and low sections operate at 12kHz and 80Hz respectively. Mid 1 may be swept from 50Hz to 3.2kHz, and Mid 2 ranges from 500Hz to 18kHz.


With an equivalent input noise of ‑127dBu, the VTP1 is capable of an overall signal‑to‑noise ratio of better than 102dB (A‑weighted), which is some 6dB better than from a theoretically‑perfect 16‑bit linear digital converter. The distortion is not particularly low at around 0.1%, but then that's the whole point of putting a valve in the signal chain — and by winding up the Pre Gain, you can get a lot more distortion than that if you want to! The line input has an impedance of around 20kΩ, so it's not really high enough to use as a DI input for electric guitars, though up to 26dB of gain is available for dealing with low‑level signals. The mic input can actually squeeze 66dB of gain, which is more than enough to cover all normal eventualities.

Set to operate at modest tube drive levels, the mic preamp is clean and open‑sounding, no doubt due in part to the wide bandwidth of the circuitry, which is only 3dB down at 40kHz. Switching in the EQ shows it to be extremely positive, and while I've heard warmer EQs, this one gives the impression that it's not going to be pushed around. It's really very good for pushing an intransigent signal into shape, while at more modest settings, it works well for fine‑tuning or sweetening.

Winding up the Pre Gain eventually causes the valve to operate in a noticeably non‑linear manner. The trick is to set the amount of coloration you want by ear, then set the Post Gain control to give a sensible output level.

The digital output works perfectly well, so long as you don't allow the output stage to clip, but I can't help wondering if including a simple limiter might have made operation more foolproof, especially in live situations where maximum levels can never be guaranteed.


There's no denying that the VTP1 does exactly as it says on the packet, though I missed not having a high‑impedance DI input. As a mic preamp, the unit is very flexible, especially with the powerful on‑board EQ, and though there's no limiter, the balanced insert points do provide the option to patch in your own compressor or limiter.

The provision of a digital output is perfect for recording to DAT or hard disk, but of course it isn't compatible with ADAT or DA88 recorders, where it would also be beneficial to have the option to pipe in signals in the digital domain. I suppose it's a bit unreasonable to expect ADAT and T/DIF interfaces as well as the more usual options, but the point has to be made in case anyone buys a VTP1 in the mistaken belief that it will plug straight into their modular digital multitrack tape recorder.

Whether it makes commercial sense to buy a VTP1 depends very much on the other components in your system, and on the type of work you're doing. Whatever people say about the input stages on typical consoles, most are surprisingly good, and any differences you achieve by buying an esoteric front end will be relatively minor, unless the rest of your kit is of really high quality; you'll need good capacitor microphones and a revealing monitor system at the very least. Similarly, if you're lucky enough to have tube microphones, it can be argued that there's little to gain by going through another tube stage.

If you do have a need for a quality preamp, then the VTP1 has a lot going for it. It's easy to operate, it looks classy, and it combines tonal warmth with transparency and the ability to interpret tonal detail. The digital output may or may not appeal to you depending on your system, but it's nice to have it anyway. If I could request more features, they would include a limiter, some means of metering the amount of valve drive and a high‑impedance input, but other than that, Digitech haven't missed many tricks.

The Digital Output

A toggle switch selects between AES/EBU (XLR) and S/PDIF (phono) output formats, while a second switch provides a choice of 44.1kHz and 48kHz sampling rates. In order to feed the digital signal into a DAT recorder or similar device, the recorder must be set to external digital sync, so that it will sync to the VTP1's clock. Most semi‑pro DAT machines do this automatically when digital input is selected, but with hard disk systems such as Pro Tools, you have to remember to set Digital Sync to 'on', otherwise you end up with a recording full of clicks and glitches, as the two clocks drift in phase relative to each other.


  • High‑quality, flexible sound.
  • Easy to operate.
  • Built‑in digital interface.


  • No limiter.
  • No high‑impedance input.


The competition in the valve hybrid add‑on mic preamp market is getting pretty fierce, but this unit has the benefit of a powerful onboard EQ and a digital output. Most esoteric mic amps are only worth considering if you do a lot of direct‑to‑stereo recording, or if the rest of your studio equipment is exceptionally good.