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SSL XLogic VHD Pre

Four-channel Mic Preamp By Paul White
Published April 2008

SSL XLogic VHD Pre

Some people love the sound of SSL's modern consoles, while others prefer the 'dirt' of their older models. The VHD Pre aims to offer you the best of both worlds...

SSL's rack mount spin-offs from their high-end console technology are proving extremely popular with the more discerning end of the private studio market, and this is a niche in which the rather lengthily named SSL XLogic Alpha VHD Pre sits very comfortably. As well as four highly specified mic preamps, you get channel-independent line (Pad) and instrument level switching, so that you can use it to get just about any form of signal into your recording system — so if you added one (or more) of these to a competent DAW system fitted with good quality converters, you should have a seriously capable recording chain...

Variable Harmonic Drive

The basic preamps are very clean and quiet, but SSL have also added in their VHD circuit. Though this may sound like an antisocial disease, it is in fact a very clever Variable Harmonic Drive circuit that introduces subtle (and, at higher settings, less subtle), character-creating distortions, in much the same way as a tube audio circuit does when pushed hard. The harder you drive a tube the more harmonic distortion you add — and VHD does the same by using FETs, with more input gain bringing more warmth. In keeping with tube behaviour, it's even OK to get the red Pad level light flashing if you want a very noticeably overdriven effect.

The VHD system was originally developed for the company's flagship Duality console, but it soon became clear that it was worth adding to other products. As the input gain is increased, the VHD circuit starts to add second- and third-harmonic distortion and the VHD control allows the user to adjust the balance of the two harmonic components. Anti-clockwise adds mainly second harmonic while clockwise adds mainly third harmonic. The even harmonics tend to sound more like tube distortion, whereas the odd harmonics have more of a solid-state crunch to them. The range of effects on offer varies from almost clinically clean to noticeably trashy and when used with restraint, the effect on vocals can be very sweet and flattering. There's plenty of output gain range and a creditable amount of headroom, so you can maintain a useful output level at either very gentle or very high input drive levels.


The VHD Pre has a very clean-cut appearance, partly down to the lack of obvious metering (which isn't a problem in practice, as the Pad button lights up red if you're in danger of overcooking the input level). There are input and output gain controls for each channel, the VHD knob, and three illuminated buttons that are styled after the fashion of partly-sucked Glacier mints. One of these activates the phantom power, one a 20dB pad, and the other switches the input to high impedance, for use with electric guitars and basses that are fitted with passive pickups. However, I was surprised to find that there's no low-cut filter, and I feel that's a serious omission.

Inputs and outputs are all on XLRs, while power comes in via a five-pin DIN socket.Inputs and outputs are all on XLRs, while power comes in via a five-pin DIN socket.Photo: Mike CameronEverything is old-school analogue, and there's no audio interface or digital converter built in. Power comes in on the rear panel via a five-pin DIN socket that connects to the included universal power adaptor, which automatically adapts to the local mains voltage. You'll also find XLR mic/line inputs and XLR outputs for each channel, but the outputs are not doubled up on jacks, as is often the case with other products aimed at the project studio market. Instrument input jacks are located on the front panel, and these override the XLR inputs, allowing the latter to be left permanently connected if required.

I have a couple of concerns over the connections, although these shouldn't be a problem if you're the careful type. One is that because the power comes in on a five-pin DIN, you could conceivably plug the output from the PSU into a MIDI socket by mistake. The other is that the mic and line inputs are effectively the same thing but with a pad for the line signals, so it is conceivable that you could accidently switch phantom power onto a line level source and possibly do it some damage.

Oddly enough, for a professional product like this one, there's no useful technical spec other than the size and weight of the unit and so on. There are no noise, distortion or other performance-related specs in the literature that comes in the box. I can only assume that SSL have taken the stance once adopted by Rolls Royce for their cars, where the horsepower and top speed was specified in the handbook as 'Adequate'.

Solid & Logical?

Tested with a variety of studio mics, the preamps came across as very neutral, providing that the input gain was kept below halfway and the output level control used to make up any necessary gain. When the VHD Pre is used in this way there's no obvious character, but then a well-designed amplifier should only make signals bigger, not change the way they sound. The VHD effect comes in only at higher input gain levels, where you can hear a subtle thickening and brightening of louder sounds — though if you set the drive level so that the red clip light flashes occasionally on peaks, the effect is far less subtle and may be more applicable to electric guitar and bass or hip-hop drums than to most vocals. I found that driving the input until the red LED started to flicker and then backing it off so the light didn't come on at all worked pretty well for male vocals, and I was able to fine-tune the character of the distortion by adjusting the VHD control and backing off the input gain where the effect was too pronounced. Anti-clockwise sounded warm and somewhat tube-like, while clockwise was brighter and grittier. By adjusting the VHD control and the input gain control I could coax some very nice sounds out of this preamp, but given the lack of any meaningful metering to indicate the amount of harmonic distortion being added, I found that I had to make all adjustments by ear.

I found the Alpha VHD very competent, but other than the harmonic distortion control there's nothing that really makes it stand out when compared with other respectable preamps in the same price bracket. In fact, in a direct shoot-out with the Aphex 207D that I was also testing at the time, I found that I slightly preferred the crisper sound of the Aphex unit on my own voice — although, to be fair, with careful use of the VHD control I was able to get pretty close. Such preferences are, of course, subjective, however, and the beauty of the VHD control is that you can emulate a range of 'flavoured' preamps as well as having a clean one. I chose the Aphex preamp for comparison as it has a similar (slightly lower) cost per channel — and because I had one to hand — but the VHD Pre also compared well with the preamps in my MOTU interface and even with my Universal Audio Solo 110.


Having four mic preamps in a 1U space is definitely welcome in today's crowded studios, and the VHD system could offer a viable alternative to owning more mics just to capture the right vocal quality for any given singer. The SSL name doesn't do the client credibility any harm either.

Although there's strong competition in this market, the VHD Pre offers good value, it is very easy to use (though you need to be careful when adjusting the VHD drive) and it offers a wide range of useful preamp characters. In fact, I've nothing negative to say about it other than that I'd have liked to see a low-cut filter included. All in all, this is a very flexible preamp that delivers a very convincing sound and would be well worth auditioning. 


There are few directly comparable four-channel alternatives, but on a cost-per-channel basis the following preamps would be worth auditioning: the Aphex 207D; the Focusrite 828; and the SPL Gold Mic.


  • Tonally versatile, from clean to flattering.
  • Sensibly priced, given its pedigree.


  • No low-cut switch.
  • Very rudimentary metering.


The VHD circuit elevates the Alpha VHD pre from 'well specified but unremarkable' to tonally very flexible. Used with care, the VHD system really does sound convincing.


£999 including VAT.

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