The SSL VHD Input Module is an X-Rack-format preamp/compressor with a variable harmonic distortion control, selectable Hi-Z input impedance, high- and low-pass filters, 70dB gain, and the classic SSL Listen Mic Compressor
SSL, particularly since their Superanalogue signal path was introduced in 1996 with the XL 9000 J-series console, is known for transparency and coaxing out digital-like consistency from analog circuits. The design philosophy being that with great transparency, comes great responsibility . . . (no, wait that’s Spiderman) . . . and adding tasteful distortion is a choice made by you, not the preamp. And when you want to make that choice, the VHD Input Module for SSL’s X-Rack gives you a best of both worlds solution: ultra-clean SSL audio capture plus the Variable Harmonic Drive circuit, which is an all-analog circuit that adds 2nd- or 3rd-order harmonic distortion. If that weren’t enough, it also features the famous Listen Mic Compressor (LMC), the result of a happy accident between Phil Collins and Hugh Padgham. The LMC is great for tucking heavy distorted guitars into a mix, or, if you feel huge, trashy (in a good way) drums coming in the air (see what we did there), the LMC will get you there with a single knob. The VHD Input Module is SSL at its rock-and-roll best and most versatile.
SSL X-Rack VHD Input Module—Just the Facts:
- Electronically balanced ultra-high bandwidth mic input with +20dB to +70dB gain control
- Hi-Z and Pad Switches on Mic Amp Input
- Dedicated Additional XLR Line Input
- Variable Harmonic Drive circuit adds a controllable blend of 2nd- or 3rd-order harmonic distortion
- Dedicated trim control after the VHD stage
- Classic Listen Mic Compressor with fixed attack and release curves... ideal for drums
- High- and low-pass filters
SSL X-Rack VHD Input Module—Under the Hood
The X-Rack VHD Mic module contains two separate input amplifiers; a microphone amplifier incorporating SSL's unique variable harmonic drive circuitry and a line-level input. It also has a set of high- and low-pass filters, switchable low- and high-impedance, phase reverse, +48V phantom power, continuously variable gain between +20dB and +70dB, and selectable left/right record output. A -20 to +20dB trim control is also included. And of course, there’s the addition of the SSL Listen Mic Compressor to round out your tone-shaping/mangling toolkit.
The VHD Pre is a proprietary design that can deliver ultra-clean SSL console-grade recordings with user-controllable amounts of harmonic distortion. The ability to add harmonic distortion to taste is made possible by SSL’s Variable Harmonic Drive VHD circuit, which can be switched in or out of the signal path—like smooth or crunchy peanut butter, except the 100% analog VHD lets you add as many nuts as you like (mind out of the gutter, now!). As you increase the input gain, the Variable Harmonic Drive circuit introduces either 2nd- or 3rd-order harmonic distortion, or a blend of the two. At lower gain settings it adds gentle tube-style warmth or a touch of transistor edge—great for sounds you want forward in a mix without having to take up headroom using EQ. As gain is increased, distortion becomes more extreme until at high-gain settings it delivers fierce trashy transistor grunge. VHD preamps give you ultimate versatility from a studio-grade preamp.
The SSL Listen Mic Compressor
The SSL Listen Mic Compressor or LMC is one of those happy accidents that remained a secret in the inner circle of studio pros until the industry changed and secrets were revealed in the form of software plug-ins modeling classic studio hardware (we all have to make a living somehow). This now-famous processor from the legendary SL4000E console was a closely guarded secret of producers, used to create sounds that both intrigued and mystified the public. A single-knob compressor originally designed to prevent overloading the return feed from a studio communications mic, its fixed attack and release curves were eminently suitable for use on ambient drum mics. (Other compressors are set to “nuke” to create the same effect.) With the VHD Input Module, you get the gamut of SSL sonics.
PAD For The Record: The sound of the SSL listen mic compressor (LMC) was made famous by Hugh Padgham on Phil Collins’ drum track for “In The Air Tonight,” heard in the pilot episode of Miami Vice. In the 80s, consoles normally had a talkback mic for the engineer to communicate with the artist, but nothing that enabled them to hear the artists. A standard trick for bi-directional communication was to set up a mic in the middle of the live room plugged into a spare channel on the desk with a compressor inserted. The compressor was set at the highest ratio with the lowest threshold. The reason for this was to prevent the band playing in the room to overload the listen-back mic and melt the musicians faces in the process. During normal speech, the compressor would be at nominal operating level, so you could hear everyone in the room. The instant the band started playing, the compressor would squash the mic’s pickup, thus preventing overload and causing the sound of the mic to disappear underneath the band’s performance in case the mic was left on.
On a session with ex band-mate Peter Gabriel for his third solo album, Padgham, who frequently collaborated with Collins, was using the SSL 4000’s listen mic. At one point, Collins stopped talking and played the drums. According to Padgham, “The most unbelievable sound came out because of the heavy compressor. I said, 'My God, this is the most amazing sound! Steve (Lillywhite), listen to this.' But the way the reverse talkback was set up, you couldn't record it. So I had the desk modified that night..." Subsequently, SSL made Padgham’s modification a standard feature so you could record the LMC at the touch of a button.
No mods, no fuss
The console surgery required to gain access to the listen mic’s compressed output was performed on many early E Series consoles before it became a standard modification on later production models. The original circuit has been added into the X-Rack VHD Mic Pre, enhanced with the addition of a front panel threshold adjustment. Due to the vagaries of the original design, the range of the threshold control is indicated as either LESS or MORE! The circuit follows after the Trim and Filter stages, enabling either microphone or line-level signals to be tamed and tailored before benefiting from the sonic possibilities provided by this compressor.
Whether it's traditional ultra-clean SSL, subtle warming, or totally trashy distortion, SSL’s VHD Input Module will change your expectations of what a preamplifier can deliver. If you’d like help configuring an X-Rack, call or chat online with your PAD Studio Specialist today.
- Reference frequency: 1kHz
- Reference level: 0dBu, where 0dBu ≈ 0.775V into any load
- Source impedance of Test Set: 50 ohm
- Input impedance of Test Set: 100k ohm
- All un-weighted measurements are specified as 22Hz to 22kHz band limited RMS and are expressed in units of dBu
- All distortion measurements are specified with a 36dB/octave low-pass filter at 80kHz and are expressed as a percentage
- The onset of clipping (for headroom measurements) should be taken as 1% THD
- Unless otherwise quoted all figures have a tolerance of ±0.5dB or 5%
Mic Input (Signal applied to Microphone Input and measured at Output. VHD and Pad switched out with Input Gain control set to +20dB (minimum) and Trim control set to 0dB indent)
- Gain: Continuously variable from +20dB to +70dB Independently switchable 20dB Pad available
- Input impedance: Switchable from ≈1.2k ohm to ≈10k ohm
- Output headroom: > +26dBu at onset of clipping
- THD + Noise (-18dBu applied, +20dB gain): < 0.2% (20Hz – 20kHz)
- THD + Noise with VHD: Adjustable between < 0.2% and 5% (1kHz – 20kHz)
- Frequency Response: ±0.3dB from 20Hz to 20kHz –3dB at 50kHz
- Equivalent Input Noise: < –126dB at maximum gain
- EIN (input terminated with 150 ohms): < –80dB at minimum gain (with pad in)
- Common Mode Rejection: > 75dB from 50Hz to 1kHz
- CMMR (-10dBu applied, +30dB gain): > 65dB at 10kHz
Line Input (Signal applied to Line Input and measured at Output. Line selected and Trim control set to 0dB indent)
- Gain: Continuously variable from –24dB to +24dB
- Input Impedance: > 10k ohm
- Frequency response: ±0.1dB from 20Hz to 20kHz; –3dB at 50kHz
- THD + Noise (+24dBu applied, 0dB gain): < 0.005% from 20Hz to 20kHz
- Equivalent Input Noise (EIN): <-90dB (input terminated with 150 ohms)