Front panel looks like glass but is actually clear perspex over laser printed gold metallic paper. The main pilot light, upper left is a subdued and rather lovely shade of pink. Blue is just so common these days. Controls are from the left: Input selector, Attenuator and then the Bass Management functions. The front panel branding bottom left is "Eleven" and for clues on where this comes from please refer to the film "This is Spinal Tap".

The other problem was that I needed a unit that was not only suitable for stereo and multi channel music as well as home theatre but also included some pretty creative bass management facilities. More on this later but suffice to say, the finished unit has 3 sets of 6 channel inputs and 10 outputs, comprising front left, centre and right, front effects left and right, rear left and right and 3 lower bass channels as I run 3 pairs of speakers for this task. A total of 28 gold plated RCA sockets on not much panel space in a standard 2U rack mount enclosure.

The rear of the unit and enough of the top panel to show the colour coded guide to what all the connectors are for because there isn't enough room on the back for it all. Besides I don't have access behind so all leads are connected from the front by reaching over the top. All my interconnects are also colour coded which makes the process of working out what goes where a bit easier.

The chassis is copper lined with single sided PCB stuck down with copious amounts of silicone which is also an effective dampening medium. There is also some soft iron shielding between the transformer and circuit areas in the form of 3 pieces of 3mm thick steel plate. All internal wiring is 0.5mm solid pure silver wire mostly run as twisted pairs except for the power supply wiring which is 0.15 x 3mm silver strip. Input selection is via a 12 pole 3 position gold plated silver contact switch.

Full height copper PCB dividers provide mounting points, shielding and aid chassis rigidity. Front, back and side panels are also shielded this way as is the lid. Power supply section is also fully shielded with mild steel. Finished unit weighs in at for a pre-amplifier a fairly substantial 9kg. All the signal wiring and buffer sections are located at the rear of the unit. Emphasis is on short direct signal paths and individual earth return paths.

After looking at every possible method of attenuating an audio signal and placing fidelity at the top of the list I ended up with what I first considered using which was a precision stepped attenuator. I choose a 24 position, 6 channel 50K DACT CT2 unit which has 2db steps down to -34db and then 3, 4 and 10db steps after that. Being a resistive divider network it is followed by the mandatory active buffer stage.

For this task I'm using Burr Brown OPA627BP op-amps for the front left, centre and right channels. Standard OPA627AP's for the rear left and right channels and for the front effects, bass management and LFE channels, much cheaper but still very good OPA134's and OPA2134's. All buffers run at unity gain and therefore have only one component in the signal path which is a Vishay 70ohm resistor in series with the output to isolate the op-amps from any cable effects and to protect against any short circuiting of the output.

Note: With the arrival of the 12AU7 tube buffer "Glowing Glass Thingies" I've ditched the output buffers on the front left and right channels so this, for stereo at least is a true passive attenuator. This is a bit of a departure from my usual modus operandi of buffer everything and low output impedance sources rule but I've been on a bit of a KISS (Keep It Simple Stupid) kick lately and this has entailed removing everything from the signal path that doesn't really need to be there. And you sure as hell don't need an op-amp buffer right after the attenuator, a short length of cable and then another valve buffer driving the cable to the active crossover.

Impedance and drive wise it works out just fine because at typical listening levels the output impedance of the attenuator is between 6k and 12k, the interconnect to the tube buffer is about half a metre long and it's input impedance is 1M ohm (one million ohms). With the extra drive available from the new tube buffer it's also allowed me to remove the input buffer from the active crossover. Less is more as they say.

Shown at top is the Left, Right, Centre, Rear Left and Rear Right audio channel buffers. All the Low Frequency Effects (LFE) processing and buffering is done on the board that sits underneath and uses identical buffer stages.

Power supply consists of the usual IEC socket/mains filter, twin fuses as there is no on/off switch, 80 watt toroidal transformer, Philips BYQ28E ultra fast recifiers, each bypassed with 10n MKP capacitors, feeding Panasonic FC main reservoir caps, +/- 18 volt pre regulator and followed by a Sulzer-Borberly precision regulator. A 10ohm non inductive resistor connects chassis earth to signal earth.

Power supply rail decoupling at the chips is a parallel combination of .01uf/50V monolithic ceramic, pairs of 0.1uf/50V Black Gate NX Hi-Q caps in Super E-Cap configuration and a 10uf/25V Rubycon ZA to provide a small current reserve right near the chips.

Power supply comprising from the right, 240V input and filter, transformer, rectifier board and +/-18V pre-regulator board which then feeds the Sulzer-Borberly precision +/-15V regulator board. Entire section is shielded with both copper and steel with an extra 10mm of mild steel plate next to the transformer and 240V socket.

Bass management consists of the abilty to derive stereo sub outs for the front left and right subs derived from the main left and right (stereo) signal and a separate mono sub signal is sent to the two rear subs. All this happens when the LFE switch is in the 9 o'clock (2.1) position with the 2.1 gain control (to the left) controlling the "mix" level for the rear subs. In short, I'm running stereo subs for two channel audio but this has only been a recent upgrade but something I should have done long ago because it's a vast improvement over mono subs.

With the LFE switch in the 5.1 position it's a whole different matter with all subs being feed with the normal .1 LFE signal present on movies and multi-channel music and controlled primarily by the AVR feeding those sources to this pre. However in addition to this the front subs also get a low frequency mix signal from the front left, centre and right speakers and the rear subs get additional low frequency input from the rear surround channels.

All filtering is 4th order (24db/octave, 6db down at cross over point) 12hz high pass and 80hz low pass Linkwitz-Riley filters incorporating all pass phase shift sections so the LFE channels are in sync with the bass output of the 5 main channels all of which also use active filters.

The front panel needs an update because the Front, Rear and EQ'd labels should now read Left, Right and Rear. These are the level controls for a pair of subs front left another pair front right and a further pair at the back of the room. The system has been balanced for two channel audio with the controls set to 12 o'clock which allows quick and easy adjustment on the fly if required.

This pre-amp has three six channel inputs, one is used for 5.1 channel SACD and by default 2.0 SACD/CD, one was for 5.1 channel DVD-A but that never happened so its currently being used as a two channel input connected to the output of my phono pre-amplifier. The remaining input is connected to the output of a multi channel AV processor/pre-amp which is used for all Dolby, DTS and MPEG material from free to air analogue/digital TV, cable digital, Blu-ray, HD DVD and DVD.

As this processor has full control over the output levels on all its six channels this effectively allows for completely different mix levels for video sourced material and for music only material at the twist of a single switch. In short, 2 channel performance without compromise, properly balanced multi channel music and home theatre with the required amount of "bass oomph" and suitably exaggerated surround sound effects.

Audio processing for all digital sources is taken care of by a Sony STR-DA5500ES 7.1 channel AV Receiver. It supports all the current audio formats including DTS-HD Master Audio and High Resolution Audio, Dolby Digital Plus and TrueHD, 7.1 Linear PCM and DSD. 7.1 analog inputs, lots of digital inputs including 6 HDMI inputs and 2 HDMI outs which is real handy when you're running to displays.