Why Passion Ultimate is the best sounding passive preamp
David Heaton, Audio Synthesis

During 1987 we produced our first passive preamp called PAS-01. This consisted of a buss of toggle switches for input selection and two Bourns linear conductive plastic controls for volume (law faked to improve low level stereo tracking) and balance. Like many of our earlier products the PAS-01 was designed in conjunction with Ben Duncan and appeared as a technical / DIY article in Stereophile (Feb. 1988).

Within a short period we replaced the inelegant toggle buss with a Swiss made rotary input selector, hardwired to switch the signal and ground of all four sources to maintain total isolation of the listening path except from the source selected. To further simplify the path all other switches were eliminated - no dim switch, no mono/stereo switch, no tape monitor switch. Three mute positions were built into the input selector to minimise the audio path's complexity within the preamp.

This purist approach improved clarity and produced a sound which was closer to the hardwired, fixed attenuated original. The new model became PAS-02 and was made available with a choice of either a 138-step dual-concentric attenuator or Penny & Giles CP continuous volume control (Penny & Giles are based in Wales and produce the highest quality mixing console linear faders for the likes of Neve and SSL).

The attenuator featured 0.5dB steps in a constant impedance design built entirely out of Holsworthy Holco H8 resistors around a Swiss six element precision switch - see Hi-Fi News May 1991 for more details. Over the next year the 138 attenuator option proved the most popular version of PAS-02 and led us to take a closer look at the design of stepped attenuators for use in sensitive minimal audio systems where sound quality takes precedence over all.

By the 1992 Heathrow Hi-Fi News Show we had a new design completed. This was our highest quality and most minimal-path passive preamp to date consisting of a mixture of Vishay Bulk-Foil and Holco resistors in a 31-step 'L-pad' attenuator with 1.5dB steps to -42dB then 6dB steps to -60dB. All wiring was solid-core, high purity silver, all connectors were Topline WBT. Inputs and grounds were again switched and no other interruptions were allowed in the audio path. For users with just one source we included a direct input, hardwired directly to the attenuator and so bypassing the input wiring and top quality coin-silver switch. Needless to say, this input always sounded the best! The new product was named 'PASSION' and became our most popular product to date and a permanent fixture in many top reviewer's systems throughout the world. See Hi-Fi News April & November 1994 (Martin Colloms); The Absolute Sound Issue 111; SoundStage October & November 1996 The key reason for using an L-pad attenuator is that at any one time there are only 2 resistors in the audio path. With careful selection of the overall impedance which these present to the source and power amplifier a passive volume control with the utmost transparent level of performance can result.

By 1994 each and every one of the resistors used within PASSION we specified as Vishay Bulk-Foil types - these gave a clear sound advantage over any alternative type irrespective of cost (and Vishays do cost plenty!). In comparison to a more conventional ladder attenuator the carefully specified L-pad sounds far more transparent at all normal listening levels in a well matched system.

The Ultimate

There have been many models of PASSION produced since the original design. Some had more inputs, most had more facilities such as dual-mono attenuators, stepper motors for remote control, balanced inputs and outputs, tape circuits etc. But all models have featured the Passion Vishay L-pad. The latest and top model of PASSION is the new 'Ultimate'. This was launched at the London HiFi News Show in September 2000 and continues the minimal approach for maximum sound quality. We believe it produces the most accurate and convincing sound of any PASSION to date. All operations are performed by remote control centred around the 60 x 1dB steps on the L-pad attenuator. Input selection, L/R balance and output muting are all included. All mechanical switch contacts, either in series or parallel with the delicate audio signal, have been eliminated with the Ultimate model.

Top quality components including Vishay Bulk Foil resistors and WBT RCA connectors are used throughout and all audio paths are kept to an absolute minimum both in terms of physical size and in the number of components and their associated solder joints.

Provided the PASSION Ultimate is used in a passive-friendly system, driven from reasonably low output impedance sources and into reasonably high input impedance power amplifiers, via a short cable from the PASSION to the power amplifier, then you can safely assume that no better preamplifier exists.

To learn more about Audio Synthesis, please visit their website: www.audiosynthesis.co.uk