The Phoenix Spatial Coherence preamp

A Short History and Operation of the Phoenix Pre-Amplifier
built by Steve McCormack circa 1979
based on the Spatial Coherence preamp by Richard Knapp

Stephen Pappin – Clifton Audio
May 29, 2015



The Phoenix preamplifier was developed by Steve McCormack in the late 70’s and is based on the “T-FET Valve” invented by Richard Knapp of Spatial, Inc. The front panel bears the “The Mod Squad” logo under Steve’s name. It was developed for the purist audiophile or audio engineer. It lacks some features that were not considered relevant, such as a power switch or indicator lamps. Some of the technology employed was cutting edge at the time and a high degree of user awareness was required to operate it. Four T-FET Valves were centrally located on the main PC board. These bricks provided core amplification for phono and line level stages. A 95VDC power supply provided B+ to the T-FET Valves. The external power transformer was located in a separate blue box. The Phoenix was a wide heavy black box. Five controls occupied the front panel which included the input selector, output selector, volume control, left gain and right gain controls. The rear panel was populated with gold plated RCA jacks and a grounded binding post. Features varied by build. The Phoenix was a custom pre-amplifier.

The T-FET Valves are truly unique in that they are a combination of high-voltage N-Channel J-FETs, film capacitors, lead shielding, and silicon carbide epoxy potting compound. The T-FET Valves are a bit unstable when first powered on and require several minutes to warm up. During that time it is dangerous to connect a power amplifier because the transients could literally destroy speakers. After a few minutes everything will stabilize and quality sound is good, but sonic quality continues to improve with time. Remember that this product was introduced at a time when purity and transparency were the goal. Non-essential circuits were not part of the objective. Transparent high quality sound for audiophiles was the highest priority. Systems were left on – no ON/OFF switch!

The Phoenix produces very rich sound. It has gain controls that provide +10 dB at the mid point settings and a central volume control. THD is less than 0.01% measured over most of the operating range. The noise floor is almost non-existent. Controls are generally noise free and well centered. Depending on the source and load the Phoenix can develop several volts at the outputs. This platform allows the user to connect almost any combination of standard or non-standard devices. It is important to leave the Phoenix powered on at all times in order to avoid start up transients. Technical support often fell on Steve McCormack’s shoulders where he explained Phoenix operation time and time again.

I enjoyed the opportunity to rebuild and upgrade a Phoenix. It was an amazing experience and the new owner is enjoying the improved version of this 1970’s technology. We were able to tame some of the less desirable characteristics by installing better parts. All of the grounds were updated and better capacitors were installed, along with a few other changes. The result is a preamp that still produces great sound, with even better performance than the original. We did not eliminate the output start up transient completely but we did reduce it. Since this device is historic we didn’t want to change its original function. That’s why we left the gain controls intact and decided not to change the start up profile.

Is there room for a Phoenix II design? Perhaps, but that’s another story. The Phoenix works as intended and that’s impressive.

Operation: Based on sn 00646 observations

1) Before powering on any attached devices allow the Phoenix pre-amp to warm up for at least 5 minutes. Failure to do this will result in loud popping noises from speakers and potential damage
2) Best gain is achieved at 10 dB. Set the left and right gain controls to 12th graticule from 0 or 12th tick mark clockwise on the dial.
3) Line inputs can develop up to 2V RMS at the outputs with controls turned all the way up.
4) The phono inputs can develop up to 10V RMS at the outputs with controls turned all the way up.
5) Avoid settings that produce high output levels, clipping, and noise. This can damage your speakers and amplifiers
6) Gain controls can be adjusted to produce different sonic responses based on music types. Higher gain will extend bass performance. Lower gain will produce greater accuracy with less noise.
7) The Phoenix does not require external cooling but should be installed in a space where air freely circulates around the chassis.
8) When connecting a turntable it is highly advisable to attach the ground wire from the pickup to the ground binding post on the back of the Phoenix.

Summary:

The Phoenix is truly an eclectic high-end vintage audio preamplifier. It does an incredible job of producing some of the best sonic performance with 1970’s technologies. It really cannot be compared to other devices since the T-FET Valve is a unique creation. That part was developed by Richard Knapp and he tried to create a package that could not be reverse engineered. Steve McCormack knows the balance of the design. There’s very little information available other than what Steve and Richard have stored in their heads. Most of the design is still proprietary.

If you own one of these historic pieces it’s worth the investment to have it serviced and / or updated. Improved capacitors, resistors, and grounds can make this design perform at its full potential.

If you have a system that must be powered OFF we also recommend adding a sequenced power distribution box that will allow the Phoenix to warm up before the power amplifiers are turned on. This also keeps everything turned OFF in the event of a power failure.