Dick Olsher: Wolcott Audio “Presence” Monoblock Tube Amplifier

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July 2000

The Wolcott Audio "Presence" Monoblock Tube Amplifier
Review By Dick Olsher
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After nearly a century of evolution it is safe to say that vacuum tube circuitry has reached a state of maturity. It appears that every conceivable circuit nuance has already been investigated; no stone left unturned by a host of ingenious engineers who grew up knee-deep in vacuum tube technology. For example, even my 1952 Radiotron Designer's Handbook (4^th Edition) already contains over 1,400 pages of information about radio receiver and audio amplifier design and application. The resultant legacy from the golden age of audio is enormous in scope and magnitude. It is therefore not surprising to recognize most modern high-end power amps for what they really are: recycled 50s designs with inferior output transformers. The only thing original about these products is the cosmetics. The associated advertising hype tends to deflect attention away from the essence of the circuitry and instead focuses on familiar audiophile esoterica such as wonder caps, kryptonite resistors, and magic wire. The bottom line for the consumer is often a price tag equivalent to the cost of a new car for an amp that isn’t even as sophisticated (or good sounding) as my vintage Harman Kardon Citation II.

The Presence power amplifier breaks new ground in audio electronics, but as Henry Wolcott will tell you, its basic topology is 40 years old, having been used in the PL-100 amplifier system he designed for JBL in 1960. It earned Mr. Wolcott two US patents at the time. His US and foreign patent total today in instrumentation and audio design stands at an even dozen. After founding Optimation Inc. in 1961, a precision instrumentation company with sales to aerospace companies and national laboratories, Mr. Wolcott further refined the original design as the PA250 series instrumentation amp. The PA250 was designed as a lab-grade instrument for precision calibration and testing. Over 1,100 units were sold worldwide, and many are still in use today, including six systems at the U.S. Bureau of Standards, now NIST. The Presence amplifier is the latest descendant of the original circuitry, introduced under the Wolcott Audio company banner in 1988. It represents a third generation version of the original design, optimized expressly to meet the needs of the high-end and professional audio markets. While each building block of the Presence amplifier is not necessarily new, the sum total of its design elements represents an original and unique statement in the realm of hi-fi design.

If you are technically minded, I encourage you to ask Wolcott Audio for a copy of their technical white paper, "A Fundamentally New Topology for Audio frequency Power Amplification." This paper does a good job of elucidating Wolcott’s design philosophy and his methodology for eliminating various types of tube distortions and limitations. Therefore, I only plan to provide a few general and technical comments to give an overall feel for the Presence amp. Wolcott believes that circuit topology is the primary determinant in amplifier performance. He finds traditional designs to suffer form several obvious failings resulting directly from the limitations and distortions inherent in transformer coupled output stages. Better load tolerance, greater stability, lower static distortion and output impedance, better transient behavior with lower noise, and higher power supply rejection are the stated performance objectives of the Wolcott circuit.

The amp is fully balanced at all stages beyond the input stage. A major feature is the use of balanced positive (feedforward) as well as negative feedback, which allows the cancellation of the amps’ internal impedance. A control on the back panel lets the user adjust the output impedance at frequencies below 100Hz from about two ohms to zero, and even about 0.5 ohm negative. Zero output impedance yields an infinite damping factor, while moving into negative territory cancels the effect of cable resistance and even some of the voice coil's DC resistance. Thus, the amp is intimately coupled to the load and is able to control transient behavior to a degree impossible with traditional designs.

A pair of 6922 dual triodes makes up the input stage. Each 6922 is configured as a White cathode follower, and the two followers are connected in parallel to create a buffer stage of high input impedance, low output impedance, and exceptionally low distortion. Negative feedback is applied to the second cathode follower from a tertiary winding in the output transformer. A pair of 6GW8 triode/pentodes makes up the second and third stages of the amp providing voltage gain and symmetrical drive signal. The pentode halves are operated as cathode followers with transistor current sources and are directly coupled to the output stage. The output stage is comprised of eight EL34 pentodes operated as pure pentodes in a conventional push-pull circuit. The output tube screen girds are actively regulated. My review samples incorporate the latest wide bandwidth output transformers with exceptionally low leakage inductance and only moderate winding capacitance. Their full power bandwidth is 20Hz to 30kHz, running flat out at 220 wpc into 4 ohms. These trannies feature a tertiary feedback winding (ala McIntosh’s unity coupled output stage) closely coupled to the primary. This isolates the feedback loop from load fluctuations and ensures that that full error correction is provided even in the presence of highly reactive loads.

A user-friendly feature, which Wolcott is justifiably proud of, is the automatic bias circuit for the output stage. A digital controller checks and resets individual tube bias at turn-on, and is then disengaged from the signal path and remains dormant for the remainder of the operating cycle. No digital clock noise is generated after the completion of the bias process. The controller is the result of months of engineering work and appears to be a truly transparent and effective autobias circuit. It also provides a gradual turn on for reduced tube stress. The entire process takes from 1.5 to 3 minutes to complete.

It should be evident by now that the Presence is the result of a sophisticated engineering effort. However, some purists might object to its design complexity on the grounds that simpler is always better. On the basis of my findings to date, I would amend that pearl of wisdom to read: simpler is not always better. A fine-tuned racing car engine is of necessity more complex than a streetcar. By analogy a high-powered tube amp must fit the standard of a complex muscle machine to cope with highly reactive real-world speaker loads. All of this brings me to the Presence Audio's ultimate challenge: the Sound Lab A-1 full-range electrostatic speakers.

Before the review process began, I made it clear to Wolcott Audio that the Presence amplifier would be judged on the basis of its ability to drive my Sound Lab A-1 in the reference room. To my mind, the A-1 is clearly the speaker load from hell. Its impedance magnitude varies from 40 ohms in the bass to 0.8 ohms at 20kHz. As far as the amp is concerned, the A-1 represents a large capacitor with a ravenous appetite for power. Of course, much of that power is not dissipated in the load but is kicked back to the output stage of the amp, creating additional stress for the amplifier. Many high-power amps, including the Jadis JA-200 and the Sonic Frontiers SFM-160 monoblocks have failed to properly energize the A-1. High-powered solid-state amplifiers have generally fared even worse. The Crown Macro reference is a case in point. On paper it should have been able to drive any load in the galaxy with lots of power reserve. However, the A-1 brought it to its knees - overload lights flashing even at moderate volume levels. I suspect that the A-1's Brilliance control was intended primarily to mitigate the effects of amplifier instability.

In so many cases, the sound of the driving amplifier would become bright, edgy and distorted, that users and dealers must have clamored for a control to simply roll off the A-1's highs. Audiophile psychology is such that they tend to blame every problem in the sound reproduction chain on the speaker. After my experience with the Fourier Components Sans Pareil and Panthere OTL tube amps, and also the Air Tight ATM-3 monoblocks, it became clear that the A-1 is not inherently a bright speaker. There is nothing in its frequency response to suggest treble emphasis. In fact, it is much more naturally voiced than other full-range electrostats, even with the Brilliance control wide open. If the A-1 sounds bright, you ought to first blame the associated amplifier. Having come to that realization, I asked Sound Lab several years ago to bypass the Brilliance control in my pair of A-1 so as to eliminate one pot from the signal path.

The Sound

I met up with the triumvirate of Henry Wolcott, the Presence amplifier, and the Sound Lab A-1 at THE Show 2000 at the St. Tropez in Las Vegas. What I heard there was very encouraging: absolutely the best sounding A-1 under show conditions in a decade of auditions. Still, I was unprepared for the impact the Presence made in my own system - no less than a sonic earthquake of magnitude 10 on the Richter scale! The Presence completely rearranged my notion of optimum amplification for the A-1 and pushed the A-1's performance envelope to new heights.

Let me start with transient control. By virtue of an extremely thin diaphragm and uniform force-over-area drive, electrostats have long been prized for their transient speed. Unfortunately, very few amps regardless of price or snob appeal have been capable of adequately controlling both the leading edge and the decay portion of musical transients on the Sound Lab A-1. The Presence is the only amp in my experience to tightly control transients to perfection. Treble detail was reproduced with startling purity and effortlessness - without any of the ringing or various other distortions imposed by lesser amplifiers. Bass information was never tighter or more detailed. Bass punch was greatly improved, allowing both pop and classical music to boogie along with greater impact. I should point out this level of performance does not automatically jump out of the box. You actually have to reach behind each amp to adjust the damping factor rotary pot. This is almost a no-brainer; simply dial in a particular setting and listen to a familiar piece of music. You should be able to hear bass precision and treble delicacy change as the output impedance is varied form negative to positive. Too much output impedance (equivalent to a lower damping factor) and bass lines become full, rich, and under damped while the harmonic tapestry’s luster and immediacy are diminished. Note that the amp is stable at all settings, so feel free to experiment. In my system, a setting of "10 AM" as viewed from the rear panel worked best. In general, however, there is no one right setting, as some speakers may actually sound better at a higher output impedance. That is the nature of the loudspeaker/amplifier interface.

Other major improvements were noted in the areas of soundstage immediacy, spatial resolution, and macrodynamics. The first area has to do with the impression of live music. The sensation of listening to the real thing was much stronger with the Presence. Both time and pitch scales were finely tuned to the music. For example, the pulsating rhythms of Rossini's overture to Il Barbiere Di Siviglia (EMI 211355) came off with such drive and precision as to become compelling. It became impossible to ignore the music, and I was swept away in the illusion of a concert hall performance.

The soundstage gained both in terms of depth perspective and focus of image outlines. The sheer size of the A-1's radiating area allows it to project realistic image sizes with very believable surface loudness densities. But because of the massive amount of reflected energy from room boundaries, it is usually difficult, especially in small rooms, to resolve spatial perspectives and specific instrumental outlines. The Presence painted a wide spatial canvass with a sufficiently fine brush, creating a surprisingly coherent soundstage for such a large speaker. Though the spatial bloom around each instrument wasn’t quite on par with the standard set by Fourier Components' Sans Pareil, image outlines were definitely in the 3-D category. Space the final frontier: the Presence boldly explores new territory solid-state amplifiers can only dream about. Note that image focus only became fully palpable after substituting the 6H23Pi-EB, a Russian military grade version of the 6922 for the stock input tubes. This wonderful tube is available for sale at $24.95 directly from LAMM INDUSTRIES, Inc., 2621 East 24th Street, Brooklyn, NY 11235 USA, Tel.: (718) 368-0181, Fax: (718) 368-0140 (web site: www.lammindustries.com).

There was much more apparent headroom relative to other 200 wpc amplifiers when the A-1 was pushed over the dynamic range from loud to very loud. The sonic character of the soundstage didn’t change in the process, as is often the case when a power amp is nudged closer and closer toward overload. As the Presence was redlined, its distortion spectrum was consistently well behaved. Thus, its sense of effortlessness remained intact during complex musical passages. No Dr. Jekyll and Mr. Hyde dual personality for this amp. It is as sure footed at resolving microdynamic contrasts as it is with all out orchestral crescendos. Finally, the tonal balance is a model of neutrality. No one particular frequency region sounded subjectively emphasized. The Presence is able to release the music’s pent up energy, but it does so without resorting to any cheap tricks. Neophyte listeners are usually, and at least initially, impressed by glare and sizzle for the same reason that heavy makeup calls attention to itself. In contrast, the Presence sounds totally natural. Its musicality is derived primarily from the virtues of clarity, resolution and transient precision.

The overall transformation of the Sound Lab A-1 fashioned by Wolcott Audio’s Presence amplifier is nothing short of stunning. Life will never be the same for the A-1. I’m now excited about the sound of the A-1 as never before. In hindsight it is easy to see that while a handful of amps came close to eliciting sonic greatness in one or more isolated respects, the Presence is the complete package. It is not only musically compelling and emotionally involving, but has the muscle and steroids to transform the "loudspeaker load from hell" into the musical experience form heaven. With the Presence in your system, you’ll be spending your audio budget on new albums rather than stressing over confusing choices from a bewildering jungle of cables and interconnects. Judging by the enormous sales volume of audio tweaks and accessories, very few audiophiles are truly enjoying the music. There is absolutely nothing more important in building an audio system than nailing down the speaker/amplifier interface. Once that is locked in, cable and other tweaks seem to make very little difference.

If you own a Sound Lab Ultimate, A-1, or even an A-3, you must audition a Presence amplifier. My advice to you is simply this: quietly sell whatever other amp you now own and purchase the Presence for your audio future. Even if you don't own a Sound Lab, but are in need of high-power amplification, give the Presence a serious listen. Its unique circuitry and variable damping factor should allow it to drive the most difficult of loads with supreme confidence. You'll find that it is capable of blending into a diverse range of systems with a minimum of fuss. Kudos is due Henry Wolcott for offering audiophiles and music lovers alike a new vision in power amplification.

Tonality	100
Sub-bass (10 Hz - 60 Hz)	100
Mid-bass (80 Hz - 200 Hz)	100
Midrange (200 Hz - 3,000 Hz)	100
High-frequencies (3,000 Hz on up)	100
Attack	100
Decay	100
Inner Resolution	100
Soundscape width front	100
Soundscape width rear	100
Soundscape depth behind speakers	100
Soundscape extension into the room	95
Imaging	95
Fit and Finish	95
Self Noise	100
Value for the Money	100