April 2000

The Cyrus Brenneman Cavalier:
A single-ended Cathode Follower
Stereo Tube Amplifier

By Dick Olsher
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   The single-ended (SE) tube amplifier renaissance that has taken shape and blossomed in the US over the past six years has introduced a new generation to the musical joys of SE drive. I feel that I was at the right place at the right time to help light the torch with my review of the Cary Audio 805 in Stereophile Magazine. The notions that the first watt is the most critical in setting a believable soundstage and that a SE amp is inherently more suitable for this task than a push-pull design took some time to sink in and gain a following. Ultimately, the truth proved impossible to suppress – even in an audio world that had been dominated for so long by the power über ales credo. With most speakers, it’s the first watt that gets us to a comfortable listening volume. And within that first watt are the low-level volume, pitch, and rhythmic modulations that allow us to analyze the auditory stream into clearly definable perceptual objects.  And because the audio signal is intact from input to output – a single output tube handles the entire signal – SE preserves such detail better than does a push-pull output stage where the signal is derived from two separate tube sections.

That’s the good news. The bad news is that SE drive imposes certain design constraints that are difficult to overcome. The most obvious one is bandwidth. The cost and sheer iron mass of an air-gapped output transformer required for extended deep bass response either drives the cost up significantly or forces the manufacturer to compromise. On the other hand, the more windings in the primary to beef up the inductance for good bass response, the greater the DC resistance and distributed capacitance which makes the treble droop. No surprisingly, many well-known commercial SE units measure poorly at the frequency extremes. They sound soft and closed-in at the top and lack convincing bass punch. The spotlight is clearly on the midrange, which typically sounds glorious. In addition, unless global feedback is used, the output impedance is typically quite high (2 or more ohms) which reduces the damping factor as well as causing amp-speaker load interactions. Since most speakers have a non-flat impedance magnitude, the amp’s output impedance introduces frequency response deviations. These deviations are most severe where the speaker’s impedance approaches that of the amp. Without global feedback signal-to-noise ratios are also quite poor with significant levels of residual hum and noise at the output. Several millivolts of hum into high-efficiency speakers are enough to drive most listeners out of the room. Finally, the character of a typical SE amp changes with drive level as distortion products rise significantly, to as much as 5% or more at rated power.

The Technology

In engineering the Cavalier (and the more expensive Reference 20 monoblocks), Cyrus Brenneman has sought to overcome these traditional shortcomings of SE designs.  In doing so he has blazed a new trail in SE amplification by incorporating an innovative cathode follower output stage. Although a rare beast, such a stage is occasionally seen in high-quality preamplifiers where its low-output impedance and inherent linearity makes it an excellent line driver. A cathode follower stage derives a signal between the cathode and ground, as opposed to between anode and ground. Instead of a plate resistance of 25,000 ohms for a typical pentode, the output transformer sees a load of 100 ohms in the cathode circuit. This is a good thing in that it lowers the amp’s source impedance thereby increasing the damping factor. The other major characteristic of cathode followers is that they develop 100% degenerative feedback. At best, such a stage has a unity gain. Such feedback is local and is not to be confused with global feedback, which introduces phase shift instabilities. The local feedback linearizes operation. Harmonic distortion at normal listening levels is said to be below 0.05%, and less than 0.25% at full power!

So why doesn’t everyone use a cathode follower stage in power amps? The problem is that since the output stage is typically operating at less than unity gain, all of the voltage gain has to be derived from the driver stage. And that’s a major design issue.  To generate 10 SE watts from a power tube such as an EL34 requires a drive voltage of more than 300 volts at the grid of the output tube. An expensive solution would be to use an interstage transformer. Another costly approach would be to use a high-voltage driver stage, which would require a second power supply. Brenneman’s solution was to a choke-loaded driver that works off the normal power supply yet is capable of the requisite voltage swings. The 6MB8 triode-pentode driver is loaded by a large choke to produce very low distortion levels. These chokes as well as the output transformers are custom-built for Brenneman by Electra Print. The power supply uses a tube rectifier (GZ-37/5U4), an unusual feature in modern tube amps, but one that I consider critical for the ultimate in tube sound.

Directly-heated triodes are awkward to use in cathode follower circuits because they don’t have separate filament/cathode structures. Brenneman has had good success in this circuit with both EL34 and 6550 power tubes, which he uses in a triode connection. The Cavalier derives 10 watts per channel from a single EL34. This tube is widely available from several manufacturers, and is certainly much less expensive than say a good 300B. Since the EL34 is conservatively used in this application, expect many years of trouble-free service. Take note that this cathode follower amp is pretty much idiot proof. With a conventional tube amp, I do not recommend that you ever short the outputs or remove a speaker connection under full power since you run the risk of destroying the output stage. In the case of the Cavalier, you will not hurt it. There is even a volume pot provided, so that you can run a CD player directly into the amp without the need of a line stage. Of course, if you have several source components to control, a line stage or a full function preamp will be required.

The Sound

The Cavalier flies in the face of what conventional wisdom dictates a SE amp should sound like. It sounds airy and extended through the treble. At the other frequency extreme if features excellent bass punch and authority. In other words, it is well balanced, from top to bottom. Yes, the midrange is nicely detailed and harmonically rich, but unlike the case with so many conventional SE amps, the mids are not the only attraction. Granted, the midrange is critical to musical pleasure, but it is no great honor to slight the frequency extremes as so many competing models do. Hey, I’m a midrange aficionado from way back. I adore a liquid and suave presentation more so than most music lovers. And I despise the modern tendency to focus on bass and treble. Yet, with the Cavalier I can have it all, and I love it! Bass definition is no longer the adventure it is with conventional SE amps.  With poor damping factors such designs give speakers plenty of gas; you know, that bloated feeling in the bass range that causes double bass to be reproduced with the grace of a dancing elephant. The Cavalier delivers pitch definition in spades. Jazz ensemble and orchestral bass lines were consistently given the sort of resolution I had not previously believed possible from a SE tube design.

This is not a dark or mellow sounding amp that can be relied upon to mask inherent loudspeaker brightness. Rather, it has a sunny disposition that shines a clear light over the entire harmonic spectrum. The Cavalier possesses an endearing sense of drive that propels it through complex passages without losing focus or resolution. You can literally “redline” it without any complaints. It can be pushed hard, just short of clipping, without the sense of stress or grossly increased distortion typical of other designs. Because of that, it sounds and behaves like a much higher-powered model.

This amp has both speed and control - no easy jobs for any amplifier. Transients are launched with a precise sense of timing, and are allowed to decay effortlessly into the blackness of the recording’s noise floor. It should be clear by now that this is not a soft-sounding amp. It stands in stark contrast to the quintessential 300B SET, which seems to liquefy harmonic textures to the state of melting butter. With the stock Amperex EL34 tubes provided with my sample, midrange textures were supremely clean, but lacked a touch of warmth. The precise character of the midrange is dependent on EL34 tube brand. My favorite substitution is the genuine British KT77. If you’re lucky enough (as I am) to own a personal cache of these tubes, then you’ll discover that their midrange voicing can become addictive. Textures are more lush (as in tropical) with the KT77 in the circuit. Again, this is a personal preference; you may in fact opt for the more direct presentation of the Amperex brand EL34. It all depends, on whether you prefer vintage or modern tube sound; the choice is completely under your control.

It can be safely stated that the Cavalier Cathode Follower amplifier represents both a technical and sonic triumph. It pushes the performance envelope of SE amplification in an exciting new direction. There’s plenty of midrange magic and startling dynamics, but unlike the competition it give you a wider window onto the sound. I have seen the light: more bandwidth is definitely a good thing. Kudos to Cyrus Brenneman Audio for following the road less traveled.

Specifications

Power Output: 10 wpc into 8 ohms

Frequency Response:  20 Hz – 20 kHz, +/- 1 dB

Harmonic Distortion: < 0.1% at 1 watt

Sensitivity: 0.6 volt

Hum and Noise: < 0.5 mV across 8 ohms

Input Impedance: 50 kohm

Dimensions: 10” x 14” x 7”

Weight: 35 pounds

Price: $2,995

Manufacturer

Cyrus Brenneman Audio
22113 S. Vermont Avenue
Torrance, CA 90502

Voice: (818) 349-5402
Fax: (818) 349-5443