The Feral Eye
by Herbert Reichart, Audio Note NYC
I have a confession to make.
By any rational standard, I am not qualified to design an audio amplifier. I have built every feasible circuit. I have read dozens of books and even studied in the classroom of my mentor Arthur Loesch. Still, after almost twenty years of trying, I feel radically unprepared for the chore.
I am a slow learner and I have the worst memory in the world, which may be an asset but it sure is embarrassment. I sound like a darn fool when discussing mathematical problems. I continue to ask the same dumb questions and rely on the advice and consul of guys like Steve Berger, J.C. Morrison, Komuro, Andy Grove, Kondo and Nobu Shishido. Without their help this amplifier would not exist.
On close examination, the only real talents I can muster are a basic ability to paint pictures and a potential to (sometimes) be quiet and listen. My (imagined?) capacity to glimpse a painting, read a poem or visualize the forces of nature is the source of my belief that I can indeed design audio amplifiers.
In order to make anything of exceptional value the maker must be untethered. As long a we truly believe that there a definite rules or concepts that govern out endeavors, as long as we are conscious of any notion that says we cant do something, we will mostly fail. To be free and create something original, we must embrace purposelessness. In fine art, when purpose is too much in evidence, the ART is longer there. The creation becomes a machine or an advertisement. Beauty is replaced and the not so beautiful hand of the artist become altogether too obvious.
In art, the simplest ink sketch- even if seemingly very crudely executed- can stimulate deep feelings and engage the focus of out whole being. Amplifiers must do the same and present music as if we were facing our own personal nature directly. When we listen to music in the home we should feel every pulsation of it as if it belonged to us. Reproduced music attracts our attention most easily when we can "identify" our own nature in it. Therefore we should let our own character resonate in the design of the amplifier.
When the new amplifier is very good, the music comes from the speakers like "the breath of life". Music reproduction should feel direct and be perceived as having a similar substance and material character as a live musical event. The presence of the musician should appear before us.
At the work bench, I feel like a student working on a science project in Dr. Zarkovs lab. The solder fumes enhance this vision. Sometimes I imagine that a tube is like a lens and the signal is a delicate energy form, like pulsing flight. I picture a field of flickering, throbbing light in front of me. Looking at it, I ask myself, what is the simplest, most effective way to build a projector. How can I make the energy bigger, how can I stimulate the loudspeaker, without losing the signals proportions and intensity?
For me, the process of designing an amplifier starts with making pictures and setting my mind at ease. I need to feel I can do this. I imagine a nearly invisible, vibrating, electromagnetic slipstream forced to pass through a series of lenses and I suppose that I can compare the image of the original on one side to the thinner, bigger expanded version appearing at the other side. I try to keep a dynamic energy model in my head. I think power and energy not sinewaves. I struggle to visualize the effects of series of partially mirrored pieces of glass (filters) on the energy. I picture a river of energy impressed on the load. I try to invent lattices of components that do the least damage to the original proportions of the energy stream. The components, the tubes and the transformers must be in good alignment. They must harmonize. I look for an ecology, a synchronous structure, a simple mathematical model and the most benign arrangement of all the physical aspects. This kind of mental imaging is, I believe, critical towards making quality amplification.
The Feral Eye is being designed as a archetype amplifier for Sound Practices readers who want to experiment with push-pull. I hope you dont find it too strange to try yourself because this design has a few excellent qualities that are seldom found in push-pull amplifiers. The goal was to maximize PPs strength and minimize its weakness.
The Feral Eye is a concept amp. It was conceived by me but several of my friends helped me bring it to fruition. Feral Eye is really an amplifier format designed for home builders as an inspiration towards their own creations. Look at its architectonic aspect. It represents my thoughts about what it takes to make a push-pull amp sound less shattered and artificial. It comes out of a desire to look again at an underdeveloped technology and see if something better can be done with centertapped primaries.
Before I ever built a SE amp, I had a "paradigm shift" while listening to records at a friends house. This unusual friend has an AR turntable, Decca cartridge, RCA type preamp and a little "Bud box 2A3" amp driving Western Electric 755As. When I asked him why I loved the music so much today he said, "The only natural way to split phase is with a transformer or a split-load inverter!". Looking closely at the Bud box I noticed the UTC LS-55s and a UTC interstage transformer.
My best amp design ever was a transformer coupled parallel single-ended 210/310. Nobus designs were the inspiration for this amp. This topology made for an even more unconstrained sounding amp than the Flesh and Blood. Tone character and dynamic contrast were extremely natural. At the time I designed this amp I was reading a lot of Norman Crowhurst. Since then, I remain convinced that a tube-iron-tube-iron lattice can reveal subtleties of musical expression that more conventional R/C (or direct) couplings miss.
The Feral Eye amplifier is a topology that can be used with almost any Class-A power tube. Believer me, you have not heard how psychedelic and colorful the 6L6 or the EL-34 can sound until you try them with this configuration. Using the "cathode feedback" secondary coil hookup with the above pentodes (in triode) will give you an amp with an extraordinary high frequency integrity and the kind of bass you never though was possible with tubes.
Most of my current amp design belief system can be seen in the schematic.
The following are some of the poetic ideas that informed this design:
1) Keep resistance values low. If you can replace a resistor with a piece of iron and a coil do it! Resistors dramatically effect tone character and resolution. The worst offenders are high value plate load and cathode resistors. This is not simply an issue of metal film vs. carbon composition. It is the value! Using plate resistors of greater than 2xRp, despite issues of measured THD, make triode gain stages generalize and sound rough and jumpy. The trick is to reduce the value of plate resistance to the point where the transconductance (and probably THD) are maximized and then reduce it a tough further.
The same principle applies to the A/C impedance of the choke in the first stage. Use as little inductance as necessary and adjust the plate current of the stage until it "opens up" sonically. Radical "henry changes" in amplifier plate loads almost always require adjustments in overall amp tuning. This kind of work can be over my head so I asked J.C. to check my arrangement on his bench. With a stage like this at the beginning of a three stage amp, you must be certain to provide adequate decoupling or you will experience some low frequency motorboating.
2) Use low mu triodes. (This should have been number one.) Low mu and plate loads between 1x and 2x Rp give whole, complete, unedited type reproduction.
3) Avoid differential amplifiers. My friend was right. Transformers and for those on a budget, split load inverters, give a push-pull amp a more "together", less shattered sound character. Transformer coupled amplifiers, with quality iron and well calculated time constants, sound considerably more intact and continuous. The most noticeable difference between live and reproduced music is that the real thing doesnt sound like it has been taken apart and put back together. Hi-Fi as most of us know it has that "Humpty-Dumpty" sound.
4) Keep the parts count minimal. The transformer as phase-splitter and driver serves several very useful ends. In this design it replaces at least four resistors and two capacitors. It allows for a greater voltage swing and extremely gentle clipping.
5) Because of the zero crossing inherent to PP designs, I probably should have called this amp The Blinking Eye. My basic feeling about push-pull vs. single-ended is that a PP amp designed by a perceptive, discriminating designer will always sound better than a SE designed by Mr. Radiotron. Bottom line, I am certain that SE has certain very tangible advantages when listening pleasure is the main criteria. PP has a slight edge when it comes to adaptability.
6) Use choke input filters. To me they give a sense of relaxation to the music making. They take the virtues of tube rectification one step further.
Many amps with fixed bias and global feedback have really violent and annoying clipping characteristics. This amp, with the single-plate 2A3s and RCA 5R4GY rectifiers has what I call a "lubricated" clipping characteristic. It has a sort of James Bond like poise when it runs out of power. The Feral Ey was built by the hands of Steve Berger (Aprilsound). He laid out the chassis, bolted it together and soldered it with care and good jazz insight. Berger, Komuro and J.C. all participated in the design of the first two stages and the power supply. The creation of this amplifier was precipitated by the design teams love of, and experience with, the three pieces of Tango iron employed. I designed this amp because I achieve when I use the Tango XE-45-5 and its center tapped secondary feedback coil.
Connecting the filament transformer center taps to this coil not only reduced distortion at the frequency extremes and lowers the output impedance but it also allows the designer to use A/C filaments in push-pull with very low values of A/C ripple left on the output.
To hook them up correctly it is important to measure the gain of the amplifier with the heater trans CTs connected to alternate sides of this coil. The connection with the lowest gain is the correct wiring and the one with the higher gain is the positive feedback version.
The driver stage, with the Tango NC-14, can alternately employ the 6L6 or 6V6 tubes. My person favorite it the 6L6. Go mad. Try the 6L6 driver stage and a 6L6 (ultra linear) output with your Quad ESLs. Do not let the current get too high on this stage or the NC-14 core will saturate on climaxes. The 6J5 is perfect with the TC-160. I love this stage! It is so good you can use it to improve almost any amplifier.
You can use other brands of iron. But! IF you lose the secondary feedback coil you might as well switch to single-ended. The beauty of this design is in the use of iron.
I promise you that, if you build an amp, with this arrangement of metal, you will experience a new type and quality of music reproduction. The Feral Eye plays music very differently than most of us are likely to have heard before. It sounds more full, whole and complete than any traditional PP amp I have experienced. It is very rich sounding. It loses a bit to quality SE amps on pure magic and texture but makes up for the loss by driving a broad range of loudspeakers. Regard it as a modest gift from the New York City Triode Mafia.
For a diagram (1,000 X 684 pixles) of the amp, click here.