With its decidedly (and studiedly) ‘retro’ physical proportions- very deep front-to-back dimensions with a narrow side-to-side width, this layout closely follows concepts of a bygone era, where the well-conceived intention was to keep the power supply as far as possible from the hum and noise susceptible octal input tubes. The output tubes themselves require careful distancing from the mains transformer and, of course, require pure DC on their filaments. Again, reverence to past times begs the use of vacuum tube rectification with its very gentle voltage rise-time. But to bring the performance up to modern demands, we use two rectifiers to enable the use of very substantial filter / reservoir capacitors… over 1300 microfarads in the B+ rail- much improved from the 4 or 8 microfarads utilized in the 1930’s.
When several other manufacturers started selling single-ended amps with 5 to 9 watts of power, we understood subjectively and objectively the good part of the story: the superb mid range, the delicate detail, the inherent simplicity. The bad news was that there were almost no speakers efficient enough to help form a dynamically viable system. The worst news was that either these single-ended amps cost more than the condo and or they had specs that could only be read as a joke. As the triode fans increased, more efficient speakers became available and several fanatics began to dismiss push-pull as a bad thing to be avoided at all costs. We objected to such blanket generalities and stressed that the push-pull done right with truly phase-balanced transformers, quality parts throughout and well designed circuitry has proven to be a superb technique. We also enjoy single-ended topology but only when it is done right, mostly because we expect flat frequency response and low distortion out of an amplifier. We knew that we could achieve this in a properly conceived single-ended design. We chose to rise to the challenge of designing special circuitry that could be switched “on the fly” From 12 watts in parallel SINGLE-ENDED to 25 watts in PUSH-PULL which, we believe is unique in the industry.
The Output transformer (hand-wound in our own factory) is of complex design employing inordinately high primary inductance to compensate for the mandatory magnetic “gap” required for the uni-direction current and voltage flow of the single-ended output stage. Another factor in the single ended triode saga (currently enjoying a wide spread popularity) is demand for little (or sometimes zero) negative feedback. We at Manley Labs believe in the judicious or sensible amount of feedback to refine and maximize performance. Our Neo-Classic SE/PP300B gives you the choice in precision switched 1 dB steps to go from ZERO dB’s to 10 dB’s of negative feedback. Even with zero negative feedback, this amp is cleaner and flatter than any single-ended amp we know of. For more information about variable feedback read Notes on Negative Feedback.
One interesting aspect of negative feedback is it gives a higher damping factor. Conventional wisdom would have us think that the higher the damping factor, the better. Our experience is that the optimum damping factor is not infinity and that it depends on the speaker, the room. and the taste of the listener. In other words, the optimum is variability, and this we provide. This amp satisfies us as long time amplifier builders, amazes the critics in several well known magazines and is bound to satisfy you with the ability to fit into your system and your expectations. BUT REMEMBER PLEASE the overall facts of power ratings versus loudspeaker sensitivity and room size when electing to venture into lower power amplifiers (less than 50 watts) and especially when contemplating single-ended topology. Please read Considerations of Amplifier Power for more info.
As highly regarded as the Retros were by dealers ,audiophiles and reviewers, the Neos take the design and the sonics to a new level. In 2000, Mitch Margolis completely overhauled the previous “Retro” model to create the Neo-Classic Version you see above.. So what’s new with this Neo-Classic version you ask?
Now… onto the techno mumble jumble… here’s all the guts improvements we made in 2001. Mitch spent six months doing all this:
The driver stage was completely re-engineered. Peak output volts of driver increased by 60V p-p in PP mode. Clever new driver circuit delivers way better drive, with less punishing plate volts, and less THD. This offers enough drive even for more exotic tubes such as VV30B, etc…
The NEO’s have WAY improved power bandwidth. NEO’s deliver much more power at higher and lower freq’s than the ol’ Retro’s did. We did rescale the B+ and reduced idle plate dissipation on the NEO’s 300Bs for better tube reliability.
New power max ratings are 12 watts in SE mode and 25 watts in PP mode. BUT because of better bandwidth delivery and better driver “drive” into 300B’s these NEO’s sound louder and “seem” to be more powerful. They distort much less early when pushing the power envelope. The power supply sags-on-demand at a much high point than before for more headroom. It is all beefier ‘n’ heftier.
The heater (300B “cathode” which is directly heated triode design) now has 40dB less ripple. NEO design has MORE capacity in heater supply and LOTS of PI filtering! So the result is a quieter amplifier. Big time.
Now in the NEO version, the B+ rectification is all-real-Full Wave Bridge vacuum tube rectification. Even more better. Especially sonically for you vacuum tube rectifier fans…
The NEO’S chassis was redesigned to accommodate a much bigger power tranny. Huger VA. Next frame size up. Cooler running. Much more able to deliver-on-demand. Less PSU sag. Kick-ass. And now, finally QUIET! REALLY! There wasn’t enough room on the old Retros for next frame size up to achieve a nice’n’quiet power transformer design. Now the NEO’s chassis is big enough to have a big enough power transformer that is really silent and running on holiday.
Bias supply is now cleverly cross-coupled so that changing one tube’s bias doesn’t whack the other one out. This is very cool.
We even isolated the input B+ for more stable input B+ supply. Again, so when pushed, the whole amp doesn’t drag everybody down with it.
We added a MUTE switch on the front panel. SE-PP switch on the other side of front panel.
Power switch at the IEC so all those nasty AC volts don’t come up and infect the input stages.
Discarded the transformer-balanced XLR input. Because WHAT’S THE POINT??? At gunpoint, we will make a balanced XLR transformer coupled input available “upon special request”… but you need to have a bigger gun than EveAnna does…
Still has those nice sounding MultiCap® coupling caps and even more of ’em….
Now NEO has nice beautiful WBT binding posts that everybody loves. Still has impedance load switch as before.
Service is now a piece of cake in the NEO design. The main PCB can now be lifted out for service without a soldering iron… if switches get banged or broken they can be user replaced with a phillips screwdriver to remove the bottom stainless-steel (now) cover and then remove faceplate with little allen key, then unscrew the switch with pliers or (better) correct nut driver, then unplugged out old switch then new one plugged in with switch assembly from us. Easy as pie!
But, just in case, we did invent newer stronger foams and packaging so hopefully no more breakies….
And bottom line, in all our listening tests, this NEO is very much improved. Now with this six months of re-engineering of the Neo-Classic SE/PP 300Bs, “THD BE-GONE!!!!”
The Electro-Harmonix Russian 300B tubes are being shipped as standard. That’s 300B-EH.