Embedded Files
The Veros PWR+ comprises a ground breaking combination of two of the best circuits in audio today: 1) LKV’s Class A, no feedback, balanced AZF line level input amplifier and 2) Purifi’s ground-breaking 1ET400A Class D power output module. Together they provide the audiophile with the musical transparency, detail and realism for which LKV’s phono and line level preamps are known, along with the amazing accuracy and power of Purifi’s premier module.
Overview
The Veros PWR+ is a big amplifier, capable of driving speakers having nominal impedances of 8, 4 and 2 ohms with power up to 460 Watts. It has three sections: a linear power supply based on one large 1000 VA and two smaller toroidal transformers; an input/voltage gain stage comprising LKV’s AZF circuit; and the Purifi 1ET400A module.
As expected of LKV’s designs, the Veros PWR+ accepts both balanced and single ended signals via XLR and RCA connectors on the rear of the chassis. Users select their preferred input mode via a toggle switch. The gain circuitry is fully balanced and any single-ended signal is immediately converted to balanced operation upon entering the unit.
The parts used throughout the amplifier are premium quality and include 1% tolerance metal film resistors, low noise HEXFRED diodes for voltage rectification, polypropylene capacitors, and hand matched JFETs. All of the DC fed to the input/voltage gain circuitry is extensively regulated and filtered to assure vanishingly low noise. The speakers and the amplifier itself are protected by circuitry that shuts down the output if excess DC voltage or heat is detected.
Design Considerations
Manufacturers like Hypex and ICE, among others, typically manufacture both Class D modules and switching power supplies to power their modules. The modules themselves contain not only the Class D output circuitry but also op amp-based input/voltage gain circuits. Thus, if you buy a power supply and a couple of modules, you will have all three pieces necessary for a power amp: 1) the power supply, 2) the input/voltage gain section, and 3) the Class D output. The manufacturer will be happy to supply you instructions on how to wire the pieces together. That’s it! Design done. Usually the sound is pretty good. Folks like those at Hypex and ICE know what they are doing, but Purifi takes the next step in the progression.
If you really want the good, state of the art sound that the Purifi modules are capable of, you need to work a bit harder. Recognizing this, quite a few high-end designers have eschewed switching power supplies and have designed their own linear power supplies. Sound improves, but there is still a weak point in such a design: commonly the input stages are comprised of op amps. They work, but they are nothing special. So we have a situation where one part of the module is truly excellent, and the other (the input section) is quite ordinary.
Recognizing that high end designers may want to use input sections of their own devising, Purifi makes one module that omits the op amp input circuitry, leaving only the cutting edge Class D output stage: 1ET400A.
The LKV AZF Input Stage
Use of the Purifi 1ET400A allows LKV to design its own input stage based on its minimalist AZF circuit, which operates fully in Class A and employs no loop feedback. Instead, it relies on meticulous testing and matching of parts used in a differential/balanced circuit to achieve both the required voltage gain and extremely low levels of distortion and noise.
Creating such a circuit to interface properly with the 1ET400A module was no easy task. Indeed, Purifi warns designers that use of such minimalist circuits is not appropriate because the 1ET400A module has an unusually low input impedance, which requires that the output impedance of the chosen input circuit must also be very low: less than 25 ohms. Very few minimalist circuits can achieve such a low figure. The 1ET400A module also requires that the DC voltage at its input (i.e., DC offset) be virtually zero, a condition hard to achieve without some form of feedback unless one puts a very large capacitor at the module’s input, but such a capacitor would threaten sound quality.
These requirements pose real challenges for a designer who want to use a minimalist circuit. LKV was able to meet these challenges by designing a special version of its AZF gain block and a very accurate, low-impedance JFET follower circuit to feed the 1ET400A, but it took some doing. One channel of this design uses, in a balanced amplifier configuration, 60 precision (1% tolerance) metal film resistors, and 33 JFET hand-measured and matched to +/- 0.1 millivolts maximum current (what engineers call “Idss”). Once built, each amplifier channel must be tested after a 48-hour warm up to assure stability of the required low impedance and DC voltage.
A lot of care and work is involved in creating and reproducing such an input circuit, but the reward is great: a very clear window on the original performance that reveals the natural timbre, detail and dynamics of the original.
The Purifi 1ET400A Module
Purifi has eliminated one of the most significant flaws found in virtually all power amplifiers: loss of feedback in the upper end of the audio spectrum. Power amplifiers need negative feedback to set gain, reduce distortion and, most critically, provide the very low output impedance essential for accurately deriving a variety of loudspeakers.[1] But, the nature of virtually all power amps dictates that the level and therefore the effectiveness of their feedback diminishes as frequency of the amplified signal increases. This means the treble frequencies are afflicted by increasing distortion that can change, often hardening, the quality of the music. This problem applies to AB amplifiers as well as to those designed in Class D.[2]
Bruno Putzey’s genius in his Purifi, and earlier NCore, designs reside in their output stages. A very clever arrangement of the negative feedback loop maintains a high and essentially constant level of feedback throughout the full audio frequency spectrum. The result is a ground-breaking, high-current output stage, one that solves the long-standing problem of high frequency inadequacy of negative feedback. We now have a power amplifier technology that provides equal feedback across the entire audio spectrum. The result is arguably the best sounding power amplifier output stage available.
All That Power??
The Veros PWR+ can deliver approximately 460 watts of power to the loudspeaker. Is that much power really necessary? Beneficial? Probably not strictly necessary in most systems for most music, but certainly beneficial.
In any amplifier, as the music gets louder, the limits of the amplifier’s ability to deliver voltage and current are approached. As that happens, distortion increases. So, in any given amplifier, we want to stay in the linear range well below those current/voltage limits. Unless we are willing always to listen at very low volumes, the only way to stay far from the amplifier’s limits is to build an amplifier with high voltage and current capability, that is, one that can deliver high power.
In a high power amplifier like the Veros PWR+, even most loud transients will be safely below the current/voltage limits. Not using all of the amp’s capability means the music remains relaxed and unstressed, even at loud levels. One might say, “It’s not the power you use, but rather the power you don’t use that maintains the life-like naturalness of the music.”
______________________
[1] The need for very low output impedance means most power amps have to use negative feedback. In contrast, low current line level preamp circuits can be designed to perform superbly without feedback because they are not called on to deliver the super-low output impedance necessary to drive loudspeakers. The 20 ohm output impedance of the non-feedback circuit LKV has designed for the Veros PWR+ is about the best achievable without feedback, but even its 20 ohm impedance is far too high to drive most speakers accurately.
[2] Class A amps that do not use negative feedback are not subject to this problem, but they are limited by practicality to very low power and have high output impedance that severely limit the speakers with which they can be satisfactorily used.
Google Sites
Report abuse