08 October 2014

Pre-RMAF Ramblings

Pre-RMAF Ramblings
The Rocky Mountain Audio Festi looms before me. The RMAF is, as they like to put it, "the largest consumer audio and home entertainment show in the United States." A big audio show, in other words. Why it wasn't instead named "RMAS, the Rocky Mountain Audio Show" is a good question. (Perhaps, the Royal Military Academy Sandhurst or Royal Maritime Auxiliary Service wouldn't let them use their acronym.) I am glad that they left off the last four letters, "ival," stopping at "Rocky Mountain Audio Fest," as a fest is a gathering or an occasion characterized by a specified activity; and "festival" implies more celebration, more rejoicing than any audio show can deliver or that anyone could reasonably expect.

Nonetheless, in two days, I will drive down to Denver, Colorado, through a mangled freeway of endless repairs, always changing speed limits, and guaranteed stop-and-go, halting progress. (This I know, as I made the exact same trip last weekend to the exact same location, but for non-audio reasons.)

At the show, I will wear my audio-press name tag, which confers a smidgen more attention from the exhibitors. But as my name tag will only state that I am the editor of the Tube CAD Journal, not a contributor to Stereophile or The Absolute Sound, I will not garner the apple-polishing, bootlicking, fawning, kowtowing, slavering, toadying displays of frenzied self-abasement—which is fine by me. Indeed, I know one audio-press personality, who wears his name tag backwards in its sleeve. On the other hand, this might not be the noble, reticent act that I imagine it to be, as all the press tags bear the same color, so by presenting the exhibitors its backside, he also presents them with an ambiguous proposition: Is he just some nobody blogger or a regular contributor to Stereophile or The Absolute Sound? Best not to take any chances they may conclude, so they crank up the sycophancy.

If you are wondering why I am writing about an event that has yet to materialize, do not worry, for there is a point, albeit an alternative-reality point. Let's imagine that I will be startled to see rows and rows of busses awaiting me at the RMAF; that I am told to get on board, as the show is being held elsewhere; that as I look out the bus's window, I am confused by our progress away from the big hotels and the cityscape, seeing upscale suburban houses instead; that the bus stops within a large new housing development and that we are told to explore the neighborhood; that I am given a map of the development, with each house labeled with a large audio company name, such as Acoustic Zen, Audio Note UK, Audio Research, Boulder Amplifiers…; that upon entering a large, modern house, I am pleased to see a high-end stereo system in the place of honor in the sparingly but nicely appointed living room; that as I sit upon the comfortable sofa I marvel at the silence between tracks, as no thumping sub-woofers can be heard leaking from other rooms; that, in fact, the loudspeakers and stereo gear now freed from the tight confines of a small, cheaply constructed hotel room, sound so good that the $70,000 price seems not too unreasonable; that after having enjoyed this acoustically-resplendent presentation, an audio oasis, I move to other rooms within the large house, where I find headphones and headphone amplifiers, cables, and other audio accessories on display; that the kitchen is filled with happy audiophiles, who sip wine and tell of their experiences; that the next house offers equal sonic wonders; that the fresh air and short walk outside perform an excellent audio reset, so my ears eagerly await the next house's auditory treasures; that I now—for the first time at any audio show I have attended—have a good idea what the stereo systems actually would sound like not in a crowded, noisy, elevator-sized room, but in my own living room... Dream on, John, dream on.

FETs and Aikido
My recent posts on hybrid power amplifiers, the ones that held a cascade of technologies: FET, vacuum tube, and then MOSFET or transistor output stages, prompted more interest in FETs than I expected. One reader asked if a FET-based buffer stage could be added to his existing Aikido line-stage amplifier. Now, the line that separates provocative, interesting questions from plain silly questions is indeed a thin one. My first impulse was, I am ashamed to say, was to place this question on the silly side of the divide, which was silly of me. The idea has real merit. How so? Well, if nothing else it would defend the volume control from the Aikido's input capacitance, which is relatively high, due to Miller-effect multiplication. The capacitance equals mu/2 times the grid-to-plate capacitance of the input triode.

One problem we face is supplying the FET with its own low-voltage power-supply rail. If you own a PS-15, there's no problem, as it offers two low-voltage, regulated power supplies, along with one high-voltage power supply. On the other hand, we could steal the low voltage from the high-voltage B+ connection.

A simple capacitance-multiplier circuit can be used, as shown above. The high-voltage transistor, say a TIP50, must dissipate the lion share of heat, as it sees the lion share of voltage.

But now that we have introduced the transistor to the Garden of Eden, why not go all the way?

Brace yourself, as there is much to cover here. First, note the Bastode arrangement of the N-channel FET and the PNP transistor. As far as the FET is concerned, it is locked in a 20V vice; it can vary its current conduction and source voltage, but not its drain voltage. This varying current flow then travels through the PNP transistor down through the second Rs resistor, which terminates into the negative power-supply rail. Because the two Rs resistors match each other's value, no gain is realized; indeed, there must be some slight loss in gain. The Aikido then accepts the input signal and does its magic, but a bit differently than with a mono-polar power supply. For example, note the absence of the usual Aikido two-resistor voltage divider that provides the bottom output triode with is sampling of the power-supply noise. Where did it go? It wasn't needed, as the natural power-supply-noise null occurs at the output, as the bipolar power-supply rails hold equal, but opposite phase ripple, which when summed equal zero.

If we need more gain, we can make the bottom Rs resistor much larger in value than the top Rs resistor, for we have about 100 volts to play with. Why would we need more gain? Say you want to use this as the first stage in a phono preamp or you plan on following this circuit with a solid-state output stage, making a fine hybrid power amplifier.

Next Time
The non-virtual RMAF, alas.

Since I am still getting e-mail asking how to buy these GlassWare software programs: