What is Rocking Your World? SS or Tube Amps? Share Here!
Ok! I've finally decided to start a friendly discussion about what is rocking your world. Is it your Best SS or your Best Tube Amps and Pre rocking for you?
Share your things here....don't be embarrassed if you get beat in this. It's just for Funs. No kness, knuckles and elbows allowed in this thread regardless of your age. (you there, RT1?)
As a guy with background in EE, I love all kinds of Electronics and all are funs to me. Anything (SS or Tubes) rocks my world as long as it has the closest representation of the actual sound and sweet to my ears. ;)
But for all of you Die Hard Veterans, let your vengeance out in this thread to Rule The World. :p
What's rocking my world Pt. 1
Even more on the design philosophy of a musically engaging SS amp.
The Pass Aleph 30 is unique in a number of ways:
Most amplifiers on the market have between five and seven gain stages in series between the input and the output. The Aleph 30 has but two, and enjoys a very direct path from input to output, further enhancing the purity of the circuit and the resulting sound.
The output stage of the Aleph 30 is a unique blend of traditional design and innovation addressing the unique requirements of loudspeakers. Previous methods of loading the output stage have used networks consisting of resistors, coils, transformers, and active current sources, all of which offer an optimal load line based on a resistive load. The Aleph 30 has a
current source topology which optimizes performance for a wide range of impedance and reactance in the load, improving all aspects of performance into real loudspeakers. Pass Labs has US patent No. 5710522 on this output stage topology.
The Aleph 30 is unique in that there are no adjustments of any kind in the circuitry. There are no potentiometers to adjust. The operating parameters of bias currents and DC offset and soon are defined by physical constants, and will not go out of adjustment. Most important, the Aleph 30 brings improvement to the recreation of subjective sonic reality. The amplifier delivers detail and subjective space rarely found in semiconductor circuits,
coupled with the authority and clarity rarely found in tube amplifiers.
The Pass Aleph 30’s ancestry lies in the highly praised Aleph 3. We have kept virtually all of the circuit design and choice of parts intact.
So far there has been a failure in the attempt to use specifications to characterize the subtleties of sonic performance. Amplifiers with similar measurements are not equal, and products with higher power, wider bandwidth, and lower distortion do not necessarily sound better. Historically, that amplifier offering the most power, or the lowest IM distortion, or the
lowest THD, or the highest slew rate, or the lowest noise, has not become a classic or even been more than a modest success.
For a long time there has been faith in the technical community that eventually some objective analysis would reconcile critical listener's subjective experience with laboratory measurement. Perhaps this will occur, but in the meantime, audiophiles largely reject bench specifications as
an indicator of audio quality. This is appropriate. Appreciation of audio is a subjective human experience. We should no more let numbers define audio quality than we would let chemical analysis be the arbiter of fine wines. Measurements can provide a measure of insight, butare no substitute for human judgment.
As in art, classic audio components are the results of individual efforts and reflect a coherent underlying philosophy. They make a subjective and an objective statement of quality which is meant to be appreciated. It is essential that the circuitry of an audio component reflects a philosophy which address the subjective nature of its performance first and foremost.
Lacking an ability to completely characterize performance in an objective manner, we should take a step back from the resulting waveform and take into account the process by which it has been achieved. The history of what has been done to the music is important and must be considered a part of the result. Everything that has been done to the signal is embedded in it,
Experience correlating what sounds good to knowledge of component design yields some general guidelines as to what will sound good and what will not:
1) Simplicity and a minimum number of components is a key element, and is well reflected in the quality of tube designs. The fewer pieces in series with the signal path, the better. This often true even if adding just one more gain stage will improve the measured specs.
2) The characteristic of gain devices and their specific use is important. Individual variations in performance between like devices is important, as are differences in topological usage. All signal bearing devices contribute to the degradation, but there are some different characteristics are worth attention. Low order nonlinearities are largely additive in quality, bringing false warmth and coloration, while abrupt high order nonlinearities are additive and
subtractive, adding harshness while losing information.
3) Maximum intrinsic linearity is desired. This is the performance of the gain stages before feedback is applied. Experience suggests that feedback is a subtractive process; it removes information from the signal. In many older designs, poor intrinsic linearity has been corrected out by large application of feedback, resulting in loss of warmth, space, and detail. High idle current, or bias, is very desirable as a means of maximizing linearity, and gives an
effect which is not only easily measured, but easily demonstrated: Take a
Class A or other high bias amplifier and compare the sound with full bias and with bias reduced. (Bias adjustment is easily accomplished, as virtually every amplifier has a bias adjustment pot, but it should be done very carefully). As an experiment it has the virtue of only changing the bias and the expectations of the experimenter.
As the bias is reduced the perception of stage depth and ambiance will generally decrease. This perception of depth is influenced by the raw quantity of bias current. If you continue to increase the bias current far beyond the operating point, it appears that improvements are
made with bias currents which are much greater than the signal level.
Typically the levels involved in most critical listening are only a few watts, but an amplifier biased for ten times that amount will generally sound better than one biased for the few watts. For this reason, designs which operate in what has been referred to as "pure" Class A are preferred because their bias currents are much larger than the signal most of the time. As mentioned, preamp gain stages and the front ends of power amplifiers are routinely single
ended "pure" Class A, and because the signal levels are at small fractions of a watt, the efficiency of the circuit is not important.
4) Given the assumption that every process that we perform on the signal will be heard, the finest amplifiers must employ those processes which are most natural. There is one element in the chain which we cannot alter or improve upon, and that is the air. Air defines sound, and serves as a natural benchmark. Virtually all the amplifiers on the market are based on a push-pull symmetry model. The pushpull symmetry topology has no particular basis in nature. Is it valid to use air's characteristic as a model for designing an amplifier? If you accept that all processing leaves its signature
on the music, the answer is yes.
One of the most interesting characteristics of air is its single ended nature. Sound traveling through air is the result of the gas equation
PV1.4 = 1.26 X 104 where P is pressure and V is volume, and whose curve is illustrated in fig. 1. The small nonlinearity which is the result of air's characteristic is not generally judged to be significant at normal sound levels, and is comparable to the distortion numbers of fine amplifiers. This
distortion generally only becomes a concern in the throats of horns, where the intense pressure levels are many times those at the mouth, and where the harmonic component can reach several per cent.
Fig. 1 shows the single ended nature of air. We can push on it and raise the pressure an arbitrary amount, but we cannot pull on it. We can only let it relax and fill a space as it will; the pressure will never go below "0". As we push on air, the increase in pressure is greater than the corresponding decrease when we allow air to expand. This means that for a given
motion of a diaphragm acting on air, the positive pressure perturbations will be slightly greater than the negative. From this we see that air is phase sensitive.
As a result of its single ended nature, the harmonic content of air is primarily 2nd order, that is to say most of the distortion of a single tone is second harmonic. The phase of this distortion reflects the higher positive pressure over the negative. Air's transfer curve also shows also that it is monotonic, which is to say its distortion products decrease smoothly as the acoustic level decreases. This is an important element that has
often been overlooked in audio design and is reflected in the poor quality of early solid state amplifiers and D/A and A/D converters. They are not monotonic: the distortion increases as the level decreases.
The usual electrical picture of an audio signal is as an AC waveform, without a DC component. Audio is represented as alternating voltage and current, where positive voltage and current alternates with negative in a reciprocal and symmetric fashion. This fiction is convenient because it lends itself to the use of an energy efficient design for amplifier power stages known as push-pull, where a "plus" side of an amplifier alternates operation with a
"minus" side. Each side of a push-pull amplifier handles the audio signal alternately; the "plus" side supplying positive voltage and current to the loudspeaker, and the "minus" side supplying negative voltage and current.
What's rocking my world Pt. 2
Problems with push-pull amplifier designs associated with crossover distortion have been discussed elsewhere at length, and one of the primary results is non-monotonicity. Class B and many AB designs have distortion products that dramatically increase with decreasing signal. This is reduced greatly by Class A mode, but crossover distortion remains as a lower order discontinuity in the transfer curve.
For reproducing music as naturally as possible, push-pull symmetric operation is not the best approach. Air is not symmetric and does not have a push-pull characteristic. Sound in air is a perturbation around a positive pressure point. There is only positive pressure, more positive pressure, and less positive pressure. Push-pull circuits give rise to odd ordered harmonics, where the phase alignment reflects compression at both positive and negative peaks and crossover nonlinearity near the zero point.
Push-pull operation in amplifiers is commonly portrayed by the analogy of a two-man saw cutting down a tree. Certainly if we are cutting down trees by hand, we would opt for this method, as it would be much more efficient.
As we are not cutting down trees, I much prefer the image of a violinist holding the bow at one end with one hand. Only in this manner does the musician gain the degree of control and precision required to produce the range and subtlety required by music. And so it is with single-ended amplifiers.
Only one linear circuit topology delivers the appropriate characteristic, and that is the single ended amplifier. Single ended amplification only comes in pure Class A, and is the least efficient form of power stage you can reasonably create, typically idling at more than twice the rated output power under the best of conditions. Single ended operation is not new. It is routinely found in the low level circuitry of the finest preamplifying stages and in the front end circuits of the finest power amplifiers. The first tube
power amplifiers were single ended circuits using a single tube driving the primary of a transformer.
Single ended Class A operation is generally less efficient than push-pull Class A. Single ended Class A amplifiers tend to be even bigger and more expensive than their push-pull cousins, but they have a more natural transfer curve.
The "purity" of Class A designs has been at issue in the last few years, with "pure" Class A loosely defined as an idling heat dissipation of more than twice the maximum amplifier output. For a 100 watt amplifier, this would be 200 watts out of the wall at idle.
As the Pass Aleph 30 idles at four times its rated output, I think we can safely think of it as “pure”. Designs that vary the bias against the musical signal will generally have bias currents at or below the signal level. This is certainly an improvement from the viewpoint of energy efficiency, but the sound reflects the lesser bias point.
This is what's rocking my world right now :D