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The VitaSound PSAP is a discreet one size fits all in-ear amplifier that boosts the natural ability to hear in normal & difficult situations. The Personal Sound Amplifier or PSAP is intended to amplify environmental sounds for non-hearing impaired consumers.
The SPA21 provides a comprehensive range of features to make it ideal as the heart of an unparalleled music or home entertainment system: Exceptional processing and amplifying performance The SPA21 uses proprietary DSP circuitry developed by Primare, providing better analog performance than the current off-the-shelf circuits used in many other surround amplifier, resulting in a very high standard of multichannel audio reproduction. For the new SP31, Primare has enhanced a design classic to deliver even greater fidelity, flexibility and reliability to aficionados of multi-channel music and lovers of the movie genre. It offers a class-beating combination of innovative DSP engineering, peerless performance, versatile connectivity, superior control and ease of use.
A fundamental component of high quality sound reproduction, whether it is movie sound or music, is amplifier power. Each of the channels within the A30.5 provides incredible current delivery at an outstanding level of quality. The A30.5's generously specified heat sink ensures excellent heat dissipation, creating the best possible conditions for perfect performance year after year.
The A30.5 is simply a glorious piece of hi-fi, configured for today's five channel requirements. Some of the information I've seen has to be considered doubtful, and advertising brochures and websites are big on hyperbole but rather low on technical detail or accuracy. Class-A is defined as an operating condition where the amplifying device conducts for the full 360° range of the input signal - namely from zero, through to the maximum positive signal peak, through zero again to the full negative signal peak, and back to zero. Virtually all valve preamp stages operate in Class-A - they use one or more single valve stages, each with a plate load consisting of either a resistor or another valve. Single valve circuits can only function in Class-A, because they cannot act as an amplifier without the plate current through the plate load resistor. For power output stages, Class-A is also used, and it can also be single-ended (using a triode, tetrode or pentode), or perhaps two of the same type in parallel.
The main disadvantage is that the output transformer's windings have DC flowing at all times, so there is a net static flux in the transformer that dramatically reduces transformer performance. Push-pull amplifiers can also operate in Class-A, and the general conditions are the same as for a single valve - the plate current never falls to zero, and preferably never falls below the minimum current that is needed to ensure linear operation.
It is sometimes mistakenly believed that all Class-A amplifiers (single-ended and push-pull) use cathode bias - a resistor in the cathode circuit that raises the cathode potential above zero, and provides the required grid bias. While cathode bias is indeed common with Class-A output stages, fixed bias from a separate negative voltage source can also be used. Although cathode bias is the most common for Class-A amps and in theory the resistor requires no bypass capacitor, it is almost always necessary to include it unless the output valves are perfectly matched in every way. When operating an output stage in Class-A, it is common that the plate current is such that maximum rated plate dissipation is the normal condition at idle.
While Class-A2; operation is possible (meaning that control grid current is drawn to get full undistorted) power, it is very uncommon.
Class-AB is the most common for push-pull amplifiers, and cannot be used with single-ended stages.
Most Class-AB amplifiers are Class-AB1, and do not draw grid current to achieve full power, allowing simple drive circuits which may be capacitor coupled. While the above are the "official" designations, almost without exception, any Class-AB amplifier that is driven into distortion will become Class-AB2. There is also an unexpected result - the coupling capacitor is charged by the grid current, and this increases the effective negative bias voltage for a short time until the excess charge is dissipated through the circuit resistances.
The definition of 'true' Class-B is that each output device conducts for exactly 180° of the input cycle. Class-B is actually very uncommon for audio valve amplifiers because of the issues with crossover (aka 'notch' distortion) that is inevitable with valves biased into (or close to) cutoff. Almost all amps that are referred to as being Class-B are actually Class-AB, but with the lowest possible bias current that ensures at least acceptable low-level distortion figures.
For high power PA usage, Class-B2 was reasonably common, because you can get the maximum possible power from a pair (or several pairs) of output valves this way.
Class-B (and Class-AB amplifiers that are biased to a low plate current) must use fixed bias. Although not relevant to audio, it's worth pointing out that a common application with RF amplifiers is Class-C. Because it is so far outside audio applications it will not be discussed in any more detail.
Although this shows the effect with a single-ended stage, exactly the same thing happens with push-pull, and it happens to both drive signals. In the diagram above, the normal saturation voltage of the valve can be lowered as shown by the grey lower tip of the waveform, but only if the drive circuit can supply an undistorted grid signal despite the relatively large grid current.
An amp that has an adjustable fixed negative bias supply can (valve dissipation and bias adjustment range permitting) be biased anywhere from Class-B right through to Class-A.
It is important to understand that all definitions are based on maximum undistorted output power. For hi-fi applications, there is no doubt that (push-pull) Class-A will give superior sound quality in almost all cases.
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By virtue of its upgraded electronic design, the SP31 widens the audio and video spectrum still further to provide ever more accurate sound quality, stunningly lifelike pictures and a more vivid contrast with lesser processors.
The SP31 uses proprietary DSP circuitry developed by Primare, to provide a far more convincing multi-channel sound stage than the standard off-the-shelf circuits used in many other surround processors. By using a range of easy to follow onscreen menus, your preferred combination of sources and outputs may be configured quickly and easily to provide simple, user-friendly operation. The explosive energy of an action soundtrack or the finely detailed complexion of the human voice must be reproduced with authenticity. Each has its own transformer and power supply, making it identical to the four other amplifiers, but also electronically independent. They are all well documented, but there is still some confusion regarding "sub-classes", and almost nothing that discusses the changes that occur with a valve amp in overdrive. The classes of operation are well established, and while some are applicable to both valve (tube) and transistor amps, others are not. This push-pull amp can be biased for Class-A, AB or B, but remember that any amp that draws grid current must always be driven from a very low impedance driver, preferably using a transformer to keep impedances as low as possible.

The benefit of this arrangement is that is will adjust itself as needed as the valves age, and no adjustment is needed. When another valve is used, it's generally used as a current source, or as part of the so-called SRPP stage. For preamps, it's generally advisable to ensure that the plate current at idle is at least 4 to 5 times the current that will be drawn by the next stage.
This is generally catered for by using a much larger transformer than might otherwise be needed, and providing an air-gap in the core to reduce the effects of saturation. Push-pull amplifiers have the advantage of cancelling the DC current in the transformer windings, so there is no static field in the core. This does not mean that the circuit is not Class-A, contrary to what you may read elsewhere. Since this is virtually impossible to ensure for the life of the valves, the bypass capacitor ensures that both valves get equal bias current and drive signal. While the peak dissipation may exceed the rated maximum when operating, the average value of plate dissipation either remains the same, or (and more commonly) is reduced slightly. Class-A amps are favoured for their fidelity, and it is extremely difficult to maintain fidelity while driving grid current. A vast number of consumer level products were built that used small pentodes in single-ended Class-A output stages - mantel radios, "radiograms" (combined radio and record player), stand-alone tape recorders and many other products used this scheme because it was comparatively cheap, and worked well enough for the intended purpose. In other words, the output devices conduct for somewhat more than a half-cycle of the input waveform. The advantage over Class-A is that the stage is more efficient, and can deliver far more power from a pair of valves of the same type as might be used for Class-A. Class-AB2 is far less common, and is generally reserved for extremely high power amplifiers as might be used as AM transmitter modulators. Typically, full power is achieved at a point where the control grid voltage on the power valves reaches (or is close to) zero volts. If you'd like to know more about this, the topic is covered in much greater detail in the Analysis article. With cathode bias, the voltage across the bias resistor(s) must remain stable throughout the cycle, so the bias point is generally set by choosing a cathode resistance that ensures that the total current is roughly equal - the average AC current component should be close to the same as the quiescent DC current to prevent bias shift during operation.
All audio Class-B amplifiers are actually Class-AB, but (as noted above) biased for a very low quiescent current. Some amplifiers intended solely for general purpose public address use (often through efficient but poor fidelity horn loudspeakers) were very close to being Class-B, because the primary goal was high volume reproduction of the speech band only (typically 300Hz to 3kHz).
For this class of service, fidelity simply was not an issue, but high efficiency and power output were both high priorities. Transistor power amps can get a lot closer to real Class-B than valves, but the same general comments exist.
Because the current varies so much between zero or low power and maximum power, a cathode resistor would cause very wide variations in the bias voltage, which in turn will change the bias voltage along with the signal. This is defined as a condition where output device current flows for (considerably) less than 180° of the signal waveform.
Once the grid is more positive than the cathode, it becomes another anode, so current will flow between the positive grid and (now) negative cathode. Note that this applies to an overdrive condition, and it happens with triodes, tetrodes and pentodes, regardless of class of operation.
The definitions provided are 'text book' - these have been established for a long time, and there is no reason or excuse for anyone to think that they might have other meanings. However, few amplifiers that are designed to be Class-AB will be able to be operated in Class-A unless the power supply voltage is reduced to prevent grossly excessive plate dissipation.
All amp classes will draw grid current when driven into distortion, and once an amp is used consistently this way (such as guitar amplifiers), the operating class is more or less meaningless. I suspect that an ultra-linear Class-A output stage could provide extremely good performance - certainly better than you'll get with triodes.
This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2009.
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Flexible range of inputs The SPA21 provides a range of inputs flexible enough to cater for virtually any combination of sources including: eight or five plus one external analog RCA inputs, two coaxial digital inputs, two TOS-Link optical digital inputs. The SP31 is supplied with a factory set-up to accommodate a standard set of sources conforming to the most usual combination of inputs and outputs, but these may be modified and enhanced according to your taste and system characteristics. This is a test of quality for any amplifier, especially in home theatre, where five channels of sound require identical resources. By taking this meticulous approach, Primare has endowed the A30.5 with optimum stability and channel separation, giving a cleaner, clearer, more accurately defined sound.
While grid current (during linear operation) is discussed, it is not usually suitable for general purpose audio amplifiers because of the requirement for a very low impedance drive circuit. Because of the huge cathode current variations in a Class-B stage, this arrangement cannot be used. At no point during normal unclipped operation does plate current fall below the minimum value needed to maintain acceptably linear operation. Either way, Class-A dictates that the valve carries current for a complete cycle of any applied waveform - at no stage is the current reduced to zero during linear operation. This can be another voltage amplifier, a tone-stack, a transformer (either to drive an output stage or perhaps a reverb tank) or an external amplifier. Rather high distortion (especially intermodulation distortion) and poor damping factor are additional disincentives.
Cathode bias has the advantage of not requiring adjustment in service, but well matched valves are essential for good performance.
The only requirement for Class-A is that the valve (or valves) conduct for the complete signal waveform, and are never allowed to be reduced to zero. This type of operation requires a very low impedance driver stage to avoid excessive distortion, and it will typically use a driver transformer. Once the plate voltage reaches saturation (its minimum possible voltage without grid current), the driver stage will continue to produce higher than needed grid voltage, and grid current cannot be avoided.
Some Class-AB amps will provide up to perhaps half the total voltage swing (a quarter of the maximum power) in Class-A, while others may make the change to Class-B at much lower power levels. A (generally quite large) capacitor is almost always necessary to maintain a constant bias voltage.

Crossover distortion can be minimised by the application of feedback around the output stage, but this can (and does) cause the harmonics caused by crossover distortion to be shifted to higher frequencies.
Normally (and unlike a transistor amp with crossover distortion), the kink around the zero crossing point is not especially pronounced, but it is still audible.
Crossover distortion is usually higher than desirable, although it should not be audible when used for the intended purpose - at relatively high volume and through speakers with a limited bandwidth. A capacitor doesn't help, because it will charge due to the much higher average current, causing the valves to be completely cut off at low signal levels until it discharges. This is an extension of Class-B, but the output device only applies a brief pulse of current to a tuned circuit load. Figure 5 shows the diode that is formed within the valve itself, and what happens to the grid drive waveform if it is from a typical high impedance source. Where an amplifier is specifically intended to draw grid current, the drive section must be completely redesigned. The drive circuit may need to supply several milliamps in a high powered amplifier, with no waveform distortion. It is important to understand that the type of bias used has no bearing on the class of operation, with the single exception that Class-B and low-bias Class-AB cannot be achieved using cathode bias.
It is also common that the required output transformer ratio will be different from that fitted. A clipping Class-A amplifier is unlikely to sound any different from a clipping Class-AB amp - while the player may well imagine a difference, it's probable that there is no audible difference at all. This is not a common topology, but there is no reason at all that it could not out-perform almost any other in terms of low distortion and output impedance.
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Basic voltage waveforms are shown in red and green, so you can see which valve is responsible for each half of the waveform. This also applies for the other classes until the valve that is turning off ceases conduction altogether. The bias is achieved by way of the voltage across Rk, and it is impossible to have a voltage equal to the valve's cutoff voltage developed across a resistor (of sensible resistance) if the valve is barely conducting.
While fixed bias may be used for a single-ended stage, it's not common to see it done that way. Regardless of whether the stage is a voltage amplifier or a cathode follower, the basic rule is still followed, as this keeps distortion within reasonable limits. A single valve is needed, and can often be used at low power without needing any additional amplification. Fixed bias is just as valid as cathode bias for a Class-A stage, whether single-ended or push-pull. The load impedance should be chosen such that the valves still conduct (perhaps 5-10% of the quiescent current) at full power, so the inherent non-linearity at very low current is minimised.
If the bias current is too low, a single-ended stage will clip, and a push-pull circuit will be converted to Class-AB.
Once the drive signal exceeds zero volts (becomes more positive), grid current flows and cannot be avoided. Some valve manufacturers insist that separate cathode resistors be used, but most cathode biased circuits use a common resistor bypassed with a capacitor. In reality, there is no way that this can be achieved with valves without gross distortion at the zero signal point - referred to as crossover distortion.
Typical applications would include PA systems for horse racing events, stadium announcements, etc. It's worth noting that both crossover and clipping distortion contain the same harmonic structures and in the same ratios.
This class of amplifier is common with FM (Frequency Modulation) or CW (Continuous or Carrier Wave - usually modulated) power stages, but cannot be used for AM final amplifiers that must handle the modulated signal. To prevent distortion of the drive current, the source (driver stage) must have extremely low impedance and resistance. The output stage will generally use a lower impedance transformer than would normally be the case, and more power can be obtained from a lower supply voltage.
Of course, it's possible that some players can hear a real difference, but this is likely to be more to do with playing style and subtle nuances that most will miss. Considering the number of Class-AB ultra-linear stages that already give outstanding results, Class-A would give it that little bit extra.
The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. The voltage and current waveforms are the same as those shown in Figure 2, but only for a single valve. This is not a hard and fast rule though - it's quite common to see circuits where the external load is the same or less than the plate load resistance. If a triode is used, a prior gain stage is generally necessary because of the power valve's relatively low gain. Because the drive signal is capacitor coupled (and is not low impedance), this causes severe distortion of the drive signal. This is where the signal crosses over from being positive or negative (at the speaker terminals). The difference is phase, and the fact that crossover distortion is worst at very low levels, while clipping distortion only occurs at very high levels. The low-current linearity of output valves is generally very poor, so even Class-B amplifiers will typically operate with a small quiescent (bias) current. Where cathode bias is used with a common resistor they are still recommended, but can be downgraded to 1W.

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