Hi Hugh, all,
I think I have some schematic of yours somewhere ...
Folks,
I have studied amps quite intensively for about eight years now. I have no particularly preference for tubes or SS, and like to build hybrids. I sell two kitset amps called the AKSA, one of 55W and the other of 100W, a new preamp called the GK-1, and a small tweakable two way speaker.
I am not an engineer, and resolutely develop my circuits by ear, using only a meter and CRO ocassionally. The 100W AKSA gives 0.045% into 8R resistive at 20KHz and 10 watts, sounds pretty good, and so I offer here my philosophy for designing amps.
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· Prevent Interstage Crosstalk – Decouple the supply rail for the low current stages.
A hard working output stage creates heavy, periodic voltage sag on the supply rail, and this has adverse impact on the voltage amplifier and to a lesser extent on the differential input stage. Interstage crosstalk can be almost eliminated by fitting a diode and small resistor in series from the positive supply rail and a decoupling electrolytic capacitor to ground. Interestingly, sonic testing reveals that a similar network on the negative rail has no beneficial effect. This simple decoupling network ensures that supply rail disturbances created by a hard-working output stage never interfere with the operation of the voltage amplifier and the input differential pair. By ensuring that complex interactions cannot occur between the output and input stages, a potential source of serious intermodulation between the two input stages and the output stage of the amplifier is eradicated. Listening tests reveal a cleaner sound, particularly on heavy musical passages, with superior resolution, clarity and a notable lack of intermodulation.
A better way yet is to use separate, fully regulated rails for the low current stages. This offers several benefits - low crosstalk, compensation for inherent voltage drops across transistors, prevents clipping from occurring the in voltage gain stages, improves detail and resolution and lets you use your high power transistors at lower voltages for the same power.
And the price penalty is really quite reasonable.
· Foster High Linearity – Operate the voltage amplifier at constant current
Remembering the sterile sound and high frequency complications of the constant current source leads to an old concept called bootstrapping. The technique uses the amplifier’s low impedance output to dynamically elevate the voltage amplifier power supply in step with the output signal, permitting the voltage amplifier to run at close to constant current. Constant current operation for any amplifying device improves linearity, minimises distortion and greatly ‘speeds up’ the amplifier by removing loading on the voltage amplifier. The bootstrap also has implications for source impedance. The alternative, the constant current source, has been tried many times but always leads to a dry, unexciting sound – and it confers an extreme high frequency bandwidth which needs to be viciously tamed to ensure amplifier stability. The taming process costs musical vitality. To its discredit, the constant current source does confer an asymmetric clip, slicing the negative half cycle long before the positive. However, although a bootstrap does suffer small current variations, it yields good (though deliberately not outstanding) voltage amplifier linearity and intrinsically confers rapid and convenient high frequency rolloff. Early rolloff helps to make the amplifier stable without resorting elsewhere to draconian bandwidth limitation thus bringing some immunity to digital high frequency artefacts and radio frequency interference, allowing the designer to reduce the size of the output inductor and lag compensation. Again, listening tests reveal that a current source powering the voltage amplifier creates a ‘dry’ sound. The bootstrap gives a subjective impression of speed, attack, pace and liquidity – all of which add impressively to the subjective musical experience and take the listener closer to the passion of the original recording.
Bootstrap also reduces inherent slew rate, and consequently transient response. If used unjudiciously, it can also lead to transient intermodualtion problems.
You don't need draconian measures elsewhere, Hugh. I agree one should not go overboard with the bandwidth, but you can solve the problem quite easily with a low pass filter at the base of the predriver or driver transistor.
· Minimise amplitude/phase Intermodulations – Split DC offset and AC feedback control.
This technique effectively changes the proportions of resistance and capacitance at the feedback node. This reduces adverse signal damping effects arising from the RC series chain in the network whilst preserving offset control. It also creates two AC paths to the blocking capacitor – usually an electrolytic – and this sets up a secondary charge path which masks the poor sonics of such capacitors by minimizing electrolytic memory effects. The result is a superior, longer lasting decay, particularly on fast percussive material such as cymbals, a considerable reduction in the size of the shunt capacitor and an improvement in sonic ambience.
This has been done for some time, e.g. Luxman's "Duo Beta" circuit, and others have used it too, e.g. Marshall Leach. It is one way of doing it, to be sure, though I don't do it that way. I mean, it can be made to work well.
· Eliminate Switching Transients – Implement charge suckout on the output stage drivers.
The output stage introduces switching non-linearities on the output as one output device hands over control of the speaker voice coil to the other. Base junction charge suckout, identified by Self in his impressive 1993 series of articles, involves placing a small resistor and capacitor in parallel between the two driver emitters in a conventional darlington output stage. This parallel RC network ensures that the drivers (and with them, the output devices) turn off more cleanly when speaker current flow passes to their opposite numbers at the crossover of the signal. This greatly reduces switching distortion, the bane of all Class AB amplifiers, by removing the usual spray of high order switching spikes generated by the handover event. It smoothes the transfer of the baton in the musical relay. It is this phenomenon which is partly responsible for the splashy, grainy and often harsh top end of most solid state amplifiers, since the ear readily registers switching disjunctions at high frequency and the feedback loop lacks the necessary speed to correct it.
It is clear that many modern amplifiers deliver poor performance because of short term stability problems which manifest only in musical passages but rarely in steady tone testing favoured by contemporary testing regimes.
As the Japanese have been doing since circa 1975. I'll have some schematics on my site soon, with some examples from the Sansui range (AU-607, AU-919, BA-5000, etc).
· Choose Semiconductors with care.
The final step in the re-design concerned the choice of semiconductors. To achieve good slew characteristics, the speed of all active devices is important, particularly the voltage amplifier. Also important is the output stage current versus voltage linearity (sometimes referred to as transconductance), since this ensures a high feedback factor under conditions of high output at high frequencies. Note that the speed of the output devices is NOT of the importance one might expect. Of course, cost and availability are also important factors.
It is important to maintain a consistent current gain in the output stage as the speaker current demand increases. This parameter relates to large signal linearity; a vital component of the amplifier’s overall transfer function. The choice finally settled on one transistor complementary pair which achieves constant gain (hfe or beta) from 100mA to 7A. This was the 2SC5200 (npn) and the 2SA1943 (pnp). These devices were developed by Toshiba specifically for push pull audio amplifiers, and are rated at a fast 30MHz. These are impressive figures for a 12 amp, 230V, 150 watt transistor with excellent SOAR ratings.
Hugh, my data sheet says 15A continuous, 30 A impulse for those transistors.
I have been using their predecessors, 2SC3281/2SA1302, also driven by Motorola's MJ15030/15031 for many years. Having run out of them, I am also switching over to 2SC5200/2SA1943.
The drivers must also exhibit high current capacity, with good linearity and matching speed. A high current rating ensures they are not destroyed if the output devices are subjected to a momentary short, and a consistent current gain across a wide range fosters linear, stable performance into difficult loads. The devices initially chosen were the ON Semiconductor (formerly Motorola) MJE15030 and MJE15031, and these are still used in the 100W AKSA. These devices are commonly used as drivers in the very high power amplifiers of professional audio, and are virtually unburstable in a small, low power audio amplifier. They can easily withstand a short term current burst of 8 amps, almost 100 times the working requirement in the AKSA circuits. This rating guarantees a long, reliable life – very important in this application, and especially in a kitset where robustness is mandatory. Recently new drivers have become available from Toshiba specifically for the chosen output devices, and if anything, their performance is superior, particularly in terms of linearity. These devices, now fitted to the 55W AKSA, are 2SC4793 (npn) and 2SA1837 (pnp) and their speed is almost triple that of the Motorola devices.
The voltage amplifier is particularly important, since the amplifier’s entire voltage gain is incorporated in this common emitter stage. Only this transistor configuration gives both current and voltage gain, and this is heavy going for a transistor owing to Miller capacitance, so it must be very fast and linear with high gain. It must be able to withstand more than twice the rail voltage of the amplifier at 10mA without a heatsink. Such a transistor with very low Miller capacitance is critical to good performance in a quality SS amplifier.
Agreed.
The input differential pair transistors are perhaps the easiest to choose. Since they are small TO-92 devices, their die is small with low parasitics and they are fast. Virtually any PNP device with a rating of at least 20mA and 100MHz is adequate. However, it is fairly important that they be closely matched for gain, as this confers precise differential pair current balance. Current mismatches affect the sonics and the DC offset of the amplifier quite significantly. Finally, the 150V 2N5401 and BF491 were chosen from ON Semi and Philips, as these are inexpensive, quiet, very fast and available in batches with consistent beta, making matching relatively easy.
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This article delineates what I have found to be important. I claim no originality; just careful, painstaking assembly of a variety of simple designs and meticulous listening. The interpretations are mine; some can be disputed on math grounds, and I don't pretend to follow some of the more obscure modes and happily eschew PSpice, which tells me almost nothing about the sonics or even the stability margin. Some of this material amounts to audio heresy, and I make no apology for that, but neither will I strenuously defend my points because I haven't the time or energy. The proof of it all lies in a good listen to my amps, nothing less.
I hope it's of some use, and have to say this is a very IMPRESSIVE forum, and my sincere thanks to JohnR, the Borg who set it all up! 
Cheers,
Hugh
Nothing to defend, Hugh, your philosophy is your own, and that's that. Nor do I see any attackers here, we cultivate a nice atmosphere here, everybody has a right to their own views.
People who design will invariably differ. In a lesser envirnoment, this might be a problem, but speaking for myself, I prefer that there be differences between us. Unison views lead nowhere, it's the difference in views which pushes us forward.
Cheers,
DVV
