Designing a matched Class A single ended pre-amp and power amp
　

 The ST-1 power amp and ST-7 pre-amp have been designed as a complimentary pair of Class A single ended amps. The bulk of the discussion below is about the power amp but a cursory glance will show that the gain boards on both are similar. The pre-amp is dealt with briefly at the end of this paper. 

 The single ended Class A amplifier is a highly respected design with a tradition going back to Linsley Hood. More recently it has been made famous by Nelson Pass and other respected audio engineers. Class A,  SE  amps have the characteristic of operating over the whole of the input cycle in such a way that the output signal is an exact scaled-up replica of the input with no clipping. Draw on the mains power supply is high and close to constant regardless of signal strength and thus Class A amps are much less efficient than, for example, class AB amps. Countering this, Class A SE amps produce even harmonics which makes them highly regarded by audiophiles for their warmth, sound stage and resolution.

 While the original Linsley Hood circuit was elegant and simple, it had a low power output. The increases in power gained by later designs like those of PASS brought with them certain problems. A differential input circuit has a tendency to reduce the SE character of the design and though it can be improved by re-tuning circuit components, the result is not as natural. Another approach used in class A SE amps has been to use bridging techniques but this is arguably an even less satisfactory solution than the differential input method.

 The design of the ST-7 pre-amp and the ST-1 power amp is as much philosophical as it is technical and brings to fruition many years of experience. Combined in these amps is the driving power of an SS amp along with the resolution, tonal colour and flavour of a SET valve amp. On top of this comes easy maintenance and low cost.

Circuit design analysis

 The design of an outstanding electronic circuit must be based not just on technical specifications but should also take into account an understanding of the acoustic fidelity of the individual components used. Beyond this, a designer has also to anticipate how these components will combine to produce a neutral and natural sound. It is much more satisfactory to make good initial choices than to have to apply add-on corrections later.

       Inherent in this classic SE amplification circuit is the problem that variations of the operating current and output voltage can occur with temperature changes. Control of these fluctuations is essential to prevent catastrophic and expensive damage to speakers. Traditional solutions to this problem have been:

       1. the addition of several thousand uF of high grade capacitance just prior to the output terminal to block DC.

        2. to employ a differential circuit to limit DC drift [as exemplified by PASS circuitry]

        In pursuit of perfection, a different approach has been taken with the design of the ST-1 and ST-7 using dc servo to lock the operating points. The DC servo circuit circuitry used here is based on a fresh examination of theory and thus breaks away from more conventional thinking. The first stage of the servo circuit uses an integrated amplifier to extract DC from the signal while reducing AC. In addition a -6db buffer is present to reduce any noise introduced by the integrated amplifier.

 The output of the DC servo again passes through the RC filter, then to the feedback coupling capacitor to get rid of the alternating component, yielding the pure dc component. When the DC level in the output is changing, the DC servo output point TP1 changes accordingly. The V/I variations pass the resistor and alter the operating point of the input stage to restore OV output. The output dc is limited to 2MV during the actual circuit test.

 Looking at the main amplifier, the circuit is quite straightforward, though it deserves explanation. If the first stage of the circuit was to use a constant current to obtain a high S/N ratio this would necessarily also reduce the of Class-A single-ended character of the amplifier. Therefore this stage does not use constant current source but instead allows other parts of the circuit to enhance the S/N ratio.

 The second part is the critical main gain stage where most of the gain occurs. The operational characteristic here significantly influences the whole sound. If this stage only uses common-emitter circuit, the sound is dense and warm, but blurred and lacking delicacy. With a desire to reproduce the mid/high frequencies to convey the feeling of running water, floating cloud and high transparency a different approach is needed.  So, a cascode circuit which has the desired timbre, becomes the logical choice here. The next challenge is to keep the cherished timbre but at the same time to reduce the wideband characteristics.

 Cascode transistors configured in common-base require biasing voltage to work properly as the reference voltage circuit audibly effects sound quality. Much time has been devoted to comparing the following reference voltage methods.

1.       Zener diode connected to the power supply loop

2.       Zener diode connected to the signal loop

3.       LED connected to the signal loop

4.       Resistor connected to the signal loop

5.  Triode BE voltage multiplier connected to the signal loop,

 Of the five options:

 1. has good body and dynamics but lacks subtlety.

2. is improved on 1) but still lacks delicacy

3. using a green LED the sound was clear and fine in the upper frequencies but weaker than the Zeners in low frequency. Interestingly, a blue LED gave different results although all up LEDs had failings and also suffer from a short life in this application.

4. using Dale resistors, low frequency volume was down and in addition the depth of frequency response was limited

5. using A970/C2240 the mid-range is warm and clear while bass reproduction is brought slightly forward with a good sense of reality.

 While it might seem to the casual observer that such distinctions are too fine to be observable, it is the nature of Hi Fi audio that attention to such tiny differences is essential to the design of outstanding equipment. The above results were the result of numerous trials under controlled conditions.

 The loading of this circuit is Constant current source that can enhance load capacity, maximize output effectively and concurrently lower the distortion. When the high current Darlington  transistor’s [MJ11032/MJ11033] operating current is 80MA, the sound quality is well balanced and delightful. Hence, this circuit is set at 80MA approx.. The output current is enough to drive the output transistors without using any pre-driver transistors.

         To bring out an open and singing nature requires low open-loop gain. This circuit’s open-loop gain is 39DB only. The -3DB frequency response is at 55Khz. Closed loop gain is 27DB. The local feed back loop is connected from the output of the voltage amplification parts but does not re-join at the front-end round to give global feedback. Audiophiles might appreciate this aversion to global feedback

 The design of this system also tested out a theory that a tubelike soft and deep low frequency presentation would be favoured by the use of a low damping factor. This was achieved by not using a pre-drive transistor. Auditioning the completed system proved the theory to be correct though interestingly, this form of design is counter to that conventionally used in hi fi with huge dynamics.

  When it is considered that many of the components in the output circuit are in parallel the simplicity of the circuit becomes apparent.  The output of the power amp can be varied from 15W at 8 Ohm to 60W at 8 Ohm by changing the number of output transistors. Board design will allow for up to six pairs of transistors. The ST-1 uses C5200’s in preference to A1943 as they offer superior linearity and the uniformity. In the output stage constant current circuit, out of 6 transistors, one is for pre-driver, 5 are for output. The 45W/8 output power needs approximately 2. 5 A electric current. To achieve this, the 1. 5 ohms resistance of the constant current flow circuit is obtained by two 1/2W 3 ohm resistors in parallel.

 The amplifier uses local feedback in every stage. All components have been selected to produce a tube like sound with superior accuracy.

 Along with the high operating current comes an increase in supply ripple and potentially hum.  An alternative to enhance SNR is to provide a voltage regulator to the output stage. The stabilized voltage supply on the left side of the power suppler diagram is for output and the supply on right side is for voltage gain circuits. These voltage regulators perform well and are of a well respected and recognized design. The circuits have been re-designed to enhance temperature stability and to suppress ripple capability.

The current consumption is as high as 2.5A for 45W at 8ohm. For future upgrade of power output, the current will be larger. For this reason the voltage regulator’s transistors use a double parallel design which provides a safe working margin with excellent linearity.

 The power supply board can house 200,000uf capacitor. When using 60,000uf the  capacity is sufficient for a 45W output. The ability of single bridge rectification to enrich  sound makes it well suited to this purpose.

 Class-A amplifier operates with a constant current draw and therefore require less current regulation. While a 350W transformer would be sufficient for a 45W output it is preferable that the transformer supplies perhaps 20 times more than the actual power consumption to reduce the problems of mechanical vibration. It is intended that a 1000W transformer will be used for each channel though it remains to be seen if audiophiles will be prepared to pay the price for this extra capacity

 With Class A-SE amplifier, audiophiles are more concerned about SNR and any noisy, remnant sound which would mask the small signal delivery as such noise would affect the mood and destroy the quality of the image. The amplifier’s output noise is only 0.1MV, the un-weighted SNR is 104DB, which is exceedingly high for any commercial product. The noise cannot be heard even the ear touching the 90DB sensitivity loud speaker.

 While a low noise, voltage stabilized power supply underpins the ST-1, the circuit board layout is important. A pair of copper wound transformers are complimented by separate ground loops applied to the voltage stabilizer and power supply. In addition the signal has individual floating signal ground.

 The SE-Class-A amplifier is designed for a fixed impedance speaker. The ST-1’s nominal rated power is the 45W/8 ohm but unlike normal p-p amplifiers, it is necessary to change some components for different speaker impedances.  If, for example the amp is to be used for 45W /4 OHM, the output stage can be increased for a 1.4 X constant flow current flow, while the corresponding stabilized voltage drops to 0. 7 times.

  No changes would be required in the parameters of the voltage gain stage.

      Final testing of the ST-1 and ST-7 using a speaker system based on 4 X Scanspeak drivers [4 ohm in parallel] brought out the full acoustic fidelity of the amps. Overall presentation is clear, detailed, flowing and soft. The full signal is faithfully reproduced with bass reaching deep but retaining a full and natural quality. Initial  listeners were amazed by the wide soundstage and excellent separation of instruments. Dynamics and driving power were also considered to be outstanding.

Single-ended pre-amplifier ST-7

      The circuit structure is the same as the voltage gain stage of the power amp, only the gain is altered to 13DB.

The volume control is of the parallel divergence type. At maximum volume there is -3db attenuation. Also at maximum volume the overall gain of the ST-7 is 10db.                                                                      

 The design of this SE-pre emphasises the characteristics of the SE-power amp with the slight differences to the circuit adding richness to the sound

 By replacing the ST-7 with the C8 re-gen. power supply pre-amp, the sound immediately changes to another flavor, clarity and sound stage display as before, bass is resilience and dynamics, but the high-mid becomes softly round and rich, female voice appears slender, tender and full of sentiment, the harp imitates like the falling bead to the jade plate, the violin sounds touching, Ancient -Zheng sounds less sharp, seems lack of reality. But the sound is joyfully easy, like dreaming. A more expensive hi end tube product could not achieve this balance.