Cascode Class A Power Amplifier

Principle

This audio power amplifier in principle is a non-feedback design. It can be converted to a normal voltage feedback design, if wanted. Since it has a noninverting (+) and inverting (-) input, the amplifier accepts true balanced signals. It is also possible to use either the + or - input alone to realize nonbalanced signals.

Schematic description

The amplifier, view the circuit diagram below, is in principle a transconductance amplifier followed by a buffer (current amplifier).


The current from the input stage is converted to a voltage by means of R25. The transconductance in the input stage together with R25 determine the voltage gain.

The input stage consists of two JFETs in a conventional differential coupling with the current generator Q10. The transconductance in the input stage is given by the JFET transconductance reduced by the resistors R7,8 and the potentiometer P9. The offset adjustment is also made up by this potentiometer. The resistor R1 and R2 set the input impedance. If one of these inputs is not used, the corresponding input resistor (R1 or R2) must be shorted.

The field effect transistors withstand a maximum of 40 V to operate properly. This limits the maximum supply voltage to about 32 V. The common base transistors Q17/Q18 are bipolar types, and together with the JFET input stage form a folded cascode stage. The folded cascode circuit gives a low distortion voltage amplifier, even without global feedback.

The gain (with balanced operation) is about 34 dB. Changing the value of R25 will change the gain. Leave the value of R76 to be about 10 times higher than R25. The distortion is not dependent on R25 alone; the voltage swing is more important. Generally a higher value of R25 gives more second harmonic distortion and a lower value gives more third harmonic distortion.

The buffer is completely symmetric, the first stage is an usual emitter follower, made by the transistors Q39 and 40. The second stage is a modified Compound emitter follower, made by the transistors Q47,48, 55 and 56, chosen from its good linearity and thermal stability. There is no ordernary bias generator, but the quiescent current is set by means of the resistors R37 and R38 and the current generators made up by the transistors Q33 and Q34 and the zener diodes D26 and 27.

The unregulated power supply to the voltage amplifier is low pass filtered by means of R57 and 58 plus C61 and 62. Together with the decoupling capacitors C59 and 60, the low pass filtering ensures that ripple and noise on the supply voltage not reach the amplifier output.

The used power supply for the current amplifier is common for the two channels. It is however used separate supply for the current and voltage amplifier, view the schematic below.


Since this is a class A amplifier, a common supply is sufficient when the filtering capacitors are large enough. These are Computer Grade type to ensure long lifetime.

The power supply voltage for the voltage amplifier is about 5 V higher than for the current amplifier. Higher power output is thus achieved without higher power dissipation worth mentioning (for an equal value of the power supply voltage to the current amplifier). Using the values in the parts list, the amplifier will perform 30 W RMS into 8 ohms. This demands a quiescent current of 1.4 A. The quiescent current may be increased above this value with the used heat sinks. In the prototype this value is set to 1.6 A, this corresponds to class A working for full output voltage down to a load impedance of 6 ohms. The values of R37 and 38 is used to adjust the quiescent current.

The layout (seen from the component side) is shown as image below.

The component placement is shown as image below.


The printed circuit board measures 139x76 mm. Two boards are needed for a stereo version. The layout is also available as a pdf-file.

The parts list applies for both the circuit board and the power supply.

Mounting description

With the exception of the power resistors, in the prototype it was used 1/2 W metal film oxide resistors with 1 % tolerance. In the prototype the JFETs 2SK170BL are used for J5/J6. The JFET 2SK147GR or 2SK147BL may be used instead. This has higher internal capacitances and transconductance and is slightly more linear. In return the price is higher. In the prototype different transistors were used for Q39, 40, 47 and 48 than those proposed in the parts list. This means that the pinning of these transistors is different from the layout. Please check the pinning of these transistors before mounting them on the board and fastening them on the heat sink. The transistors Q39 and 40 could also be mounted on the output power transistors Q55 and 56 for best thermal tracking (use thermal conducting glue).

The output transistors used for Q55/Q56, are the well-known couple 2SA1216/2SC2922 from Sanken. They are relatively linear, fast and at the same time very rugged, a seldom combination. These transistors have been on the market for quite a long time now and are relatively often used in commercial amplifiers, and they are relatively cheap. They come in a rare plastic housing, something that makes it necessary to mount them directly to the heat sink. In the prototype the printed board is fastened to the heat sink, and the transistor leads are fastened to the board on the solder side.

For the power supply it is used a common copper plate as ground plane. One terminal of each electrolytic capacitor is screwed on this plate (Remember the polarity!). The transformer mid point (normally it is two conductors) is also screwed on this plate. From the plate it is necessary to have a good connection to chassis. The phono socket ground terminal (for unbalanced input) is connected to chassis (near the input). Remember to shorten the input resistor not in use. For balanced input socket the 0V terminal is connected to chassis. The (ground) shield of the signal cable is connected to the circuit board point marked with 'SG' (Signal Ground). The inner conductor(s) of the signal cable is connected to the circuit board point marked with 'IN+' (and/or IN-). The loudspeaker output minus socket is connected to the chassis at the output. From the loudspeaker output the two conductors are twisted and fastened to the circuit board in the two points marked with 'OUT' and 'PG' (Power Ground). The last is connected to the minus conductor. All connections should be as short as possible. If some sort of instability or noise should occur, the probability is high that the reason is bad wiring (e.g. earth loops).

Start-up and adjustment

It is recommended to use a variable transformer or variable DC voltage generator first time the amplifier is started up. When the power supply voltage is increased, adjust the output-offset voltage by means of the potentiometer P9 to be close to 0 V DC (shorten R72 temporary). Also check the quiescent current. If necessary, adjust the values of the resistors R37 and 38. If possible, look at the output with an oscilloscope, there should not be anything but noise here if everything is OK. When the temperature is increasing, it is necessary to re-adjust the offset voltage and monitor the quiescent current (min. 1.40 A). The offset voltage at the output varies, but should not exceed 100 mV (with R72 shortened).

The amplifier is provided with a servo coupling made up by the operational amplifier E74. When adjusting and monitoring the offset voltage, the resistor R72 should be shorted. In normal operation (with R72), there should be an offset voltage not greater than the operational amplifier offset.

For the 30 W prototype about 0.8 V RMS unbalanced input voltage is required for full output power. This should be sufficient for the most modern signal sources. If higher gain is wanted, R25 is increased (and vice versa).

It is two known problems with this amplifier. Firstly the bias setting is rather 'tricky' as resistors are used. It could be an idea to mount these resistors on solder towers to make them easy changeable. The other problem is the stability of the output stage. The capacitors C45 and 46 should cure this problem, but higher values may be used. It is not a good idea to use fast transistors for the drivers Q47 and Q48.

The author has no measured values of bandwidth, distortion etc to present, as these measurements not are retained.

It is possible to convert this amplifier to a normal voltage feedback amplifier by using the inverting input as a feedback input. This calls for a feedback resistor (in parallell with a compensating capacitor). The values of the resistors R7 ,8 and 25 should be adjusted together with the value of C24 to set the open loop gain and amplifier stability. It is left to experienced designers to do this modification.


 
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Knut Harald Nygaard