Bomb Proof 150 Watt MOSFET Power Amplifier
Posted on Apr 15, 2012 5177
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Used in a power amplifier this self regulating effect can be desirable as it offers a high degree of immunity from misuse or abuse according to your musical taste perhaps! So this amplifier use these particular qualities of the MOSFET which allow for much simpler design to be produced, as complex biasing and quiescent current compensation arrangements are not required,which is usually the case with bi-polar transistors. Also the Total Harmonic Destortion (T.H.D.) of the MOSFET is very low for output levels up to 50 watts. At higher frequencies and power levels approaching 100 watts the T.H.D. levels increase, although the levels remain fairly insignificant in practical terms. Circuit description: In the circuit diagram TR1 and TR2 are configured as the well known differential pair front end for the specified power supply, each transistor is biased at 500uA. 2SA872A devices are used here because of their very low noise and high voltage specifications. The circuit around TR5 and D1 form the constant current load for voltage amplifier TR4. Potentiometer RV1, positioned between TR5 and TR4 collector, is recuired for setting the quiescent current in the MOSFET output stage. At the fully clockwise position RV1 becomes a short circuit and the gates of MOSFETs TR6 and TR7 are effectively at 0V. By turning RV1 anti-clockwise a voltage potential is developed between each MOSFET Gate and both devices will start to conduct. This type of output stage is commonly known as class B as the MOSFETs will not generate any output current until AC signals are present. The AC open loopgain of the amplifier is very high, therefore it becomes necessary to introduce a feedback loop in the form of R6,R7 and C3. These two resistors set the AC gain at 33 (times) and C3 provides DC blocking to prevent symmetry and biasing problems at the output. The Amplifier gain naturally determines the input signal level requirements and for example, with an RMS input signal of 0.86V the output signal will be 0.86x33=28.38V (80.2V peak to peak). When the amplifier output is driving into an 8 Ohm load the power available will be (28.38x28.38) /8= 100.7 Watts (RMS). The amplifier frequency responce is determined at the LF end, by capacitors C1 and C3 and rolls off at 20 Hz. At the HF and C2,C5 and C6 begin to roll off at 40Khz but due to the simple circuit design, waveform distortion becomes more apparent here. Resistors R13 and R14 introduce gate resistance to the MOSFET to helpprevent HF oscillation - which MOSFETs are very prone to! C7 and R15 form a Zobel network which, together with L1, ensures the best quality and stability when driving into reactive loads at higher frequencies. Construction: Construction is quite straight forward but attention must be paid to MOSFET and bracket mounting and also to inductor L1. Do not forget to place a bracket for heatsink and a good heatsink (or smaller with small fun)There are instuctive images in this page that can help you out. No big deal. Power Supply: A split rail supply is required here for powering the amplifier and you will probably find that this item costs more than the amplifier itself! The suggested circuit of the PSU is suitable for most requirements. Toroidal transformer T1 is an audio grade device designed for use with this amplifier, and has two secondary windings fitted: 39V-0-39V for powering the amplifier an 12-0-12V for additional preamplifier stages if required or for heatsink fan etc. Once powered up the PSU will develop 55V-58V DC on each supply rail and this is the ABSOLUTE maximun allowed for the MOSFET we use here. Whatever type of PSU is to be used DO NOT exceed the maximum voltage given as the risk of component breakdown will be increased. MOSFETS are capable of switching heavy currents very fast and this can lead to stability problems. Supply rails must offer both low impedance and low inductunce, therefore all wiring between PSU and Amplifier must be kept as sort as possible ( wire lengths up to 300mm normally present little problem). Use heavy duty wire of 30A rating, preferably with a solid conductor, and cooker cable is ideal for this job. The copper PCB tracks themselves can also offer a high inductance and this can be reduced by making each -/+V supply connection direct onto the MOSFET Drain terminal instead of pins 2 and 5. There have always been arguments for and against fitting fuses in the supply rails. Much depends on the quality, type and reasons for using them. Anti surge fuses are prone to "pop out". It is good idea to fit lower value (0.6A) fuses when powering up a newly built amplifier for the first time- just in case! The transformer primary should also be fused to protect the mains connecting cable. It is important to use Anti-Surge fuses (3A) as torroidal transformers draw a very high surge current at switch-on. Runing Hot 'n' Cold: "What type of heatsink should be used with this MOSFET amplifier?" This is a simple question, often asked, which does not have a simple answer; as this totally depends on the application and environment where the amplifier is to be used. For maximum continuous power availability, a MOSFET must run cool and the largest possible heatsink should be used.Of course the size of the case is important. Use big one and place a quiet computer fan on it and you are done. Setting Up: Fit low current fuses in each rail prior to powering up and do not connect a loudspeaker yet. Connect a multimeter, set to read current in series with the +V supply rail, ensuring the current range selected is at least that of the fuse fitted, and turn on the supply. Assuming that all is well thus far, select a lower current range on the meter and adjust RV1 for a quiescent current reading of 100mA DC. The actual value of 100mA is not critical on this amplifier and 80mA or 120mA is fine. Turm off the power and after allowing a minute or so for the PSU electrolytics to discharge (CHECK!) remove the meter and reconnect the +V supply rail to the amplifier. Re-connect the meter, set to read 50Volts DC or more. Between the speaker output pin1 and 0V. Power up again and there will probably be a small DC offset voltage of -/+20 to 50mV here. If 0.5V or more is present then switch off and look for an assebly fault or check if one of the test fuses have blown. If any component appears to be getting hot or the quiescent current reading cannot be lowered then it is possible that the amplifier is oscillating. This effect usually manifests itself by R15 'cooking' due to the Zobel circuit absorbing generated RF energy. Re-check the PSU wiring and also capacitors C4 and C6 as they are easily broken during installation. Power down and install the 3A fuses for normal use. In Use: Always take 0V connections from one place on a PSU including the speaker 0V return. This is often referred to as "star earthing" and ensures that eddy currents do not flow between different 0V points thus reducing hum and noise. The 0V point at which capacitors C101 and C102 join together, is the best place to take connections from. When choosing a loudspeaker, ensure that it is capable of handling continuous power levels of 100 watts RMS or more for 8 ohm versions and 150-200 watts for 4 ohm versions. Multi-banked speaker assemblies should not total less than 4 ohms and must be connected together in parallel/serial combinations to achive this. Remember that many speaker impedance figures are nominal and will vary according to frequency and applied power levels. Over-driving the amplifier by aplying input signals greater than 0.86V will cause the output waveform to 'clip' or square off. Square waveforms are rich in harmonics and can cause excessive excursions of a loudspeaker cone or more commonly, tottaly destroy a tweeter. Guitarists may like the sound produced, but speakers do not! When using any amplifier at high power levels due consideration must be given to the application. Bass guitars and miked-up drum kits, for example, generate huge transients as strings are plucked or heads are struck. For these applications, devices such as compressors or limiters have to be introduced at the amplifier input, to protect speakers as well as the amplifier. That MOSFET amplifier is robust and fairly "bomb proof", but not invulnerable. If even greater power outputs are required then two amplifiers can be bridged together in preference to driving one unit to its maximum. Finally a word about earth loops. A correctly working MOSFET and PSU will not produce audible 50/100Hz hum by itself, but unscreened low level signal wires can introduce this. If the PSU 0V rail is connected to mains earth and other equipment connected to the mains earth have their inputs and outputs connected via screened wire to the amplifier, then a loop exists between the screen and earth wires. The mains supply generates an electromagnetic field that is very easily induced into the earth loop and hence the 0V rail will be carrying 50Hz signals, which are then amplified and heard at the loudspeaker. One method of reducing this common problem is to terminatethe signal wire screen at one end of the wire only. The other end of the screen is then left unterminated. Another way is to connect mains earth to the amplifier case (containing amplifier and PSU). But not to connect 0V to the case. On no account should any earth be removed from equipment connected to the mains. unless it has been designed for this purpose or double insulated.
file1: Click here to download 1N4001.pdf