Creating an Audio Power Amplifier that can deliver impressive output power while maintaining simplicity in component count and preserving audio quality posed a challenge for this project. The Audio Power Amplifier section of this design relies on a minimalistic setup comprising just three transistors, along with a handful of resistors and capacitors, arranged in a shunt feedback configuration. This configuration is capable of producing over 18 watts into an 8 Ohm load with a low Total Harmonic Distortion (THD) of 0.08 percent at 1KHz, near the onset of clipping (0.04% at 1W – 1KHz, and 0.02 percent at 1W – 10KHz). Furthermore, it can deliver up to 30 watts into a 4 Ohm load.
To achieve such performance and ensure the overall stability of this remarkably simple circuitry, it is crucial to have a well-regulated DC power source. Fortunately, this requirement does not pose a challenge, as it also contributes to minimizing noise and hum in the preamp section while guaranteeing consistent output power across various load impedances. Additionally, since the amplifier only necessitates a single rail supply, it is feasible to construct a high-quality DC voltage regulator capable of delivering more than 2 Amps at 40V using just a handful of components.
Audio Power Amplifier Parts:
R1 = 2K2 1/4W Resistor
R2 = 27K 1/4W Resistor
R3 = 2K2 1/2W Trimmers Cermet
R4 = 2K2 1/2W Trimmers Cermet
R5 = 100R 1/4W Resistor
R6 = 1K 1/4W Resistor
R7 = 330R 1/4W Resistors
R8 = 330R 1/4W Resistors
C1 = 22µF 25V Electrolytic Capacitor
C2 = 47pF 63V Polystyrene or Ceramic Capacitor
C3 = 100µF 50V Electrolytic Capacitors
C4 = 100µF 50V Electrolytic Capacitors
C5 = 2200µF 50V Electrolytic Capacitor
Q1 = BC550C 45V 100mA Low noise High gain NPN Transistor
Q2 = IRF530 100V 14A N-Channel Hexfet Transistor (or MTP12N10)
Q3 = IRF9530 100V 12A P-Channel Hexfet Transistor (or MTP12P10)
Setting up the Audio Power Amplifier:
- This amplifier’s setup must be done properly and without haste:
- Connect the Power Supply Unit (separately tested) to the Power Amplifier but not the Preamp: the Power Amplifier’s input must be left open.
- R4’s cursor should be fully rotated towards the Q1 Collector.
- R3’s cursor should be in the middle of its travel.
- Connect the amplifier output to a suitable loudspeaker or an 8 Ohm 20W resistor.
- Connect a multimeter to the positive end of C5 and the negative ground, calibrated to measure roughly 50V fsd.
- Turn on the power and carefully spin R3 until the Multimeter display reads around 23V.
- Turn off the power, unplug the Multimeter, and rejoin it in series with the positive supply, set to measure at least 1Amp fsd (the possible use of a second Multimeter in this place will be very welcomed).
- Turn on the supply and slowly rotate R4 until a reading of roughly 120mA appears.
- Check the voltage at the positive end of C5 once more, and if required, adjust R3.
- If R3 was readjusted, R4 will very certainly need to be readjusted as well.
- Wait around 15 minutes, then check to see whether the current is changing and make any required adjustments.
- Please keep in mind that R3 and R4 are quite sensitive: even minor movements can result in significant voltage or current fluctuations, so proceed with caution.
- Those with access to an oscilloscope and a 1KHz sine wave generator can set R3 to achieve symmetrical clipping of the sine wave exhibited by driving the amplifier to maximum output power.
Preamplifier Section:
The preamp’s sensitivity and overload tolerance have been engineered to accommodate the majority of modern music sources, including CD players, tape recorders, iPods, computer audio outputs, tuners, and similar devices. The selection and arrangement of source switches and input connectors have been left to the discretion of the builder. The volume control has been strategically placed near the preamp input to ensure a substantial input overload margin.
Following a unity gain impedance converter stage (Q1), a negative-feedback Baxandall-type Bass and Treble tone control stage has been introduced. This stage necessitates some gain (approximately 5.6 times) with minimal noise, and it utilizes a “bootstrapped” two-transistor circuit with a FET-input configuration. This stage also boasts impressive Total Harmonic Distortion (THD) figures, delivering clean performance up to 4V RMS output, and it maintains a low output impedance, vital for effectively driving the Mini-Mosfet Power Amplifier. However, it can also be employed for other purposes.
Preamplifier Parts:
P1 = 50K – Log. Potentiometer
P2 = 100K – Linear Potentiometers
P3 = 100K – Linear Potentiometers
(twin concentric-spindle dual gang for stereo)
R1 = 220K – 1/4W Resistor
R2 = 100K – 1/4W Resistor
R3 = 2K7 – 1/4W Resistor
R4 = 8K2 – 1/4W Resistors
R5 = 8K2 – 1/4W Resistors
R6 = 4K7 – 1/4W Resistor
R7 = 2K2 – 1/4W Resistors
R8 = 2K2 – 1/4W Resistors
R9 = 2M2 – 1/4W Resistor
R10 = 47K – 1/4W Resistor
R11 = 47K – 1/4W Resistor
R12 = 33K – 1/4W Resistor
R13 = 2K2 – 1/4W Resistors
R14 = 470R – 1/4W Resistor
R15 = 10K – 1/4W Resistor
R16 = 3K3 – 1/4W Resistor (See Notes)
C1 = 470nF – 63V Polyester Capacitors
C2 = 470nF – 63V Polyester Capacitors
C3 = 47nF – 63V Polyester Capacitors
C4 = 47nF – 63V Polyester Capacitors
C5 = 6n8 – 63V Polyester Capacitors
C6 = 6n8 – 63V Polyester Capacitors
C7 = 10µF – 63V Electrolytic Capacitor
C8 = 22µF – 25V Electrolytic Capacitors
C9 = 470nF – 63V Polyester Capacitors
C10 = 22µF – 25V Electrolytic Capacitors
C11 = 470µF – 25V Electrolytic Capacitor (See Notes)
Q1 = BC550C – 45V 100mA Low noise High gain NPN Transistors
Q2 = 2N3819 – General-purpose N-Channel FET
Q3 = BC550C – 45V 100mA Low noise High gain NPN Transistors
Power Supply Section:
By simply adding a PNP power transistor to the outstanding LM317T adjustable regulator chip, a very good and powerful Regulated Power Supply section was created. This circuit was able to deliver far more power than was required to drive two Mini-MosFet amplifiers to full output (at least 2Amp @ 40V into a 4 Ohm load) with little effort.
Power Supply Parts:
R1 = 3R9 – 2W Resistor
R2 = 22R – 1/4W Resistor
R3 = 6K8 – 1/4W Resistor
R4 = 220R – 1/4W Resistor
R5 = 4K7 – 1/2W Resistor
C1 = 4700µF – 50V Electrolytic Capacitor
C2 = 100nF – 63V Polyester Capacitors
C3 = 10µF – 63V Electrolytic Capacitor
C4 = 220µF – 50V Electrolytic Capacitor
C5 = 100nF – 63V Polyester Capacitors
D1 = Diode bridge – 100V 4A
D2 = 1N4002 – 200V 1A Diode
D3 = LED – Any type and color
SW2 = SPST – Mains switch
IC1 = LM317T – 3-Terminal Adjustable Regulator
PL1 = Male Mains plug with cord
Q1 = TIP42A – 60V 6A PNP Transistor
T1 = 230V Primary, 35-36V (Center-tapped) Secondary,
50-75VA Mains transformer (See Notes)
Notes:
- The Power Amplifier’s Q2 and Q3 must each be installed atop a finned heatsink of at least 80x40x25mm.
- In the Regulated Power Supply, Q1 and IC1 must be installed atop a finned heatsink with dimensions of at least 45x40x17mm.
- If you want to use Loudspeaker cabinets with an 8 Ohm nominal impedance, you’ll need a power transformer with a secondary winding rated at 35-36V and 50VA (i.e. roughly 1.4Amp). A 70-75VA Transformer (2Amp at least) is a superior alternative for driving 4 Ohm loads at high power levels. The central lead on these transformers is normally left open because they are center tapped.
- R16 and C11 in the Preamp will be shared by both channels in the stereo version of this project, therefore just one component is required for each channel. R16 must be a 1K5 1/2W resistor in this scenario. C11’s value will remain unchanged.
Technical data:
Output power:
- 18 Watt RMS into 8 Ohm (1KHz sine wave) – 30 Watt RMS into 4 Ohm
Input sensitivity of the complete Amplifier:
- 160mV RMS for full output
Audio Power Amplifier Input sensitivity:
- 900mV RMS for full output
- Power Amplifier Frequency response @ 1W RMS:
- flat from 40Hz to 20KHz, -0.7dB @ 30Hz, -1.7dB @ 20Hz
- Power Amplifier Total harmonic distortion @ 1KHz:
- 100mW 0.04% 1W 0.04% 10W 0.06% 18W 0.08%
- Power Amplifier Total harmonic distortion @10KHz:
- 100mW 0.02% 1W 0.02% 10W 0.05% 18W 0.12%
- Unconditionally stable on capacitive loads
- Preamp Maximum output voltage:
- 4V RMS
- Preamp Frequency response:
- flat from 20Hz to 20KHz
- Preamp Total harmonic distortion @ 1KHz:
- 1V RMS 0.007% 3V RMS 0.035%
- Preamp Total harmonic distortion @10KHz:
- 1V RMS 0.007% 3V RMS 0.02%
- Bass control frequency range referred to 1KHz:
- ±20dB @ 40Hz
Treble control frequency range referred to 1KHz:
- +18dB/-20dB @ 20KHz
