Mini-MosFet Audio Amplifier

Power Amplifier Circuit diagram:

Power Amplifier Parts:

R1_______________2K2 1/4W Resistor
R2______________27K  1/4W Resistor
R3,R4____________2K2 1/2W Trimmers Cermet or Carbon (or 2K)
R5_____________100R  1/4W Resistor
R6_______________1K  1/4W Resistor
R7,R8__________330R  1/4W Resistors

C1______________22µF  25V Electrolytic Capacitor
C2______________47pF  63V Polystyrene or Ceramic Capacitor
C3,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)

This project was a sort of challenge: designing an audio amplifier capable of delivering a decent output power with a minimum parts count, without sacrificing quality.
The Power Amplifier section employs only three transistors and a handful of resistors and capacitors in a shunt feedback configuration but can deliver more than 18W into 8 Ohm with <0.08% THD @ 1KHz at the onset of clipping (0.04% @ 1W - 1KHz and 0.02% @ 1W - 10KHz) and up to 30W into a 4 Ohm load.
To obtain such a performance and to ensure overall stability of this very simple circuitry, a suitable regulated dc power supply is mandatory. This is not a snag because it also helps in keeping noise and hum of the preamp to very low levels and guarantees a predictable output power into different load impedances. Finally, as the amplifier requires only a single rail supply, a very good dc voltage regulator capable of supplying more than 2 Amps @ 40V can be implemented with a few parts also.

Setting up the Power Amplifier:

The setup of this amplifier must be done carefully and with no haste:

1. Connect the Power Supply Unit (previously tested separately) to the Power Amplifier but not the Preamp: the input of the Power Amplifier must be left open.
2. Rotate the cursor of R4 fully towards Q1 Collector.
3. Set the cursor of R3 to about the middle of its travel.
4. Connect a suitable loudspeaker or a 8 Ohm 20W resistor to the amplifier output.
5. Connect a Multimeter, set to measure about 50V fsd, across the positive end of C5 and the negative ground.
6. Switch on the supply and rotate R3 very slowly in order to read about 23V on the Multimeter display.
7. Switch off the supply, disconnect the Multimeter and reconnect it, set to measure at least 1Amp fsd, in series to the positive supply (the possible use of a second Multimeter in this place will be very welcomed).
8. Switch on the supply and rotate R4 very slowly until a reading of about 120mA is displayed.

Preamp Circuit diagram:

Preamp Parts:

P1______________50K  Log. Potentiometer (or 47K)
                     (twin concentric-spindle dual gang for stereo)
P2,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,R5____________8K2 1/4W Resistors
R6_______________4K7 1/4W Resistor
R7,R8,R13________2K2 1/4W Resistors
R9_______________2M2 1/4W Resistor
R10,R11_________47K  1/4W Resistor
R12_____________33K  1/4W Resistor
R14____________470R  1/4W Resistor
R15_____________10K  1/4W Resistor
R16______________3K3 1/4W Resistor (See Notes)

C1,C2,C9_______470nF  63V Polyester Capacitors
C3,C4___________47nF  63V Polyester Capacitors
C5,C6____________6n8  63V Polyester Capacitors
C7______________10µF  63V Electrolytic Capacitor
C8,C10__________22µF  25V Electrolytic Capacitors
C11____________470µF  25V Electrolytic Capacitor (See Notes)

Q1,Q3_________BC550C  45V 100mA Low noise High gain NPN Transistors
Q2___________2N3819   General-purpose N-Channel FET

1. Check again the voltage at the positive end of C5 and readjust R3 if necessary.
2. If R3 was readjusted, R4 will surely require some readjustment.
3. Wait about 15 minutes, watch if the current is varying and readjust if necessary.
4. Please note that R3 and R4 are very sensitive: very small movements will cause rather high voltage or current variations, so be careful.
5. Those lucky enough to reach an oscilloscope and a 1KHz sine wave generator, can drive the amplifier to the maximum output power and adjust R3 in order to obtain a symmetrical clipping of the sine wave displayed.

The Preamp sensitivity and overload margin were designed to cope with most modern music programme sources like CD players, Tape recorders, iPods, Computer audio outputs, Tuners etc. The source selecting switches and input connectors are not shown and their number and arrangement are left to the constructor's choice.
To obtain a very high input overload margin, the volume control was placed at the preamp input. After a unity gain, impedance converter stage (Q1) a negative-feedback Baxandall-type Bass and Treble tone control stage was added. As this stage must provide some gain (about 5.6 times) a very low noise, "bootstrapped" two-transistors circuitry with FET-input was implemented. This stage features also excellent THD figures up to 4V RMS output and a low output impedance, necessary to drive properly the Mini-MosFet Power Amplifier, but can also be used for other purposes.

Regulated Power Supply Circuit diagram:

Regulated Power Supply Parts:

R1_______________3R9 1 or 2W Resistor
R2______________22R  1/4W Resistor
R3_______________6K8 1/4W Resistor
R4_____________220R  1/4W Resistor
R5_______________4K7 1/2W Resistor

C1____________3300µF  50V Electrolytic Capacitor (or 4700µF 50V)
C2,C5__________100nF  63V Polyester Capacitors
C3______________10µF  63V Electrolytic Capacitor
C4_____________220µF  50V Electrolytic Capacitor

D1_____Diode bridge  100V 4A 
D2___________1N4002  200V 1A Diode
D3______________LED  Any type and color

IC1___________LM317T 3-Terminal Adjustable Regulator

Q1____________TIP42A  60V 6A PNP Transistor

SW2_____________SPST  Mains switch

T1_____________230V Primary, 35-36V (Center-tapped) Secondary,
                    50-75VA Mains transformer (See Notes)

PL1____________Male Mains plug with cord

---

Comments:

A very good and powerful Regulated Power Supply section was implemented by simply adding a PNP power transistor to the excellent LM317T adjustable regulator chip. In this way this circuit was able to deliver much more than the power required to drive two Mini-MosFet amplifiers to full output (at least 2Amp @ 40V into 4 Ohm load) without any appreciable effort.

Notes:

• Q2 and Q3 in the Power Amplifier must be mounted each on a finned heatsink of at least 80x40x25mm.
• Q1 and IC1 in the Regulated Power Supply must be mounted on a finned heatsink of at least 45x40x17mm.
• A power Transformer having a secondary winding rated at 35 - 36V and 50VA (i.e. about 1.4Amp) is required if you intend to use Loudspeaker cabinets of 8 Ohm nominal impedance. To drive 4 Ohm loads at high power levels, a 70 - 75VA Transformer (2Amp at least) will be a better choice. These transformers are usually center tapped: the central lead will be obviously left open.
• For the stereo version of this project, R16 and C11 in the Preamp will be in common to both channels: therefore, only one item each is necessary. In this case, R16 must be a 1K5 1/2W resistor. The value of C11 will remain unchanged.

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Circuit Electronics Car

Travel Touch Alarm

The Travel Touch Alarm can be used to provide a audible alarm when someone touches the door knob or handle of your hotel room. The door knob or handle must be made of metal for the circuit to work. The main chip in the circuit is a 555 timer which will be triggered if a hand comes close to or touches the door knob. The circuit attaches to the door knob at the end of the 1 meg ohm resistor. Once the timer is triggered the LED will light and the UJT will output a tone to the speaker. The timer will time out in 5 seconds. The sensitivity of the trigger can be changed by changing the 1 meg ohm resistor to another value. The 5 second time out can be adjusted by changing the value of the resistor connected between pin 8 and pin 7. The output tone can be changed by changing the RC values on the base of the UJT.

Sun - Up Alarm

The Sun - Up Alarm can be used to provide a audible alarm for when the sun comes up or it can be used in a dark area and detect when a light comes on. It can also be used to detect a light beam, headlights etc. The circuit works as follows. The phototransistor is very sensitive to light. (Any phototransistor will work fine) The sun shining on this device will provide a high to one of the NAND gates. This will cause another NAND gate to oscillate which will drive another gate to output a 100hz tone. The transistor provides drive for the speaker.

Stereo Mixer

With this circuit you can mix four separate audio inputs. Each input will accept high or low impedance microphones, phonograph, tape or aux signals. You can adjust the gain of each input by adjusting each respective pot. With the Stereo/Mono switch you can combine just inputs 1,2 and 3,4 or in the Mono position combine all four inputs. Signal response is 20 Hz. to 20kHz. with very high input impedance and very low distortion with complete output short circuit protection. This circuit will drive any power amp with an input impedance of 2,000 ohms.

Variable DC Power Supply

This power supply is based on the LM317 Variable Regulator. The input of the regulator needs to be around 28 Volts DC and it will output a DC voltage from 1.25vdc to 25 vdc. To adjust the output voltage simply turn the 5k ohm pot. The regulator will supply 1.5 amps of current.

Bike Light Flasher

I ride my bike allot at night and sometimes I'm not sure if people can see me. This circuit will flash an incandescent light that you can purchase from Radio Shack. Adjust the VR's for your flash requirements and make sure that the lamp you use is designed to put out high brightness. The enclosure should be water tight. The Batteries can be replaced by a bike wheel generator and a diode. If you add the generator then you can still use rechargeable batteries and the batteries will charge while you are riding your bike.

Audio Auto Shutoff

Circuit electronic car

I have had many requests to design or provide circuits for graduation projects. Yes, I had to build a graduation project too before receiving my degree. Even though I have always refused to design circuits for this situation, I thought it might be helpful for you to see what I built for my project many years ago when I graduated in 1978. I will also provide a more detailed description of the circuit than I normally do. Hopefully, this may help some of you with your projects.

The purpose of this circuit is to automatically turn off any device plugged into its power outlets after a certain period of time. Shutoff is activated by an absences of an audio signal or by a standard timer function. This unit would be connected to speaker terminals of a stereo system, tape outputs, TV earphone jack or audio outputs etc. Once the audio signal drops below a predetermined level a timer function will turn off any device powered by the outlets on the back of this unit. For example, if you like to fall asleep listening to your CD player. This unit could be hooked up to your speaker terminals. After the CD is done playing the audio signal would drop below the threshold value and the unit would turn off your stereo system after a certain timer interval had passed and therefore you would not have to worry about falling asleep and having your stereo being on all night. This controller can also be used as a standard timer control without the audio input and turn devices off after a preset time interval. The outlets can handle loads of up to 1200 watts and the unit is fused with a 10 amp fuse for protection.

This is how the unit works. The audio signal goes into IC1a for amplification and filtering with a roll off occurring about 1.25khz at -6db per octave. The second stage, IC1b is a 2 pole filter with a cutoff frequency is approximately 1 khz with unity gain. The 2 stages have a combined roll off of 18db per octave to remove any noise and filter out any high frequency hiss if a radio station of TV station goes off the air. This is helpful if the audio source is either FM or TV. The filtered DC level of D1, D2 and C6 with R10 "bleeding" the charge from the capacitor C6 when the signal is not present. IC1c is used as a comparator having fast "snap action" (positive feedback) so that when the rectified signal applied to the non-inverting (+) input exceeds the level set by the sensitivity control R11, the output switches off very rapidly. When IC1c output is high (audio signal is present), LED1 is turned on and current limited by R15. IC1c output signal turns on the OR gate formed by IC1d which in turn, causes Q4 to saturate and draw current through the coil of reed relay, Relay1. With the reed relay contacts closed, gate power is applied to the triac, and power is present across the multiple AC outlets. This turns on any device connected to these sockets. When the audio signal either disappears or falls below the pre-set sensitivity threshold, comparator IC1c switches off rapidly. This action also starts one of the timers in IC2 whose output pin (pin5) keeps the OR gate operating until the timer times out. Power remains on the AC outlets. If another audio signal should appear within the time-out interval, the second timer within IC2 will generate a 5 millisecond pulse which will turn on Q3 and discharge the main timing capacitor. This resets IC2 back to zero and ensures that the last audio signal is always the one that begins the time out delay. Transistors Q1 and Q2 act as a "quench" circuit by grounding the comparator signal an instant before shutdown. This is necessary because some audio power amplifiers generate a "thump" when turned off and this may retrigger the timer and never allow the system to shutdown. Reed relay Relay1 is necessary for complete isolation between the circuit and the triac. The triac should be heat sinked. The timer function is determined by the setting of normal/timer switch S3, which disables the input circuit by turning on the "quench" transistor, Q1 and connecting the a larger capacitor in parallel with the main timing capacitor. Potentiometer R17 sets the timer delay in either case, although the range for normal and timer positions of S3 is different. To operate the unit set S3 in the normal position and both pots in the mid position and then press the rest button S1. LED1 should turn on ad any device plugged into the AC outlets should turn on. With no audio signal present the LED will go out and the unit will turn off any devices plugged into after the timer expires. The time length is set by R17. Connect an audio source to the unit and adjust the sensitivity control R11 until the LED remains on continuously. The time delay for normal is maxed out at around 20 minutes and in the timer mode it is maxed out at 2 hours.

Auto Burglar Alarm

This alarm circuit is based on two 555 timers. The alarm will sound your car horn if anyone opens the car door while the circuit is armed. The timers will allow you to leave the car without sounding the horn. To turn the circuit on S1 must be closed. To set the alarm, open S2 ( it is normally closed ) this will give you about 5 seconds to get out and close the door. The exit delay time is set by R1 and C1. If anyone opens the doors for more then two seconds the horn will sound until power is removed from the circuit. The 2 second time is set by R2 and C2. If you open the door, you must deactivate the alarm by closing S2. This very basic circuit could be used for a home also.