Tube Li Amplifier

The unit is powered directly from the 120 volt AC line, with no power transformers. Filaments are wired in series, with the total adding up to 117 volts (35 + 35 + 35 + 12). The 35W4 forms a half-wave rectifier, which is filtered by a three-stage RC network. The B+ for the output stage plates and screens are taken from the second capacitor, and the B+ for the preamp and phase inverter from the third capacitor in the filter. 


The input signal to the amplifier is applied directly to the volume control pot, from whence it passes through a variable high-pass filter (the "Treble" control). When the wiper is set to minimum, response is approximately flat (though actual frequency response will depend somewhat on volume control setting). When it's turned to maximum, higher frequencies are favored, with the lower 3 dB corner at around 1500 Hz. and the higher pole (plateau) around 4000 Hz. For testing purposes, the volume control was set to maximum and the treble control to minimum, to minimise the effect of this control. 

click on this image to enlarge

The output of the volume/treble control circuit is applied directly to the grid of the first section of a 12AX7 twin triode. A partially bypassed cathode resistor supplies grid bias, while providing a modest amount of local negative feedback to help linearise the stage's response. A 100k resistor provides the plate load for the preamp stage. The cathode also has a 10k resistor to the "Bass" control connected to it; we'll discuss the function of these components a little later, as they are part of the global feedback network. 

Read rest of this entry.

Download ful Report

Read more »

High 800Watts Amplifier-using MOSFET

The 800 Watt AV amplifier is based on My 1kw Amplifier and shares the same topology and basic PCB layout. The only real difference is the number of Output devices that the unit uses. The 1kw design has 20 O/P devices, while the AV amplifier has 14 O/P devices. This amplifier can be used for practically any application that requires High power, low noise, distortion and excellent sound. Examples would be Sub-woofer amp, FOH stage amplifier, One channel of a very high-powered surround sound amplifier etc. The AV amplifier has four main stages of amplification. We will begin by looking at each stage in reasonable detail.

click on image to enlarge

                                                                                                         circuit of 800w MOSFET amplifier

The Error Amp Stage

The first stage is what I call an asymmetrical balance input error amplifier. It is a design, which allows only one single differential stage and yet has the ability to accept a balanced I/P source. An unbalanced source can be used if either the inverting or noninverting I/P is tied to signal ground.

Now I will explain how each device in this stage works together. Q20, Q21, R51- R54, form the main differential error amplifier, which then has its collectors connected to a cascode load. Q18, Q19, R49 and ZD2 form the cascode stage which provides a constant 14.4 volts on the collectors of Q20, 21. Q17, R48, R50, ZD1 and C12 form a constant current source, which supplies 1.5milliamps to the first differential stage. These modules form the first stage of the amplifier and basically set up how the whole amplifier is biased from front to back.

 Read rest of this entry..

Download full Report

Read more »

Phone Amplifier

While talking to a distant sub- scriber on telephone, quite often we feel frustrated when the voice of the distant subscriber is so faint that it is barely intelligible. 

click to enlarge

To overcome the problem, circuit of an inexpensive amplifier is presented here. It can be assembled and tested easily. There is no extra power source needed to power up the circuit, as it draws power from the telephone line itself. The amplifier will provide fairly good volume for the telephone conversation to be properly heard in a living room. A volume control is included to adjust the volume as desired. The circuit is built around IC LM386. Diodes D6 and D7 are used to limit the input signal strength. Transformer X1 is a transistor radio’s output transformer used in reverse. As original secondary (output) winding is connected in series with the telephone lines, the speech signals passing through the lines cause change in the magnetic flux in the core of transformer and thereby induce signal voltage across the primary winding. This audio signal is used as input for IC LM386. Diodes D2 through D5 connected in bridge configuration constitute a polarity guard so that the amplifier is powered with correct polarity, irrespective of the line polarity, Zener diode D1 may have any breakdown voltage between 6 and 12 volts range. There is no need of a separate power switch as the circuit energises (via the normally open contacts of the cradle switch) when one lifts the handset. The circuit may be wired on a general-purpose PCB or by etching a PCB for this circuit. The circuit can be easily tested by connecting a 6 volts supply to line terminals 1 and 2. A hissing sound will be heard from the loudspeaker. Now connect 6V AC from a transformer to terminals 1 and 2 and observe hum in the loudspeaker. The volume of the hum can be changed through potentiometer VR1. Diodes D6 and D7 limit the input below ± 700 mV. The circuit is to be connected to the telephone lines in series with the telephone instrument, as shown in the figure.

Read more »

Audio amplifier output relay delay

Component list

C1    100 uF 40V electrolytic

C2    100 uF 40V electrolytic

D1    1N4007

D2    1N4148

Q1    BC547

R1    33 kohm  0.25W

R2    2.2 kohm 0.25W

RELAY 24V DC relay, coil resistance >300 ohm

click to enlarge

Circuit operation

Then power is applied to the power input of the circuit, the positive phase of AC voltage charges C1. Then C2 starts to charge slowly through R1. When the voltage in C2 rises, the emitter output voltage of Q1 rises tigether with voltage on C2. When the output voltage of Q2 is high enough (typically around 16..20V) the relay goes to on state and the relay witches connect the speakers to the amplifier output. It takes typically around 5 seconds after power up until the relay starts to condict (at absolute time depends on the size of C2, relay voltage and circuit input voltage).

When the power is switched off, C1 will loose it's energu quite quicly. Also C2 will be charged quite quicly through R2. In less than 0.5 seconds the speakers are disconnected from the amplifier output.

Notes on the circuit

This circuit is not the most accurate and elegant design, but it has worked nicely in my small homebuilt PA amplifier. This circuit can be also used in many other applications where a turn on delay of few seconds is needed. The delay time can be increased by using bigger C2 and decreased by using a smaller C2 value. Note that the delay is not very accurate because of simplicity of this circuit and large tolerance of typical electrolytic capacitors (can be -20%..+50% in some capcitors).

Read more »

Car Stereo Player Circuit

Using a mobile phone while driving is dangerous. It is also against the law. However, you can use your mobile phone as a  powerful music player with the help of a stereo power amplifier. This does away with the need of a sophisticated in-dash car music system. 

click to enlarge

 Most mobile phones have a music player that offers a number of features including preset/ manual sound equalisers . They have standard 3.5mm stereo sockets that allow music to be played through standard stereo headphones/ sound amplifiers. Nokia 2700 classic is an example. A car audio amplifier with 3.5mm socket can be designed and simply connected to the mobile phone output via a shielded cable with suitable connectors/ jacks (readymade 3.5mm male-tomale connector cable is a good alternative).

 Fig. 1 shows the circuit of car stereo player. It is built around popular single-chip audio power amplifier TDA1554Q (IC1). The TDA1554Q is an integrated class-B power amplifier in a 17-lead single-in-line (SIL) plastic power package. IC TDA1554Q contains four 11W identical amplifiers with differential input stages (two inverting and two non-inverting) and can be used for single-ended or bridge applications. The gain of each amplifier is fixed at 20 dB. Here it is configured as two 22W stereo bridge amplifiers.

 The amplifier is powered from the 12V car battery through RCA socket J2. Diode D1 protects against wrongpolarity connection. LED1 indicates the power status. Connect stereo sound signal from the 3.5mm headset socket of the mobile phone to audio input socket J1. When you play the music from your mobile, IC1 amplifies the input.

 The output of IC1 is fed to speakers LS1 and LS2 fitted at a suitable place in your car. Electrolytic capacitor C5 connected between pin 4 of IC1 and GND improves the supply-voltage ripple rejection. Components R2 and C4 connected at mute/standby pin (pin 14) of IC1 eliminate the switch on/off plop. The circuit is quite compact. A good-quality heat-sink assembly is crucial for IC1. Fig. 2 shows the stereo socket and stereo jack.

Assemble the circuit on a generalpurposePCB and enclose in a suitable cabinet. Small dimensions of the power amplifier make it suitable for being enclosed in a plastic (ABS) case with vent holes. Signal input socket, speaker output terminals, on/off switch, indicator, fuse holder and power supply socket are best located on the front panel of the enclosure as shown in Fig. 3.

Read more »

Pre -amplifier for FM Radio

FM transmissions can be received within a range of 40 km. If you are in fringe areas, you may get a very weak signal. FM DXing refers to hearing distant stations (1500 km or more) on the FM band (88-108 MHz). The term ‘DX’ is borrowed from amateur radio operators. It means ‘distance unknown’; ‘D’ stands for ‘distance’ and ‘X’ stands for ‘unknown.’ For an FM receiver lacking gain, or having a poor signal-to-noise ratio, using an external preamplifier improves the signal level.

click to enlarge

  The dual-gate MOSFET preamplifier circuit shown in Fig. 1 gives an excellent gain of about 18 dB. It costs less and is simple to design. Field-effect transistors (FETs) are superior to bipolar transistors in many applications as these have a much higher gain approaching that of a vacuum tube. These are classified into junction FETs and MOSFETs. On comparing the FETs with a vacuum tube, the gate implies the grid, the source implies the cathode, and the drain implies the plate. In a transistor, the base implies the grid, the emitter implies the source, and the collector implies the drain. In dual-gate FETs, gate 1 is the signal gate and gate 2 is the control gate.

 The gates are effectively in series, making it easy to control the dynamic range of the device by varying the bias on gate 2. The MOSFET is more flexible because it can be controlled by a positive or negative voltage at gate 2. The resistance between the gate and rest of the device is extremely high because these are separated by a thin dielectric layer. Thus the MOSFET has an extremely high input impedance.

  Dual-gate MOSFETs (DG MOSFETs) are very popular among radio amateurs. These are being used in IF amplifiers, mixers, and preamplifiers in HF-VHF transceivers. The isolation between the gates (G1 and G2) is relatively high in mixer applications. This reduces oscillator pulling and radiation.The oscillator pulling is troublesome particularly in shortwave communications. It is a characteristic in many unsophisticated frequency-changer stages, where the incoming signal, if large, pulls the oscillator frequency slightly off the frequency set by the tuning knob and towards a frequency favourable to the (large) incoming signal. A DG MOSFET can also be used for automatic gain control in RF amplifiers.

 DG MOSFET BF966S is an n-channel depletion-type MOSFET that is used for general-purpose FM and VHF applications. In this configuration, it is used for FM radio band. The quadratic input characteristic of the FET input stage gives better results than the exponential characteristic of a bipolar transistor. Gate 1 is meant for input and gate 2 is for gain control. The input from the antenna is fed to gate G1 via C1 and L1. Trimmer VC1 is used to tune and select the input frequencies. Capacitor C4 (100 kpF) at the gain control electrode (gate 2) decouples any variation in G2 voltage at radio frequencies to maintain constant gain. Set preset VR (47k) to adjust the gain or connect a fixed resistor for fixed gain. The output of the circuit is obtained via capacitor C5 and fed to the FM receiver amplifier.

  For indoor use, connect a ¼- wavelength whip antenna, ½-wavelength 1.5m wire antenna, or any other indoor antenna set-up with this circuit. You may use a 9V battery without the transformer and diode 1N4007, or any 6V-12V power supply to power the circuit (refer Fig. 1). The RF output can be taken directly through capacitor C5. For an improved input and output impedance, change C1 from 1 kpF to 22 pF and C5 from 1 kpF to 100 kpF.

 For outdoor use at top mast, like a TV booster, connect the C5 output to the power supply unit (PSU) line. Use RG58U/ RG11 or RG174 cable for feeding the power supply to the receiver amplifier. The PSU for the circuit is the same as that of a TV booster. For TV boosters, two types of mountings are employed: The fixed tuned booster is mounted on the mast of the antenna. The tunable booster consisting of the PSU is placed near the TV set for gain control of various TV channelsMount the DG MOSFET BF966S at the solder side of the PCB to keep parasitic capacitance as small as possible. Use an epoxy PCB. After soldering, clean the PCB with isopropyl alcohol. Use a suitable enclosure for the circuit. All component leads must be small. Avoid shambled wiring to prevent poor gain or self oscillations. Connecting a single-element cubical quad antenna to the circuit results in ‘Open Sesam’ for DXing.

 You can use a folded dipole or any other antenna. However, an excellent performance is obtained with a cubical quad antenna (refer Fig. 2) and Sangean ATS- 803 world-band receiver. In an amplifier, FET is immune to strong signal overloading. It produces less cross-modulation than a conventional transistor having negative temperature coefficient, doesn’t succumb to thermal runaway at high frequencies, and decreases noise. In VHF and UHF, the MOSFET produces less noise and is comparable with JFETs. DG FETs reduce the feedback capacitance as well as the noise power coupled to the gate from the channel, giving stable unneutralised power gain for wide-band applications.

 This circuit can be used for other frequency bands by changing the input and the output LC networks. The table here gives details of the network components for DXing of stations at various frequency bands.

Read more »

High Watts Amplifier

The main changes made in this design was the addition of a clipping detector circuit and bias circuit modifications for the use of International Rectifier HEXFET MOSFET’s. Later modifications where made to the error amp and VAS stages which improved the overall sound of the amplifier. The power supply components for this amplifier are as follows and are expressed for Two Channels. 1 x Toroidal Transformer with a Core rating of 625VA. Primary windings are made to suit your local mains supply. Eg: for Australia One single primary winding with a 240VAC rating. For USA, 110VAC, 115VAC and I believe there is a 220-Volt AC mains supply in some areas of the United States. For the UK it would be 220 VAC to 240 VAC.

click to enlarge

The secondary windings are as follows.

2 x 50 volts AC at full load.

One 400 Volt 35 Ampere, bridge rectifier.

2 x 4.7K 5 Watt ceramic resistors

Minimum filter capacitor requirements would be 2 x 10,000uf 100 volt electrolytic.

Ideal capacity would be 40,000uf per voltage rail.

A suggested power supply schematic is shown below with the schematic of

The amplifier.

When using this type of MOSFET in the AV800 amplifier is strongly recommended that the output stage devices be matched. As it has been found that if this is not done then there is no guarantee that they will share the current under load. The Source resistors provide only a bit of local feedback and don’t in any way force the devices to current share. The best method I have found to work very well utilises just a 150 Ohm 1 watt resistor and a +15 volt DC power supply. If you look at the schematic below it shows how to connect and measure the N-channel devices and the P-channel devices. With the devices connected, as shown measure across R1 with a multimeter set to DC volts and measurement of between 3.8 volts and 4.2 volts will be shown. Simply match the device in-groups to a tolerance of +-100mv. Please note that you only have to match the n-channel to the n-channel devices and the pchannel to the p-channel devices, not the N-channel devices to the P-channel devices.

Download full Project

Read more »

STEREO TAPE HEAD PRE AMPLIFIER FOR PC SOUND CARD

Here is a stereo tape head preamplifier circuit for your PC sound card that can playback your favourite audio cassette through the PC. Audio signals from this circuit can be directly connected to the stereo-input (lineinput) socket of the PC sound card for further processing. 

Click to enlargec

 The circuit is built around a popular stereo head preamp IC LA3161. Weak electrical signals from the playback heads are fed to pins 1 and 8 of IC1 via DC decoupling capacitors C1 and C6, respectively. Components between pins 2 and 3 and pins 6 and 7 provide adequate equalisation to the signals for a normal tape playback.

 The amplified and equalised signals available at output pins 3 and 6 of IC1 are coupled to the inputs of line amplifier circuit built around transistors T1 (via capacitor C5, potmeter VR1, resistor R8, and capacitor C12) and T2 (via capacitor C10, potmeter VR2, resistor R19, and capacitor C16), respectively. Left and right playback levels can be adjusted by variable resistors VR1 and VR2. The audio signals are finally available at the negative ends of capacitors C13 and C17.

 The circuit wired around relay driver transistor T3 serves as a simple source selector. This is added deliberately to help the user share the common PC sound card line-input terminal for operating some other audio device as well.

 When the preamplifier is in ‘off’ state, switching relay RL1 is off and it allows connection of  external signals to the sound card. When the preamplifier is turned ‘on’, the relay is energised by transistor T3 after a short delay determined by the values of resistor R21 and capacitor C23. On energisation, the relay contacts changeover the signals to internal source, i.e. the head preamplifier.

 After constructing the whole circuit on a veroboard, enclose it in a mini metallic cabinet with level controls and sockets at suitable points. Use a regulated 1A, 12V DC power supply for powering the whole circuit including the tape deck mechanism. (A  A, 18V AC secondary transformer with 4700μF, 40V electrolytic capacitor and 78M12 regulator is sufficient.)

You can use any kind of tape deck mechanism with this circuit. Use of goodquality playback head and well-screened wires are recommended.

Read more »

Simple Intercom Circuit

The circuit of a two-position intercom is presented here. This circuit is very simple yet it functions quite satisfactorily. The circuit does not involve any complicated switching. The switches S1/S2 must be fixed in such a way that when the handset is resting on the cradle, the switch is OFF and when it is taken off the cradle, the switch turns on. 

Click to enlarge

 Both the sets used are identical in construction. When one set (say, party 1) is switched on, the other set’s (party 2’s) bell energises. When party 2 turns on his own set, his bell automatically stops and he can talk to party 1 via his mi c r o phone . One can substitute the BEL 1895 IC based amplifier and bell circuit with any other low power amplifier and bell circuit. The block diagram clarifies the connection of the two sets. Only three wires are required to connect the two sets if separate battery is used in each set. However, if the battery is common for the two sets, it requires four wires for interconnections.

 The circuit can be easily assembled on a general-purpose PCB. Intercom cases are also available in the market which may be used for giving it a professional outlook.

Read more »

Power supply with audio Amplifier Circuit

The circuit in Figure 1 can help if you must transfer dc power and audio over a pair of copper wires. One application for such a circuit is a low-cost door-opening system with speech input. 

click to enlarge

The circuit uses only one IC, the wellknown LM317, a low-cost powersupply regulator. Using this chip, you can modulate the adjustment-pin input with the audio signal from an electrets condenser microphone, connected between the output and the adjustment terminals of the IC. The LM317 regulates the output in such a way that the voltage on the microphone is always 1.25V dc. This application uses a WM34 electret microphone, which comes in a standard 10-mm capsule from Panasonic and is common in low-cost equipment.You can use nearly any electret capsule, because the well-regulated voltage on the microphone never exceeds 1.25V.Every electrets capsule contains an integrated JFETbased impedance converter that translates speech into a current flowing from the source to the drain terminal. This current through the microphone modulates the voltage on the variable resistor, RP. Because the output of the LM317 must follow the voltage on RP, you obtain a low-impedance audio signal riding on the output dc voltage.

 The microphone directly modulates the adjustment pin, so a smoothing capacitor, such as C1, for noise and hum does not influence the level of the audio signal. C1 shunts some of the audio signal to ground, but the LM317 compensates for the loss with internal gain. To avoid excessive losses in the LM317, use a capacitor with as low a value as possible. The circuit works well without a capacitor, but values as high as 47 _F do not present a problem.Using RP, you can adjust the dc output voltage and the gain for the microphone signal. For proper operation, the LM317 needs to deliver a minimum current of 4 mA from its output terminal. If your design uses no loudspeaker, you can connect a load resistor to sink this 4 mA. Designs using low-impedance loudspeakers must also have load resistors. You must add the ac current in the audio signal to the minimum current requirement of 4 mA. For an 8_ loudspeaker, you need a minimum resistive load of 470_ to avoid distortion.

Read more »

PRECISION AMPLIFIER

This circuit is similar to the preceding circuit of the attenuator. Gain of up to 100 can be achieved in this configuration, which is useful for signal conditioning of low output of transducers in millivolt range. The gain selection resistors R3 to R6 can be selected by the user and can be anywhere from 1 kilo-ohm to 1 meg-ohm. Trimpots can be used for obtaining

any value of gain required by the user. The resistor values shown in the circuit are for decade gains suitable for an autoranging DPM. 

 Resistor R1 and capacitor C1 reduce ripple in the input and also snub transients. Zeners Z1 and Z2 limit the input to ±4.7V, while the input current is limited by resistor R1. Capacitors C2 and C3 are the power supply decoupling capacitors.

Op-amp IC1 is used to increase the input impedance so that very low in puts are not loaded on measurement.

  The user can terminate the inputs with resistance of his choice (such as 10 megohm or 1 meg-ohm) to avoid floating of the inputs when no measurement is being made.

  IC5 is used as an inverting buffer to restore polarity of the input while IC4 is used as buffer at the output of CD4052, because loading it by resistance of value less than 1 meg-ohm will cause an error. An alternative is to make R7=R8=1 meg-ohm and do away with IC4, though this may not be an ideal method. Gains greater than 100 may not be practical because even at gain value of 100 itself, a 100μV offset will work out to be around 10 mV at the output (100Μv x 100). This can be trimmed using the offset null option in the OP07, connecting a trimpot between pins 1 and 8, and connecting wiper to +5V supply rails. For better performance, use ICL7650

(not pin-compatible) in place of OP07 and use ±7.5V instead of ±5V supply. Eight steps for gain or attenuation can be added by using two CD4051 and pin 6 inhibit on CD4051/52. More steps

can be added by cascading many CD4051, or CD4052, or CD4053 ICs, as pin 6 works like a chip select. Some extended applications of this circuit are given below.

1. Error correction in transducer  amplifiers by correcting gain.

2. Autoranging in DMM.

3. Sensor selection or input type selection in process control.

4. Digitally preset power supplies or electronic loads.

5. Programmable precision mV or mA sources.

6. PC or microcontroller or microprocessor based instruments.

7. Data loggers and scanners.

Read more »

SINGLE STAGE,R-C COUPLED,COMMON BASE AMPLIFIER

The circuit of asingle stage resistance capacitance-coupled amplifier employing the common base configuration.The circuit sometimes called ‘Grounded Base.

 This amplifier provides voltage gain of 80 when operated into a high impedence load.Power outpot is 1.8milliwatts.Its input impedence is 180ohms and output impedence 5,000 ohms.Themaximum input signal voltage which may be applied before output voltage peak clipping appears is 0.1 volts rms.The corresponding maximum output signal is 8 volts rms.

  

     Two batteries are required in the common base amplifier,B1(1.5 volts) for the emitter bias and B2(6 volts) for colector bias.battery B2 supplies approximately 0.8 milliampere dc and battery  B1,0.85ma.The dpts switch (S1-S2)makes and breaks connections to the batteries simultaneously.

   This is an alternative,single battery circuit.Here,voltage divider R3-R4 is operated with a single 7.5 volts battery.current flowing through the divider develops the emitter voltage as a drop across and collector voltage across R4.The bleeder current is 9.62 ma.All resister in the circuit 101 and103 are half watt.Capacitors C1 and C2 may be miniature,low voltage tantalum electrolytics if subminiature is desired.

Read more »