diagram ingram

This design for a headphone amplifier arose after the purchase of commercial equipment with separate pre and power amplifiers without a headphone output.

It is based on designs for a headphone amplifier by John Linsley-Hood, and an active volume control, using a linear pot, by Doug Self (the "pot" circuit was originally designed by P.J. Baxandall), both published in Electronics and Wireless World in recent years.

Its advantages are ...

ow output impedance to drive several pairs of phones

the active gain stage is, almost, perfectly logarithmic and ...

• is independent of the absolute value of the pot
• has excellent channel tracking
• the O/P noise reduces with gain reduction.

Description

The intention is to permanently insert the headphone amp between pre and power amps, although it can be used as a stand-alone item. The input relay is operated by auxiliary contacts on the headphone sockets through a transistor driver (with a small delay) so as to mute the power amp input when listening on headphones.

The relay contact arrangement enabling it (the headphone amp) to be left switched off when normally not in use. The relay is a high quality, sealed, gold plated contact, TQ signal switching type, reputedly with a very long life expectancy.

The gain control being used to pre-set the gain so that the pre-amps gain control is normally used for setting the listening level.

Figure 1 - The Headphone Amp Circuit

One channel only is shown, so two units are required for stereo. The gain control pot must be a dual-gang linear type, as the circuit configuration provides the logarithmic law required. This is similar to the circuit shown in Project 01 (except that this version provides a useful reduction of noise). A value of 47k or 100k should be fine in this circuit. Diodes should be 1N4148.

The first stage is a conventional series feedback circuit using the ubiquitous NE5534, the gain being set by the feedback AOT (adjust on test) resistor to suit individual needs, this stage provides the necessary low impedance output for the variable gain stage. The resistor/ capacitor networks around the input stage may seem a little extravagant, but are necessary to reduce any possible RF pickup especially the 470 pF between the two IC + and - inputs.

The complete second stage consists of a zero gain follower, an inverting gain stage and the output emitter followers, volume control gain being set around these three stages. The x10 gain of the inverting stage gives the closest approach to a logarithmic law, stability being ensured by the 27pf capacitor in this stages feedback. The output complementary pair runs in Class-A at about 80 mA and should be mounted on a small heatsink.

Dissipation is about 1.8 Watts for each device, and they must be isolated from the heatsink with mica washers and mounting bushes to prevent short-circuiting the power supply (the collectors are connected to the case). Make sure that heat-conducting paste is used, or use sil-pads for mounting - these require no thermal compound and are very convenient for low power operation.

Figure 2 - Alternative Relay Driver, and Component Pinouts

The OPA2604 was chosen because its high, FET based, input impedance provides better DC conditions for setting the O/P at 0V DC than the NE5532 alternative, its low output impedance has no problems in driving difficult loads, but it is still relatively cheap.

The power supply is a fairly conventional split variety, the regulated O/Ps feeding the ICs - note the decoupling arrangements - and the 22V pre-regulated supply feeding the O/P transistors, the relay supply being rectified and regulated separately for the necessary isolation, separate signal and supply star earthing being essential

Figure 3 - Power Supply

The output jack sockets, with independent changeover contacts, are obtainable from Maplin Electronics and have proved extremely reliable over many years of regular use. If these are not obtainable a circuit is included for use with conventional break contact jack sockets.The LED series resistors will need to supply a current of about 7.5mA, so 2.2k should be used. Diodes for the supply should be 1N4004 or equivalent.

If desired, the 12V regulator may be dispensed with, and suitable value resistors placed in series with each relay coil to retain the correct operating voltage. It is the constructors responsibility to determine the value of these, as the relay current cannot be predicted as there are so many different types available. Use of 15V relays is also possible, if available.

If this arrangement is used, a slight amount of noise may be introduced as the relay operates, because of the sudden application (or removal) of the additional load. It is not expected that this would be a problem in use.

My thanks to Richard for submitting this circuit - it is sure to provide a very high sound quality, and is not overly complex. The active gain control (originally designed by Peter Baxandall) is very effective.

As always, resistors should be 1% metal film types for all signal paths. Their use in the power supply and relay circuits is not necessary, but will not do any harm, either.

Source:www.sound.westhost.com

It shows the connection and wiring between each parts and component of audio system of the vehicle such as the alternator, ignition switch, antenna meter, tail, audio, front speaker, rear speaker, tweeter speaker, and many more.

Crowbar circuits are so-called because their operation is the equivalent of dropping a crowbar (large steel digging implement) across the terminals. It is only ever used as a last resort, and can only be used where the attached circuit is properly fused or incorporates other protective measures.

A crowbar circuit is potentially destructive - if the circuitry only has a minor fault, it will be a major fault by the time a crowbar has done its job. It is not uncommon for the crowbar circuit to be destroyed as well - the purpose is to protect the device(s) attached to the circuit - in this case, a loudspeaker.

Description
Theres really nothing to it. A resistor / capacitor circuit isolates the trigger circuit from normal AC signals. Should there be enough DC to activate the DIAC trigger, the cap is discharged into the gate of the TRIAC, which instantly turns on ... hard. A TRIAC has two basic states, on and off. The in-between state exists, but is so fast that it can be ignored for all intents and purposes.

 Figure 1 - Crowbar Speaker Protector

The BR100 DIAC (or the equivalent DB3 from ST Microelectronics) is rated for a breakdown voltage of between 28 and 36V - these are not precision devices. Needless to say, using the circuit with supply voltages less than around 40V is not recommended, as you will have a false sense of security. The supply voltage must be higher than the breakdown voltage of the DIAC, or it cannot conduct. Zeners cannot be used as a substitute for lower voltages - a DIAC has a negative impedance characteristic, so when it conducts, it will dump almost the full charge in C1 into the gate of the TRIAC. This is essential to make sure the TRIAC is switched into conduction.

The TRIAC is a common type, and may be substituted if you know the specifications. Its rated at 12A, but the peak current (non-repetitive) is 95A, and it only needs to sustain that until the fuse (or an output transistor) blows. A heatsink is preferred, but there is a good chance that the TRIAC will blow up if it has to protect your speakers, so it may not matter too much. The 0.47 ohm resistor is simply to ensure that the short circuit isnt absolute. This will limit the current a little, and increases the chance that the TRIAC will survive (albeit marginally). Feel free to use a BT139 if it makes you feel better - these are rated at 16A continuous, and 140A non-repetitive peak current.

The peak short circuit current will typically be about 90A for a ±60V supply, allowing ~0.2 ohms for wiring resistance and the intrinsic internal resistance of the TRIAC, plus the equivalent series resistance of the filter capacitors. Thats a seriously high current, and it will do an injury to anything thats part of the discharge path. Such high currents are not advised for filter caps either, but being non-repetitive they will almost certainly survive.

Construction & Use
Apart from the obvious requirement that you dont make any mistakes, construction is not critical. Wiring needs to be of a reasonable gauge, and should be tied down with cable ties or similar. C1 must be polyester. While a non-polarised electrolytic would seem to be acceptable, the circuit will operate if the capacitor should dry out over the years. This means it will lose capacitance, and at some point, the crowbar may operate on normal programme material. This would not be good, as it will blow up your amplifier!

Make sure that all connections are secure and well soldered. Remember that this is the last chance for your speakers, so it needs to be able to remain inactive for years and years - hopefully it will never happen. The circuit doesnt have to be mounted in the amplifier chassis - it can be installed in your speaker cabinet. Nothing gets hot unless it operates, at which point no-one really cares - it just has to save the speakers from destruction once to have been worthwhile.

Remember that the crowbar circuit absolutely must never be allowed to operate with any normal signal. A perfectly good amplifier that triggers the circuit because of a high-level bass signal (for example) will very likely be seriously damaged if the crowbar activates. To verify that no signal can trigger it, you may want to (temporarily) use a small lamp in place of R2, and drive the amp to maximum power with bass-heavy material.

A speaker does not need to be connected. If the lamp flashes, your amp would have been damaged. If this occurs, you may want to increase the value of C1. Note that bipolar electrolytics should never be used for C1, because they can dry out and lose capacitance as they age. This could cause the circuit to false-trigger.

Source :www.extremecircuits.net

The NAND gates use CMOS 4011 chip, a low power component that is suitable for this battery-operated circuit. You can see that this chip is supplied by a 5V voltage coming from an LM7805L regulator. You might wonder what the purpose of this regulation is, since the power supply come from a  9V battery and the CMOS gates can handle the voltage of 3-15 Volt. The main purpose of the regulator is to keep a constant voltage source for the reference oscillator frequency stability, since the frequency is affected by the power supply voltage variation as the battery voltage drops in the long time of usage.

This circuit uses parts as follows :

•     U1: CD4011
•     U2: LM389
•     U3: 78L05
•     R1: 2.2k 5%
•     P2: 4.7k lin.
•     R3: 330k 5%
•     R4: 270k 5%
•     R5: 1k 5%
•     C1: 390pF (NPO)
•     C2,C3,C4: 10nF
•     C5: 10uF 16v electrolytic
•     C6,C8: 220 uF 16v electrolytic
•     C7: 100uf 16v electrolytic
•     C9: 100nF ceramic
•     P1: 4.7k log
•     L1: 22cm in diameter with 14 turns AWG 26
•     K1: SPDT toggle switch
•     J1= Headphone jack 1/4 or 1/8 inch
•     Other parts: 9v battery connector, speaker or headphones

In this article, an RC oscillator is used as a baud rate generator. If you can calibrate the frequency of such a circuit sufficiently accurately (within a few percent) using a frequency meter, it will work very well. However, it may well drift a bit after some time, and then…. Consequently, here we present a small crystal-controlled oscillator. If you start with a crystal frequency of 2.45765 MHz and divide it by multiples of 2, you can very nicely obtain the well-known baud rates of 9600, 4800, 2400, 600, 300, 150 and 75. If you look closely at this series, you will see that 1200 baud is missing, since divider in the 4060 has no Q10 output!

If you do not need 1200 baud, this is not a problem. However, seeing that 1200 baud is used in practice more often than 600 baud, we have put a divide-by-two stage in the circuit after the 4060, in the form of a 74HC74 flip-flop. This yields a similar series of baud rates, in which 600 baud is missing. The trimmer is for the calibration purists; a 33 pF capacitor will usually provide sufficient accuracy. The current consumption of this circuit is very low (around 1mA), thanks to the use of CMOS components.

It’s always a bit difficult to measure the current in the positive lead of a power supply, such as a battery charger. Fortunately, special ICs have been developed for this purpose in the last few years, such as the Burr-Brown INA138 and INA168. These ICs have special internal circuitry that allows their inputs to be connected directly to either end of a shunt resistor in the lead where the current is to be measured. The shunt is simply a low-value resistor, across which a voltage drop is measured whenever a current flows. This voltage is converted into an output current Io by the IC.

This current can be used directly, or it can be converted into a voltage by means of a load resistor RL. In the latter case, the ‘floating’ measurement voltage across the shunt is converted into a voltage with respect to earth, which is easy to use. The value of RL determines the gain. A value of 5 kΩ gives 1×, 10 kΩ gives 2×, 15 kΩ gives 3× and so on. It all works as follows. Just like any opamp, this IC tries to maintain the same potential on its internal plus and minus inputs. The minus input is connected to the left-hand end of the shunt resistor via a 5-kΩ resistor.

When a current flows through the shunt, this voltage is thus lower than the voltage on the plus side. However, the voltage on the plus input can be reduced by allowing a small supplementary current to flow through T1. The IC thus allows T1 to conduct just enough to achieve the necessary lower voltage on the plus input. The current that is needed for this is equal to Vshunt / 5 kΩ. This transistor current leaves the IC via the output to which RL is connected. If the value of RL is 5 kΩ, the resulting voltage is exactly the same as Vshunt. The IC is available in two versions.

The INA138 can handle voltages between 2.7 and 36 V, while the INA168 can work up to 60 V. The supply voltage on pin 5 may lie anywhere between these limits, regardless of the voltage on the inputs. This means that even with a supply voltage of only 5 V, you can make measurements with up to 60 V on the inputs! However, in most cases it is simplest to connect pin 5 directly to the voltage on pin 3. Bear in mind that the value of the supply voltage determines the maximum value of the output voltage. Also, don’t forget the internal base-emitter junction voltage of T1 (0.7 V), and the voltage drop across the shunt also has to be subtracted.

Nearly always when a remote control doesn’t work, the underlying problem is elementary: the unit does not emit light. The cause may be dry solder joints, defective LEDs etc., but also a flat battery (perhaps due to stuck key). The human eye is unable to perceive infra-red light. By contrast, an ordinary photo transistor like the BP103 has no problems working in the infrared spectrum, so in the circuit here it simply biases the BC558 which, in turn, makes LED D1 flash in sympathy with the telegram from the remote control. The preset in the circuit determines the sensitivity.

This is a simple  egg timer circuit , which is both simple and functional, shows once again that it is not essential to use a microcontroller for everything these days. The circuit consists of only two ICs from the standard 4000 logic family, a multi-position rotary switch and a few individual components. The combination of a 4040 oscillator/counter and a 4017 decimal counter is certainly not new, but it is an ideal combination for timers that are required to generate long intervals that can be programmed in steps. The circuit can be directly powered from a 9-V battery, without using a voltage regulator. The signalling device is a 12-V buzzer, which generally works quite well even at a much lower voltage. We won’t explain the operation of the two ICs here; if you would like to know more about this, we recommend consulting the device data sheets.

The RC configuration has been selected for the oscillator circuit of the 4060, since the frequencies of standard crystals and resonators would be too high (even 32.768 Hz is much too high), making it impossible to achieve the desired times. With an RC oscillator, it’s also easier to modify the times to suit our purposes. For instance, if the oscillator frequency is reduced by a factor of two, we obtain a range of 1 to 16 minutes in steps of 1 minute. The range is split into two by taking advantage of the fact that the 4017 has an AND gate at its input (with an inverted input).The two ranges overlap by two steps. The oscillator has been dimensioned such that the 23 divider output (pin 14) has a period of 30 seconds, so IC2 receives a clock pulse every 30 seconds. This means that the oscillator frequency must be set to 8.5333 Hz.

The first output of IC2 is active after a reset, so it cannot be used. If S1 is in position I, pin 14 of IC2 is connected to the positive supply line. This input is used as an enable input. Directly after the first pulse from the 4060, the second output of IC2 goes high (which means after exactly half a minute). The sub-sequent outputs become active in turn at intervals of one clock pulse, and thus generate the states for 1 to 4.5 minutes. In the second range (II) of S1, the ‘enable’ pin of IC2 is connected to the 212 divider output of the 4060 (pin 1). This output goes high 4 minutes after the reset (which is why it is labelled ‘240 s’, instead of the period time of 480 s). Since the 4060 is an asynchronous counter, this output goes high a short time after the 23 output goes low. This delay provides the proper condition for an extra clock pulse for the 4017. The outputs of the 4017 will thus count upwards once. This means that the second output will become active after 4 minutes, with the rest of the outputs becoming active after 4.5 to 8 minutes. The desired timing interval is selected using switch S2.

The output of S2 is connected directly to emitter follower T1, which energizes the buzzer when the level on the wiper of the switch is high. At the same time, the counter of IC1 is disabled via diode D1 by forcing the oscillator input high. The buzzer thus remains active until the circuit is switched off. The first counter output of the 4060 is connected to an LED (D2), which indicates that the circuit is active and the battery not yet exhausted. The blinking rate is approximately 0.5 Hz. The current through the LED is set to a modest 1mA, since this current represents the majority of the current drawn by the circuit.

This ranges from 0.5 to 1.5 mA, with the average current consumption being approximately 1mA while the timer is running. The buzzer used in our prototype increases the current to around 13 mA when it is energized, but this naturally depends on the actual type used. In principle, the circuit will work with any supply voltage between 3 and 16 V. However, the actual supply voltage should be taken into account in selecting the buzzer. The value of the supply voltage also has a small effect on the time interval, but in practice, the deviation proved to be less than 5 percent - which is not likely to matter too much to the eggs.