Electric Guitar/Violin Preamplifier

Magnetic pick-ups in musical instruments have a relatively high output impedance. This can result in a reduction in treble response when connected via a long cable run or to equipment with a low input impedance. This preamplifier provides a high input impedance and a low impedance output, solving both issues. It has adjustable voltage gain and can run off a battery or DC plugpack. The input signal is AC-coupled to the non-inverting input of IC1a, part of a TL074 quad op amp. This has JFET input transistors and the input impedance is set by a 330kΩ bias resistor which also sets the DC level at this input to half supply (Vcc/2).

This is generated by a voltage divider comprising two 10kΩ resistors and bypassed by a 47µF capacitor to reject noise and hum. IC1a is configured as a non-inverting amplifier with a gain of between 2 and 20, depending on the setting of VR1. IC1a’s output is fed to VR2 via a 22µF capacitor, allowing the output volume to be set. The audio then passes to the non-inverting inputs of the remaining three op amps (IC1b-IC1d) which are connected in parallel to provide a low output impedance; it will drive a load impedance as low as 600Ω. The 100Ω resistors in series with the outputs provide short-circuit protection for the op amps and also prevent large currents from flowing between the outputs in case they have slightly different offset voltages.

The buffered signal is then AC-coupled to two output connectors using 47µF electrolytic capacitors. For Output 1, a 47kΩ resistor sets the output DC level to ground and a 220Ω series resistor provides further short-circuit protection. Output 2 is similar but includes another potentiometer (VR3) to allow its level to be set individually. Note that this means the impedance of Output 2 can be high (up to 2.5kΩ depending on the position of VR3’s wiper). The total harmonic distortion of this circuit is typically less than 0.01% with the gain set to six. If a TL064 is used instead of a TL074, the current drain will decrease but there will be more noise at the output. Finally, the input impedance can be increased by increasing the value of the 330kΩ resistor to suit high-impedance pick-ups.

Read More

Phone Line Polarity Checker

Do you suspect your broadband speed (DSL) is slower than it should be? It could be as simple as incorrect polarity in your phone wall socket. Build this very cheap, very simple device to find out whether you need to change your wiring!

It might not seem that telephone line polarity is important, since the ring and voice signals sent over telephone lines are AC. However the lines are actually biased to 48V DC (less when in use) and so the polarity can matter. The main problem with incorrect polarity is that some DSL (Digital Subscriber Line) modems and routers can perform poorly in this circumstance. Since telephone wires are color-coded, it should be possible to simply check that the sockets are wired correctly. Unfortunately, there are multiple wiring colour schemes and they have changed over time. Believe it or not, the old color scheme (from around 15 years ago) is identical to the current colour scheme except that the polarity of both lines is reversed! This is why so many homes have this problem and yours may well be one of them.

The common telephone line is simply a copper pair, ie, two wires. As mentioned, there is usually a 48V DC bias across the pair which drops to around 8V when a telephone is “off-hook”. The ring voltage (around 90V AC) and the audio signal voltage (also AC) are overlaid on this DC bias. The DC power is “rectified” by each telephone on that line to run its own circuitry. Note, though, that this does not include cordless phones which usually use a plugpack, as their power requirements are far in excess of what the telephone line can deliver. (As an aside, that is the reason it is important to keep a line-powered telephone in your home so you can still make and receive calls if the mains power goes out.

Telephone exchanges can usually supply power from their backup batteries for up to some days, even if they are blacked out). Usually, telephone lines are run with 4-core cable. This allows up to two lines on the one cable. The first line is on the inner pair (pins 2 and 3) and the second line, if present, is on the outer pair (pins 1 and 4). Modern telephones use modular plugs, specifically RJ11 (6P2C, one line), RJ14 (6P4C, one or two lines) or RJ25 (6P6C, 1-3 lines). By the way, 6P4C stands for “six pins, four connectors”. Incidentally, “RJ12” connectors are physically compatible – and commonly available – so that is what we have used in this project.

Because modern phones rectify the DC voltage from the telephone lines before regulating it and because the ring and voice signals are AC, for voice communications the polarity doesn’t really matter.

Read More

Gentle Battery Regulator

This small but very effective circuit protects a lead-acid battery (12-V solar battery or car battery) against overcharging by a solar module when the incident light is too bright or lasts too long. It does so by energizing a fan, starting at a low speed when the voltage is approximately 13.8 V and rising to full speed when the voltage exceeds 14.4 V (full-charge voltage). The threshold voltage (13.8 V) is the sum of the Zener diode voltage (12 V), the voltage across the IR diode (1.1 V), and the base-emitter voltage of the 2N3055 (0.7 V). In contrast to circuits using relays or IC amplifiers, the circuit has a gradual switching characteristic, which avoids relay chatter and the constant switching on and off near the switching point produced by a ‘hard’ switching point.

The circuit does not draw any current at all (auto power-off) below 13 V. Pay attention to the polarization of the Zener and IR diodes when building the circuit. The transistor must be fitted to a heat sink, since it becomes hot when the fan is not fully energized (at voltages just below 14 V). A galvanized bracket from a DIY shop forms an adequate heat sink. The indicated component values are for a 10-W solar module. If a higher-power module is used, a motor with higher rated power must also be used. The circuit takes advantage of the positive temperature coefficient of the lamp filament. The filament resistance is low at low voltages and increases as the voltage rises. This reduces the speed of the fan to avoid generating an annoying noise level. The lamp also provides a form of finger protection.

If you stick your finger into the fan blade, the lamp immediately takes over the majority of the power dissipation and lights brightly. This considerably reduces the torque of the fan. An ordinary 10-W or 20-W car headlight (or two 25-W headlights in parallel) can be used for the lamp. Don’t try to replace the LED by two 1N4001 diodes or the like, replace the ZPY12 by a ZPY13, or fit a series resistor for the LED. That would make the ‘on’ region too large.

Read More

Sixties-Style 40W Audio Amplifier

In the early 1960s RCA brought out a transistor that was to become truly legendary: the 2N3055. With a pair of these devices, you could put together an audio power amplifier that could deliver a healthy 40 W into 8 Ω. The circuit described here is fully in tune with the spirit of that era. For example, there are only seven active components in each channel, which reflects the design simplicity typical of that era (and actually a timeless quality). This ‘retro’ power amplifier pumps 45 W into 8 Ω with an input signal level of 0.5 Vrms. It works as follows: the input signal is applied to the base of T1, while negative feedback from the output, attenuated by voltage divider R5/R6, is applied to the emitter of T1. The collector current of T1, which is proportional to the difference between the input and feedback signals, is fed to the base of T2.

Amplifier Circuit Diagram:

This transistor draws its operating current through R8 and R9 and provides voltage gain. Capacitor C6 is a bootstrap capacitor that hold the voltage across R9 nearly constant, so that the current through R9 is independent of the amplifier output signal level in the audio band. Transistors T4–T7 form a quasi-complementary push-pull output stage. In the early 1960s, there simply wasn’t any PNP transistor available that was truly complementary to the 2N3055. Designers came up with an ingenious way to get round this problem, which was to use a complementary Darlington pair consisting of a PNP driver transistor and an NPN power transistor. The schematic diagram clearly illustrates what is meant by a quasi-complementary push-pull output stage. Diode D1 provides balanced biasing for the output stage, which helps reduce distortion.

The operating point of the output stage is set and stabilized by transistor T3, which for this reason should be thermally coupled to the output transistors. The amplifier is powered from a single supply voltage at approximately 65 V, which is also ‘typical sixties’. Capacitor C1, with a value of 4700 μF, transfers the signal from the output stage to the load and provides a bit of protection for the speaker in case one of the transistors fails. The amplifier does not have output current limiting. Although this is not a critical shortcoming, a certain amount of caution is advisable. The only protection in this regard is provided by a slow-acting 1.6-A fuse in the supply line, which is intended to limit the damage if anything goes wrong.

Power Supply Circuit Diagram:

The power supply consists of a transformer, a bridge rectifier, four small capacitors and a 4700 μF electrolytic capacitor. This is enough to power a two-channel stereo amplifier. The LED is a power-on indicator and is intended to be fitted on the front panel. Assembling the circuit is very straightforward. Transistors T3, T4 and T5 should be fitted with heat sinks suitable for a TO126 package and with a thermal resistance less than 20 K/W. Transistors T2, T6 and T7 should all be fitted on a single heat sink with a thermal resistance of 2 K/W or less, using insulating washers and thermal paste. Before applying power to the circuit for the first time, set P2 to its maximum value, temporarily replace the fuse with a 47 Ω, 5 W resistor, and connect a voltmeter across R17.

Then switch on the power. The voltmeter should indicate 0 V. Now carefully adjust P2 until the voltmeter reads 15 mV, which corresponds to a quiescent current of 50mA. Then switch off the power and install the fuse in place of the power resistor. After this, check the voltage across R17 again (with the power on) and if necessary adjust it to 15mV. This is fun DIY project, cheap and unpretentious. Nevertheless, the sound quality of this amplifier is respectable. The distortion level gives no grounds for complaint. Of course, it’s not a figure with an incredible number of zeros after the decimal point, but the idea here is to brush up on sixties technology.



Author: Joseph Kreutz (Germany) - Copyright: Elektor Electronics 2011

 

 

Read More

Voltage Limiter For Guitar Amplifiers

Guitar amplifiers using output devices such as the TDA7293 (100 W) or LM3886 (68 W) are surprisingly often damaged as a result of excessive supply voltage in the quiescent state. The transformers are often used so close to their specification that this problem can even be caused by a high mains input voltage. In most countries the domestic AC outlet voltage is permitted to rise as high 10 % above the nominal (published) value. Since replacing the transformer is not an attractive proposition, the author developed a relatively simple electronic solution to the overvoltage problem: a voltage limiter for the symmetric supply to the amplifier. The circuit is based on the classical voltage regulator arrangement of a Zener diode connected to the base of a pass transistor.

However, in this version we replace the conventional bipolar transistor with a power MOSFET. The circuit is symmetrical with respect to the negative and positive supplies, and so we shall only describe the positive half. The input voltage (at most 50 V) supplies the chain of Zener diodes D1, D2 and D3 via resistor R3. The resistor limits the current through the Zener diodes to about 5mA. The series connection of Zener diodes has the advantage that their dissipation is divided among them, as well as giving more options for the total voltage drop by judicious selection of individual components. The sum of the diode voltages (39 V with the values given) must be greater than the desired limiting output voltage by the gate-source voltage of the MOSFET.

Circuit diagram:

C1 smooths the voltage across the Zener diode chain. The circuit therefore not only limits the voltage, but also reduces the ripple (hum component) on the supply. The gate of the HEXFET is driven via R1. In conjunction with C4, this prevents the FET from oscillating. Without any load the output voltage is rather higher than expected. With a small load, such as that presented by the output stage in its quiescent state, it falls to the desired value. The circuit then does not provide regulation of the output voltage, but rather a stabilization function. The operation of the negative half of the circuit is identical to that of the positive half apart from the polarity of the voltages, and hence a P-channel MOSFET must be used there.

It is worth noting that there is a relatively large degree of variation (up to a few volts) in the gate-source voltage of the HEXFETs used. This can be compensated for by selecting the Zener diodes in the chain and the current through them, but for most applications the exact voltage at which limiting begins to occur will not be critical. The HEXFETs must be provided with adequate cooling. If possible, they can be attached to the heatsink already present in the amplifier; otherwise, a separate heatsink will be required. A thermal rating of 2.5 K/W will be suitable.



Author: Alfred Rosenkränzer (Germany) - Copyright: Elektor Electronics 2011

 

 

Read More