IC-7905 Stereo Power Amplifier Circuit Project with schematic

79xx is a widely known series of low-cost, fixed-negative-voltage regulators. These integrated circuits are available with output current of 100-150 mA (L series), 0.4-0.5A (M series), up to 1A (standard series), etc. They can be used in many applications other than regulators, audio power amplifier being one of them.  As shown in the circuit diagram, a simple stereo audio amplifier is built around two 7905 negative-voltage regulators (IC1 and IC2) and a few discrete components. The 7905 IC (a -5V regulator) used here is readily available. However, the circuit will also work with other 79XX regulators if appropriate power supply is used. Both channels shown in the diagram are identical. Hence the description below is only for the first channel. The quality of the output signal is within acceptable limits.

Stereo Power Amplifier Circuit Diagram:

Regulator IC 7905 works as an amplifier for the voltages applied to common pin2 (Ground or GND). The minimal voltage drop over the standard 7905 is around 2V and it depends on the output current. Feedback resistors in the IC set the gain of the channel internally. The amplifier is a class-A audio amplifier. The regulator IC produces the negative output signal.

Resistor R3 provides the positive signal. It limits the maximum output current of the regulator during the negative half period of the amplified sinusoidal signal. The minimal applicable value of R3 for the regulator 7905 is 8.2 to 10 ohms per 5W.  Optimisation of the value of R3 depends on the output voltage of the regulator, negative power supply (–5V) and load resistance of loudspeaker (LS1). If the required output current for LS1 is below 100 mA, the value of resistor R3 can be 33 to 51 ohms per watt.

Normally, the load resistance of the loudspeaker should be higher than of R3 in order to obtain a large peak-to-peak amplitude. But this can be neglected in order to obtain lower power dissipation on R3 and the IC. The circuit works with any load resistance (R3 in parallel with LS1 as the load) under the condition that the regulator is not overloaded with current and power dissipation. However, it is preferable to use a loudspeaker with a high resistance (8 ohms, 16 ohms or more). The amplifier works well with low-impedance headphones having a resistance of 24 to 32 ohms. The voltage difference between the ground pin of 7905 and the output pin is fixed internally.

The input resistance of the amplifier is relatively low and depends on potentiometer VR1 and input resistance of the ground pin. Practically, any stereo output capable of driving 24- or 32-ohm headphones and loudspeakers can drive the input of the stereo amplifier with 7905. If VR1 is removed, the amplifier will still work but there will be more distortion. Therefore potentiometer VR1 is used to provide sufficient variable audio signal.  The values of output capacitors C10 and C11 are usually between 0.1 µF and 1 µF. A small resistance can be connected in series with them if needed. S2 is the on/off switch. Switch S1 is for mono/stereo selection. When switch S1 is closed, the amplifier works as a two-way mono amplifier. If S1 is open, the amplifier works as a stereo amplifier.

The circuit is powered by a 12V battery. The positive terminal of the battery is the common node. The negative terminal is connected to pin 2 of IC1, which is the –12V supply line. The maximum operating voltage can be up to –35V. If no input signal is applied, the DC voltage on the output of the regulator 7905 should be around –5V, which depends to some extent on the value of VR1. The maximum output current of 7905 can be up to 1A and the maximum power dissipation is up to 15W. IC 7905 has internal thermal protection.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Fix the stereo female jack on the front panel and speaker to the rear side of the cabinet, and the 12V battery inside the cabinet. Fix LED1 and switches S1 and S2 too on the front panel of the cabinet. Mount the regulator IC 7905 on a heat-sink with thermal resistance below 15°C/W. The metallic part on the case is internally connected with the input pin of the regulator.

Author : Petre tzv. Petrov - Copyright : EFY

K295 stereo tube sound line buffer Amplifier kit(Oatley Electronics)

The latest kit offering from Oatley Electronics is the K295 Stereo Tube Sound Line Buffer Amplifier Kit which retails for $36AU. The low cost, simple to construct, stereo tube buffer, lets you experience "tube sound". The kit is based around JAN 6418 sub-miniature vacuum tubes and designed for 100-120 or 200-240V 50/60Hz AC operation. These inexpensive tube kits can sound quite good, see Mark's review of the K272 Headphone Amplifier Kit and the recent review in audioXpress. Via - Mark [K295 stereo JAN6418 tube buffer / preamp kit].

Oatley Electronics K295 Stereo Tube Sound Line Buffer Amplifier Kit

RIAA stereo preamplifier amplifier, preamplifier, RIAA, RIAA stereo, RIAA stereo preamplifier, stereo, stereo amplifier

Records are cut with a frequency response such that when they are replayed with a magnetic pickup and a preamplifier with RIAA equalization (Recording Industry Association of America) the reproduced sound will be as similar to the original as possible.

The disc is cut at constant amplitude, except from 500Hz to 2120Hz where it is cut at constant velocity. When this disc is replayed with a magnetic pickup, the relative output voltage rises with frequency, due to the fact that the magnetically generated voltage is proportional to the velocity of the stylus as it moves sideways in the groove. To restore the original sound quality, a preamplifier with afrequency response that, gives decreasing output with increasing frequency is required. This response curve is known as the RIAA equalization and it is tailored accurately to fit the cutting and replay processes. The signal level from a magnetic pickup is low, generally 20mVpp and so a low noise pre- amplifier is needed.

The circuit shows a realization of this requirement. The low noise amplifier is the LM381 -made by National Semiconductors. A DC bias control is included (RV1, RV2), and the feedback components generate the RIAA curve. Use screened cable for the wiring to the pickup, keep the circuit away from transformers (and the pickup and its wiring) and connect all the earths together, near to the IC.