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Audio Volume Control Attenuator with IR Control
Posted on Saturday, May 28, 2011   •   Category: Amplifiers


The circuit provides both audio volume and input channel selection. A stepwise volume control is implemented with a set of small relays and resistors. In a high-end audio system, a noticable sound improvement over potentiomeneters can be obtained, also over 'audio grade' potentiometers. Clearly, the IR remote control provides convenience over solutions with stepped attenuator rotary switches. The sealed relays will maintain contact quality over a practically endless lifetime.


75 Meter SSB Transceiver
Posted on Friday, May 27, 2011   •   Category: RF Radio Frequency


Here's 75 Meter QRP SSB Transceiver. In general, the transceiver switches the 4-element 1500 ohm xtal BPF ends between the inputs and outputs of the two SA602s to reverse the signal flow for R/T operation. Since no IF amplifier is used in the design, 20 dB of additional receiver gain is produced by the 2N2222 receiver RF amplifier, while automatic gain control (AGC) is produced by the peak DC swing of the LM386 output passed through a rectifier and filtered by a capacitor and fed to the gate of a BS170 enhancement mode FET acting as a variable resistor across the input of the LM386. Both receive and transmit band pass filtering are done by the same half-pi BPF. The diode pair in the mic circuit reduce the "chirp" that occurs during the R/T transition. Additional BS170s could easily be used to mute both the mic and audio instead of the R/T switch directly. These BS170s would be controlled by the +R and +T voltages on their gates while their drains would be tied to 1) the mic circuit between the two coupling capacitors and 2) pin number 1 (audio in) of the LM386 (BS170 sources to ground). Additional power output (perhaps 60 mW) could also be attained by connecting the RF output transistor's collector choke (10 uH) to a 9 V supply instead of the 5 V. Additional biasing current might also be required for this change.


8W PLL Stereo Transmitter with LCD
Posted on Friday, May 27, 2011   •   Category: FM Transmitters


This is 8W PLL Stereo Transmitter with LCD. It is a stable PLL FM transmitter based on TSA5511 synthesizer. Frequency is performed with three buttons through PIC16F84 microcontroller. Frequency of the transmitter is displayed on 16x1 LCD. Transmitter oscillator is based around BF981, BFR91, BFR96 transistors. 2SC1971 RF power transistor can be replaced with 2N4427 or 2N3553 but they will provide less output power.


LM4780 Gainclone Amplifier
Posted on Thursday, May 26, 2011   •   Category: Amplifiers


LM4780 gainclone amplifier with a design similar to the National Semiconductor BPA-200 (Bridge/Parallel Amplifier) which uses 4x LM3886 per channel and an input buffer. The total effect is (2x LM3886's paralleled amplifiers) 2x Bridged and should give approx 225 watts into 8 ohm and 335 watts into 4 ohm speakers when used with a sufficient power supply.


LM3875 Gainclone Amplifier
Posted on Thursday, May 26, 2011   •   Category: Amplifiers


Gainclone amplifiers have very few components and this one is based on the National Semiconductor LM3875 IC. The PCBs and components are very simple and quick to make, only took about 20 mins to assemble both amps and rectifier board. DC offset was about 80mV on one channel and about 40mV on the other. I used the optional Ci capacitor in the national datasheet for the IC which reduced it to between 0-4mV: This is the capacitor I chose, its an Elna Starget (expensive). The case was MUCH more time consuming and difficult to make though. I bought all the aluminium from a scrap metal yard including the heatsink. I got my aluminium panels cut at a sheet metal shop as I cant make straight cuts with a hack saw.


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