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LM3886 Power Amplifier 100W
Posted on Friday, June 24, 2011   •   Category: Amplifiers

This is 100W LM3886 power amplifier. Since my DIY speaker is 4-ohm and somewhat difficult to drive, I want to have a more powerful amplifier to match with it. Therefore I designed this amplifier which uses two LM3886 per channel, in parallel circuit. This amp can deliver about 50W into a 8-ohm speaker and 100W into a 4-ohm speaker. This is a stereo amplifier and therefore 4 LM3886s are used. The LM3886 circuit is in a non-inverted configuration, so the input impedance is determined by the input resistor R1, i.e. 47k. The 680 ohm and 470pF resistor capacitor filter network is used to filter out the high frequency noise at the RCA input. The 220pF C4 and C8 capacitors are used to shot out the high frequency noise at the LM3886 input pins. I used high quality audio grade capacitors at several locations: 1uF Auricap at the input for DC blocking, 100uF Blackgate for C2 and C6, and 1000uF Blackgate at the supply filter.

LIPO Battery Overvoltage Protection
Posted on Friday, June 24, 2011   •   Category: Battery Chargers

Simple LIPO saver board to throw onto lithium based batteries for overvoltage protection. It consists of a TLV3012 comparator with onboard reference, an voltage divider, load resistors, indicator LED and a transistor. It should turn on the transistor, and load LIPO battery cell when its voltage is >4.25V Ive attached LIPO saver schematic. The diode represents the onboard reference in the comparator, eagle didn't have the device in its library.

LIPO Saver
Posted on Friday, June 24, 2011   •   Category: Battery Chargers

The presented battery saver or TL431 LiPo Saver is designed to prevent deep discharge of lithium polymer batteries. Decreases the cell voltage of a LiPo batteries below 2.4 volts will permanently damage the cell. When LIPO battery is about to reach the critical voltage a buzzer alarm is activated. I have built several of these for use as Tx alarm and they do SCREAM especially if you fit twin piezo sounders. TL431 is adjusted to sound off at pretty much any low voltage level you wish.

RF Field Strength Meter
Posted on Thursday, June 23, 2011   •   Category: FM Transmitters

Most transmitter has several variable capacitors which are used to match impedance for transistors and antennas. I know people hate trimmers and so did I. The reason is that it is difficult to trim a system if you can't measure the performances. To trim a transmitter you need to measure the output power. Most transmitter are tuned with a dummy load of 50 ohm to substitute an antenna of 50 ohm. Not everyone has a power meter, and how can you know that the antenna you connect is purely 50 ohm. If not, the hole trimming is waste of time! What you would like to do is to measure the radiated power out from the antenna you actually are going to use. If you can measure the radiated energy field you can easy tune the system for max output field strength (max power). So, how can we measure the radiated energy field? The block diagram at right show you one easy way to measure the RF filed strength. To the left you find a dipole antenna. The antenna should be cut to match the receiving frequency ...

80W RF Amplifier 88-108 MHz
Posted on Wednesday, June 22, 2011   •   Category: FM Transmitters

This is 80W RF power amplifier that boosts FM Transmitter's power using 2SC2782 bipolar transistors in a tuned class C circuit. RF amplifier can be driven to full 80W power with less than 1 watt driving input power, so that a large gain margin results in this FM transmitter. To obtain stability in this RF amplifier, I employed several techniques, such as placing the resonances of base and collector chokes far apart, damping the chokes with resistors, using RC combinations for absorption of unwanted frequencies, using feed trough capacitors for bypassing on the board, etc. It took some tweaking, but the amplifier ended up unconditionally stable.

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