EME206 500mW Audio Amplifier Kit
The Mini-Kits EME206 500mW Audio Amplifier Module is part of the M1 series building blocks to construct a high performance M1 HF Transceiver. The M1 HF Series Kits have been designed to allow easy construction due to the use of mainly leaded components for those that don't like surface mount devices. Modern SMD components are only used when required due to availability of components, or for better performance. The RF modules are all designed with a 50ohm impedance making it easy to interconnect with the other M1 series modules, and your own homebrew circuit designs.
The module board incorporates audio switching, a pre-amplifer with low pass filter, and low power audio amplifier to provide up to 700mW output into 8ohms. Up to 3 audio inputs are available using onboard analogue DPDT switches, that can be swithed with simple logic on a header connector. This allow various audio signals from other modules including the audio from the SSB/CW BFO demodulator, and audio from the AM demodulator on the I/F AGC modules to be easily switched when changing modes. The audio is then processed using a TLO71 pre-amplifier IC with a low requency boost, which reduces high frequency noise from the speaker for communications quality audio. The audio then passes through the volume control, and is amplified by a LM386-N3 audio amplifier IC. The amplifier uses feedback to set the gain and for stability, and produces up to 700mW into 8ohms. An onboard mute circuit can be used to mute the audio if the module is used in a Transceiver application, and there is an optional audio input for a sidetone oscillator for CW.
The module is controlled through a single 16 Pin IDC header connector, making it easier to interface to logic circuitry. See the Block diagram for the connections to this module.
1/ A mistake has been made with boards marked Ver1.2 with the 100uF output coupling capacitor C18. The PCB overlay on the board is incorrect and has the capacitor around the wrong way. The +ve connection should be connected to Pin 5 on the LM386, IC4.
2/ Intermittant audio in AM mode will require a pull down resistor fitted on the gate of the 2N7002 FET T1 to ground. This can be soldered between pins 1 and 2 of the F2 connector.
The images below show both sides of the board. The amplifier is very easy to construct as it mainly uses conventional leaded components. There are a few SMD components on the bottom side of the board which are quite easily fitted.
1/ To fix the audio plop from the speaker when going from TX mode back to RX, the discharge time of the audio mute has been increased by changing Capacitor C20 has been to 2.2uF.
2/ A mistake has been made on boardss that are marked Ver1.2 with the 100uF output coupling capacitor C18. The PCB overlay on the board is incorrect and has the capacitor around the wrong way. The +ve connection should be connected to Pin 5 on the LM386, IC4.
3/ Intermittant audio in AM mode will require a pull down resistor fitted on the gate of the 2N7002 FET T1 to ground. This can be soldered between pins 1 and 2 of the F2 connector.