Audio Amplifier
This side will explain my construction of a simple AF-amplifier for general purpose.
The amplifier has a squelsh control three different gain set and a sharp lowpass filter.
All contribution to this page are most welcome!

Most of my radio construction needs a AF-amplifier of some kind.
Some construction also need squelsh-control wich turns of the sound when there is no RF-signal.
I needed a general construction for all my project, so I have made a simple card for this purpose.

Block diagram

The Digital volumecontrol unit (DS1869) can choose the input signal from two ways. One is the direct input line the other is through a lowpass filter. I have made this filter very sharp, so if there is any side nois from synthesizers (12.5kHz wine) It will be filtered away, se figure below. I have used a simple two-ways switch to select the inputs.

The DS1869 is a digital potensiometer. This circuit can be configured in several different ways. I use only one input to set the volume control. Se datasheets for futher information.

LM386 is a well known audio amplifier. The gain in this circuit can be set with jumpers. I have made 3 different gain levels.

The LM311 circuit is a comparator. I compares the Ref in with the RSSI in. The Ref in is a variable voltage from a potensiometer. If the RSSI level goes higher than the preset value, of Ref in, the output will go low and trigg the monostabil circuit (NE555). When NE555 is trigged, its output goes high for a preset time (0.5s), and the realy will draw and let the audiosignal go out to the amplifier and to the speaker.

The Ref in is set the 0.5 Volt. The is no RF-signal, so the RSSI level will be around 0-100mV depending on the circuit.
THe output from the LM311 will be high and the NE555 will not be trigged, so the relay will disconnect the audiosignal and the speaker will be quiet.
When a RF signal is present, the RSSI level will rise above the 0.5V and the LM311 will now trigg the NE555 wich will draw the relay and the audio signal will appear in the speaker.
The signal will continue until the RSSI level drops below the 0.5V

Why do I have a relay, when many other constructions uses a transistor ?

Many construction use a transistor to pull down the signal to ground to quiet the speaker. I have used a very small relay instead wich will not pull down the level to ground, it will make the input 3-state instead. I find this much better because I will not get any clicking sound when the signal is turned on and off. The relay I use draws only 8mA so there is no heavy load for the circuit.

Click on the image at right to view the full schematic of the amplifier.
I will not attach a PCB to this construction, because it is so easy to make one yourself.

The respons of the filter

As you can se the filter drops very fast from 1kHz and up.

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Copyright © Last modified on 5 june 2001.