Unit 0. A ? It is quite simple to obtain this function using a microprocessor Fig. At first St-by signal from mP goes high and the voltage across the St-by terminal Pin 7 starts to increase exponentially. When this voltage reaches the St-by threshold level, the amplifier is switched-on and the external capacitors in series to the input terminals C3, C5 start to charge.
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There is an LED indicator for power, one showing when a station is tuned in, and another indicating when a stereo signal is being received. An auxiliary external audio input is provided that is currently connected to a Raspberry Pi running Moode Audio.
Broadcast stations are mechanically tuned by means of an inexpensive k linear potentiometer. The tuning range is set by selecting resistors based on the part of the world you are in.
The majority of the remaining required external components are inexpensive resistors, capacitors, and one inductor. A RF transformer is required if an external air loop antenna is chosen as I did for AM broadcast reception. The Si provides the necessary connections for the tuning and stereo front panel indicators, the inputs for AM and FM antennas, and stereo audio output. The Si is only available in a SMD package and the application note AN has some specific recommendations on component placement and grounding.
I stayed with SMD for all of the parts in an attempt follow those guidelines. Some time prior to envisioning this receiver I had invested in a small Qinsi QS reflow oven.
I chose the QS over the less expensive T models based on online reviews such as this one at Dangerous Prototypes. Unlike the T models, which apparently require rework prior to use, the QS is ready to use right out of the box.
I intend to post a blog entry on that process at some point. As it turns out, AM reception is poor even with the external air loop antenna. I suspect that the transformer T1 on the schematic I selected for coupling the antenna to the Si is not ideal.
I had success with that device in a previous project see TDA Amplifier. However, in this application, the TDA presented very high distortion enough that it was easily audible at all but the lowest volume levels.
The first thing I discovered was that the steady 15VDC power I was expecting was fluctuating between 2 to 3 Volts, dropping as low as 12V at times. I was also using two uF filter capacitors which could have been a bit much for that little transformer to keep up with. Looking back, I think that using just one 47oouF, uF or even a uF filter capacitor would have been more that adequate. So, instead of investigating the exact cause of the power fluctuations with the TDA, I built another amplifier PCB using the lower power TDA the pin outs of the two devices are identical.
This time I used a single uF capacitor for the DC filter and left everything else in the circuit the same. DC power remained steady at all volume levels and the amplifier sounded great. I stayed with the TDA In the above photo one can see the transformer secondaries coming in at the bottom of the PCB feeding the bridge rectifier and the filter capacitor locations to the right of it.
At the left of the rectifier is a 5V regulator circuit that supplies power to the tuner PCB. The additional circuitry required for power input and power distribution is also included.
The internal wiring is shown in the following photos. The artwork was created in Inkscape. Front of Receiver Powered On The calibration of the radio dial is linear and spans degrees. The rear panel was also manufactured by Pololu. For it I chose their 1. Again, Inkscape was used to create the artwork for the laser. Rear View I had originally intended to provide two auxiliary inputs.
I never did produce an overall wiring diagram for this project. If I had done so, the rotary switch issue mentioned above would have been discovered much sooner and definitely before the rear panel was made.
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