This project is an AM/FM stereo receiver. It incorporates an AM, FM, FM Stereo tuner for the commercial broadcast bands along with an integrated stereo amplifier.
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.
The AM/FM Stereo Receiver Circuit
The radio receiver circuit is centered around the Si4831 from Silicon Labs. This small, 24 pin, SSOP IC contains all the RF circuitry for an AM/FM stereo tuner. Broadcast stations are mechanically tuned by means of an inexpensive 100k linear potentiometer. The tuning range is set by selecting resistors based on the part of the world you are in. These details are explained in the Si4831 datasheet and Silicon Labs Application Note AN555.
The majority of the remaining required external components are inexpensive resistors, capacitors, and one inductor. A 5:1 RF transformer is required if an external air loop antenna is chosen (as I did) for AM broadcast reception.
The Si4831 provides the necessary connections for the tuning and stereo front panel indicators, the inputs for AM and FM antennas, and stereo audio output.
It’s worth noting that the tuner PCB, with the exception of connectors, uses all surface mount devices. The Si4831 is only available in a SMD package and the application note (AN555) has some specific recommendations on component placement and grounding. I stayed with SMD for all of the parts in an attempt follow those guidelines.
Although I’ve hand soldered small surface mount devices such as the Si4831, it’s not something I enjoy. Some time prior to envisioning this receiver I had invested in a small Qinsi QS-5100 reflow oven. I chose the QS-5100 over the less expensive T962 models based on online reviews such as this one at Dangerous Prototypes. Unlike the T962 models, which apparently require rework prior to use, the QS-5100 is ready to use right out of the box. However, it is necessary to characterize the soldering/heating profile. I intend to post a blog entry on that process at some point.
The schematic for the tuner PCB was developed while studying the Si4831 datasheet and the AN555 Application Note.
I chose to use an external air loop antenna for AM reception since an internal ferrite antenna would most likely be ineffective as it would be mounted inside the steel chassis. 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 Si4831 is not ideal. However, the only AM broadcast I ever listen to comes in nice and clear – Wheels with Ed Wallace on 570kHz. It’s on every Saturday morning here in the Fort Worth/Dallas area.
The Stereo Amplifier
As noted in the title of this post, the amplifier is based on the TDA7266 from STMicroelectronics. Originally, I had used the TDA7297 which is capable of delivering approximately twice the output power of the TDA7266. I had success with that device in a previous project (see TDA7297 Amplifier).
However, in this application, the TDA7297 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. It’s my belief that the 50VA transformer I was using (the largest I could fit in the case) had insufficient regulation for the TDA7297 power requirements. I was also using two 4700uF 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, 3300uF or even a 2200uF filter capacitor would have been more that adequate. I still have that completed PCB with the TDA7297 installed and one day I may return to it to understand exactly what was going on.
So, instead of investigating the exact cause of the power fluctuations with the TDA7297, I built another amplifier PCB using the lower power TDA7266 (the pin outs of the two devices are identical). This time I used a single 2200uF 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 TDA7266.
The amplifier PCB contains the DC rectifier, DC filtering, and power distribution for the receiver. 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 schematic for the amplifier portion of the amplifier PCB follows the TDA7266/TDA7297 datasheet example with some added filtering at the inputs and an additional 100nF decoupling cap at the device power input pins. The additional circuitry required for power input and power distribution is also included.
The receiver is powered by 110VAC mains through the rear IEC connector. An Antek AN-0512 transformer provides 12VAC which is rectified and filtered on the amplifier PCB to provide approximately 15VDC to power the tuner and amplifier. The 15VDC is tapped by a 5V regulator to supply power to the AM/FM tuner PCB.
The internal wiring is shown in the following photos.
The front panel is laser cut / engraved by Pololu and follows the silver face trend I’ve been on lately. I used their brushed aluminum surface/black core, 1.5mm thick acrylic. The artwork was created in Inkscape.
The calibration of the radio dial is linear and spans 240 degrees. While working on the dial calibration for this receiver, I now understand why commercial radios with analog tuning (knobs) don’t have very precise tuning readouts. My strategy for aligning the knob marker with the frequency markings on the dial quickly developed into “it’s good enough.” The photo above shows the receiver tuned to WRR 101.1 FM, broadcasting out of Dallas, Texas.
The rear panel was also manufactured by Pololu. For it I chose their 1.5mm thick black face/white core acrylic. Again, Inkscape was used to create the artwork for the laser.
I had originally intended to provide two auxiliary inputs. That intention didn’t account for the limited number of switchable sections available on the 3 position, 4 pole rotary switch I was using for source selection. I had neglected to account for the fact that I needed to power off the tuner PCB when an auxiliary input was selected (to silence the front panel TUNE and STEREO LEDs). So one of the external Aux outputs had to be sacrificed so I could switch the 5V power lead to the Tuner PCB. The local ACE Hardware store had the correct size hole plugs to fill the voids left by the now missing auxiliary input #2 RCA jacks.
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.
Although this is the second receiver I’ve built I still gave it the SR1 model number (Stereo Receiver #1).