Emil's Projects & Reviews

OpenHardware & OpenSource

PreenFM2 SMD Version 13th November 2017

This project is a redesign of the open source FM synthesiser PreenFM2 created by Xavier Hosxe.

Improvements from the original version:

The project new schematic

These are the design files for this project (version 1.1):

Below is a simulation of the audio signal path:

Simulation schematic of the Analog Path to Line Out
Frequency DomainTime Domain

and the mixer and headphones amplifier:

Mixer and Headphones Amplifier Schematic
Time Domain

Bellow are real captures of sine waves at different frequencies. The FFT is computed from the scope trace so the noise floor is much higher than in reality (extra noise is added by the scope's ADC sampling error). The DAC's output capture is taken after the 160kHz filter so the real output voltage is twice what's shown in these captures.

DAC Output
Note C1 SineNote C6 SineNote C8 SineFFT at 4.1kHz
Line Output
Note C1 SineNote C6 SineNote C8 SineFFT at 4.1Khz

Even extreme frequencies of 18kHz and 24kHz look very close to sine waves although we only provide the DAC 3 respectively 2 samples per period. This is of course thanks to the FIR filter inside the PCM5102.

The datasheet measures distortions at -1dBFS and some other people reported some performance loss at 0dBFS. The capture bellow still shows a perfect sine wave even for the 6.12V peak-to-peak voltage swing. However I will limit the range in software to -1dBFS ; for 24 bits this equals to (1<<23) * 10^(-1/20) or +/-7476354.

Extreme Frequency or Amplitude
18kHz DAC out24kHz DAC out1kHz DAC out 0dbFS
18kHz line out24kHz line out

All components have been placed manually and the footprints checked.

The PCB was routed with 10mils traces and 48mils vias, minimum clearance 8mils.

Analog and digital ground planes are not displayed.

Front SideBack Side

Below are links and prices (including shipping) to components I've purchased from Aliexpress (for 10 kits):

ComponentQuantityPriceTotal [USD]
10pcs PreenFM SMD green PCB manufactured by EasyEDA1$37.00$37.00
5pcs STM32F405RGT6 QFP64 STM32F405 QFP ARM2$26.55$53.10
10pcs STM32F103C8T6 STM32F103 LQFP481$11.89$11.89
5pcs MCP42010T-I/ST MCP42010 MCP42010-I/ST TSSOP14 Single/Dual Digital Potentiometer SPI4$4.14$16.56
10pcs OPA2337UA OPA2337 OPA2337U SOP82$3.47$6.94
10pcs PCM5102APW PCM5102 PCM5102A PCM5102APWR SOP2$15.38$30.76
5pcs LCD 2004 20×4 Character LCD Display Module HD44780 Controller Green screen backlight2$18.06$36.12
10pcs HCPL-063L HCPL63L A63L 63L SOP8 new original1$3.87$3.87
10pcs 12mm Rotary Encoder Switch W. Keyswitch HQ with knobs5$9.56$47.80
10pcs 6.35mm Stereo Audio Microphone Female Jack sockets Audio2$8.14$16.28
10pcs USB Type A Standard Port Female Solder Jacks Connector PCB Socket USB-A type1$0.78$0.78
200pcs 2.2uF X5R Error 10% 16V 0603 225 SMD Thick Film Chip Multilayer Ceramic Capacitor1$1.99$1.99
20pcs DIN5/DIN Female Jack/Plug Connector Panel Mount solder type adapter1$7.44$7.44
200pcs 0603 SMD Chip Multilayer Ceramic Capacitor 100pF1$1.39$1.39
50pcs 6*12mm 220uF 25V 6*12 Electrolytic capacitor2$1.78$3.56
50pcs SMD Power Inductor CD54 22uH 5mm5.2×4.5mm1$2.68$2.68
100pcs Tactile Push Button Switch Momentary 12*12*7.3MM Micro switch button + Black Caps1$8.50$8.50
100pcs 10K 5% 0603 8P (2*4P) SMD Thick Film Chip Resistor Network Array1$1.05$1.05
50pcs WH148 knob knob plastic knob AG2 15×17mm type potentiometer power amplifier knob1$2.58$2.58
100pcs Black Plastic Cap Hat G62 for 6*6mm Tactile Push Button Switch Lid Cover1$2.28$2.28
50pcs DC-005 Black DC Power Jack Socket Connector DC005 5.5*2.1mm1$1.97$1.97
1000pcs 0603 104k 100nf 0.1uf 10% X7R 50V chip SMD Ceramic capacitor1$3.32$3.32
500pcs 0603 SMD Resistor 1% 33K ohm1$1.25$1.25
500pcs 0603 SMD Resistor 1% 12K ohm1$1.25$1.25
500pcs 0603 SMD Resistor 1% 47K ohm1$1.25$1.25
500pcs 0603 SMD Resistor 1% 1.5K ohm1$1.25$1.25
500pcs 0603 SMD Resistor 1% 390 ohm1$1.25$1.25
Grand Total$304

So without a case the components price for one PreenFM2 (SMD version) comes to a little more than 30 USD.

Tags: fm, synthesiser.

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