This is an article about my new PCB design for CMoy headphone amplifier. Main feature is that the new PCB fits exactly to the classic Altoids tin-can. This new CMoy incorporates moreover bass-boost circuit and constant current charger for 9V battery.
Few weeks ago I decided to modify my first version of PCB for CMoy amplifier. I wanted to implement following improvements:
+ use new nice small stereo potentiometers with On/Off switch
+ increase capacity of input coupling capacitors
+ add possibility for bass-boost circuit
+ add blocking capacitors for opamp
+ add constant current charger for 9V battery
+ add power jack connector for charging
+ use double side PCB with ground plane
+ modify the shape of the PCB
After few hours of thinking new schematic have arrised.
P1 is the volume potentiometer with On/Off switch. C2 is coupling capacitor. R2 defines the input impedance of opamp. R3 and R4 define the gain of the amplifier (5x in this case). Rb and Cb form bass-boost circuit. These components increase the gain for low frequencies. For the low frequencies the gain is 14. By increasing the Rb resistors you can increase the gain for low frequencies and vice verse. You can decrease the cutting frequency by increasing the Cb capacitors. Bass-boost can be deactivated by shorting out Rb resistors. As a opamp I use OPA2132, but many others can be used as well like OPA2134, OPA2227, NE5532, ... C3+ and C3- are blocking capacitors for power supply for opamp. R5 is optional, but to avoid some bouncing at the output the value should be around 22 ohms.
V1 is a Rail Splitter which creates virtual ground. C1 is high quality blocking capacitor with low ESR. Resistor R1 limits the current through LED. The value should be modify according the type of LED. D2 limits the maximum voltage in the input to 12V. U2 together with resistor R0 create constant current source for charging the 9V battery. In my case the R0 has value 47 ohms and then the current is around 25mA. Cin is tantalum blocking capacitor for U2. D1 is Schottky diode for avoiding any damage when the voltage with opposite polarity is connected.
I designed the PCB with two rounded corners. The PCB fits exactly to the Altoids tin (see pictures below). On the PCB are also nice pictures of input jack and headphones. The resistors and diodes must be assembled vertically to reduce occupied space.
As you can see on my first prototype there are some mistakes and modification:
1) pins 1 and 3 of volume potentiometer P1 must be exchanged, otherwise if the amplifier is switched on the volume is set to maximum
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