My stereo headphone amplifier project is ideal for GCSE students learning Design and Technology.
I decided to call this Loads-a-money because this is the kind of circuit one would build if one had loads of money.
After designing the Tiny Tim Headphone Amplifier, a teacher emailed me to ask how I would design it if I had loads of money. Of course, if I were a rich man, and not limited by money, then the circuit would be a little better.
If I were a rich man, then all day long I would biddy bum, making electronic projects fun. I would not have to work hard either. I would put one socket right in the middle of the PCB, and another one on the side just for fun.
After designing the circuit, I realised that it was still too conservative. I suppose poverty is in my blood! That is what happens when you grow up in Croydon under the heel of the imperialistic... :-)
The Rite of Passage
This op amp circuit is one of those projects that beginners make when they want to learn how operational amplifiers behave. This is a useful circuit that one can make at home, and does not need any specialist equipment such as an oscilloscope. The great thing is that you just connect a pair of headphones, and feed an input signal from a radio or a Vintage Walkman, and you can hear how the circuit behaves.
You can notice the difference in sound by changing the feedback resistance, or the difference in bass and treble response by adding a capacitor. This is all useful stuff that can come handy one day.
As Cheap as Chips
Generally, operational amplifiers are not suitable as power output drivers due to the low slew rate; however, one low-cost operational amplifier chip works well. The BA4560N is a low-cost dual operational amplifier chip available in an eight-pin DIP package. This chip is ideal as an audio amplifier due to its high slew rate of 4 V/us, and a gain bandwidth of 10 MHz. However, this chip was a little too pricey for me and costs around 2.00 on eBay.
However, the BA4560N also has a lesser-known equivalent, which is the JRC4560D, and the NJM4560D. These chips are even cheaper on eBay; hence, if you are in charge of a school budget, then you might be interested.
I then remembered the LF353N chip, which was on the Sharp Optonica SM-4100 Input Selector Board. This is a dual operational amplifier chip made by the Texas Instruments Corporation. This chip has a wide bandwidth, and a slew rate of 13 V/us. It also has extremely low power consumption. A great feature of this chip is that it has JFET inputs thereby eliminating impedance matching issues. Moreover, this chip is low cost on eBay. I managed to get five chips for 1.20 from a seller in the Far East.
In the following sections of this multi-page article, I will show how I designed the various stages of my operational amplifier circuit and the reasoning behind some of the components used.
This Article Continues...GCSE Headphone Op Amp Project
Operational Amplifier Circuit Design
GCSE Op Amp Project Build
GCSE Op Amp Circuit Test
GCSE Op Amp Circuit Modification