OPA2134 Stereo Headphone Amplifier
This is an amplifier project to boost the output of an MP3 player or mobile phone that otherwise would not have enough power to drive certain headphones to an acceptable level, which is increasingly the case due to manufacturers wanting better battery life and the EU wanting to protect our hearing. So in the spirit of rebellion here is a little headphone amp that goes loud and sounds great!
The circuit is based around IC1, a Burr-Brown OPA2134PA dual opamp. This chip has a superb specification anyway but with the added bonus of being able to drive relatively low impedance loads such as headphones, it lends itself perfectly to this project. The circuit is fairly standard and is configured for non inverting mode, with the gain being set to 7.8 by Rf and Ra (GAIN = Rf / Ra + 1). A gain of 7.8 was chosen because it sounded just right! Another favourite opamp of mine that's worthy of a listen is the LM4562. To me it sounded a little more laid back in this circuit than the OPA2134 but it's all really a matter of taste. The schematic is shown below
Because this is an AC coupled design that uses a single 9 volt power source, a reference voltage equal to half the supply is required at the + inputs, so that the outputs will also be at half the supply due to the feedback to the - inputs. This will enable the outputs to swing up and down equally and 'clip' equally if they should ever reach the positive or ground rails. This reference voltage is achieved by two 47K resistors in series connected across the power supply which form an equal potential divider. The voltage at their junction will always be half the battery voltage (minus the drop across the protection diode) which in this case is approximately 4.5 volts. Their junction is decoupled by a 100u capacitor which has no effect on the DC conditions of the circuit, but looks like a short circuit to ground at audio frequencies. The half supply reference voltage is fed via two 10K resistors to each + input, which is also the point where audio is fed in, so in effect, the audio source will see 10K to ground. The 47K voltage divider is shared between the left and right amplifiers to reduce the component count, but this also has the advantage that it will supply exactly the same bias voltage to each half of the chip. A veroboard layout is shown below
The amp is protected from accidental battery reversal by a 1N5817 schottky diode. Schottky diodes have a much lower forward voltage drop across them than standard diodes so less power is wasted. Bipolar capacitors are used for input coupling as they have a relatively high capacitance for their size. These are effectively electrolytics that have no polarity
Construction is not critical and any small enclosure is fine, but preferably it should have a battery compartment to make replacement as easy as possible. The one I used was purchased from a UK supplier called Spiratronics which also supplied most of the other parts too. You may be wondering why there is no volume control. Well... one could be added by placing a 10K dual pot at the input, but I decided that as most of the gear that this amp will be used with already has a volume control, another one wasn't required
It's universally accepted that AC coupled amps are not as good sonically as DC coupled ones due to the audio having to pass through capacitors which cause distortion and phase errors. But I still believe that good quality can be achieved with AC coupling... after all, your music source will most likely have passed through output coupling capacitors anyway, so the sound that you're hearing will already be AC coupled! Having said that, it's still better to keep the number of capacitors to a minimum. The advantage of AC coupling is that a simpler single rail power supply can be used and the series capacitors will protect your headphones from being damaged by DC
Using an External Power Supply
The headphone amp can also be powered from an external 12V DC supply by using a simple adapter made from a length of twin flex with a PP3 battery clip at one end and a standard 2.1mm line socket at the other. A 100uF electrolytic capacitor is connected across the junction where the PP3 clip joins the flex. This was omitted from the main circuit to save space and because it's not really needed when the opamp is located so close to its power source... the battery. But now the power source (mains adapter etc.) will most likely be some distance away, it is good practice to include it. See photos below
The red (+) wire actually goes to negative not positive as expected due to the clip now being used in reverse to supply power to another battery clip instead of taking power from a battery as it normally would. The assembly is protected with heat shrink sleeving to prevent it shorting out
A slot is filed into the battery cover to allow the cable to exit, which can be tied in a knot just inside the battery compartment to add strain relief
WARNING: This amplifier can go loud and may damage your hearing if used at high volume for long periods. Please be safe!