Another buffered opamp headphone amplifier

For no particular reason I built another headphone amp. This one is similar to the Emmeline XP-7 clone I built, in that it utilises op-amps with buffers. The complete setup comprises three components; the amplifier itself, a dual power supply, and a delay circuit, all of which are slowdiyer designs.

The amplifier is relatively straightforward and uses two op amps and the BUF634 buffer. The op amp recommendation is for LM49710 but as these are no longer available I looked for an alternative. The ICs mentioned on the slowdiyer website are all very expensive at the moment, upwards of £10 a chip. I originally fitted LM4562 as I thought these were a rebadged LM49710, however, after building and testing, I realised that they are instead the same as LM49720, the dual version. This meant that the positive supply is assigned a different pin and would not work on this board. Trying to find single chips with the correct pinout was quite difficult, with most of the reputable suppliers out of stock with long lead times. I eventually found some OPA134 and these are now in the amp. The photos below were taken before swapping out the incorrect LM4562. If you have any thoughts on using the BUF634, I would buy some now as this is now an End of Line (EOL) product, at least in its DIP8 package. There is provision for an onboard Alps RK27 potentiometer, check resistor values for the resistance of the pot you use. I set the gain at 5 and this provides full use of the volume control when played through my HD600 headphones. I did notice the buffer ICs get quite hot when I ran the power tests through the 25Ω load. The power output was measured at 675mW into 25Ω and 177mW into 330Ω, very similar to that found with the Emmeline XP-7 and perhaps unsurprising given their similar topologies. The scope readings can be found here.

The amplifier uses a dual polarity supply and this gave me an opportunity to use another new PSU design. This one uses dual IRM 20-12 transformers to provide the positive and negative supplies and each are filtered by a CLC circuit to reduce the switching noise. The board has a nice output setup of two positive and two negative terminals and three grounds, which means I can access both the required 12-0-12 for the amplifier and a single 12V for the headphones delay board.

The headphone turn on delay circuit, based around the 555 timer, mutes the headphone outs for a short time to provide protection for the headphones. To calculate the time delay you can use the formula 1.1 x Rt x Ct or use a Resistor-Capacitor (RC) Time Constant Calculator such as the one provided here. By example, a resistor of 100k and a capacitor of 47uF gives a delay of 4.7 seconds with 12V. The two LEDs show the current state of the circuit. When power is supplied, both light up but the delay LED switches off after the required delay interval, which I thought was a nice touch.

The chassis (2204E Type) was sourced from AliExpress and included the volume knob and front power switch but not the IEC socket, RCA and headphone jacks. The three boards fit quite nicely inside. Listening using a wide range of music and my HD569/600 headphones reveal a very capable amplifier. Thanks to the Slowdiyer for making the boards available.

---