I bought a mixed bag of boards recently and came across this one knowing nothing about it, apart from it appearing to be a headphone amp. I finally found out that it is a clone of a commercial project – the Emmeline XP-7 which has been around for a number of years. A cursory search yielded little information about this particular board, beyond that given in a short post by the Slowdiyer, who kindly gave me some help with the capacitor types.
As the board came from a mixed bag, there was no schematic but most of the relevant values are printed on the silkscreen. It uses a high-spec, low noise opamp, the AD797, with a BUF634 buffer in the opamp feedback loop to boost current. The datasheets for both ICs recommend that the supply pin bypass capacitors be of solid tantalum types – 10uF for the BUF634 and 4.7uF for the AD797. Further bypassing is achieved by using either 0.1uF X7R multi-layer ceramic caps (MLCC) or film capacitors (I used Wima MKS). The 220pF caps are TDK C0G/NP0-type MLCC and I used 250mW 1% metal film resistors throughout. The volume pot is a 100k log and provision is made on the board for different pot sizes, I went with the Alps RK27. The enclosure is a Hammond 1455 series, available from many of the big components sellers. I opted for slightly better quality RCA connectors than normal, using REAN NYS367.
The original was designed as a portable amplifier run from batteries that could also be hooked up to an external PSU. I’m not interested in portability for this as I just want it as a standalone desktop amp. It requires a split DC supply so I purchased another ESP P05D preamplifier power supply board from Rod Elliott to use within the enclosure. The P05 board is powered from a 16V 1A AC wall transformer. It uses LM317/337 voltage regulators and can be configured to obtain the necessary 12V by resistor choice.
The construction was straightforward and everything mostly worked as expected during the initial tests, always conducted with ‘safety’ resistors in line with both the PSU from the mains and from the PSU to the amp board. I also use a cheap pair of headphones for the initial tests. There was a noticeable buzzing apparent which increased in volume when the volume control was touched, but this disappeared once I connected a wire from the amp PCB ground point to the enclosure.
As for listening, first I fed a phono cable directly from my phone to the amp. The sound was good and the full scope of the volume pot could be utilised. I then sent audio from a Pi streamer via my Topping D30 DAC. Again, sound was excellent but the much higher output from the DAC means the volume control operates over a narrower range.
Some of the images below were taken before the volume knob arrived and the rear panel is held in place temporarily by plastic plugs.