Using a headphone amplifier allows you to listen to decent home hifi sources like a turntable or CD player, which are likely to sound a lot better than portable devices. Here's the one I made.
The case for this amp is a Modushop Economica (which are really decent quality). The front panel is a piece of teak salvaged from an old church pulpit and the volume control is the tuning dial off an Akai AA-1125 receiver.
The amplifier itself is based around the CMoy Pocket Amplifier and uses Burr Brown OPA2132 dual op-amps and Vishay/Panasonic capacitors. The copper block is scrap from an old consumer unit and forms a "star" earthing point.
The power supply is a regulated +/-15v split supply which runs off a 16vac wall-wart transformer (the power supplies for Pro-Ject turntables are a good source of these) and is based on the older version of the ESP Project 05 preamp power supply.
All circuits are built on perf board with solder bridge connections. This is a slow way of making circuits and I wouldn't recommend it, certainly not with through plated holes as the solder blobs through to the other side of the board where you don't want it.
The amplifier circuitry is in 3 modules:
First (at the top of the stack) is a fairly standard non-inverting op-amp circuit. I used SIL sockets to allow resistors to be swapped out so the gain can be changed if needed (I doubt that's ever going to happen).
On the bottom layer is a unity gain output buffer made with 2 dual op-amps. This is probably unnecessary most of the time, but could help provide that extra bit of current should I ever use some particularly low impedance headphones.
Sandwiched in the middle is a crossfeed circuit.
This feeds a bit of the left channel into the right (and vice versa) to mimic the stereo image that you would normally get when listening to speakers. The amount is frequency dependent - higher crossfeed at low frequencies, gradually reducing as frequency increases (which you would normally get due to your head being in the way - low frequencies with long wavelengths can bend around your head but higher frequencies can't). This article explains more, and is where the crossfeed circuit I used comes from.
The image above shows the left and right channel output from the headphone amp with only the right input channel plugged in. The signal is a frequency sweep (20Hz-20kHz).
The top stereo pair is with crossfeed turned off - all frequencies are played on the right channel, nothing comes through on the left.
The bottom pair is with crossfeed turned on. At low frequencies the level fed to the left channel is about 3dB lower than the right and the level drops progressively as frequency increases (at roughly 6dB/octave after 700Hz or so).
The frequency spectrum above shows the fall in level with frequency of the crossfeed signal. I'm not sure what the drop below 150Hz is all about - it's probably something to do with the crude test setup.
The crossfeed effect is very subtle. In fact a problem with this amplifier is that I've not yet found a way to label the switches nicely so I've a hard time knowing if the crossfeed is set to on or off!
It does reduce the extreme channel separation you normally get from headphones and it's supposed to make a difference when listening for long periods (something I never do).
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