A while ago I posted details of my first experiment with the TPA6120, a high-performance headphone amp IC from Texas Instruments.
More recently I have shown a design using multiple NE5532s in parallel, combined with a PGA2310 stereo volume control, a chip which again has an impressive performance on paper.
For some time now I have been working on a rechargeable headphone amp to fit in a quality, easily-available enclosure.
This design has gone through many iterations, I have drawn up versions with both analog and digital volume controls, some fully DC coupled with a DC servo, some employing the AD8397 or the LT1206 as drivers, sometimes in a composite arrangement with global feedback, some with dual DC wallwarts and some with a single AC wallwart as supply.
In this case I have departed from my usual practise of making the PCBs myself, doublesided home-made PCBs at this level of detail are very labour-intensive, although some prototyping was still done before committing to placing an order. The large majority of parts are SMT, the exceptions are the sockets, switches, trimmers and the handwound inductors.
This particular version has AC blocking at the input followed by a unity gain buffer to ensure that the PGA2320 (in this case) is driven from a source of <600 ohms impedance. Following the volume control chip is the output driver, a TPA6120, chosen, in this case, because of its output short-circuit protection. The main board fits into rails built into the enclosure.
6 CR123A protected lithium cells with a nominal capacity of 880mAH provide power when on the move, These are carried on a mezzanine PCB which also engages in the rails in the enclosure. Fully charged they provide a measured +/- 12.3V. The PCB has a dual- configurable layout, jumpers mean that it can be configured to run either from 2 unregulated DC wallwarts, or from a single AC wallwart. For this first build I have gone the single AC route. CRCRC filters precede the adjustable regulators which are configured to provide a constant-current / constant -voltage charge regime to the batteries or alternately power the amplifier directly. On-board ferrite beads following the socket are intended to keep PSU switching noise off the supply cable..
On/off switching is by pushbutton, one button for on, one button for off. The switching is performed by a relay, arranged to latch its own coil. The protected cells have an internal circuit which interrupts their output when their voltage falls below 3V. Consequently, if any cell interrupts its output momentarily the relay coil is no longer energised and the device switches off with no possibility of a restart unless the 'ON' button is pressed. This SMT relay was chosen for its very low coil power. The on and off buttons and the power socket are positioned at the rear of the unit.
A vertical daughterboard carries the uprocessor and display. The main board layout follows TI's recommendations for a split groundplane with analog circuits on one side of the split and digital circuits on the other. The PGA2320 straddles the split.
Three additional pushbuttons provide control inputs. The right-hand button causes the display to count up, or wakes the uproc from sleep. The left-hand button counts down. The centre button switches control to left, right or both channels controlled. Decimal points on the inverted displays indicate which channel is being controlled. When both channels are controlled the higher of the 2 volumes is displayed. The uprocessor clock shuts down after a few seconds to save power and to avoid introducing noise into the system. A single red LED provides an indication that the device is switched on.
The front panel is pierced in 5 places to accommodate the buttons and the input and output sockets, transparent acrylic has been used for the first build, but a smoked fascia will be cut in due course, this will permit the display and LED to be seen while hiding the rest of the interior.
When turned off and on the ampifier sets the volume to the same level as it was when it last went to sleep. The display wakes from sleep when the right button is pressed. The up/down count is measured at first but ticks over faster after the first five counts.
The assembly altogether weighs a substantial 400 grams, which detracts from its portability a little, it measures 112*90*48mm but the weight contributes to a reassuring impression of solidity and quality.
The main PCB:-
How it goes together:-
How does it sound?
It sounds exquisite. The amp is silent as far as I have been able to discover so far. Volume control is smooth and noiseless. Bass is powerful and distinct without any trace of pitch information being subsumed into boominess. Vocals are natural sounding without sibilance. Rough tones carry no additional edge. High pure tones are sweet and clear. There is an excess of volume available with all the phones I have available and about 100 steps of attenuation remain unused with the sources I have tried so far.
More trials and some measurements next.