USB Headphone Amplifier for the ObjectiveDAC

Introduction

The Objective O2 amplifier and Objective DAC have generated a lot of interest recently, due to their excellent objective performance, and low cost. I built up a combined ODAC/O2, and was very happy with it. Unfortunately, the O2 requires an AC power supply at all time and bringing laptop, laptop power adaptor, the O2/ODAC, and it’s adapter leads to not a lot of space for anything else.

One of the main draws of the ODAC is its native 24-bit interface, which means that digital volume control doesn’t lead to reduced audio performance, as is the case with 16-bit audio. With this in mind, it should be possible to design a small board that is powered by USB without any need for a volume control onboard. Audio potentiometers are often quite large, expensive, and their tracking between channels isn’t particularly good. Also, the DAC used on the ODAC doesn’t have the output current capacity or voltage swing to drive headphones, and so a buffer (or buffer with gain) is required afterwards. I happened to have some of the excellent EL2002 buffers (sadly now no longer made) left over, although I will release a second board based on the O2’s driver chip, the NJM4556A.

Switched Power Supply

Grounding provides the complication: USB only gives +5 V, and audio amplifiers, without resorting to large output coupling capacitors, need bipolar supplies. The usual method of providing a bipolar supply from a single polarity supply is to construct a virtual ground. However, in this case the ODAC provides a ground at a different potential and with only +5 V available, this would only give ±2.5 V, which is not typically enough to drive headphones to a reasonable level.

Switch mode power supplies offer a way round this, and a number of high performance, easy to use, and inexpensive controller ICs are now available. I chose to use the Linear Technologies LT3467A for this design; available in a small but easily solderable SOT-23-6 package, with a number of good reference designs. This chip, with a small number of external components, allows the +5 V from USB to be transformed to the required bipolar supply. Two variants are available, the LT3467 and LT3467A, which have switching frequencies of 1.3 MHz and 2.1 MHz respectively. Higher switching frequencies allow the use of smaller value inductors, which are often the largest and highest cost component of switch mode supplies.

Design and construction

The full schematic and layout are shown below (click to enlarge, nb R10 should be 500R not 1K). The power supply is in the bottom left, showing the LT3467A controller chip configured to give ±9 V for the amplifier, energy storage components, feedback, and sundry components. The amplifier section is in the centre. The component values and selection choices are included in the readme.txt file in the download .zip file. I haven’t included an input low-pass filter or compensation capacitors for very fast opamps, but as this is SMD you can easily stack these on top of R6/R11 and R8/R13 respectively.

ODAC_EL2002_schematic

EL2002

The board is easy to build if you have some experience with SMD components; the LT3467 is small but can be soldered pin-by-pin, and I would do that first followed by resistors, capacitors, diodes, inductor, and then the through-hole components. The unloaded -ve voltage seems to be a little higher than expected, but corrects back when the other ICs are installed. The output jack is slightly raised, so needs a fair bit of solder to attach to the board. The board is sized to fit with an ODAC in a Hammond 1455B802 case without any additional mounting hardware.

I’m waiting on some new ODAC boards and the cases, but in the meantime here are a couple of pictures of the constructed board. As before, the boards were made by OSHPark – for the price and delivery, totally unbeatable so far! You can download the Eagle files (Eagle v6.1.0) along with the readme file. Look forward to any comments or questions. Quick update: works beautifully! Casing to come…

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