Headphone amp

This headphone amp is a budget affair and came about beacause a change in the DAC source in the hifi set-up resulted in the loss of a headphone socket. It is configured with a relay, such that with the power off the signal is routed directly from the input to the main power amplifier. With the power applied the headphone amp is powered on and the input signal is routed to this amp.

The circuit is a very simple single ended source follower loaded with a voltage regulator. The circuit was tested in ltspice and showed adequate results with headphone loads between 20 and 300 ohms.

basic circuit showing the relay
ltspice circuit for clarity

PCBs

Designspark pcb layout

Most of the amps shown on this web site have involved point to point wiring on a mixture of matrix board, paxolin or terminal blocks. This amplifier became an exercise in investigating design and manufacture of professional PCBs. The circuit was drawn in Designspark which produced a pcb layout which in turn produced a set of gerber files. The gerber files were sent to a pcb manufacturer who returned the finished pcbs.

finished pcb

Construction

Left side. Heat sinks are OTT but were in the parts box so pressed into use.
From the front. Panels were cut from a piece of 2mm aluminium bought on ebay.
Right hand side. Mosfets and voltage reg are bolted to the heatsink with silpads for insulation.

Measurement

LTSpice plot with 56 ohm headphone load
Actual frequency response 56 ohm load
Harmonic distortion 56 ohm load

Summary

Considering the extremely modest approach, i.e very simple budget project, this amplifier produces quite exceptional results. With 56R headphones(Superlux 682) the presentation is extremely revealing, particularly of hitherto hidden detail. To be entirely ruthless the presentation in this set up could be described as verging on the bright side, but not enough to be fatiguing. In reality it is thought that at least 50% of a listening audience would find this attractive.

The total outlay for this amplifier was in the region of £25(excluding the wallwart power supply) so it wouldn’t be misleading to describe the value for money as outstanding.