Headphone Amp with Crossfeed
This headphone amplifier is based on a design by Toni Kemhagen published on the Headwize projects page. It uses Burr-Brown (TI) OPA627 opamps in the voltage stages and a Burr-Brown BUF634 as the driver. This circuit includes adjustable/defeatable crossfeed to help bring the stereo image in front, instead of inside your head. More details can be found in Toni's write-up. The BUF634 can supply up to 250mA of current and should be able to drive any dynamic headphone with ease. I use mine with Sennheiser HD-580's and Etyomotic Research ER-4S.
The parts list is available as an Excel file here. Components without reference designators do not mount on the PCB. If an item is marked as zero quantity, it means that there are multiple parts that can be used of varying quality.
The OPA134 or OPA604 can be substituted for the OPA627 if you don't want to pay for an expensive opamp. Pins 5 and 8 are tied together to support DC offset adjustment for both parts.
You can download the schematic here in PDF format.
The PCB's are double sided with both surface mount and leaded components.
Files for download in PDF format (V2.3):
Top silk screen, bottom silk screen, top layout, bottom layout
These photos are for version 2.2:
This is a version I made as a gift:
This figure was from the Project Addendum of the Toni Kemhagen page on Headwize.com. It no longer seems to be available on the website (Feb 2013). The PWB is labeled +1, +2, +3, +4, corresponding to switch positions 2, 3, 4, and 5 respectively. They each represent a level of “stereo expansion”. Your switch needs to be 6-position 2-pole. There are 2 types of rotary switches: non-shorting and shorting. A “non-shorting” switch breaks the connection before switching to the next. A “shorting” type makes the next connection before breaking the first. Either type should work since there are no DC voltages or high currents being switched.
If you don't care about changing or disabling the crossfeed, you can choose to simply hardwire the points so that a switch is not needed. The following picture shows how to wire up the Lorlin CK2394 switch:
It will be easier to assemble if you start with the surface mount components first. This will allow you to lay the board down flat. When working with surface mount components you must be very careful not to apply too much heat. It's best to use a very fine tip iron and small diameter solder. I usually set up my iron to 650-700 degrees and use 0.01" or 0.015" solder. A set of good tweezers will help too. I personally use a microscope when soldering surface mount components, but the 1206 parts should be large enough to do with the naked eye.
Since it's difficult to hold tweezers, solder, and iron all at once, I like to first put a dab of solder on one pad, then hold part in place and remelt the solder. You can then go back later and solder the other end.
The design uses ±15 volt regulators so you will need to supply it with at least 18 volts depending on the type of regulator you choose. Some are low dropout and can tolerate a lower input voltage. Standard types typically need around 2-3 volts higher than the output voltage. The power source needs to be DC rectified.
The potentiometer is for nulling the DC offset at the opamp output. I've found in practice that this can be used to null the offset at the circuit output without requiring a lot of DC offset at the first opamp output. In fact there is so little DC offset that the circuit will be just fine without the pot. But the adjustment is there for those who want to use it.
The output series resistors R25 and R31 may be adjusted to change the sound with your particular headphones. Series resistance changes the damping factor and will affect the quality of the low frequencies. The BUF634 has current limiting so it's safe to omit these resistors if you want maximum damping factor. Increasing the value will result in a "warmer" sound.
This webpage last updated Wednesday, February 20, 2013 11:49:56 AM