Wyn Palmer Headphone Amplifier
This webpage describes the construction of a high performance headphone amplifier, designed by Wyn Palmer in an AudioKarma thread from 2020:
This thread contains schematics, the bill of material, test data, and other documentation related to this project.
Wyn Palmer, the designer of the amplifier discussed here, is a retired Senior Design Fellow at Analog Devices where he, among many other things, designed integrated circuits. So, a very well credentialed engineer! Many thanks to Wyn and all those involved in this project for doing the R&D work and sharing their results.
A summary of the design, from the above thread, by the designer, Wyn Palmer:
The goal was to provide an unbalanced headphone amp that would produce essentially unmeasurably low distortion at 2v RMS output, could drive headphones from 15-300 ohms, and would operate using the +/15v supply that was used in the preamp.
Specifications:
+/-15v - SMPS is fine. 0.6A per supply rating.
>90dB power supply rejection over the audio band.
Peak output drive current set at 270mA/channel
0.4W max into 15 ohms.
0.8W max into 30 ohms.
0.15W max into 300 ohms.
Stable into any load capacitance.
Output impedance <0.1ohms.
DC offset <2mV.
AC coupled input (0.47u PP cap, 220K ohm input resistance).
1.5Hz - 1MHz 3dB bandwidth.
Thermally protected.
Protected against shorts.
Fault LED indicating overheating condition.
PC board designed using TI guidelines to act as heatsink, and can accommodate 15ohm and higher loads without shutdown at rated power levels.
Stable into any capacitive load.
6dB gain.
10k volume pot can be added.
TH Distortion plus noise @1kHz, 2v RMS output 80kHz measurement bandwidth: <-114dBc (0.0002%) A weighted.
Not appreciably different at 20kHz, not appreciably different for 30-300 ohm loads.
This photos shows the parts before assembly. The switching power supply is the upper right, and the headphone amp PCB at the center with all the parts to be stuffed below, including the ICs on the lower right.
Since this was a 2022 project, supply chain issues impacted the schedule. Availability (or lack thereof) of specific parts required a bit of detective work to track down alternative sources as well as some patience. Some parts had several months lead time.
Note that this project was originally built into the chassis shown ihere, but later moved to a custom chassis to match my Wyn Palmer Phono Preamp, here. The new chassis is shown in all the other photos.
These are the heart of the amplifier: the LMH6321 IC, a unity gain high speed buffer. It's a surface mount component with a heatsink tab on the rear that must be soldered to the PCB for heat sinking.
This is a closeup of the amp PCB. The pads for the LMH6321s are at each side, with another surface mount IC, an OPA1656 used as the input stage, in the center of the board. The surface mount components just require a small tip on your soldering iron for the leads and a large tip for the heatsink, and for me at least, a magnifying light.
This shows the surface mount components in place.
And the completely stuffed PCB. The red parts are bypass and coupling caps, all resistors are 1% metal films.
The inputs to the amp are at the top in this photo, and the output to the headphone jack lower right.
The power supply is a Meanwell PD-2515, +/-15VDC switching supply, capable of 0.8A per leg, or 24 watts total. The same unit is recommended for the Wyn Palmer Phono Preamp, details here.
For the volume control, I used an Alps RK27 50K stereo volume pot, mounted on a small PCB to simplify connections. Shown here pre-wired before installation in the chassis.
Top view of the assembled amplifier, front at the bottom, headphone amp PCB on the right, and power supply on the left.
This is a view of the inside from the rear after assembly. The signal comes in from the RCA jacks on the rear panel (red/black and blue/black wires on the bottom left) to the volume pot on the front panel. The headphone jack is in the center, and power switch and LED on the right.
I installed an aluminum wall to isolate the AC and switching power supply on the right. The green/black/yellow power supply wires go thru that wall to the headphone amp PCB on the left.
I added a PCB for a 12V remote trigger relay circuit, shown here. If no speaker amp is connected to the system, the preamp trigger will turn this headphone amp on and off automatically. If a speaker amp is plugged into the 12volt trigger output, the trigger signal will turn on that amp but not the headphone amp, until the headphone amp front panel power switch is turned on, then the 12volt trigger is cut to the speaker amp, turning it off, and the headphone amp is powered on.
Front view of the completed amplifier. As usual, FrontPanelExpress.com for the faceplate.
Rear view of the completed amp. Audio inputs on the left, AC input and remote trigger jacks on the right.
You may note that there is no visible AC line fuse: its integrated into the Meanwell power supply internally.
Finished unit atop my Wyn Palmer Phono Preamp, next to the Benchmark gear whose cosmetics it was designed to mimic.