I've built quite a few basic CMOY portable amps over the years for friends and family. All of these were the basic CMOY design, powered by a single 9-volt battery, normally using an AD823 op-amp due to its tolerance for low battery voltage. All of these have run really well, but I always wanted a desktop CMOY amp that would operate at 18 volts on either battery power or AC. I also wanted to incorporate a Tangentsoft modified Linkwitz crossfeed kit that I built up years ago but had never used. Tangentsoft no longer makes the crossfeed kits, unfortunately.
I decided to use an external 18 VAC wall wart to power the amp to avoid any possible AC noise coupling inside the enclosure. I built the rest of the AC power supply section right on the board with the amp circuit itself. I used a small bridge diode unit from Radio Shack, followed by a 440 mf capacitor for filtering. The output of the filter cap goes to an LM317 adjustable voltage regulator programmed with resistors for 19 VDC output. The output of the LM317 has a small 1 mf capacitor to ground to improve transient response. I feed the power supply output via a diode to the main power rail. I also wired a pair of 9-volt battery clips in series through another diode. This allows seamless switching between AC and battery power.
The CMOY circuit and rail splitter circuit are entirely standard. I did upgrade the split-supply filter caps to 440 mf, but kept the simple resistive divider circuit. I opted to use the Burr-Brown OPA2132 over the AD823. The AD823 will only source 15 ma/channel, which can cause issues driving low-impedance headphones. The OPA2134 will source 30 ma/channel. Since operation at low battery voltage isn't really a concern in this dual-battery unit, I wanted the higher current capacity, although I like the sound of the AD823 better.
I wired the modified Linkwitz crossfeed board in between the input jacks and the volume control. The version of the crossfeed kit I ordered and built is the high impedance version, so needs to go in the input path. The kit has provisions for two DPDT toggle switches, one to enable/disable the crossfeed, the other to adjust the level of crossfeed from low to high. I brought these two switches out to the front panel.
I also wired in 1/4". 1/8", and RCA jacks from the input to the crossfeed for maximum flexibility. On the output side, I provided both 1/4" and 1/8" jacks to avoid the need for headphone adapters.
The build came out very nicely! There is absolutely no audible AC hum when operating from the wall transformer. The crossfeed cuts the input signal down a bit. The x11 gain of the stock CMOY circuit easily compensates for the loss without resorting to higher than normal levels from the source device.
At first, I could barely notice the effects of the crossfeed on the listening experience. I did verify that it was working as expected by using the balance control of the source device to cut out each channel with the crossfeed engaged. After several hours listening with the crossfeed at its lower setting and then disabling it, I could clearly hear the difference. Listening to most material with the crossfeed engaged is very pleasant and easy on the ears. I found the lower setting to be ideal for most modern pop mixes. The high setting narrows the sound stage too much for my liking, unless listening to older material, like the Beatles "Rubber Soul" or "Revolver" albums that use extreme left-right channel separation.
Nothing really exciting about the design, but my new Grados are certainly happy with the result.
Here's a few pictures of the build: