I did a little more formal temperature testing of the Delta Power Supply Module, model AA04D1515A.
I arranged a bunch of 10W resistors into four different pairs of loads (350Ω, 250Ω, 150Ω, and 116Ω). I tested the module with both the positive side and negative side of the power supply connected to identical loads. I measured the module's temperature with a Fluke 62mini infrared thermometer. The ambient temperature for these tests was 67°F. Informal empirical testing did not find any particular hot spots on the exterior of the Delta module, so the temperature was taken off the top surface of the module.
I switched on the power, and measured the temperature every five minutes, and recorded the measurements below. At the 40mA output needed to run the Rhodes Janus preamp, the temperature rise was minimal. It rose to 79°F and stayed there. This is just barely warm enough to detect by touch (it started out cold to the touch, and five minutes later felt "not cold anymore"). I stopped measuring after 90 minutes. I think it is safe to say that it would run at that temperature indefinitely.
These tests were performed in free air, so maybe the module would get slightly warmer when confined inside a small box. The spec sheet says it can run at full capacity in an ambient temperature of 122°F (50°C). The spec sheet also says the module itself will shut down at 194°F (90°C), but restart at 153°F (67°C) (I assume they mean internal temperature). In my high-current tests, I got the module up to 101°F without any apparent problems.
So I think we should feel comfortable using this 4-watt module to power the Rhodes Janus preamp.
Here is the data for my four tests:
Target Load Current
Current Ohms Measured Start 5min 10min 15min 20min 30min 60min 90min
42mA 350Ω 42.9 mA 65 °F 73 °F 78 °F 78.5°F 79 °F 79 °F 80.5°F 79 °F
60mA 250Ω 59.2 mA 70 °F 76 °F 80 °F 83.5°F 84.5°F 87 °F gave up after 30min
100mA 150Ω 99.5 mA 67.5°F 80.5°F 88 °F 90.5°F 93.5°F 95 °F "
129mA 116Ω 127.1mA 70 °F 86 °F 93 °F 95.5°F 98 °F 101 °F "
The 116Ω loads proved that the module could easily produce the current claimed on the spec sheet. The module got warm, but still comfortable to hold in your hand (barely more than body temp). The load resistors, however, got impressively hot. The 100Ω resistor carrying all 129mA of current got up to 155°F - quite painful to play with.
Sean