I have to thank you for your thorough help on this and investment of time! It’s great on a forum to get intelligent, thought-out responses rather than stabs in the dark. I note you refer to plugging into the laptop. The desktop system is the main one I want to work with. However, since you’ve mentioned the laptop, I’ve conducted the tests below on both laptop and desktop.
Plug headphones into the Xenyx 1202.
Plug in the power to the Xenyx 1202 and switch on.
Turn up the level controls a bit and the headphone level to about half way - does it hum?
No. With Main volume slider fully up and one channel fully up, there is only a small- to medium-level hiss similar to a radio that is totally off station. But definitely not a hum.
Same as test 1, but also connect the “Tape out” (RCA) to the input (RCA) of the UCA 202.
No. Precisely the same result as in Test 1.
As for test 2, but also plug a mono jack lead (guitar lead) into the jack socket of channel 1 of the mixer.
Touch the tip of the other end of the guitar lead, and while touching the tip, increase the gain knob, and channel level slider until you can hear a hum clearly through the headphones (headphone volume level about half way).
Now remove your finger from the tip of the jack lead and the hum should stop. Does it?
Yes. Having done this test using three different guitar leads, the result does produce the following: Plugging in (but not touching the end) produces a very, very slight hum - about 5% of the hum volume that is noted when touching the end. I’m assuming this is just lead quality as each of the three produced marginally different ‘sounds’. If I turn the amp volume up at this stage (without touching the end), the hum can be heard there too. Disconnecting the lead between the mixer and amp eliminates hum.
Turn down the gain on channel 1 and disconnect the jack lead.
Connect the output (RCA) of the UCA 202 to the Tape In of the Xenyx.
Connect the USB plug of the the UCA 202 to a USB port on your laptop.
Boot up the laptop (running on batteries).
Laptop: No. Desktop: No. (Note that the hiss mentioned in Test 1 is still present if the volume controls are increased.
Open Audacity and set the playback device to the USB option (the UCA 202).
Import an audio file into Audacity and start playback.
Adjust the levels on the Xenyx so that you can clearly hear Audacity playing through the headphones.
Does it sound good? Any hum?
Laptop: Sounds good; No hum. Desktop: Sounds good; No hum.
Repeat test 6 with the laptop plugged into mains power.
Does Audacity playback still sound good?
Still sounds good; No hum.
As test 7, then:
Connect the “Main Out Left” (1/4" Jack) of the Xenyx to the 1/4" jack input of the Crate. (Crate power cable not plugged in)
No hum in either laptop or desktop.
As test 8, then:
Plug in the power cable to the Crate (Crate switched off).
Laptop: Yes. Volume of hum is not affected by position of main volume slider. Unplugging laptop power supply removes hum.
Power up the Crate.
I guess there must be some hum going on by now?
Desktop: Yes, but very slight through headphones; loud through Crate speakers.
Laptop: Yes, but very slight through headphones; VERY loud through Crate speakers (includes electronic buzz). When laptop on batteries, hum is present also but not as loud as when plugged into desktop.