Guitar Amp to Computer Hum

[steve]

The UCA 202 has a pair of line inputs and a pair of line outputs, so you can use it for playback.
You are likely to get less noise from the power supply when playing back via the UCA 202, but you will probably still get some noise.
The problem is that PC laptop power supplies are very inexpensive, and electrically noisy. Apparently Macbooks do not have this problem, but a Macbook power supply costs about 4 x more than a PC power supply.

[philby1]

Thanks for your help.

[kozikowski]

a Macbook power supply costs about 4 x more than a PC power supply.

Unless you get one on eBay and take that one to work so you can leave the regular one home.

Koz

[philby1]

This is where we usually send people off to buy a UCA202 USB sound device.Koz

Well, the UCA202 arrived, but the hum remains when connected to eitehr the desktop or laptop. Not only that, The computer is receiving no signal from the amp/mixer. I have Audacity set to recording and nothing shows on the screen. I'm lost.

[steve]

he UCA202 arrived, I plugged it in, and the hum remains.

You plugged what into what and the hum comes out where?

You have a lot of potential sources for this hum - and reading back through this topic it looks like you are talking about two different hums - one when recording and one when playing back. The only way we are going to get anywhere with this is to break the problem down into individual steps.

When you switch on the amp (nothing connected) does it hum?
You ten plug the amp into the UCA202, does it hum?
Keep going, one step at a time until the hum starts.
At that point, what can you disconnect without "curing" the hum?

[philby1]

Hi Steve

Thanks for your interest in helping here. I'll endeavour to explain things concisely and clearly. At the outset, a clarification: The hum is primarily coming from the guitar amp. The volume of the hum is dependant on the volume setting on the amp but is still present at low volume even when the volume knob is set to "0". I don't generally notice the hum coming from the computer speakers but Audacity definitely picks it up. The hum is heard when I'm trying to record but is present the moment ANY physical connection is made between the amp and computer.

Here is the situation:
1. Mixer "Main Out" into amp "Input" = no hum.
2. Add "RCA Out" from mixer to "Mic In" on computer = hum. (Crude, I know)
3. Replace No. 2 above with UCA202: "RCA Out" from mixer to "USB Input" = hum.

Efforts to correct:
1. Powered everything from same powerpoint (and different points out of desperation).
2. Moved amp to different room.
3. Checked for any nearby equipment that could interfere (none).
4. Switched off LCD monitor.
5. Used different cables to make connections.
6. Turned off energy-saver light globes.
7. Powered down mixer (making it a passive connection).
8. Removed mixer altogether.
9. Ensured no audio cables were crossing power cables.

Other issue:
1. ON UCA202, I changed RCA's to "Output" to send signal from Media Player to amp. Signal is sent but hum present then also.

I look forward to your input. This is frustrating. I just want to sit down and record a few songs for myself but it seems nearly impossible!

Thanks,
Mark.

[steve]

Let's try approaching this from a different direction.
Do you have headphones?

In the following tests, if some item is not specifically mentioned, then it means that item is not connected for that particular test - for example, if I say "UCA 202 plugged into the laptop", then I've not mentioned plugging the laptop power supply in, so leave it disconnected unless the test specifically says "laptop power supply connected to laptop".

Test 1
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?

Test 2
Same as test 1, but also connect the "Tape out" (RCA) to the input (RCA) of the UCA 202.
Any hum?

Test 3
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?

Test 4
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.
Any hum?

Test 5
Connect the USB plug of the the UCA 202 to a USB port on your laptop.
Boot up the laptop (running on batteries).
Any hum?

Test 6
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?

Test 7
Repeat test 6 with the laptop plugged into mains power.
Does Audacity playback still sound good?
Any hum?

Test 8
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)
Any hum?

Test 9
As test 8, then:
Plug in the power cable to the Crate (Crate switched off).
Any hum?

Test 10
Power up the Crate.
I guess there must be some hum going on by now?

How far did you get before hitting a problem?

[philby1]

Hi Steve

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.

Thanks,
Mark.

Test 1
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.

Test 2
Same as test 1, but also connect the "Tape out" (RCA) to the input (RCA) of the UCA 202.
Any hum?
No. Precisely the same result as in Test 1.

Test 3
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.

Test 4
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.
Any hum?
No.

Test 5
Connect the USB plug of the the UCA 202 to a USB port on your laptop.
Boot up the laptop (running on batteries).
Any hum?
Laptop: No. Desktop: No. (Note that the hiss mentioned in Test 1 is still present if the volume controls are increased.

Test 6
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.

Test 7
Repeat test 6 with the laptop plugged into mains power.
Does Audacity playback still sound good?
Any hum?
Still sounds good; No hum.

Test 8
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)
Any hum?
No hum in either laptop or desktop.

Test 9
As test 8, then:
Plug in the power cable to the Crate (Crate switched off).
Any hum?
Desktop: No.
Laptop: Yes. Volume of hum is not affected by position of main volume slider. Unplugging laptop power supply removes hum.

Test 10
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.

[steve]

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.

Yes, that'll be due to the lead. Long leads are more susceptible to this than short leads. High quality (shielded) instrument cable is better protected against this.

Test 9
As test 8, then:
Plug in the power cable to the Crate (Crate switched off).
Any hum?
Desktop: No.
Laptop: Yes. Volume of hum is not affected by position of main volume slider. Unplugging laptop power supply removes hum.

Test 10
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.

The difference between the laptop and the desktop is probably because the desktop computer is Earthed, but the laptop computer isn't.

The problem is almost certainly due to the Crate rather than the other equipment, but is probably not a fault with the Crate.
You are not using the Crate in the way that is has been designed to be used and that's why you're getting this problem.
Typically a guitar amp would be used to amplify the input from a guitar, which in the case of an electric guitar will be a magnetic pick-up (essentially just a coil of wire). The signal that is amplified is the difference between the tip of the jack plug and the sleeve of the jack plug. It does not matter if the Crate has is a bit of AC voltage on the sleeve connection, because there will be a near identical AC voltage on the tip contact. As the amplifier is amplifying the difference between the two contacts, and AC voltage that is present on both contacts is irrelevant (common mode noise).

The problem is that when you plug in the mixing desk, the sleeve contact on the mixing desk side is properly Earthed. This will pull down any AC ripple on the sleeve contact on the Crate side down to Earth, which means that the AC ripple is no longer common to both tip and sleeve contacts. Because the AC ripple (hum) is now only present on the tip contact, it will be amplified by the Crate, and that's what the problem is.

Possible solutions:
1) Use a different amp. (probably not the solution that you're looking for).
2) Isolate the Crate input from the other equipment using an audio transformer. This is essentially what a "hum killer" does. I've not used one, but the ART DTI looks like it should do the job. ART have a good reputation for solidly built, good quality accessories at very reasonable prices. You could possibly make one using a Neutrik NTE1 (or similar) for about $15, but it'll cost more than that if you put it in a nice metal box with input/output jack sockets.

What I'd recommend, is that if you have a friendly music shop nearby, either ask if you can take your gear into the shop and try an isolation transformer "hum killer", or ask if you could buy one and return it if it does not fix the problem.

By the way, the reason that the fault does not appear if you just connect the mixer to the amp is because until you have closed the circuit by plugging in the computer, the outputs of the mixer will just be "floating". The mixer output will be getting its "ground reference" from the input, so while there is no input, the output can just drift up and down with the hum from the Crate.

Do keep us informed of your progress. I'm hoping for a happy and inexpensive outcome

[philby1]

Thanks Steve for your detailed reply . As I progressed through the tests you gave me, I was starting to think that it might be the amp - not that I understand all the technical aspects of your explanation. Give me a few days and I'll see what I work out. But I'll definitely give a reply so you know how it all works out.

Thanks,
Mark.