Hi all!
I've been experimenting a lot lately on patches that could turn my guitar into something that's a little bit more than a plain electric guitar. ALthough i have been quite successful on the soft side of this endeavour, i am faced with a few problems regarding the connection between my laptop and my amplifier. One is the latency, but i m not too worried about this since i m running PD on an old HP with Ubuntu Studio 9.04 (it can only get better when i switch to a newer machine, right?). Another problem was that of the "manning" of the machine, which i managed to solve by hacking a usb game controller, just like anybody would.

Now here's the real issue. My gear setup is roughly as follows :
guitar --> usb soundcard --> PC (PD) --> usb soundcard --> Amp
The sound card is an EMU 0404 (usb), and all in all ive been pretty satisfied with it's sound. The problem is that for some reason i can hear some noise coming from the amp, not like a constant hum like a ground loop would create, more like the "voice" of the computer. The noise actually changes with something that is correlated to the CPU load of the laptop. Yet there is definitely a ground loop - like noise as well, which can be greatly reduced by unplugging the pc and letting it run on the battery.

So here's my question : has anyone ever encountered such a problem? Do you know of an electronic device that could reduce this noise?

Thank you!

perhaps look at a 'ground hum eliminator'

Does the noise remain when the laptop is running on battery instead of mains? I had this problem too and "fixed" it using an old (seventies) power-socket(don't know for sure how to call this in english) which has no ground pin and effectively cuts all laptop-related groundloops. I know it's probably not really smart to cut the ground, but hey, i've been a noise-free laptop musician ever since

Thank you both for your replies!

Yes, the noise is much lower when i run the laptop on the battery. I ll try pluging it without the ground.
I ve also been doing some research since yesterday and it turns out that the output of my EMU is balanced, whereas the input of my amp is unbalanced (as any guitar amp i suppose). It seems like i might need a DI box, or a reverse-DI, to change the signal from my EMU. Has anybody ever used such things with a laptop?

By the way, i appologize in advance if you think that this topic is only remotely related to PD, but i do believe that we need to find reliable ways of fully integrating a computer into our conventional gear.

maybe something like this would help

http://www.audiomidi.com/Hum-X-Ground-Hum-Exterminator-P4803.aspx

I'll go to the shop i bouht my sound card from this afternoon to see if they sell such things.
But seriously, nobody has ever tried to plug a pc into an amp?? I can't seem to find anything on the net about this.

For what it is worth, I recently resolved a hum by muting everything unnecessary (mic, cd/dvd player, etc.) in the "master-volume" window of my Win-XP laptop.

Good luck, and keep well ~ Shankar

I bought an impedance matching transformer yesterday, it is supposed to change a low impedance and balanced signal into a high impedance unbalanced one. This should idealy give a much better noise/signal ratio. I m gonna try this today and let you know if it works.

Hi,
so here's the update. I have tried this impedance matching tranformer, and the sound is now definitely better. I don't get that annoying grzzz-bzzt-tzzz-grzz noise from the laptop, and i don't need to run it on the battery anymore (which is cool because it's quite old now and the battery is kinda dead).
BUT...
I do get a very nice ground noise, or a constant hum much like the noise one gets from a fuzz pedal when not playing. I don't find it too disturbing as i general grab my guitar with an intention to rock and roll, but it could be a problem in case one needs to process the sound of a quieter and cleaner instrument with Pd (such as an accoustic guitar for instance). I ll let you know if i find a way of solving this issue.

Anyways. For those interested in trying a similar setup, here's what i m using now :

guitar --> EMU 0404(USB) --> Laptop --> Emu 0404 (USB) --> Shure impedance matching transformer --> Guitar Amp

The main problem came from the fact that a guitar amp only accepts an unbalanced high impedance ("hi-z") input, whereas the output of my soundcard spits a balanced low impedance signal. This means that without the transformer i had to crank the output volume all the way up, increasing the volume of both the signal AND the noise. The transformer wasn't that expensive by the way (30 euros in a store in France, you could probably get a better deal on the internet).

I ll post a new reply as soon as i get rid of the ground hum.