I had my brand new VPC1 and PTQ hooked up for the first time yesterday, and had a whole lot of trouble with an extremely annoying and loud hum through my Behringer keyboard amp/speaker, and crackling through any speaker (although not noticeable with headphones).  The amp may not be the best choice for a powered speaker, but I can't afford to replace it right now. Pretty sure the loud hum was ground loop noise. Using a Behringer PCA202 audio interface. Laptop running Intel Dual Core, so it's halfway decent. What solved it was a combination of reducing the sample rate from 512 to 256 and inserting a Behringer hum destroyer (MicroHD HD400, about $25) after the audio interface. Now I can play even at high volume with neither hum nor crackle, and I can't say I've noticed any degradation of the sound quality.

Any comment on this would be welcome... this was an ad hoc "Trial and error" solution on my part.

I had a similar problem with a headphone amplifier. The HUMX device
worked for me--the hum is completely gone. Also, unlike other units
the HUMX does not go into the signal path so doesn't alter signal quality; rather
the HUMX device goes between PC power plug and outlet. I tried a radio-shack in-line hum remover and pianoteq timbre was definitely altered a bit. Very low latency on my system, BTW, with
an echo-indigo expresscard for D/A I can usually get 64 sample rate. Great sound
card, though many recent laptops don't have expresscard slots. You can find
them on EBAY though, like DELL Studio laptops etc.

http://www.amazon.com/Ebtech-Hum-Voltag...B0002E4YI8

Laptop running Intel Dual Core

I had the same kind of problem.
Try to use your laptop on his battery without connecting the external power bloc.
Do you have noise in this condition?

I had my brand new VPC1 and PTQ hooked up for the first time yesterday, and had a whole lot of trouble with an extremely annoying and loud hum through my Behringer keyboard amp/speaker, and crackling through any speaker (although not noticeable with headphones).  The amp may not be the best choice for a powered speaker, but I can't afford to replace it right now. Pretty sure the loud hum was ground loop noise. Using a Behringer PCA202 audio interface. Laptop running Intel Dual Core, so it's halfway decent. What solved it was a combination of reducing the sample rate from 512 to 256 and inserting a Behringer hum destroyer (MicroHD HD400, about $25) after the audio interface. Now I can play even at high volume with neither hum nor crackle, and I can't say I've noticed any degradation of the sound quality.

Any comment on this would be welcome... this was an ad hoc "Trial and error" solution on my part.

How's the delay with all that equipment hooked up?
I tried using a portable DAC I have for my headphones and it introduced some lag. It was pretty annoying

Any comment on this would be welcome... this was an ad hoc "Trial and error" solution on my part.