KVR Audio

bharris22 · Post by **bharris22** » Fri May 29, 2015 1:23 pm

Hi Everyone,

I recently purchased a Laney IRT Pulse (from Uncle E - thanks, E!), and I think it may be my answer to getting a high-gain real tube sound into my DAW. However, I am getting an annoying RF/EM hum that I can't seem to isolate. I get this hum when I plug the Pulse into a wall outlet (it does not run on batteries) and listen through my headphones using the included jack. I have tried unplugging almost everything else in the house to get rid of the hum, with no luck. I have also tried using a different guitar, different cable, and different outlet, all without luck. The hum disappears when I turn the volume of the guitar all the way down, and there is no hum when no instrument cable is plugged into the unit.

This morning, I plugged the Pulse into my laptop, hoping to run the hum through a spectrograph to see if it was at any particular frequency. The laptop was plugged in to the wall (not on battery power). When I plugged the USB cable in to the Pulse and the laptop, there was no hum. I tested this both with my headphones plugged in to the headphone jack on the Pulse, as well as the headphones plugged into the computer. The laptop was plugged in to the same outlet as the Pulse. At first I thought that plugging in the USB cable grounded the Pulse. However, if I unplugged the power cable from the laptop (taking it off mains power) and had the computer running on battery power, the hum returned, even though the USB cable was still plugged into the computer. Do you have any idea why this would happen, what it means about the source of the hum, and what it might tell me about a solution for using the Pulse without hum when it is not plugged into a mains-powered computer?

Thank you in advance for your help!

lfm · Post by **lfm** » Fri May 29, 2015 4:43 pm

Did you try to ground yourself - putting your hand on some equipment that has protective ground.

Since it behaves a bit like when a guitar is not grounded through strings and onwards - volume down no hum - it comes into guitar as I see it.

Does just pluggin in cable - no guitar - make a difference compared to when guitar is also plugged in(and volume on 10)?

bharris22 · Post by **bharris22** » Fri May 29, 2015 4:48 pm

Thanks for your response. I agree with you - I think the hum is coming through the guitar somehow. Do you have any idea why it would disappear when the Pulse is plugged in to the same outlet as the laptop?

The hum does go down when I touch the strings, but doesn't go away completely. I have not yet tried putting my hand on something else (other than the guitar). If I just plug in the cable, with no guitar, I believe that the hum is still present. In that case, would a ferrite bead on the cable do anything helpful?

Deep Purple · Post by **Deep Purple** » Fri May 29, 2015 4:53 pm

bharris22 wrote:Do you have any idea why it would disappear when the Pulse is plugged in to the same outlet as the laptop??

Because you removed the ground loop that is causing the hum. If you have a common ground for all equipment it should reduce the hum.

Another option is to use a DI box with a ground lift.

bharris22 · Post by **bharris22** » Fri May 29, 2015 5:01 pm

But the hum was there even when I played through the Pulse (using headphones) without connecting to the laptop or any other equipment. I thought you needed two connected pieces of audio equipment to create a ground loop - is that not the case?

Deep Purple · Post by **Deep Purple** » Fri May 29, 2015 5:34 pm

You don't need them directly connected, you just need them both to be grounded which means they are connected.

bharris22 · Post by **bharris22** » Fri May 29, 2015 5:43 pm

But I use the Pulse without even turning on the laptop in most cases - in other words, as a standalone unit - and the hum is still there. Could that still be a ground loop hum created with something else plugged in to another outlet somewhere in my house?

Deep Purple · Post by **Deep Purple** » Fri May 29, 2015 6:04 pm

Yes, it's possible - anything could be causing it. It could also be the wiring in the house.

One other thing - are you sure your guitar lead is balanced? If not, then that could be a source of noise.

bharris22 · Post by **bharris22** » Fri May 29, 2015 7:58 pm

OK - thanks. I'm pretty sure my guitar cable is unbalanced - aren't most instrument cables unbalanced? Since the guitar output jack is an unbalanced output, as far as I understand it, would using a balanced cable have any beneficial effect?

Deep Purple · Post by **Deep Purple** » Fri May 29, 2015 8:06 pm

Yes, you're right - DI box to mixer would be balanced, but guitar to DI box would be an unbalanced cable.

I was thinking too far ahead to you using a DI box. If you could borrow one, you could see if I ground lift helps - it would tell you for sure if a ground loop is the problem.

bharris22 · Post by **bharris22** » Fri May 29, 2015 8:31 pm

Excellent - thanks!

Guitar Pre-Amp/Interface Hum Question