Hum from amp through snake

[MrBlackthorne]

This isn't really a DIY/Homebrew question per-se, but I wan't sure where to post this, and I'm interested in the techie answer to my issue.

I just picked up a pair of Behringer HA400 headphone amps for my studio. The price point on these is nice, and hey, they're just for monitoring while recording. I intended to run the stereo output from these amps through my snakes to the 1/4" TRS jacks in the recording rooms. I've currently been using a mishmash of splitters from my mixer to my patchbay, and this seemed like an elegant solution to give everyone the volume they desire on their cans.

Here's the problem: It doesn't work with most (but not all) of the headphones in my studio. Most pairs of headphones just give a hum and very low sound level, crackling if I turn up the volume. A couple pairs of headphones work, though! The amps work great if I plug the headphones directly into them, and they also work fine if I run over 100' of extension wire from the amp to the headphones, so I'm presuming it's not the cable distance.

Weirder still, if I re-amplify the signal after it goes through the snake, the headphones work fine. So the audio signal is making it to the output on the snake fine, just not in a way that most of the headphones can output.

My thoughts were: Losing voltage over the snake? (But then why do SOME sets of headphones work? Why does boosting the level just crackle?) Impedance issue? Grounding issue? (But again, no hum if I re-amp the signal, and the signal sounds great on some headphones.)

Obviously, I'll be returning these amps and getting something else. Still, I'm interested in why they behave this way. Also, I wonder if I might have a similar problem with another brand of headphone amp. I don't have this problem with the phones output on either of my mixers...

Thanks for the help,

Rick

 

[mshilarious]

Could be an impedance issue, if the snake is real light gauge. Headphones vary in their impedance, so a headphone with a lower impedance will cause a larger voltage drop across a wire, which is dependent also on the gauge of the cable.

For example, if you have a 100 ft run of 28 gauge cable, that's 12 ohms; your headphones could be 32 ohm or 24 ohm, so you will lose a lot of power along the way. Let's say you want 42mW out of the cans, that's 1VRMS into 24 ohms. Send the same 1VRMS through that cable, the total impedance is 36 ohms, for power of 28mW, but of that 9mW will be lost to the cable.

I don't get why you would lose all of your signal, or the crackling, but maybe somehow there is a really bad connection somewhere in the snake or the wall plates which is not noticeable at line-level impedances, but becomes a severe problem with a low-impedance signal.

Try putting the headphone amp in the recording room, and running a line-level signal through the snake to the headphone amp.

 

[MrBlackthorne]

Thanks for the input. I'm pretty sure you're right -- this is impedance-related.

Another interesting thing is that I hooked two sets of headphones (one that works, one that doesn't) into the jack in parallel, and that caused both sets of headphones to not work. Interesting...

I'm perplexed, because I've sent the headphone output from two different mixers over these snakes before. The sound is always crystal clear. But, I suppose that's what I get for spending $30 on an amp.

Sending the signal line-level and amping it post-snake probably makes the most sense. I was trying to avoid that from a neatness perspective (no place but the middle of the floor to put it), and it's nice to have the ability to change the levels from the control room. Either way, it'll serve the purpose, though.

Thanks again,

Rick

 

[MrBlackthorne]

Sorry... inadvertent double post.

 

[MrBlackthorne]

Just an update on this issue... I decided to return the Behringer amps and give the ART HeadAmp4 a try. Big difference. The ART feels sturdier, the knobs don't feel as chintzy, and it has no problems playing through my snakes. I guess it's true that you get what you pay for. (Though the HeadAmp4 isn't THAT much more expensive. I should have bought it to begin with.)

Rick