Weird and unwanted 80hz hum???

[bleuciel]

I've experienced a weird problem few days ago. I just installed in my rack a new 4 channels mike preamp (Vintech 473). In the rack there is 3 other pre, 2 DBX 386 and an Art Dual MP, and a Rolls headphone amplifier. Everything was fine till I sent a signal in the channel 4 of the Vintech (I didn't have the same problem with three other channels). As soon as I've cranked the gain I heard a bizarre hum of about 80hz too loud to record through that channel. I turned crazy about that and I've wanted to find what causes that noise. The first thing I did was to take off the pre from the rack and the hum stop completly. Now I work with that pre off the rack but I don't know the reason that caused that hum and I don't have obviously a solution. If someone can suggest something, please go for it! It's a problem I've never seen in my life.Thanks,Jean Custeau / Bleuciel@mail.com

[mazz]

Hi,I'm going to venture a guess that it might be one of these things:If your rack has metal rails then all of the chassis of your gear are connected to each other via the rack rails. A bad shield on a cable on any one of the other pieces could cause what you are hearing. 2 solutions: find the bad cable. After that, use plastic washers to isolate your gear from the rack.It's obviously not a problem with the preamp since it works outside of the rack.Put the preamp back in the rack, recreate the hum and then start disconnecting cables from all the gear in the rack, one at a time.Hope this helps,Mazz

[bleuciel]

Thanks a lot Mazz. I'll try this tomorrow and I will come back.Jean Custeau

[nickbatzdorf]

It's the fourth channel, which is likely to be on the right side right above the power supply of the unit below it. That's where my money's going: induced hum. It's going to be 60Hz, and it's probably more of a buzz, right? Chances are it's not a ground loop caused by the metal rack rail, or you'd hear the hum in all four channels, no?I'd try rearranging the order of the units in your rack. The good news is that due to the old inverse square law, doubling the distance from the induced hum source reduces the power to 1/4, tripling it reduces it to 1/8, etc. - in other words you only have to move it a short distance to get rid of it.

[mazz]

Quote:It's the fourth channel, which is likely to be on the right side right above the power supply of the unit below it. That's where my money's going: induced hum. It's going to be 60Hz, and it's probably more of a buzz, right? Chances are it's not a ground loop caused by the metal rack rail, or you'd hear the hum in all four channels, no?I'd try rearranging the order of the units in your rack. The good news is that due to the old inverse square law, doubling the distance from the induced hum source reduces the power to 1/4, tripling it reduces it to 1/8, etc. - in other words you only have to move it a short distance to get rid of it.Good call Nick! My next question was going to be "does anything use a wall wart?" cause I've gotten induced hum before from one of those being next to an audio cable. The fourth channel only threw me, I'll keep that one in the memory banks!!Great to have you monitoring the topic!Mazz

[bleuciel]

Million thanks Nick and Mazz. I really appreciate your support. I will try everything tomorrow to solve that problem (it's 5 o'clock here) and I will get back. If I can't I will launch another SOS. Thanks againJean CusteauP.S. TAXI: What a marvelous organization!

[nickbatzdorf]

Well, let's see what the problem turns out to be. There's always a good chance that I'm completely off-base and it's something totally different.

[bleuciel]

Hi Nick!Still a mystery. I tried a lot of things, plug & unplug everything I can. When the Vintech is in the rack that buzz keeps on except if all the rest is off power. The only way for me is to putting it out of the rack but it works fine when it's on the rack, no buzz!!! It's probably the little distance that unit need to works. May be that unit is a loner and doesn't like company...Thanks again for your message.Jean Custeau, bleuciel@mail.com

[ddusty]

Hmmm,I would go back to Mazz's original post. sounds like a ground issue. did you say the only way it works in the rack is if everything else is turned off?and it works fine outside the rack?Go with the plastic washers on the rack, see what that does.Rob

[nickbatzdorf]

This is why God created wooden rack rails, whether or not that turns to be the problem in this case. I don't know why everyone has switched to steel, because the wooden ones were much better.