VMP-4000e PS Hum?!

Posted: Mon Jul 21, 2008 11:05 pm

Just got my VMP-4000e today. We got a great deal on a used unit. I wanted to buy a nice, shiny, new one, but our budget is soooooo tight right now.

An initial test with a Shure SM7 had me -literally- giddy. We also summed a mix through channels 1 and 2 with the -25db pad engaged. It really, and I mean REALLY, "glued" the mix together. It wasn't subtle either. Me like !!

I do have a question for you Sebatron owners. Does your unit induce noise into neighboring gear? We installed out VMP-4000e directly above an SCA chassis containing two, N72, preamps. The N72s are picking up a low frequency hum from the Sebatron. Our Elixirs (2Us up) pick it up too but not nearly as bad. The low frequency hum can also be heard in the Sebatron channels themselves as well with just the Sebatron on, no mic plugged in, and the outputs going to our console. This happens at all gain setting but it is obviously more prominent at higher gain settings with no pad engaged.

Let me know how yours effects other gear. Since mine is used, perhaps there is a newer version of the VMP-4000e by now and I can get mine serviced or... something. Perhaps better PS shielding? I guess that is a question for a different forum.

We have a session coming up on Saturday so any help would be greatly appreciated!

Posted: Tue Jul 22, 2008 4:37 am

I can't speak to your particular pieces of equipment but this is not an uncommon problem. I mean even with my API 3124m's, I really can't use a low output level microphone into input 4. That input is directly next to the internal power supply and if gain is cranked, I'll get some 60 hertz. So I avoid doing that with input 4. This is another reason why so many "old-fashioned" consoles don't have the power supplies built in. They resigned in racks, elsewhere in the control room away from the console electronics. Electromagnetic interference is all around us. And even the equipment that is thought to be properly shielded, still picks up stuff. It can become particularly compounding when you work for a large television/radio station. Talk about interference!

If you're hearing junk coming from your device and it has no microphone terminating the input, you need to terminate the input. No microphone preamp is quiet without an input load or, microphone. So if that's your criteria? Your test results will be flawed. And yes, if you stack equipment upon each other and there are built-in power supplies, you may need to rethink your equipments proximity to each other. No other way around that other than "mu-metal" shielding. And that's generally not an option. Heck, we couldn't even use an MCI analog tape recorder, an NBC control room,, due to a 300 amp AC line under the control room! Of course, I came with the fix because, MCI couldn't. And it involved mu-metal shielding around the playback head. There was no other way around it at all. MCI didn't think it was necessary until they met our control room. Then they believed me. So sometimes audio is like a bad buffet.

A new concept! McDonald's buffet!
Ms. Remy Ann David

Posted: Tue Jul 22, 2008 10:16 am

Thanks Remy ! Very Happy

Well if you have purchased the unit second hand it's difficult to say but yes we have been upgrading the layouts to optimize for lowest possible interference.
It sounds as though your unit could use a Faraday Cage
around the power supply.

The very latest 4000e have the input transformers
lifted up 90degress and off the motherboard and located back left.
We have also introduced internal walls to isolate fields .
The front face plates are now 2.5/3.0 mm mild steel instead of
Aluminium which we were using up to three years ago...
Cases are 1.2mm mild steel powder coated 'Textured Black'
for increased durability.

I must add , craming everything in there ,, the valves running at
Proper High Tension putting out heat and all Class A electronics and transformers we always recommend keeping a distance from any other gear... Wink

Posted: Wed Jul 23, 2008 12:29 am

Ran a session with the Sebatron this weekend. While the hum is there if you CRANK the gain on my console it just wasn't an issue in tracking - even with acoustic guitar! It really is way, WAY, down so low that it doesn't matter in real use.

So, everything's cool here! Thanks everyone for your replies.

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