Longest tube cable runs

Posted: Fri Feb 15, 2008 8:40 am

FifthCircle wrote:

We placed a universal power supply at the edge of the stage and ran the cables to each mic from there.

It's probably a cliche by now, but Einstein once said, “If at first the idea is not absurd, then there is no hope for it”. When I first read Ben's post (above), I actually misread it and thought he had suggested an *uninterruptible power supply*.

That's absurd, I thought to myself. Then I realised that, in theory at least, the idea offers a lot of hope for dealing with a situation like yours. Imagine that you had a battery powered AC supply, as used in uninterruptible power supplies, that was powerful enough to drive your tube mics for the duration of the rehearsal and gig. You'd probably need one or two lead acid batteries (deep cycle type is preferred, although a car battery might get you by for occasional use) hooked up to a true sine wave inverter, with an appropriate ampere/hour capability and maximum current capability to power your tube mics for the day.

I think that would be a great solution; you can place the AC where you need it, and where the fire marshall's book doesn't need it. And apart from the fabulous autonomy, you can feel smug in the knowledge that the AC produced by your true sine wave inverter has (typically) 2% lower THD than the mains power - so maybe cleaner audio!

It's a very simple system. All it needs is a mains powered battery charger, the battery itself, and a pure sine wave inverter. The whole assembly could be mounted on a small trolley and wheeled to where you needed it - a bit like the lower half of a film sound recordist's rig. And, just to be on the safe side, between sound check and gig you could unplug it, wheel to a fire-marshall-approved-AC-powering-position, and top up the charge.

Okay, okay, I'm going back to my box in the patent office now. Sorry...

Posted: Fri Feb 15, 2008 9:15 am

Interesting...that's not a bad idea, albeit a little on the Rube Goldberg side. Wink

Okay, so I haven't really weighed in on the many replies thus far, so here goes:

Re: running extension cables -

One of the things I'm trying to avoid is having power supplies scattered all over the place. I'd like to have them in one convenient location (such as in the wings, etc) and have all the leads come from there. In some of the locations I deal with (200+ year old churches) there isn't grounded power or sometimes any power within 100 feet (approx 30 meters).

So, if I were to have say 6 tube mics in use, I'd have 6 extension cords running from a single point to tube power supplies scattered in a hall (and in many cases, no place to visually "hide" the power supply, so there'd be a big ol' box sitting in the floor next to the mic stand.)

I'm not worried about the voltage or current running across the lines (although the local fire marshall is). It's more of a convenience thing for me. (and a big factor that is!)

Re: the fire marshall:

Yes, he's a jerk and an idiot. However, there's no getting around him. Fortunately, the only FM I've ever had a problem with is the guy that does stuff here locally. Since that makes up less than 5% of my business, I can get over that. I've never had problems with FMs in any of the major cities in the US that I've travelled to.

Remy - it's actually not the guys at Schlesinger. Despite their relatively stringent rules about leads, etc., they're a breeze to work with (but you know that...) Yes, I did actually get the contract for George's ensemble and I do feel bad that I "took" it away from you and Bruce. Please understand that, while I did indeed thirst for their business, I was under the impression that it was their bass-clarinetist that was doing the recording and didn't know until 1 hour before we were both on site that it was otherwise. I will gladly make it up to you with free beers for life whenever we have the fortune of frequenting the same watering hole at the same time.

Boz -
I love the idea of the AC box. It fits my sarcastic profile to a T. Of course, practicality goes out the window there, but hey, it's humerous none the less.

My whole reasoning behind all of this is that, for the longest time, I've been using Schoeps CMC 6 bodies and various caps for most of my recordings (along with some odd-ball Gefells and AKGs and of course various ribbon mics). However, in a move that I'm considering right now, I may be shifting to quite a few Schoeps, Neumann, and Mojave tube microphones to use along with some of my existing stuff.

That being said, I don't want to be leaving little droppings all over the various halls consisting of tube power supplies, extension cords and all the other lovely acoutrements.

Given that about 80% of my business is live-concert recording, this just simply isn't an option.

I do appreciate all of the advice and information that you have provided. Ben and Tom - your empirical evidence is enough to convince me that I shouldn't have a problem.

I will likely attempt to use 100' cables on the Mojaves when I have them here for review later this year and see how they work out.

Thanks-

Jeremy

Posted: Fri Feb 15, 2008 11:58 am

If you were using the same mics, one could build a central HV+heater supply. This would be a practical and elegant solution. Clean on stage or in the cloud, easy set-up and low hump factor.

*****************************

The UPS idea is kind of neat. You need to figure out the power req's. Lets say the tubes are going to pull 12v @ 300mA for the heaters and 300V @ 3mA for the B+...

12*.3= 3.6W
300*.003= .9W
_____________
Total Power per mic = 4.5W

and for Six Mics = 27W

27W from 115V will pull 250mA. Assume you want this thing to supply for 6 hours to cover reh and perf.

6*.25=1.5Ah battery

Posted: Fri Feb 15, 2008 12:53 pm

Wow...that's even less power consumption than I originally thought.

The battery idea might not be a bad one! Considering that rate, I could get by with a relatively light battery.

Posted: Fri Feb 15, 2008 1:16 pm

Sorry, I had to pull away before I finished my last post.

That 1.5Ah figure is wrong. The 250mA is the A/C current draw. Likely the battery connected to the inverter will be 48V. 27/48=600mA. * 6hours = 3.4Ah from the battery. I always like to at least double my requirements to cover stray losses and for safety. So that leaves us with 7Ah minimum from the battery.

Posted: Fri Feb 15, 2008 2:52 pm

Zilla wrote:

If you were using the same mics, one could build a central HV+heater supply. This would be a practical and elegant solution. Clean on stage or in the cloud, easy set-up and low hump factor.

It was awesome in the "hump factor." The way this system was designed (I didn't design it- rather my colleague who was somewhat of a tube design genius did) was that the power supply output whatever the top filament voltage required for any of our mics was. Then, each cable was modified (usually with a diode) to knock that voltage down for a given microphone to what it required. No changes were made to the actual signal path of the mics. For microphones that had variable patterns (say an SM-69 or a UM-57), a small adapter was made to adapt that single feed into what the microphone needed to operate with multiple patterns.

He made a variety of power supplies that all sounded pretty awesome that ranged from 2 channels all the way up to 8 channel supplies. Believe me, it saved a huge amount of effort in the setup and tear-down of gigs where multiple tube mics were used as the rig was really not much more difficult than placing a set of preamps on stage for the mics.

--Ben

Posted: Fri Feb 15, 2008 10:30 pm

Zilla wrote:

Likely the battery connected to the inverter will be 48V. 27/48=600mA. * 6hours = 3.4Ah from the battery.

Right... but before going too far with that, we need to factor in the efficiency of the inverter, which is typically 90%. So to get 27W out of the inverter, we'll need to put 27/0.9 = 30W into it, from the battery.

If a standard 12V DC car battery was used (not the best choice for regular work if deep discharging is required, but okay for occasional use, and cheap and readily available), you would be drawing 30/12 = 2.5A.

If you wanted, say, 6 hours of autonomy the 12V battery would have to be rated at 6 x 2.5A = 15A/h.

Zilla wrote:

I always like to at least double my requirements to cover stray losses and for safety. So that leaves us with 7Ah minimum from the battery.

Agreed. So for six hours autonomy with a 12V battery you'd want something around 2 x 15 = 30A/h capacity. Theoretically, you could run your microphones for six hours and the battery would only be 50% discharged - this is reassuing if things end up going a bit longer than six hours. It's also good for the battery life.

Then there's the 12V DC to 115V AC inverter to consider. The equipment requires 27W. A true sine wave inverter rated at 50W or more would do the job with ease.

Of course, all of this assumes that the total power drain from the system to power the tube mics is 27W. If you wanted to get serious about this idea, you'd need to go back and figure out how much power those mics are actually drawing. I reckon Zilla's earlier estimates would be pretty close, however...

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