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Gear / Technical Help => Remote Power => Topic started by: kirk97132 on January 04, 2011, 05:19:06 PM
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So I have had the chance to run the DVD battery stepped down to 5volts for the USBPre2. Under loud conditions I am getting a tiny digi drop then relock. so fast my 680 never says digi unlocked but shows that it has acquired lock again. I suspect the Radio shack voltage regulator cannot supply enough current under a big load. It only seems to happen when it is loud. The other issue is I cannot verify the load my TOA KY mics put on it. So be warned that the ratshack voltage regulator is probably not a viable option for the Sound Devices USBPre2. I'll report back anything else I find out, Kirk
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to bad, keep us posted.
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That preamp is "bus powered" right? The spec for USB says 500ma max, so the box can't expect more than that. A 7805 should handle twice that. If you don't have a 10uF capacitor across the output to act as a "shock absorber" then do that, or use whatever circuit is recommended on the package you got. If you don't have the VR well heat-sinked, find a chunk of aluminium such as the CPU heat sink from an old PC, and smear some of that heat sink goop on there (comes in a small tube like toothpaste).
Any 5V/500ma bus powered box is at a big disadvantage compared to something like a UA5, V3, mini-me which has 9V and 1 amp. SD's website says it can supply 10ma of power to each mic. Although most mics only use something like 3ma, Earthworks will use 10, and I'm not sure about TOA. 20ma of 48V phantom power uses a big chunk of the power budget for the entire box, which doesn't leave much for the rest of the box. It's not unthinkable that if you have mics drawing a lot of juice, when the analog circuitry hits clipping that it could starve the SPDIF output connection voltage for a couple of hardbeats.
What I would suggest:
- put heat sink, goop, and caps on the VR if it's not already.
- put a voltmeter on the VR while it's running. If you can see it dipping below 4.75V then your power supply is "out of spec". http://en.wikipedia.org/wiki/Universal_Serial_Bus#Power
- Try it with a Denecke AD-20 power supply and see if that makes the problem go away. Not forever necessarily, but for a test. I can loan you one.
- Try other mics.
If the voltage is dipping below 4.75, then I would focus on improving your power supply. If it doesn't dip, then your preamp isn't quite what it says it is, or your mics draw an unusual amount of phantom power. But the idea of using a 7805 VR is fundamentally sound.
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I'm outa town for a week , I'll check things when I get back home.
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Back home and sorting a few things out. The capacitor idea is one that has got my attention. I had a heat sink but maybe I didn't have one big enough. so I'll redo the heat simk, add the cap and see if I can't get some kind of real world reading on the mic draw. Like I said it only seems to be at really loud peaks( which I assume will be max draw for current) and it happens so quick that I neve see a DIN unlock message on mt 680 even when I am staring at the screen, just a Lock message pops up. So the drop out of the digi lock is very very quick. I will also try it with other mics that I know the draw on, before I make changes to see if I can't narrow it down to mics or the VR box. Thanks for the input!
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OK so a little update. I was able to get a full on tech sheet from the National Toa headquarters in USA. The Toa KY mic does in fact draw 10ma per mic. So yes it is high current draw mic. I am waiting on reply to find out exactly what the preamp draws. The packaging says that the 7805 is good for up to 1.0 amp of current. So even if we say that it can only supply 80% that is still 800ma. So I do not expec that to be an issue. After all it is designed to run on 5v/500ma. I'm sure that sound Devices has got there stuff all within spec for those voltage/amperage ratings.
I did not have a very big heatsink on the unit. So I have put a much larger heatsink and now and I am only using the metal back plate on the box. I ditched the plastic cover. I also made sure that the top of the heatsink is in contact with the metal backing plate to help draw off any heat. I am also going to drill some holes right where the heatsink is to add the ability to circulate air around the heatsink as opposed to being enclosed in a sealed box. The only thing that stinks is that I can only test this setup at a show where I can get high volume levels. I have no added the capacitor in yet. That's next. My gut feeling is the larger heatsink/metal plate AND some ventilation will solve the problem IF overheating was the cause. Adding in the capacitor just makes good sense to me. More reports as I find out more.
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I don't know how a 7805 works internally. But I predict it works better with a constant load than a fluctuating load. A capacitor stores charge, which could buffer that fluctuation a bit. Every packaged 7805 I've seen at Rad Shack has a little circuit on the back which is how they recommend you use it. And I think a lot of people ignore it and get away with it, but if you aren't, maybe "when all else fails, read the directions."
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Yeah no particular circuit shown on the package nowadays, just pin out assignments. It does list:
internal thermal overload protection,
stable fixed output voltage,
up to 1.0 amp output current,
internal short-circuit limit,
max operating temps of 0-70 C and a
max junction temp of 150 C.
I am still thinking that I did not have enough heat sink and that was contributing to possible overheat condition. I think I was right on the edge and the heavier draw when it was loud would push it into over temp condition momentarily. With the changes to heat sink and ventilation I'm willing to try it without a cap in line. BUT I do really like the capacitor idea and saw the same type of setup on the eco-charge sla batteries so I will most likely add that in. My thought is even if the USBPre2 needs all 500 mA and the mics each draw a full on 10mA that I am still within the 80% range of rated power. So even if it is a little shy of the full 1 amp of current there should be plenty of current. I know that the internal thermal protection will reset quick if it is not too far over temp. I found out that even when I was trying to build a circuit for an Apogee AD-1000 to step down 18 volts with extremely high temps it would reset rather quick....depending on how fast the heatsinks could bleed of heat of course.
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Does a Tekkeon solve this problem, since the 3450i has switchable voltage? I assume yes... albeit at $$$$
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Right, I was trying to utilize the 9 volt DVD battery that I already owned. My plan was that when the battery I had died I could upgrade to something that delivered the 5V specified. I wish there was some way to test this VR out without having the preamp. Joe do you know if a larger capacitor can be used? Would there be any benefit in doing so?
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Just in from SD:
With two condensers on the inputs, gains at a healthy level and headphones with headphone gain at a normal level, you can expect between 175mA and 275mA draw(approximately).