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Old 11th January 2003, 07:06 PM   #1
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Default Drawing power from USB or Firewire

Hello,

Short story long(...):
I produce electronic(-ish) music using the amazing 'sequencing instrument' called Ableton Live. It's basicly a loop sequencer designed to be played like an instrument. More info at www.ableton.com; I can highly recommend it to anybody fed up with MIDI and composition, who'd rather just jam!

To get my sounds from my iBook to the outside world, I use a six mono/three stereo out USB audio interface (the EMI 2/6), which I'm very pleased with. This is what I'd like to do:

I'd use one channel to pre-listen my loops, on the fly. The other two stereo pairs would come into a small two channel 'mixer', consisting of an insert point (a stereo jack, where you'd use a Y-cable, left as 'send', right as 'return'; for adding outboard effects) and a gain/trim knob (going to 0 dB, no amplification needed) pr. channel. Then I need a crossfader for the two. I'd like a master insert point as well.
This should not be a complex design should it? Would it be possible to build this, in an acceptable quality, as passive circuitry (no EQing, no amplification). If not, and this is the actual question, how would I go about drawing power from USB or FireWire (I prefer the latter, as I don't use my FireWire port for anything)? And what would this thing cost: RCA plugs, jack plugs, knobs, crossfader, solid casing (metal? this'd see plenty of stage use), USB/FireWire plugs, etc.
Would I be better off getting a proper DJ-mixer? I'd prefer being able to run from my iBooks batteries, as this is more than enough for the average live gig. In that way, I don't have to worry about bad power on stage - a common problem... or maybe I could just use regular batteries? I'd need some sort of 'how-much-juice-I-got-left'-meter, then.

Hope this makes sense, and thank you!
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/Niklas
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Old 11th January 2003, 08:20 PM   #2
haldor is offline haldor  United States
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Hi Niklas,

To maintain a 0dB output you are going to need some gain.

You can get 5V at 100 mA from USB (actually 500 mA if you ask permission first). The problem is that the USB power is not particularly clean and could drop downwards of 4 V (passive hub with two cable drops at 500 mA). That's too low to you give you much head room for your mixer circuitry.

It is possible to step the USB power up (charge pump would be the simplest) and then run the output of the charge pump through a linear regulator to clean it up, but that is quite a bit of work. Plus this will add an electically noisy circuit inside your mixer which could end up causing you sound quality problems.

Depending on your circuitry you can get a lot of use from a 9V battery. I have built active electronics that get installed inside bass guitars and they get about 200 hours of operation from one 9V alkaline battery. One key issue for battery life is the op-amp choice. I like the MC33178 for very low power applications (200 uA per channel). This is a very low noise, unity stable part that has 600 ohm load drive capability.

If you go with a single 9V battery source then you will need to capacitively couple the inputs and outputs and create a virtual ground reference for the signal at 1/2 of the battery voltage.

Phil
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Old 11th January 2003, 10:44 PM   #3
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I'm not sure about the older iBook but the new one provides a 6 pin IEEE1394 port, which means it is powered. As for the voltage provided, the current, and if you need to request power via software, I don't know. Being it's an IEEE standard the documentation probably isn't available to the public, only to members of the IEEE.
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Old 12th January 2003, 07:37 AM   #4
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Default Just like SAE...

Wow! You can have the IEEE 1394 standard as a 10.5MB pdf file or the 420 page document...your choice of format, for $161.

Wouldn't you think a basic summary could be available?

Then again, you wouldn't like to know what NFPA standards cost...oh my!
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Old 12th January 2003, 11:59 AM   #5
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Quote:
Originally posted by haldor
[B]
To maintain a 0dB output you are going to need some gain.

The problem is that the USB power is not particularly clean and could drop downwards of 4 V (passive hub with two cable drops at 500 mA). That's too low to you give you much head room for your mixer circuitry.

It is possible to step the USB power up... this will add an electically noisy circuit inside your mixer which could end up causing you sound quality problems.

Depending on your circuitry you can get a lot of use from a 9V battery.
Sounds like you're with the V9, then? 200h of life is more than plentiful, and the USB/FireWire solution sounds a bit out of my league - I prefer making music to soldering, you see... Also, I do know a bit of active circuitry in instruments-theory; if I could apply that, so much the better.

But thanks very much for the advice, I'll probably bug you again soon!
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/Niklas
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Old 12th January 2003, 11:55 PM   #6
haldor is offline haldor  United States
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Hi Niklas,

Unless you are just really eager to get into USB or Firewire development then a 9 V battery is probably a whole lot easier and more likely to get you a satisfying result.

I have developed a couple of USB interfaced weight scales that operate from USB power and am very familiar with the issues involved. It is doable, but for a one off application that doesn't require much current I would just use a 9V battery.

I recall that Radio Shack use to sell a little 9 V battery powered line mixer (I believe it was 2 or 4 channels) for less than $40. This might be worth investigating also.

Phil
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Old 14th January 2003, 12:16 AM   #7
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As these guys have pointed out, USB power can be had but it's not easy to get at. I've built some USB powered hardware... ...9V is much easier. (Actually, you can just steal the power, but but that's not recomended)

If you're going to use a battery, why not use two. One for each rail of the op-amp. (You might even want to think of a lithium). A 9V alkaline is rated at around 600mAh. If drawing 1mA, you'll get 600 hours of operation. (Of course, you'll only get 60 hours at 10mA).

-Dan
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Old 17th May 2003, 05:03 PM   #8
jwb is offline jwb  United States
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Well I'm jumping on this thread late, but I just built some FireWire-powered hackery, and you can pull 12V at 2A (!) from the firewire port. I think 1A is the spec, but I was able to pull 2A without blowing anything up (yet).

Yeehaw.
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