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Old 15th November 2004, 06:55 AM   #1
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Default unbalanced (consumer) to balanced

Okay, I posted a little while ago about this, and you guys pointed me in the right direction (thank-you very much). I've since found a schematic that I'd like to build a device to do this from, located here.

I was wondering what you'd recommend as far as where to get components, and if I would be better off substituting other components (for example, better op amps?). Also, the power LED isn't strictly necessary, though they are really cheap so I can't see much of a reason to eliminate it. In addition, I'd (as the title implies) only be building the unbalanced to balanced circuits.

Keep in mind that this is for car audio on an un-spectacular system, and that the source will be a lowly Mp3 CD player. On that note, will feeding it 12V (more like 13-14 when it's running) direct from the car be fine, and can I just ground everything that needs grounding (neglect the XLR jacks - won't be using those, will be spliced) to the chassis?

So, to recap: what to get and where to get them, do you think there are any modifications that should be made?

Thanks in advance, this site is spectaular

Matt

P.S. A little help interpreting the diagram would be appreciated - some of the capacitors are not marked very clearly (i.e. what does "C15 .047 and C12 47U" mean), I'm used to microfarads, though I appreciate the fact that mu doesn't show up real well on most comps so they use U, as in C7 1k UF, which is clear to me.
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Old 16th November 2004, 03:04 PM   #2
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Have you looked at this?

http://www.jensen-transformers.com/an/an003.pdf

The transformer is expensive but is state of the art and won't degrade the sound.

It also requires no power supply and will likely overall be cheaper than to diy what you are considering.

The only consideration would be does the output stage of your MP3 player have enough drive. It probably does, but I can't answer that for sure.

Car power is horribly noisy and not too good for audio. Avoid if possible.
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Old 16th November 2004, 05:14 PM   #3
markp is offline markp  United States
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C15 .047 mean capacitor location number 15 has a value of .047uF (microfarads) look on the circuit board for a C15 written on it to find the cap.
The U is just fir microfarad.
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Old 16th November 2004, 05:23 PM   #4
macboy is offline macboy  Canada
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Quote:
can I just ground everything that needs grounding (neglect the XLR jacks - won't be using those, will be spliced) to the chassis?
The circuit creates a virtual ground (which they call +VB). This is the ground reference, so the chassis becomes the negative supply (-6 V, WRT +VB). However, they use DC-blocking caps on all inputs and outputs, so it is safe to connect the audio plugs to ground (i.e. chassis ground). In fact you'll notice that the plugs in the schema are connected to ground, not +VB.

Make sure that the capacitors on the inputs and outputs are rated at least 12 V. I'd use 50 V ones. They will have substantial DC on them virtually all the time (around 6 or 7 V). Also make certain to polarize them as shown in the schematic. Because of the large DC offset, this does matter.

You are right in assuming that 47U means 47 micro-farad (I'd write 47 uF). 47U is actually a standard notation. Likewise 4U7 would mean 4.7 uF, and 470n would mean 470 nF (or 0.47 uF). Unfortunately their notation is all over the place; the 47 nF caps are listed as 0.047 (with no units!) instead of 47n. Also note that although some resistors are not listed as 1%, you should use 1% for most of them, specifically R12, R14, R16, R19.

Replace the 0.047 uF caps at the inputs with something an order of magnitude larger, like 0.22 to 1 uF. With 0.047 uF, the circuit creates a low pass filter with a -3 dB point at 34 Hz! Say goodbye to half your bass!
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Old 17th November 2004, 10:22 PM   #5
lgreen is offline lgreen  United States
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Default differnetial box

Or you could make yourself a box with 8 batteries and 2 DRV134s like i did here.
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