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Old 16th May 2011, 08:34 PM   #21
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You're right (about the capacitor); I usually use a 1F 160 volt polyester (because I've got some, I suppose) but I'm generally stepping down the voltage and up the impedance. But since what I said about the low impedance drive helping (drastically) the low end on small transformers, and this presumably being because the inductance isn't high enough to keep the impedance up at low frequencies, so it takes more current drive. And a capacitor in series, increasing the drive impedance, is only going to intensify the problem.

As long as you're driving line inputs, 10k-10k should be fine. The only things with bridged 600Ω inputs are older than I am, or telephones. Mic inputs might be a bit low, but you'll be cutting down the output level, anyway.

Unfortunately, planning impedances and levels when you're on the road is not very effective, unless you bring all the gear yourself. So you design for massive overkill,and turn it down. I could design the optimum matching transformer for interfacing different systems, giving a simultaneous isolated mic level and a full spec 300Ω +22dBm German broadcast line level out; but nobody would want to pay for it (or, for that matter, carry it around) Your passive box should work fine in ninety-five percent of situations, and adequately in most of the rest of them. How big a box are you using? Could you fit more switches, and XLRs, if later this turns out to be useful? (one advantage of DIY is that, knowing what you've done, you can change it later.

And you say you've got transformer/direct switches; I hope you've got ground break (preferably with a third position for "resistive") switches.

The output impedance of your mixer is about 120Ω, but it's happiest driving into six hundred or higher, so the 10k should suit it fine. That's the jack output; there are no details on the RCA recording out.

Thinking about the capacitor problem I opened up my Canford line isolator (good, but XLRs everywhere, so you'd need adaptor cables in profusion, and if you think your 10k transformers are heavy you should heft a couple of 600Ω-600Ω) and discovered – no DC blocking capacitors. Just a transformer. So you're probably safe.
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Old 16th May 2011, 08:47 PM   #22
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Well thanks again Chris. At least i don't feel foolish thinking it a very complex thing to plan for with so many unknowns. I think I have as many bases covered as I can for now, and I'll have to see what Murphy has up his sleeve as the system is put to the ultimate test.
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Old 19th May 2011, 08:22 AM   #23
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No problem, Lets assume a negligible output impedance, most op amps are less than 100 ohms.

10K is a common input impedance, so lets do the numbers for 5K just to be safe (I could do 600 ohms but your 10K transformer might not work properly on 600 ohms anyway) with a 1:1 turns ratio the impedance seen by the mixer output is roughly 5K, Lets assume a 20Hz lower rolloff frequency.

5000 = 1/2PiFC rearanging 1/2PiFx5000 = C so the required capacitance for this example is 1.59 uF doubling it to reduce the rolloff at 20Hz from 6 dB to 3dB gives 3.2uF so either a 3.3 or a 4.7 uF would be fine for any normal mixer unless you get a rare 600 ohm input one in which case the rolloff frequency will be nearer 50Hz, not a show stopper.

IMHO you made the right choice getting a quality 10K 1:1 transformer
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Old 22nd May 2011, 10:10 AM   #24
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Somtimes but not always lifting the ground on the signal line feeding the board can solve the hum problem I have had to do this on several ocasions.

One in particular was when I was trying to feed my mackie 32-8 from a little mackie mixer in another room I had a bad hum problem.

I spent many hours on the tech line with them and even they couldn't figure it out.

Then it dawned on me that we had the same problem in a friends studio that we spent hours doing the custom wiring only to find out that we had to redo all six 30 point patch bays and take off the signal grounds on one side of each signal line going in one direction.

After that the whole system was compleatly dead silent unless of course there was a signal.

jer
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Old 22nd May 2011, 10:29 AM   #25
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Just the paper for this thread : http://jhbrandt.net/Grounding,%20wir...oop%20area.pdf

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