Topic: AT822 into balanced preamp how? (plus, any way to mod it balanced? or phantom?)

[DSatz]

spaceboy_psy, the white papers on Rane's Web site are far better than the usual on-line opinion pieces that masquerade as expertise on this subject. But in the particular article that you linked to, they don't include an example of the type of hookup that I'm suggesting. They show something that's partway similar: the use of a transformer at the output of an unbalanced source component, which then drives a balanced line (cable) with a balanced input at the far end.

That would be a possible solution in your case, though it would be less then optimal in several respects. Just briefly, if you're going to use transformers, you'd get considerably better noise rejection by using input transformers at the inputs to your UA-5 than you would by using output transformers on the microphone end of things. (The two types of transformers are built differently; input transformers can use a type of shielding between windings that blocks the capacitive coupling of RFI, while output transformers can't.)

But that wasn't the approach that I was talking about. I was suggesting a less expensive way to create a balanced output--essentially the way Neumann does it in their transformerless microphones. Only one wire (say, the one connected to XLR pin 2) is directly driven by the microphone's output circuit, which has a certain output impedance. The other wire (say, the one connected to XLR pin 3) is connected to a resistor that matches that output impedance. To improve the match at higher frequencies (which is important, since any interference is rejected only to the extent that the hookup is balanced at the frequency of the interfering signals), a series capacitor can also be used.

I understand why you feel that I haven't said very clearly what you should do--mainly because I haven't! But the more I think about this, the more I wish that one or more of the people on this board who build stuff and modify circuits for people here would look into building small, low-cost, transformerless microphone balancing boxes, because a shielding enclosure and some panel-mounted connectors would be needed, and that takes a little experience to design and assemble. The value of the resistors and capacitors inside the box would also have to match--as closely as possible for maximum noise rejection--the actual output impedance of the microphones that the box would be used with.

So, unfortunately, this couldn't be a "one model works with all types of microphones" kind of thing. Someone would have to find out what the exact, actual output impedance of an AT822 is, for example--not just the spec sheet value. That's not too hard, but it takes a bit of doing (or a bit of asking--I'd expect A-T to cooperate gladly; the engineers that I've met from that company have been very nice and very smart).

Just as an aside I'd say that the majority of the technical problems that come up on this board involve making consumer-type equipment work with professional-type or "semi-pro" equipment. People should be aware that the standards of the two realms weren't designed to "interoperate" and that all these contortions and anxieties result from trying to fudge distinctions that were never designed to be fudged.

It's not fair; even if some people are always going to have less money than others, on some level everyone should have an approximately equal right to be able to use the equipment that they can afford. But the industry standards that try to guarantee that stuff will work together properly only define the expected behavior of professional equipment on the one hand, and consumer equipment on the other hand. There are no standards for how those two "hands" should work together, and different equipment manufacturers make widely different (and sometimes totally conflicting) assumptions about how that should be done. So the solutions are often ephemeral and piecemeal.

--best regards

[chris319]

http://www.dself.dsl.pipex.com/ampins/balanced/balanced.htm

[spaceboy_psy]

 

Incredible DSatz, thanks for the white paper of your own!

Let's get stuck in...

Only one wire (say, the one connected to XLR pin 2) is directly driven by the microphone's output circuit, which has a certain output impedance. The other wire (say, the one connected to XLR pin 3) is connected to a resistor that matches that output impedance. To improve the match at higher frequencies (which is important, since any interference is rejected only to the extent that the hookup is balanced at the frequency of the interfering signals), a series capacitor can also be used.

Aha! Aha Aha ahaha ha! I get it now

reading Chris319's link helped a lot too (cheers chris!). So THIS is what we're looking for (just with an added capacitor?):



So the clever bit (one thereof...) is that pin 3 [cold] is being subtracted from pin 2 [hot], but the cold wire isn't carrying any signal, just noise and interference, which will be cancelled! Great! One question - could both channels share one resistor and capacitor, or would I need one set for each channel?

So yup, alas the consumer/professional audio dialectic has not been throwing up any kind of positive synthesis, only trouble, trouble, and more trouble!

In terms of implementation then, it's got to be the Balance Box doesn't it. No way to really implement this in a cable as such..? Unfortunately I think building a boxed unit is probably above my present ability in these things, and certainly beyond my time and money resources, for now. But great to know theoretically how it would be done, really appreciate the knowledge imparted here, thanks.

I'm thinking I might go for the pseudo-balanced lifted sleeve approach mentioned before. I figure get a 5pin XLR to 2 x 3 pin XLR cable and swap the 5pin for a 3pin with the two sleeves disconnected. Add TRS adaptors for Microtrack. Hopefully, from what I've read, that will avoid the nasty MTII "sprinkler" noise, and give me a bit less noise going into the UA-5.

To be honest I'm even starting to worry whether I can afford to buy cables for this setup (was banking on getting a grant to go and conduct research this summer, but the grant fell through. Still going, just with little to no money!)

Whatever I end up doing, I'm damn glad to know all my options! If I do end up getting into building myself a Balance Box, or even trying to get these cable shenanigans to work, I'll post back to update (and most likely to weep "WHY WON'T IT WORRKKKK???")

Many thanks for the valuable info everyone, particularly DSatz, ya star

[spaceboy_psy]

Hi mshilarious, thanks for chipping in. Apologies for late reply, been very busy finishing uni (and trying to research your PFA), but still, how rude!

This PFA sounds great. I couldn't find a schematic floating around anywhere, don't suppose you'd be willing to share a copy with a friendly scot who wants to modify it slightly? Would want to remove the phantom power section as the AT822 is battery powered of course, and maybe build it in XLR -> 2 x XLR...

However, many of these type of mics have an output impedance in the 1k-2k range, so I don't typically stick with the impedance-balanced arrangement.  I'd prefer a circuit that lowers the output impedance to something more reasonable.

So impedance-balanced would be fine for circa. 200 Ohm AT822?

Cheers!

[spaceboy_psy]

and no capacitor?

[spaceboy_psy]

Right! Again, many thanks for all the help. I jet off for Nepal on tuesday so this little effort will be something to look forward to upon my return in August

Looks like I'm now going without my UA-5!! As the stupid Walmart battery died on me 

So just AT822 and MTII, not gonna be anything spectacular then.

Adios lads

[eman]

Are you aware that the Walmart battery has a sensitive overload/ short circuit switch? To reset it, just plug it back in for recharging. I've tried to use them for powering guitar pedals, but there is something about the pedals that the battery doesn't like. The battery appears dead until you plug it back into the wall. It's too bad- the battery is perfect for that application and the cable is the same for a UA-5 as a Boss pedal. Check your battery cable for shorts when you move it around.

I just got AT853's and I've had an 822 for years. The 853's completely smoke the 822, and cost less. It does make a nice interview mic for using with a video camera, though. People will talk to someone holding a mic much more easily than someone holding a camera, plus you get both sides of the conversation on video.

[spaceboy_psy]

hmm, plugging it in to recharge does seem to make it 'work' normally, but only until you plug somethingin to be powered! Then it dies

As I'm away now I'll have to wait until August for further testing...

Cheers for the help

[Windwman]

I know this is an old post, but I have to give thanks to all who made suggestions....I used the 200-ohm cold to gnd trick & it worked great. I noticed it was noticably quieter on a long 30ft run compared to "stock". Thanks again!