Topic: line transformer DIY plans?

[unclelouie]

I was wondering if anybody could advise me on how to build a line transformer(s?) that will take 2 mono TRS jacks, raise the line impedance, and combine it into a stereo 1/8" TRS jack. 

I'll be using this to boost the mic levels from an AT822 on a travel rig without a preamp. 

I realize this could be tricky    so if anybody knows of a company thats building something like it, I'd rather just buy the thing.  thanks in advance

[DSatz]

unclelouie, make sure that the leads at the outputs of the transformers are the minimum length possible (certainly no more than a foot or two), since any capacitance in that part of the cable will cost you in terms of audible high-frequency response.

Since the AT 822 has unbalanced outputs, I realize that it's tempting to place the transformers near the microphone so that your signal lines will then be balanced, giving you much better immunity to hum and buzz and hash (of the wrong kind) being coupled into the cable. That would be a big mistake. Instead, to get that improved noise immunity you can use the same trick that Neumann does in their FET 100 microphones: You can convert the unbalanced signal to a balanced signal at the microphones without a transformer, then feed that signal via balanced cable to the inputs of your transformers. If you're interested, I can explain how this is done in a separate reply--it would take a little research and a little skill, but it's not overly complicated.

Also, don't go for subminiature transformers with large voltage step-up ratios such as 1:15 (or even 1:25) even though these may be available from some pretty good manufacturers. Transformers like that may seem to offer you large amounts of "free voltage gain," but such transformers saturate rather easily, especially at low frequencies. You may find that suddenly, the recorded sound "chokes" completely and no music can be heard. Stick with a more modest step-up ratio, and/or use transformers of more substantial size that can handle the full signal strength that you need.

--best regards

[unclelouie]

excellent. It sounds like balanced lines -> transformers is a better solution.

how do i go about converting to balanced cables?

[DSatz]

mshilarious, in my earlier message I referred to "a little research" that needs to be done. This is because 200 Ohms is the microphone's nominal impedance (i.e. the quick-and-dirty general category that it's in), while a balanced connection needs to have the same actual impedance on both its "legs." The noise-repelling ability of a balanced line only gets really good (60, 70, 80 dB or better) when the impedance of the two modulation leads are a really good match.

That's always the rub with this approach. Sometimes someone in the company will be nice enough to divulge the information; it can be measured indirectly, of course, but that's a little tricky. Also, I think an RC arrangement will likely get you much closer than just an R, if there's room wherever you're building these components out.

On the other hand, compared to an unbalanced connection, even the 200 Ohm nominal value will give some useful amount of protection that wasn't there before. 10-15 dB less hum and buzz is still 10-15 dB less hum and buzz, and I've heard plenty of recordings where I'd be grateful for that.

I would hope that any serious taper would eventually graduate to higher-quality microphones with balanced outputs. The AT822 is a venerable oldster by now; the world has begun to use an awful lot more personal radio-frequency devices since it was invented. Live recording with unbalanced microphone-level signals, where there are lots of people around, just isn't a reliable approach any more. It's bad enough with professional equipment sometimes.

--best regards

[unclelouie]

I do understand circuits and basic electronics.  However, I'm mostly familiar with DC stuff, and while I know I can use a multimeter to measure the DC resistance, to measure the impedance I know I would need a bit more.  So, in agreement with DSatz, I'll accept 200ohm nominal impedance, and build from there.  Yes, it may not help that much, but it's better than nothing. Besides, I'm really only looking for a fun project for my vacation, and don't care too much about getting ultra quiet levels on this particular rig. You guys have been a big help, giving me lots to get started.  I appreciate it.