Balanced Mic Preamp suggestions

[W7TFO]

RDL, or Radio Design Labs sells a bunch of "Fix-its", known as stick-ons.

They have a couple mic preamps that work really well and are very inexpensive.

Here is a list of some products:

http://www.rdlnet.com/series.php?series=13

73DG

[WD5JKO]

   With my Flex 3000 station I use a balanced Dynamic mic and feed it to a Presonus Tube Pre. This is discontinued but a Tube Pre II is available.

http://www.presonus.com/products/TubePRE

   This little box has a 12AX7 in the signal path using 45V Phantom power for tube B+. It does sound real nice to me. I then feed a Presonus Blue Max compressor which feeds a 600:600 ohm isolation transformer (UTC A-26) that feeds the unbalanced rear input on the Flex.

  The Tube Pre is susceptible to RF. The attached schematic shows how the power ground is not in any way attached to case ground. I first tried bypassing pins 2, 3 at the input XLR to case through two .002 uf tiny disc ceramics. The XLR pin 1 floats at analog common, so it is RF HOT. Next I tried grounding the case to my Flex ground just inches away. That caused the typical SDR latency delayed feedback. So I use another .002 cap from case to Flex ground. That makes 80-40-20 behave. If I go to 15-10M, that case to ground cap is best at 220 pf since the .002 uf in there caused feedback. This is pretty insane, and I do not understand what is happening. I do like though how the thing works and sounds when I tame it as described.

  Maybe some of the other guitar store boxes have similar issues such as the floating XLR pin 1. By floating I mean it is not grounded, but instead tied to analog power common...which floats.

Jim
Wd5JKO

[WA2SQQ]

So last night I was about to head out to Guitar Center and pick up a Yamaha mixer that has balanced in / out. $90 was not so bad.

Decided to get on 75m and was talking to some guys up in New England. One suggested something I had not considered. I went to Radio Shack and purchased an XLR to 1/4" adapter (transformer) for $20. Tonight I'll hook it up, backwards, so I can use my existing Behringer 502 mixer which only offers unbalanced out. That will allow me to connect the mixer (acting as my preamp) to the Behringer 2496's balanced input.

I'm far from an audio expert, and I suspect there is some downside to this approach, but in theory it should work fine. BTW, in my case no RF problems what so ever. All my cables have multi-turn ferrite chokes right behind the connector.
One concern I have is if there is any low frequency roll-off in the miniature xmfr RS uses. I had few of their 600:600 isolation xmfrs and that turned out to be a big issue.
What do you think?

[N2DTS]

It should be no problem going from unbalanced to balanced, if you plug in a 1/4 inch unbalanced plug into a balanced input, the shield/ground connects one side of the unbalanced audio to the shield/ground and its unbalanced.
You do not need a transformer to do that, but it might help to isolate things some.
They made hifi transformers, UTC used to make some really small ones like a pencil eraser with 10 to 20,000 Hz for microphone level stuff.

[Steve - K4HX]

You cannot go from unbalanced to balanced without a transformer or an active circuit. Connecting an unbalanced item to a balanced one will just make BOTH unbalanced.

[WA2SQQ]

Yup, which is why I went the transformer route. We'll see how it works this evening! Thanks

[N2DTS]

Yes, but there is no reason to use a transformer when connecting a balanced to unbalanced input/output.
If something is unbalanced, adding a transformer is not going to make it balanced.

[w1vtp]

You cannot go from unbalanced to balanced without a transformer or an active circuit. Connecting an unbalanced item to a balanced one will just make BOTH unbalanced.

Agree.  This transformer has good shielding and permits the transition from unbal --> bal or the other way.  Using an underpar line to line transformer might just confuse the issue.  A good one may solve the problem

http://amfone.net/Amforum/index.php?topic=37478.0

a

[W7TFO]

The Triad A-67J was the best of the line-to-line transformers around for the $$.

It could be configured either side for 150/600, and was totally shielded.

They show on the auctions from time to time.

73DG

[Steve - K4HX]

There is a reason and a very good one. Using a transformer will keep the balanced side balanced and greatly reduce the chances of hum and RFI. It's done all the time in the pro audio world and that's why millions of such transformers have been made.

Yes, but there is no reason to use a transformer when connecting a balanced to unbalanced input/output.
If something is unbalanced, adding a transformer is not going to make it balanced.

[N2DTS]

On a long run that might help, but most audio in the shack is very short, no?

[Steve - K4HX]

All pro audio uses balanced ins and outs, regardless of the length of the cables. Why? Immunity to noise and hum. There's a reason well known and prestigious companies like Shure and Altec-Lansing made unbalanced to balanced transformers. 

In my own station, I've used such transformers to get rid of hum and RFI.

[WA2SQQ]

So it's time to close this chapter of moving in to the new shack.

Last night W2NBC confirmed what I had suspicions of, that the $20 Radio Shack adapter's transformer rolls off a good chunk of the low frequency content. At 60hz it was ~ -1.5db, and progressively getting worse. I decided to brave the 9 degree weather and returned the adapter to Radio Shack. Stopped at SamAsh Music and picked up the Yamaha MG09 mixer for $99. Two balanced in / outs plus a few more unbalanced that I probably won't use. Hooked it all up and it sounds fine. Never realized how much crud was coming out of that 12AX7 mixer until I compared it to the new preamp method. Thanks for all the assistance and suggestions.

[Steve - K4HX]

Good to hear you got it fixed.

Not unless your voice is very unusual, -1.5 dB at 60 Hz is not worth worrying about.

[WA2SQQ]

I agree, but this makes for a much neater installation. The reversed adapter required extension cables and gender adapters. I got approval from "the boss" for the new shack based on the condition that it's neat. She sees cables as clutter! It's all about compromise and keeping her happy!

[w4bfs]

I got approval from "the boss" for the new shack based on the condition that it's neat. She sees cables as clutter! It's all about compromise and keeping her happy!

oh yes!  they also respond well to small clear carbon crystals in precious metal settings