The AM Forum

Been doing some upgrading to my audio chain that will eventually feed the Flex 5000 on AM. I’m looking for an inexpensive mic preamp that has balanced audio in and out. All of the Behringer / Mackie small mixers do not offer balanced outputs. Any suggestions on what I should look for on the used market? Tried using one of the $50 variety 12AX7 preamps offered by guitar center, but this just has too much 60 hz crud coming out of it.

My vx2000 has a balanced output...
I also have an ART tube preamp someone posted about, a small thing, has power for the mic, phase reversal switch, a nice meter, input and output level pots, and a limiter. Its supposed to be balanced in and out but I am not sure it really has a balanced output or not. Was $25.00 I think, has no hum on it.
TRS and XLR input and output jacks on it, heavy metal case, backlit meter, color changing LED's.

The price is a bit high, but...

http://www.ebay.com/itm/Behringer-Ultra-Voice-VX2000-/161546883885?pt=US_Preamps_Channel_Strips&hash=item259cf1d32d

Are you saying my Behringer MIC100 doesn't have a balanced output? HMMM.

If I may, how about sharing your setup details eg.. mic cable length general hookup detail.  Maybe we can help you debug things.  I'm quite satisfied with my Behringer MIC100 and use it with a legal limit AM transmitter.  Gotta be something we all can debug.

Do you get the same hum problems going into a dummy load as you do on your antenna?

GL  Al

PS:  While I'm at it why don't I put up the manual

The bidding hasn't moved off $20.00 but yes, the shipping is a bit high at $25.00...  

I use one of these to modulate my TS440 and yes, it has balanced output
going direct to the balanced modulator ...

A quick reminder, in order to eliminate any ground loop (hum)  problems, the shield of the VX2000 or any other balanced preamp line should BE isolated, IE, not connected to the casing of the audio output line connecter.  Just run the audio to the ring & tip to the B. modulator input xformer...

 A Behringer VX 2000 and a Behringer PEQ 2200 would do good that was my plan for my Flex 1000. I changed my mind ??? ???. I am going for more of a ( TUBE ) sound. I plan to use a HHB RADIUS 40.  I will swap out the cheap rough sounding ruskie tubes for  European   60s vintage tubes.   If you are interested the Behringer VX 2000 and Behringer peq 2200 are  4/sale.

This might help with ground loops in a balanced line system:

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

Currently using the same ART tube preamp that N2DTS mentioned. Actually purchased and returned it twice as each unit has 60 hz crud on the output. Pretty sure it’s not a ground loop as I can see the 60 hz on the output with nothing connected to it. Also tried using an isolation xmfr, just in case, with no change. At this point everything points to the preamp.
Mic is a Heil PR40, feeding a Behringer 2496. My goal is to keep audio balanced all the way through to the Flex 5000. Cables are short, longest about 4 ft – all double shielded.

Difference between using a dummy load and using the ant?

Definitely not RF. Turned output down to 0 - same.

A quick reminder, in order to eliminate any ground loop (hum)  problems, the shield of the VX2000 or any other balanced preamp line should BE isolated, IE, not connected to the casing of the audio output line connecter.  Just run the audio to the ring & tip to the B. modulator input xformer...

Going to try this tonight... thanks

If the shield isolation does not fix the problem then try Al's isolation xformer
scheme, that has been a cure for crud elimination for years in computer sound card
inputs, etc...

Good luck.

Good reading regarding the "Pin 1 problem" and grounding with balanced audio:

http://www.rane.com/note151.html

I see that your ART unit has a supplied "9 volt adapter" (wall-wart).. Is it a switching supply, and do you have you any other clean 9 volt source to power it up?

It's a wall wort, but it's 9V AC. I'm almost tempted to find a schematic and build a DC supply to bypass the crazy circuit they use. I'll try the shield trick later tonight and post results.
Thanks!

My day job is selling broadcast gear.  I can get you a DBX 286S balanced in and out with EQ and compression for way under $200 easily. Dont want to be commercial, just trying to help.
Bob

+1 for the 286S. Not as many "bells and whistles" as some but does the job very well.

I was just messing with mine last night.
I never use it, never have, only got it because it was so cheap.
I plugged my mic into it, and plugged it directly into the line input on the screen modulator and for no EQ and no compression or limiting, it sounded and worked quite well.

I do not think its a balanced output though.

I was testing it because one of the switches on the VX2000 is dirty and it gets scratchy sometimes, not very high quality...

ART Tube MP..

hummmm:

http://amfone.net/Amforum/index.php?topic=16673.0


Seems like hum may just be a documented issue with your unit..  :o

Tried lifting pin 1, without any improvement. Have to just accept the fact that this unit is a piece of excrement! May just bite the bullet a pick up a Behringer mixer that has balanced out.

Absolutely correct and everyone should be aware of this issue.   Many audio boxes just pipe in crud into the boxes.

p

Note, the output xlr is balanced output, but the TRS is NOT.
The TRS input IS balanced.
For fun I put an RCA 12ax7 in mine and got a firmer lower mid range and a crisp high end with tight siblance response.
I just made all that up but it sounds good, and no hum at all.
I think its interesting I can plug it right into the screen modulator (line level) and it does not sound real bad.
Seems like it needs a high end boost, but I hear nothing over 3K so it might not sound bad at all.

My 286s developed noise in the mic preamp stage so I hooked up the MIC-100 that I used to use into the line input and my stations sounds good again.  Don't understand why the MIC-100 got such a ho hum reception - it is balanced both input and output and affords both XLR and 1/4" TRS option - NO hum and is cheap.  Oh well.

My class E station has absolutely no hum and here's what I use:  EV RE-15 / Behringer MIC-100 / dBx 286s --> line level out..

If you can find some vintage Bogen solid state stuff at a good price, they work well.

I've used the M67 and the model CFC over the years. The CFC can be configured many ways and it is immune to RFI, and the M67 was widely used in the broadcast field.

Al VE3AJM

Did some searching and think I'll pick up a Yamaha MG06X. It's a small mixer with two channels of balanced in / out plus some other unbalanced that I might be able to make use of. Guitar Center has it on sale at $119. All the on line comments seem trouble free. I can always return it if it has problems. Thanks for all the suggestions.

Its not rack mount, but I like the reverb.
I used to have a spring reverb that I turned up when I said long as in a llllloooooonnnnnnggggg transmission.

Someone had a design of a balanced mic preamp using both triodes of a dual triode tube.
I thought that was da bomb.

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

   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

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?

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.

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.

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

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.

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

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

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.

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

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.

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.

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.

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!

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