Audio Chain/transmitter Grounding Question

[KF1Z]

Hi all,

Trying out a new thing...

That is, a equalizer and compressor/limiter combination...

All is fine, right up until I keyed the rig....
Then BUZZZZZZZZ!

All kinds of RF in the audio chain....

Well, even when I completely isolate the EQ and compressor from the Transmitter, and just listen to the EQ output with headphones....
It's just awfull buzzy.....


So, how do I deal with this..?
Do I have to run an elaborate grounding system, to everything?
Or is it possible that the EQ just isn't going to handle being around the RF?

Haven't done too much experimenting yet....
And thought I'd throw the question out here, before I wasted a bunch of time!


Thanks for any comments.....

Bruce
KF1Z

[KF1Z]

Ok....

Yes, 2 seperate pieces.....

I am convinced it's RF.... just because I only hear it when the transmitter is keyed.....

Now that dinner is over.... I hoked it all up again.... and noticeably less buzz...

Hmmm loose connections maybe.....

So maybe I'm ok.....


But, that leads to another question....
Which is the proper order in the "chain"?

Preamp....EQ....Compressor/limiter...
or
Preamp....compr/limit....EQ..

Does it make any difference?


tnx

[Mike/W8BAC]

With seperate pieces I'd go with preamp, EQ, comp, limiter sequence. Some of the pros on here may know better and say differently-go with their word if different from mine! I've been learning about this stuff for 40 years now and still don't understand all that I've learned?

I agree with placing the EQ and any other future audio shaping equipment in front of the compressor/limiter. I'm open to learning a different way as well but I have had my best luck this way and it is a logical choice. Use the equalizer to shape, and flatten the audio than compress and limit.

Look at how the shields in your audio system are hooked up. If your using a 600 ohm, balanced system I start with three conductor (mic high, mic low, ground plus shield). I pick a central point  closest to a ground buss in the chain, say the compressor/limiter. At that point and only that point the shield of the entire audio chain is tied to ground. Lets say from the compressor up stream to the mic you go through the EQ the preamp and than to the mic. Jump the ground shield at each piece of equipment.

At the mic the shield is floating. At the pre amp the shield is jumpered without contact to chassis from the input plug to the output plug and on to the EQ. At the EQ the shield is not connected to the EQ but transfered from the input plug, XLR (or whatever kind of plug) to the output plug without touching ground or chassis. At the compressor the shield is jumped out of the input plug to the ground buss. The output of the compressor has it's shield broken out to the ground buss as well. The audio than goes to the transmitter right? Don't tie the shield to the transmitter. Let the shield float here as well. They call this a starfish ground system. Think of the ground buss at the compressor as the starfish body and the shielded cable going to the audio chain and transmitter as it's legs. A central point ground providing the shield. It is a pain to work this type of grounding sometimes but it is a tried and true method of protecting yourself from ground loops in a 600 ohm, balanced audio system. Keep the brake outs or jumpers as short as possible.

Passing audio lines close to AC cords and appliances can cause pickup of 60 cycle hum and should be avoided. If the AC cord of the transmitter is to close to the audio cables you might be hearing it as well. Isolate audio and AC cabling.

Good luck and don't give up.

Mike

[n3lrx]

The chain of command as it were may vary. Some frequency response can be lost during compression which you may want to tweak a little after the compression stage. Although you may never notice it with spoken voice it depends on how much of a purist you are. It also depends on the compressor limiter as well some do a better job than others. The only problem with adding an EQ after compression is you could effectively defeat the limiter if you add too much gain with the EQ. With proper adjustment it can still work.

I personally am not that much of a purist and prefer the C/L be at the end of the chain. Some people I've talked to like to add an EQ as the final stage. Chances are I'd never even hear the difference either way unless too much gain has been added after compression.

[w3jn]

Oh boy...  I've battled RF in the shack problems constantly.  What finally cleared it all up was running the mike cable thru a length of hollow braid - it's like ground strap braid but  you can run a wire thru the middle of it.   Other things that you can try that could improve the situation include putting an RF filter on the power for the audio chain, using those snap-on ferrite chokes around your gozintas and gozoutas, etc.

[KF1Z]

Ok, thanks for the comments...

I MAY have the RF under control....( at least toned down a bit ).
Seemed like th EQ was just loaded with it.... to the point if I lifted the compress/limiter off the to of the EQ, the Buzzies got better....

Anyhow, on the order of components, I had assumed and order of  MicPre, EQ, Compress/limiter....

Seemed the most logical choice.....

There were some on the air locally, that though otherwise....
And I wanted more opinions!!


Thanks,
Bruce
1Z

[Steve - WB3HUZ]

I've run an eq before and after the compressor. It doesn't make much difference, if you get the settings right for each case.

Consider putting 0.001 uF caps across all inputs and outputs of your audio gear, to include the AC line. Also make sure everything is properly grounded. Finally (maybe first), if your gear is not running balanced line in/out, start running it that way, or get gear that has balanced in/out. This alone can often get rid of most/all RF problems.

Good luck.

[WA1GFZ]

You can also load the audio signals with resistors to lower the Z of each line. This increases the audio current so it takes more rf signal to mess it up.
Don't feel bad I watched 2 guys work on an aircraft mockup for a year trying to eliminate 400 hz. buzz on the intercom audio. PITA.

[flintstone mop]

Hopefully your EQ and compressor/limiter are not consumer type boxes. If they are consumer, then wrapping the power cords and audio lines in toroid cores and the .001's across the ins and outs might clear it up.
If they are b'cast, you'll have a fighting chance. And balanced audio lines throughout lessens RFI dramatically. You can try to use CAT 5 data cable. The twists in the pairs greatly reduces RFI. The extra shielding John mentions cleans up real nasty RFI. 40M legal limit might start some problems once again. There was a thread for 40M RFI to audio equipment a few weeks ago. If you can keep your aerials 100 feet from the house helps a lot too.
G'day
Fred

[KB2WIG]

As a real wild gues...  you can always try flex conduit......  but the ballanced line using SJT cordage is the way to go..   klc

[N6WDR]

Check out this web site it may answer all your questions.

www.nu9n.com

I used this site to set all my audio gear up for the Valiant, I am using all Berhinger gear for the chain with a Heil PR40 for the Mic.  The only thing I did different was I added 3 ferrite chokes to each cable to assure I wasn't going to get any stray RF in the audio.

Richard

[W1GFH]

Optimum grounding and equipment layout schemes can solve the problems. However the Instant Gratification solution is to buy 10 or 15 of the snap-on ferrite chokes and put 'em on all the audio signal path cabling. Just keep doubling up on 'em until the buzzies stop. Expensive, but it works.

[Mike/W8BAC]

Hay Joe,

Who makes the snap on choke you showed in your post and how much is "expensive"? One more thing, How big is the opening?

Mike

[W1GFH]

Hay Joe,

Who makes the snap on choke you showed in your post and how much is "expensive"? One more thing, How big is the opening?

Mike

http://www.radioshack.com/sm-snap-together-ferrite-choke-core--pi-2103222_tb-accessories.html

Radio Shack, $5.29... each.  Others sell them too, but I don't recall offhand who they are.

ID of the opening....is big enough to fit standard audio cables, it seems.

***EDIT: I just re-read KF1Z's original post. If you're detecting a constant HUMMMM or BUZZZ at the output of the audio chain (Mike--->Preamp--->EQ--->output) when NOT transmitting, then RF is not the culprit, and chokes are not the answer. Sorry. Follow the advice of others regarding balanced audio lines and filtered power supplies.

[KF1Z]

Yes, it was Only while the transmitter was keyed....

Have recently learned that my Equalizee is one brand that is particularly suceptable to RF problems......
(Rane RE27)

But, also finding out that my simple little homebrew preamp, compressor/limiter (QIX design) is just as good, if not better do to simplicity than the Alesis 3630, and the Rane EQ....

Though with a 27 band EQ, at least I thought, I would have a little more "tailoring" options....


So, I tried them out, and PROBABLY just going to sell them off.....

As far as I'm concerned..... the simplest option must be the correct one.....


Thanks all for the input.....

Bruce

[flintstone mop]

The Alesis and Rane sound like good sound system brands, not for broadcast environment.
Fred

[KF1Z]

Yes, I have found out that the Rane is about the worst choice for an RF application...

But, quite a few guys are using the Alesis in their AM audio chain with great results...


Oh well, that's all it was, was a test... just to see if I wanted any other audio equipment...

Now I know..... simplest is best !


Bruce

[WA1GFZ]

I just modified a Motorola condenser desk mike to work into the V2-CDC.
I had bad RF in the audio problems so I put a .001 uf across the output right inside of the mic connector on the mic side. The output is way too high so need to change the gain of the tow transistor stages in the mic. The whole case is palstic so may need to put some foil inside to act as shielding.
The Cap got rid of most of the RF. I need to load the line down to reduce the level and line Z. this should also help. Go to amidon to get sleeves. The ones without the split case are a lot cheaper.

[Steve - WB3HUZ]

Though with a 27 band EQ, at least I thought, I would have a little more "tailoring" options....

It will give you lots of options, but in reality only 5 -7 of the bands fall within the useable range for amateur AM use. With the right mic and/or a simple RC preemphasis, no eq is needed.

As far as I'm concerned..... the simplest option must be the correct one.....

KISS - Keep It Simple Stupid. It works.

Hope to hear your new and improved audio on the air.