RF Getting into Audio - Symetrix 528 is culprit - Any Suggestions?

[K1JJ]

Hi,

When running QRO, I am getting RF in my audio using a Symetrix 528 processor. When at 100w it's OK. When I take it out of the circuit completely, the audio is fine.  When I bypass it using the internal bypass button, the RF problem is still there.  I am coming in at 1V to the line input of the 528.

I  have tried torroids on the audio cables and AC plug, as well as adding 1k resistors and .001 bypass caps to the input and output audio  cables, but still NG.  I have all audio boxes well bonded to ground with heavy braided cable.

I see a long row of inputs and outputs on a terminal strip on the rear. Not sure if they are causing RF input.

Anyway, I want to stay with unbalanced audio - been using it for years with no problems. I have four audio boxes in line and they are all FB when QRO except for the 528. (RE-20, mic preamp, EQ, echo, etc.)

Has anyone had experience with the 528 and curing this RF-in-the-audio problem?

Thanks,

Tom, K1JJ

[k3zrf]

Hi Tom,

I suppose you may be driving the line out with the Symetrix 528 processor. I am not using one however I ran into a similar rf situation you are experiencing. I go from mic to Rane vp12 to console to DAP 310 or Urei 422 to dbx160. The dbx drives the line out to the various rigs, valiant, ssb rig, thing, etc. On 75M low I would experience rf getting in....tried everything. I now take the dbx line out into a Deane Jensen 10k:10 xfrmr and switch the output side to the various rigs. RF went away! Sescom makes some good xfrmrs also. Pricey but you get the job done w/o the rf.

Anyway my $.02

[K1JJ]

Hi Dave,

That's a good idea.  I do have a tiny 1:1 audio transformer left over from my PDM rig experiments and will give it a go.

Yes, the 528 does drive the line to the rigs and is the last box in the chain.

Thanks for the idea, OM!  I wouldn't have thought of trying that.  I'll give it a go tonight or tmw.

T

[WA1GFZ]

Also try putting a load resistor in the audio cable connector. I put a 1 K in the mic line to my V2 and the problem went away. You need extra audio to drive the resistor. 

[w4bfs]

hi Tom .... the 528 I tried was VERY rf sensitive and the 528E I tried (and am using) did not show any sensitivity to rf ... I will be curious to see what it takes to de-rfi thisun in your application ...73 ... John

[KL7OF]

Tom...Using similar boxes, I too had RF problems with unbalanced audio when QRO......Going to balanced audio cured the problems...

[K1JJ]

hi Tom .... the 528 I tried was VERY rf sensitive and the 528E I tried (and am using) did not show any sensitivity to rf ... I will be curious to see what it takes to de-rfi thisun in your application ...73 ... John

That's what I wanted to hear - that the 528 was prone to RF problems.  At least it's not my layout. 

I'll try a tiny 1:1 audio transformer on the output first.   Then play with some in and out R/C filtering again.  I don't want to go with balanced, Steve, cuz everything else I have is unbalanced.  It's just the box itself, it looks like.

I cud always sell it to a QRP station and get the 528E.. ;-)     Or build my own compressor last resort.

[stevef]

Tom,

Have you looked into the so-called "pin-1 problem"?

http://www.audiosystemsgroup.com/Pin_1_Revisited.pdf

I cured an mild RF problem in my older Behringer gear by properly grounding pin 1 in the XLR connections.

Steve, KK7UV

[Steve - WB3HUZ]

Put the tranny on the balanced output of the 528. Then connect the secondary unbalanced to your rig. You may even want to try breaking the ground at the point, if the transformer doesn't have isolated grounds.

hi Tom .... the 528 I tried was VERY rf sensitive and the 528E I tried (and am using) did not show any sensitivity to rf ... I will be curious to see what it takes to de-rfi thisun in your application ...73 ... John

That's what I wanted to hear - that the 528 was prone to RF problems.  At least it's not my layout. 

I'll try a tiny 1:1 audio transformer on the output first.   Then play with some in and out R/C filtering again.  I don't want to go with balanced, Steve, cuz everything else I have is unbalanced.  It's just the box itself, it looks like.

I cud always sell it to a QRP station and get the 528E.. ;-)     Or build my own compressor last resort.

[K1JJ]

Tom,

Have you looked into the so-called "pin-1 problem"?

http://www.audiosystemsgroup.com/Pin_1_Revisited.pdf

I cured an mild RF problem in my older Behringer gear by properly grounding pin 1 in the XLR connections.

Steve, KK7UV


HuzMan said:
" Put the tranny on the balanced output of the 528. Then connect the secondary unbalanced to your rig. You may even want to try breaking the ground at the point, if the transformer doesn't have isolated grounds."

Steve, (squared)

Those are both good ideas and I will try them.  I would rather get the unit working than get a new one. I'll let ya know if any works out...

Thanks.

T

[ka1cde]

Tom,


You may have a ground loop. eliminate the ground on the plug that goes to outlet from the 528.

I have also had success by grounding all equipment to a single point such as antenna tuner, then running the ground out to the ground rod.

Make sure the 528 has its own ground wire also.

I was surprised to hear the torroids did not help, as they are typically my last resort and usually cure what is left, use plenty of them and wrap them as many times as the opening allows. Don't forget the torroid on the ground wire to the 528.

Good luck,

[K1JJ]

Hola,

I've tried a number of suggestions made here and finally came across one that works to eliminate RF into the audio of my Symetrix 528. I am using unbalanced audio cables.

Steve, HuzMan suggested to place an audio isolation transformer on the output of the BALANCED line drive from the 528. I had already tried it on the unbalanced output with poor results.

The balanced output connection worked beautifully.  I connected the transformer primary across the balanced output and the secondary into the normal unbalanced cable to the rigs.  No more RF in the audio, even at QRO levels.

I used a tiny inexpensive Radio Shark 1:1 isolation transformer # 273-1374. They cost about $5 or so.  I swept it and found it to be very hi-fi on both ends. Listening to the rig on the station monitor, it sounds great. There are very expensive ones out there, but I don't think I will bother.  It says on the package, "300-5000 hz," but it checks out to be much better than that - not sure why.   

* Update: I just swept the FT-102 using the new xfmr and find the waveform is very clean. The response of the rig is now 30 hz - 9khz. It used to be about 2 hz - 60 khz.  So, nothing lost for voice audio, really.

Steve, did you try this method before or was it just a guess? (Or did it come to you in a meditative trance as a vision, like Guru Trogladite?) 

Thanks to all who have contributed ideas. Now I don't have to go out and spend any $$ on a new processor !!

T

[Steve - WB3HUZ]

Yes, it came to me as I was coming out of a coffee coma.

I've done it myself. I think it's generally best to use the balanced in and outs on gear that has such. If you need unbalanced in the set up, use the external tranny.

Glad you got the problem fixed. Hope to hear you on the air soon with that "new" 528. I'm sure you'll be much louder.   

[flintstone mop]

HEY VU/TOM
This thread started with a Symmetrix and then we drift to the copy-cat Behringer box. So, what dost thou have in thy possesion?? The real thing or a copy?

Glad that your RFI is clean sailing now.

Fred

[K1JJ]

HEY VU/TOM
This thread started with a Symmetrix and then we drift to the copy-cat Behringer box. So, what dost thou have in thy possesion?? The real thing or a copy?

Glad that your RFI is clean sailing now.

Fred

Hi Fred,

It's been a Symetrix 528 all along - I just slipped up calling it a Behringer.

However, a good friend of mine cornvinced me to give a Behringer 9024 a try.

Anyone have one for sale under $100 ?

T

[WA1GFZ]

I needed a real good line level transformer at work once. I ended up with a 5 watt part that extended way beyond spec at line level. Maybe that is why the RS part works well.

[K1JJ]

Just to update the epitaph of this thread...

The RF in the audio came back on 40M. Thought I cured it, but only on 75M.

Picked up the used Behringer 9024 and hooked it up tonight.  No RF in the audio now!

Works and sounds good with 6 bands in the monitor.  Menu driven - the manual is a bear - will read this weekend.

The old Symetrix 528 was the definate culprit for RF in the audio.  I swapped it even for the 9024.  Both parties are happy campers.

T