FM RFI in Console

[museic]

Just tried out my friends behringer mixer and............ no radio.

 

[abbey road d enfer]

Just tried out my friends behringer mixer and............ no radio.

I'm not surprised. Behringer have to sell their products in all countries and conform to EMI/RFI regulations not only for legal reasons but also because they sell mainly to unskilled operators who would put the blame and badmouth them if their mixer picked up Slayer in the middle of Sunday sermon.
OTOH, Toft rely on their mystique to receive absolution if their mixer picked up Perry Como in the middle of a Keith Jarrett session.
I'm not impressed by Toft's designs, particularly under the tutelage of PMI.

 

[abbey road d enfer]

  Yes I hear the radio with just the stereo bus around 0db and the monitor turned up. This is with every channel muted. So im afraid it is demodulated in the stereo bus somewhere. If I solo every channel on the board and monitor through the solo monitor the interference is much less.

So it seems like looking at the schemo is on order. Do you have them or is it Ian Fleming's territory? But I still think that adding a 100uH in series with the summing amp input would be the first thing to try. That would be a tad more complicated if the bus were balanced. Toft designs are notorious for being EMI/RFI unprotected, so applying the basic techniques should improve things.

 

[JohnRoberts]

If you determine that a virtual earth sum amp is causing the rectification I repeat my earlier suggestion to try a low noise bifet opamp in that location.

Adding an inductor in series can help if bus capacitance is a problem, or bus is acting like an antenna. Another topology for the simple inverting opamp is MFB 2 pole LPF.

Think of R1 as the sundry bus resistors and C1 as the stray bus capacitance. R3 is a new added resistor. Caveat, this is esoteric and you may get most of the benefit just by putting 100 ohm resistor in series with sum amp - input.  Of course the filter poles are tuned well above audio range.

Another thought Is the mixer chassis grounded? Do the channels have inserts or direct outs? If yes are they noise free?

JR

 

[museic]

  Yes I hear the radio with just the stereo bus around 0db and the monitor turned up. This is with every channel muted. So im afraid it is demodulated in the stereo bus somewhere. If I solo every channel on the board and monitor through the solo monitor the interference is much less.

So it seems like looking at the schemo is on order. Do you have them or is it Ian Fleming's territory? But I still think that adding a 100uH in series with the summing amp input would be the first thing to try. That would be a tad more complicated if the bus were balanced. Toft designs are notorious for being EMI/RFI unprotected, so applying the basic techniques should improve things.

I emailed toft again the other day looking for the schematic to no avail. I was assured a year ago they would help me out with this problem. Kind of disappointed on that note.

All FM transmitters also generate an AM parasitic signal. Usually that's not a problem because it's very small in relative ratio, but you are so close to the very high power transmitter (50kW), so that can be your problem. Also, sometimes the AM parasitic level is higher if the transmitter's modulation circuitry is overloaded with the modulation signal  (deviation is more than 75kHz), or the transmitter doesn't work correctly at all (for example, the band pass filter is too narrow, or the antenna isn't properly matched to the transmitter so SWR is poor). There might be also IM interference related to the link type between transmitter and studio, so IMO it will be a good idea to call inspection to check the transmitter. Especially if you receive the signal in the console and bass cab individually. 

On this note I did try to contact a technician for this radio station a while back but didn't have any luck.


Thanks everyone for their help so far. It will be interesting to see what works. I'll have to find some time to open it up and take some pictures.

 

[museic]

Great news! No radio! Kicking myself that I made this so hard. Should have opened up the console a long time ago.

Basically I opened it up and found a unconnected cable that was soldered to the power connector on one end. This was supposed to power the digital converter option that was never implemented. So long story short opened it up and found a antenna!

Thanks everybody for the input. Got me going on finally just opening her up!

And pucho812 gets the prize, although I'm not sure what that is.

:beer:

 

[bruce0]

nd pucho812 gets the prize, although I'm not sure what that is.

King of the Lab?

 

[pucho812]

And pucho812 gets the prize, although I'm not sure what that is.

:beer:

Wow, awesome. Glad to hear it it got fixed. As for me getting a prize, I an not worthy especially with the smarts of PRR, JR and others but glad I could help out. IF there is one thing I learned a long time ago, dive in especially if it is already not working as you can't make it any worse then it already is. Of course that can happen to.