tube mic picking up radio

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

gang of elk

Well-known member
Joined
Jun 16, 2004
Messages
257
Location
tn, usa
Hmmm. I'm having a problem with a pair of tube mics picking up the radio. Both mics are connected to the same PSU and each mic picks up a different station. Moments ago, one was picking up jingle bells, the other talk radio news. What a nightmarish combination!

What's the usual cause of this problem. Bad ground? Bad cable? Tube? Wondering if anyone has experinced this and if so, how did you solve the problem?

I was thinking of starting with new cables from the PSU to the mics anyway since the ones that came with the set seem really flimsy, and moving the cables around does effect the radio noise. The mics sound fine otherwise.

BTW, the mics are a pr of Lomo 19A19.
 
Forgot to mention, they're both connected to the same dual PSU.

PSU is 220v running off a radio shack 110->220 step-up transformer.
 
You could try a really small capacitor at varoius locations to ground, like the input of the tube or fet.
Start with 500 pf and work your way down, 500 will chew up too much highend, but it should short out the rf.

The capacitor will have a relatively low resistance to rf, but a high resistance to audio, thus not shorting it out.

Could make a cool Karoke machine, the mic plays jingle balls and you guys sing into it at the same time. :shock:
 
First clean and check the mil style connectors.

Second I did not know there was a dual supply for 19a19s

I would use a variac to adjust the AC to be exact as possable. You don't want to overheat the fils

Have the microphones worked OK before or are they new to you?

The 19a19 is well shielded I would suspect the cable or dirt at the connectors.
 
Thanks for the suggestions.

I just got these today, were shipped from Moscow. I suspect the PSU is a custom job but maybe not. I'll try to post some pics. The mics did not have the problem before, as I was told by the seller.

I ordered some new cable from redco this afternoon so will try that, and will clean the connector pins. I forgot until now that I had a similar problem once with a Schoeps CMC4 where a few dust particles between the capsule and body caused what sounded like a shortwave sound. Drove me crazy til I found it. Different problem, but not totally unrelated.

CJ's idea is something to try if all else fails.
 
Finally, problem solved. It was a termination problem. The sheild was connected to the connector shell but when the the connector collar was fully tightened to the mic body (and psu), the continuity of this connection was interupted. I loosened the collar ring and burnished the inside edge where it contacts the outer surface of the shell. Did this on both ends. Continuity! and no more interference.

:guinness:
 
Thats good to know. I did suspect it was "just" a bad contact(s) at the mil type russion connectors.
 
CJ...your comment with respect to RFI (RF interference).  My primary work deals with RF interference, "hardening" of equipment to keep RF out, and on the other hand, make antennas radiate RF energy efficiently.

Bypassing inside a microphone has to be done carefully.  Bypassing the grid of a tube or gate of a FET with the high impedance involved will affect the audio.  If T = RC (time = resistance x capacitance), the RC constant of 250 meg ohms or more paralleled with 500 pf changes the response of the circuit; the paralleled capacitor will also attenuate the output since it is in parallel with the capsule.

TN, how far are you from the radio station transmitter?  If the RFI is AM broadcast and you live near the transmitter (3 km or so), you may get RFI and you probably can hear the audio in your equipment.  You may get RFI from a nearby TV or FM transmitter but the programming will not be heard.

My suggestions:

1.  Clean all connections on cable connectors.  Oxidized connections rectify RF.
2.  Use AC line filters (Ebtech's Hum-X, Corcom, etc.) on sensitive equipment.  The purpose of this is to keep RF conducted from the AC power line out of your equipment.
3.  Try a AC isolation transformer to power your audio equipment.  Doesn't hurt to use a AC line filter on the primary side.
4.  Are any cables 1/4 wavelength (or a multiple) at the transmitter's frequency?  984/(F in Mhz) approximates the wavelength; divide this by 4 for 1/4 wavelength.  Resonant cables act like a tuned frequency receiver.  Modifying cables can get time consuming so try 1-3 first.
5.  If none of these works, then deal with the mic itself.  Some mics have an output transformer; others are unbalanced, and some of them are differential out.  Study the schematic first.
5a:  Output transformer (normally 200 ohms):  Connect a 500 to 1000 pf capacitor across the output transformer at the output XLR connector.  If this doesn't work, connect 500 to 1000 pF capacitors to ground from pins 2 and 3 of the output XLR connector.  Alternatively, you can put ferrite beads on the leads to pins 2 & 3 of the output XLR connector; make sure the ferrite beads are suitable at the nearby transmitter's frequency.
5b:  Unbalanced audio (like AKG C60, Altec M30, etc.)  No output transformer in microphone; audio from cathode follower is sent unbalanced to the power supply where there may or may not be a transformer.  The impedance at the cathode may be 2 k or less.  You can try 500 pf of bypass (to ground) but note if doing this "colors" the audio.  If there is a output transformer in the power supply, bypass the XLR connector as above first.  Alternatively, you can add a good quality output transformer in the power supply if one is not there.  Most good transformers stop RF.
5c:  Differential mode:  Solid state transformerless balanced output:  Recommend ferrite beads first.  All connections at the mic should have beads since the audio rectification is probably occurring in the mic.  Study the schematic.  Be careful with these circuits...at the high impedances involved, bypass capacitors in particular may set the whole works into oscillation.  Most of these mics are phantom powered.  This means that beads are required on each lead of the XLR connector.
5d.  If you live very close to the radio station (within the 1V/meter contour), this is called the "blanket contour" for AM, and one or more of the suggestions listed above is needed to keep the RF out of your gear.  Speaker wires and RF:  Suggest bypassing speaker leads with 0.1 uF 400 v mylar capacitors at the amplifier output.  The twin lead running from the speaker to the amplifier acts like a doublet antenna.  The inductive reactance of the speaker coil is high at AM broadcast frequencies so this looks like a doublet antenna terminated at the amplifier.  You want to "short out" the RF at the amplifier or bypass it to ground.  You don't want high levels of RF getting into the amplifier...in worst cases the RF rectified in the output transistors is strong enough to upset the bias on the solid state devices and cause failure.

CONSULT THE MANUFACTURER for suggestions also, especially for equipment in warranty.  Mods to equipment in warranty usually void the warranty.  You may need to call or email "up the food chain" at some of these companies to locate someone who understands the problem.  Fortunately, I have encountered only one company in almost 45 years that was uncooperative.
 

Latest posts

Back
Top