[Sursound] Mike RFI

[Richard Lee]

IMHO, practical RFI performance (as opposed to meeting EU specs) is one of the major factors that separate the good from the great mikes.

The only thing I can say is that the guys who made the recommendation were practitioners, not just theorists, being both designers of instrumentation

dealing with (often very) low level signals like those from insect muscles
and nerve cells and also people who were running a commercial EMC testing
facility. Mot sure if they ever did any mic designs, tho' ;-)

Once upon a time, before the days of EC RFI standards, there was the Calrec CB radio, an electric motor driving a car coil, distributor and spark plug which sparked every second with a 1m aerial on top.

Everyone hated when I started it up, cos every meter at Calrec would do a blip blip blip including (I'm told) all the passive multimeters so I could only test RFI at night. Pity TV reception in Hebden Bridge.

We got back a VERY dented Calrec stick mike from BBC Manchester with a note that it seemed very prone to lighting buzz. I opened it up and found the earth connection to the case was cracked; not surprisingly really. Since it was Saturday, I could fire up the Calrec CB radio and played around a bit ending up with 2 extra ceramic caps and slightly different earth connection.

I sent the dented souped up mike back with a letter saying we had done a full investigation and proposed modification for an exorbitant sum. About half the cost of a new mike for 10min work to move the earth and 2 caps.

The next thing, a small crate arrives by courier with some 200 mikes, the complete BBC spare inventory of Calrec mikes for the North of England and all the mikes from Manchester. Then an irate phone call asking why we hadn't done them and they had to all come back by Fri cos they were the only mikes of any make which didn't buzz in their new TV studio.

Wharfedale had some HiFi stuff tested at the York University EMI centre in the early EC standard days and found the gurus there very competent. But thyristor stage lighting in dem days required much more than meeting the EC RFI standards. I haven't tested old Calrec mikes with mobile phones but I'd be surprised if there was a problem.

[William Sommerwerck]

25 years ago, when I was making recordings with the then-new Sony PCM-F1 (as
it was designated in the US), I ran into some RFI problems (buzzng,
apparently from TV sync) with my Pearl mics. I didn't know what to do, until
another recordist at the concert suggested "pigtailing" the mic cable around
the mic stand. That did it. The small added inductance "choked" the RF.

[Dave Malham]

Post by Richard Lee
Once upon a time, before the days of EC RFI standards, there was the Calrec CB radio, an electric motor driving a car coil, distributor and spark plug which sparked every second with a 1m aerial on top.
Everyone hated when I started it up, cos every meter at Calrec would do a blip blip blip including (I'm told) all the passive multimeters so I could only test RFI at night.

I bet! I used to use a little transmitter based on a 27mHz centre
frequency LC tuned Colpitts oscillator, using a UHF transistor, with its
collector overmodulated by an audio signal derived from the base of one
of the transistors in a two-transistor multivibrator (asymmetrically
tuned, just to be even nastier). The combination of FM and AM modulation
with both sharp edges and sloped ones from the transistor base meant
that this transmitter spewed rf just everywhere. Unfortunately, I leant
it to a theatre technician at the Theatre Royal in York many years ago
and it never came back - I suspect he used it to get his own back on the
taxi drivers who were forever causing him breakthrough problems....

Dave
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[Dr. David A. Pickett]

Post by Dave Malham
I bet! I used to use a little transmitter based on a 27mHz centre
frequency LC tuned Colpitts oscillator, using a UHF transistor, with its
collector overmodulated by an audio signal derived from the base of one
of the transistors in a two-transistor multivibrator (asymmetrically
tuned, just to be even nastier). The combination of FM and AM modulation
with both sharp edges and sloped ones from the transistor base meant
that this transmitter spewed rf just everywhere. Unfortunately, I leant
it to a theatre technician at the Theatre Royal in York many years ago
and it never came back - I suspect he used it to get his own back on the
taxi drivers who were forever causing him breakthrough problems....

Funny zou should have done that: I made a similar little box to block
transistor radios on the beach in the 70s!

david