[Amps] RF in the Audio
jim at audiosystemsgroup.com
Sun Sep 25 00:51:59 PDT 2011
On 9/24/2011 11:29 PM, Jim Thomson wrote:
> Date: Sat, 24 Sep 2011 13:50:16 -0700
> From: Jim Brown<jim at audiosystemsgroup.com>
> Subject: Re: [Amps] RF in the Audio
> ### Brown, if ur gonna insert a choke BETWEEN the coaxial output of your titan
> 425 amp...and ...and the input of the tuner, why mess with a bifilar choke in the 1st place??
It was a TEST -- i wanted to know the extent of heating by a
differential field, which turned out to be small.
> [inserted at the same point]
> ## I'm assuming there is a 1:4 balun between the output of the tuner and the OWL.
I don't use open wire line.
> ### A buddy of mine had SEVERE 'RF in the audio' problems....on 6m, while
> running 5 kw output...esp with the yagi pointed anywhere near the house. My suggested
> 'fix' was a string of large type 43 beads
Not bad, but BETTER would be a string two-turn chokes through #43 beads,
which moves the resonance (and thus the peak of the choking impedance)
from about 150 MHz (a single turn with #43) to 50-80 MHz (two turns with
#43). Where that resonance lands depends a lot on the dimensions of the
core. So you use a string of those two-turn chokes in series.
> ## It was blatantly obvious to me that 4-5 wraps of scramble wound coax ain't gonna cut it on 80m. The short term
> 'fix' was again, to slide a mess of large diam type 43 beads over the heliax output of the amp. [ well> legal limit].
> Again, problem solved, no more RFI.
You had a very mild problem if you fixed it with a string of beads.
You're right that a coil of coax isn't going to cut it, but a lot of
turns on some #31 cores will solve far more serious problems.
> ### I see very little, if any pro-audio gear these days with a pin 1 problem. In fact, I haven't seen any pro-audio
> gear, made by anybody with a pin 1 problem. You wouldn't be in business very long building gear with pin 1 problems.
In 1994, when Neil Muncy's paper on Pin One was published, virtually ALL
pro audio gear, including nearly all broadcast gear, had Pin One
Problems. Now, the pros have mostly gotten the word, but the mfrs of
the pseudo-pro, computer, consumer audio, and video gear have not. I
haven't done the work to prove it, but I strongly suspect that most of
the noise coupled out of TV sets, onto cable systems, and radiated to
our antennas is the result of a Pin One problem in those sets (at the RF
> ## I got around the pin 1 problem in the yaesu simply by using the chassis instead....and installed a new RCA connector
> on the rear AL portion of the chassis.
I got rid of my MPs a few years ago, but solved the Pin One Problems at
the mic connector with a multi-turn choke. The K3s are much better in
that regard, although they do some really dumb things with shield
bonding at analog I/O.
> From mic to all the various audio gear is all balanced line level. At the output of all
> this mess is the 20 db pad... then into a jensen balanced to unbalanced xfmr. [with dual shields]
Jensen is the best.
> A real short piece of
> coax goes from jensen xfmr Un-balanced output to new rca input on yaesu FT-1000 MP MK-V.
> ## each balanced audio jumper is made from scratch, using shielded [braid like 58-u] balanced cable.
That cable is a good choice. Have you seen Muncy's work on Shield
Current Induced Noise, or mine showing that a foil/drain shield CAUSES
RFI issues? The bottom line is that a braid shield is the way to fly. I
like either RG58 with a copper braid shield or a mini version for
unbalanced audio interconnects. Impedance doesn't matter, but the
quality of the shield does.
> Neutrik brand XLR connector's are used, male at one end, female at the other end.
Good stuff. I've visited their headquarters in Lichtenstein, and their
people are very active in AES Standards work. Top of the line
Switchcraft is also quite good.
Jim, the only thing I'd do differently is that I am convinced that
balanced interconnects are overkill if you're doing everything else
right, like chassis to chassis bonding, good cable, and chokes as needed
to kill any RF. In other words, there's nothing wrong with doing
balanced I/O, and I wish that all mfrs would, especially for audio,
because it gets rid of hum/buzz from AC leakage currents. But it's more
complex and costly than those other simple measures that are plenty good
73, Jim K9YC
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