On 2014-06-21 1:49 PM, Paul Stamler wrote:
> On 6/21/2014 9:21 AM, Ron Roscoe wrote:
>> Radio stations are detected at low level inputs by the stray
>> inductance and stray capacitance [and of course the diode] at the
>> base-emitter junction or equivalent. This L and C resonate at some
>> frequency often exactly at the frequency of a local broadcasting
>> station. One of the most effective ways of stopping this kind of
>> interference is to insert a low value resistor, say 470 ohms, in
>> series with the input leads and as close as you can get it to the base
>> emitter junction of the input transistor. This louses up the "Q" of
>> the resonant circuit and reduces the sensitivity of the circuit to RF
> This does work. Adds noise, though.
I think there are some other things to consider.
With proper treatment of Pin One (thanks Neil Muncy--I spent much of
yesterday further helping distribute his estate, how sad) and good
shielded cable, much of the RF goes into the ground and never gets to
the input. At RF it is not so much the balance that is useful, but
rather the signal-in-a-pipe that helps.
In theory, with the shield properly addressed (as suggested by FCC
regulations for digital video interconnects -- both ends solid grounded)
even unbalanced interconnects can be effective against stray fields. The
picked-up RF tends to ride on the outside of the shield (skin effect)
and stays out of the audio/video.
Proper grounding where there are no common impedances between the ground
connection and the signal (so that the ground noise signal can modulate
the desired signal) is a huge benefit. Neil built a "generic mixer" with
one input with common impedance and another without (now owned by Chris
Brown) which clearly demonstrates this. Before Neil broke this down and
analyzed it we were all playing "blind people and the elephant" with the
It has been mentioned previously in this thread, but the common mode
choke on the differential input is very effective as are ferrite
beads/cylinders on both unbalanced and differential inputs.
The ferrite bead can be as effective as the series resistor without the
Case in point, I was living in Glendale California and I bought an RTS
phono preamp (a good friend was working there at the time). I complained
that I was getting RF interference on the preamp. I was line of sight to
a couple of transmitters (while I still find it difficult to believe it
was an FM station that was breaking through--at least we thought so).
Anyway, the friend put two ferrite cylinders (generically called
"beads") on the input leads and the problem was solved (one cylinder per
This was in the early 1990s. In the mid 2000s, I had my last issue with
RFI. We had installed shielded runs of cable in a church here in Aurora,
Ontario, for recording mics. The cables had about a 35 foot vertical
drop from the peak of the ceiling to the mic location and the other end
went down the same distance at an angle along beams. There was never an
audible problem with neither AKG C451 nor DPA 4006TL mics hanging there,
but we bought some low-priced Audio Technica ATM10a omni condenser mics
that worked with either an AA cell or 48 V phantom, and we could clearly
hear an AM station in the recordings. These were purchased due to budget
and their relatively good sound for the price. The AKGs and DPAs are my
We finally installed a pair of Shure A15RF In-Line RF Interference
Attenuators and they worked like magic when installed at the mic.
The ATAM10a apparently had a reputation for RF sensitivity which, I
think may come in part from its ability to be dual-powered
It is all about keeping the RF away from the "diodes"!
Richard L. Hess email: [log in to unmask]
Aurora, Ontario, Canada 647 479 2800
Quality tape transfers -- even from hard-to-play tapes.