Inductive Loop Amps
DangMeSH
scenarios for inductive loops.
What sort of power should I be looking for in terms of driving a floor loop ?
Does the length of the the loop affect the power needed to drive the earwigs or
is it not a factor ?
Thanks,
Scott Harber CAS
John Garrett
Crown DC300 will do the trick. Also, raising the loop above the floor may
improve your results. I imagine large loops take more power, but I don't know
(a) the sensitivity of earwigs nor (b) the field strength per watt per meter of
x gauge wire inside a closed loop. While we're on the subject, does -anybody-
know where this info is, or how to calculate it?
G. John Garrett, C.A.S.
Phillip Palmer
the ones that aren't so heavy!
John, here is a repost from Roger Stevenson, regarding ohm/wire/length.
Details can be found in the handy Pocket Reference.
1) Although it is essentially a dead-short across the amp's terminals,
it's not simply the "LONGEST possible run". At least, the way I learned
it, what you want is a length of wire that approximates a 4 to 8 ohm
load to the output of the amplifier. You can look it up...I like my
handy Pocket Ref for this...16 ga copper wire is 244.5 feet per ohm (at
77 degrees F.), yielding a whopping 1000 feet for 4.09 ohms...but if
you use something skinnier, like 21 ga., which i think is what phone
wire uses, the figure is 76.3 feet per ohm (at 77 degrees F.) which
yields 305.2 feet for 4 ohms, or 610.4 feet for 8 ohms...well, that's a
little more manageable. Then, if you take about 100 feet of 6 strand 21
ga cable, with 7-pin connectors to accomplish the daisy-chaining, then
you have your 600 feet in one easily managed cable and save your self
the hassle of walking six times around the circle laying out your loop
on the set. I've used both Glenn's system and rented the same from
Location Sound many times and it really is the most sanitary and
user-friendly induction loop i've come across.
2) In my experience, it doesn't seem to make much difference how many
times the loop loops around the set, as long as you have enough wire to
keep you amp from exploding. The loop seems to work at arbitrarily
large diameters on the set (maybe more amp-power is needed for bigger
loops), or quite small diameters (I've put the ENTIRE LSC loop coiled
inside the compartment of a piano bench to hide it from camera and it
works swimmingly for anyone who sits at the piano).
3) Hygiene: Ear infections are BAD NEWS...and not good for your
reputation...I like to have a supply of pre-moistened alcohol pads on
hand and clean the earpieces EVERY time they are inserted into the ear
canal. Good Karma at least...and cheap.
Thanks to Roger Stevenson for that post.
--
Phillip Palmer
ph...@palmeraudio.net
www.palmeraudio.net
"John Garrett" <jgar...@world.std.com> wrote in message
news:3A8F5A26...@world.std.com...
Glen Trew
It has always worked well and the amplifier has never been damaged. Most of
today's speakers amps will deliver plenty of power for the induction
earpiece with loads of 2 ohms or even less. A load limiting resistor to
protect the amp is NOT needed and pretty much renders the loop useless.
If your amp has 1/4" speaker connections, an adapter can be made so that you
can use standard speaker cable to loop the room. The adapter is a short Y
cable with 1/4" male and two 1/4" females. The tip of the male goes to the
tip of one female and the sleeve of the male goes to the tip of the other
female. Using standard speaker cable to loop the room with both ends plugged
into this adapter, the amp's output is essentially shorted. We make a nice
version of this using thin, flexible, Canare cable that lays flat and is
easy to hide.
Glen Trew
"Phillip Palmer" <ph...@palmeraudio.net> wrote in message
news:u6Tj6.5$x_3....@nnrp3.sbc.net...
John Garrett
on Wednesday; its staying in my shoulder bag forever!
G. John Garrett, C.A.S.
Ian Sands, Work, Pipex
up a pair of adaptors from the output of your amp like this. Loop your cable
round the set, high as possible for good coverage, then take one leg of the
amp output to pin 1 (screen) and 'send' it round the set, at the 'return'
end of the loop cable take pin 1 and connect to pin 2 (hot) to go round the
set again, at the 'return' end take pin 2 and connect to pin 3 (cold) and
send it round the room yet again, then at the 'return' end take that pin 3
with its dizzy signal) and connect it back to the other leg of the amp
output. 3 loops for 1 cable. Don't cook it though.
is
Ian Sands
Location Sound Mixer / Recordist
email: i...@iansands.co.uk
Web page: www.iansands.co.uk
Phone: +44 (0)7860 753717
John P. Pytlak
John Pytlak asked:
Could you do the same trick with a multiconductor cable like 4-wire or
8-wire telephone cable? For example, wire the cable loop so the signal
goes around the room 4 or 8 times to improve the magnetic strength?
These cables are likely much less expensive than shielded mike cable.
Glen Trew
Have you tried this to verify that in increases the loudness at the
earpiece? I have not try this but I have avoided it because it seems like
doubling the circuit back over itself would cancel out and reduce the
signal. No?
Glen Trew
"John P. Pytlak" <john....@kodak.com> wrote in message
news:3A9196DA...@kodak.com...
David Patterson
The set-up you describe would in fact radiate a signal, but only
about a third as strong as you probably think. Out of the three
conductors you mention, only the shield would radiate a signal.
The inner conductors would be severely attenuated by the shield.
To work well, all the conductors need to be unshielded.
The setup that I have used before is to take a piece of 25 pair
telephone cable, which is unshielded, and jump each conductor over
to the next until all 50 wires are in series with each other. I was
able to completely cover the interior of a fastfood restaurant
with a 25 watt rms amplifier driving the loop. I strung the loop
all the way around the interior (about 150 feet). Worked great.
David Patterson
Atlanta, Georgia usa
Ian Sands, Work, Pipex wrote:
>
Stephen King
The canceling effect doesn't happen with loop antennas for RF
reception. I think that you might be correct if you wired up a
'termination' xlr so that the signal went back and forth in the cable
with the amp connected at one end. That might have a cancellation
effect like an impedance matched two wire r.f. transmission line;
however, I doubt it would at audio frequencies. But, if the signal goes
round 'n round it should work fine. Theory not supported by empirical
evidence ;-)
Steve King
Bill Drucklieb
Some good info, some great ideas and a bit of bunk. That problem has been
solved several times over.
Ok folks, here's one for ya... How would you wind the following low current
(200 watt) mains power transformer:
primary = 110~117 VAC
secondary 1 = 60 VAC
secondary 2 = 36 VAC
secondary 3 = 18 VAC
secondary 4 = 18 VAC
Direct email or group discussion is appreciated...
To start the discussion, center tapping the 36V coil could produce the (2)
18V legs. If you feel this is a good solution, please describe the phase
relationships of each 18V leg to the 60V leg.
BD
ken
Very interesting thread about inductive loop feeds. I'd forgotten about that stuff.
What are the popular pro receivers for this? Anybody home-brew these by adding a
pickup coil to a small practice amp or modified radio?
Thanks,
ken
Larry Fisher
The shield will not cancel out the currents on the inner wires when wired in the way described by Ian. This is a confusing subject because when mic cable is used for its intended purpose the shield does cancel out
the currents on the inner wire(s) but that is due to the way it is wired. The currents are in opposite directions and they cancel out. But this isn't true for Ian's wiring method in which the currents are all in the same
direction. David's 25 pair telephone cable will work well also. A good precaution (somebody else may have mentioned this already) is to put a 10 watt or so 4 Ohm resistor in series with the amplifier output. Not
only does it protect against shorts and miswiring but it also protects the amplifier against a very inductive load with lots of distributed capacity. Poorly designed amps as well as some audiophile units will oscillate
into such a load. If the wire has high enough resistance anyway, the resistor may be unneccessary, but I've always liked a smoke free enviroment.
Larry Fisher
Lectrosonics
JnyVee.
Larry Fisher wrote:
>
> David and Ian,
> The shield will not cancel out the currents on the inner wires when wired in the way described by Ian. This is a confusing subject because when mic cable is used for its intended purpose the shield does cancel out
> the currents on the inner wire(s) but that is due to the way it is wired. The currents are in opposite directions and they cancel out.
The shield does NOTHING to 'cancel out' currents (or common mode
inductive EM noise field pickup) in the inner wires, the ballanced
aspect of simply the two wires alone is the cause of that and
two-wires-twisted does it even better... and for most such things, you
really could do just dandy without the shield entirely if the gear at
both ends of the cable were properly ballanced. The shield is there as
an extra protection and common grounding line (as well as to allow
special power supply transmission to gear at the live end) and helpful
with some forms of special interference.
JnyVee.
indeed, except for the lack of stealth look, Rad Shack makes (made?) a
simple little all-purpose intercom-like box... 1/8"
input-plug/amp/speaker/9vbatt that, with a headphone jack added and a
pair of walkman cans and a classic suctioncup phone coil makes a dandy
portable pickup/checker for loops.
Bill Whitlock
current flows must be large (i.e., large open area inside the loop
conductor). If the same cable carries current in both directions (i.e., to
and from the amplifier) the magnetic fields will cancel because the two
conductors are so close together (as in an XLR cable). The field from the
"to" wire opposes the field from the "from" wire because the current
directions are opposite. If they are far apart (large loop), the radiated
magnetic field is maximized. And the frequency makes no difference at all.
Bill Whitlock
Jensen Transformers, Inc.
"Stephen King" <stevek...@home.com> wrote in message
news:3A91F3ED...@home.com... (in part):
Bill Whitlock
It has a sensitivity of about 1 millivolt. A great all-round troubleshooting
tool, too!
Bill Whitlock
Jensen Transformers, Inc.
"JnyVee." <moc....@ybmurbrevlis.com> wrote in message
news:3A92AA...@ybmurbrevlis.com... (in part):
Ian Sands, Work, Pipex
Looping the feed around the set on a regular mic cable works just fine. So
far no problems with volume ~ so I guess everything is getting through. I
haven't done any field strength measurements though. We just get the gig,
don't we. As Larry Fisher mentioned the circuit is continuous ~ not doubled
back from the end of the cable ~ so you end up with three turns on the set.
It did occur to me when I first tried it that the *screen* would act in just
that way ~ but it doesn't seem to reduce the effect. If it worries you then
just use hot and cold ~ still gives you two turns on the set for the trouble
of laying only 1 cable that we can all find in our kit. Easy enough to
extend with another mic cable if when you measured the perimeter of the set
you got it wrong and didn't buy enough mulitcore!! Dooooh! "Now did I pace
it out in meters or yards"? Good point on the load resistor for protection
though ~ need to be fairly beefy. I've been lucky on the few occasions I've
done it.
is
Ian Sands
Location Sound Mixer / Recordist
email: sand...@dial.pipex.com
Larry Fisher
magnetic interference, other than keeping the wires physically close together and the "receiving" loop small. The shield will do a lot for electrostatic pickup, adding to the rejection inherent in a balanced system.
I think it is fascinating though, that you can have a wire inside a wire (shield) and if the currents are in the same direction, as in Ian's hookup, the magnetic field will be increased.
Best Regards,
Larry "Mumbles" Fisher
Lectrosonics
Glen Trew
Thanks once again for the RF savy. But, as posted previously:
Most of today's speaker amps (all?) will safely deliver plenty of power for
the induction earpiece with loads of even less than 2 ohms. Small gauge wire
of the lengths that we are talking about (100' minimum) will have enough of
a voltage drop to be verly different from a direct short. So, a load
limiting resistor to protect the amp is not needed. It is my experience that
a resistor also renders the loop nearly useless for inductive earpiece use.
Glen Trew
"Larry Fisher" <lar...@operamail.com> wrote in message
news:1103_982688990@larryf...
Larry Fisher
If you get a big change in level (more than 3 dB) putting in a 4 Ohm resistor,
then the load that the amp is seeing is much less than 4 Ohms and the amp
is delivering fairly hefty currents which most amps can do before the output
protection begins to curent limit. The safest approach, if you don't want to
use the resistor, is to measure the DC wire resistance with an ohmeter and
make sure it is above the minimum rated load for the amplfier. Having
blown up 3 commercial amplifiers, with weird loads, I would still go for the
belt and supenders approach. Of course, I was much younger
and the amps were much older.
Regards,
Larry Fisher