Loop Current &
Circuit Loss Technical Bulletin
Click here to go right to Circuit Loss Information (below),
to fix "Can't Hear" problems
Loop Current is the amount
of electrical energy flowing through the telephone and line, as opposed to the voltage
which is the force behind the energy. There is a definite
correlation between the Loop Current and Line Voltage (Ohm's Law), but the loop current
reading is often what indicates the problems in telephony... not the voltage reading. The
carbon transmitter used in telephones has been the controlling factor for years, since it
needs over 20ma to sound good.
- THE PROBLEM
I was made aware of the high loop current problem by Don Jordan
of Newmont Gold Co. in Las Vegas, NV (a long time phone man), a number of years ago. I
didn't have a clue what was causing these problems before that, but as soon as he
explained it to me it was like a light bulb lit up, and I started remembering all of the
weird cases that I couldn't fix, that were probably loop current problems.
Until about the mid 1980's, the big problem with loop current
was that it was often too low. That was when the only way to get from point A to point B
was a pair of copper wires. Now, with the proliferation of electronic Central Offices and
electronic pair gain equipment: T1, Fiber Optics, Remote Central Offices and SLICs in
every suburban and even rural area (you can see these Huts and Buried Vaults scattered
around everywhere), over 90% of the problems are high loop current. This is because the
manufacturers of the "far end" pair gain equipment have adhered to a very old
specification for loop current, but one that is still valid, that says between 23ma and
120ma are OK - but the CPE is much closer to the source of the talk battery than the old
days. When the phone company tells you that they are within specs (while smoke wafts off
your trunk cards at 80ma of loop current), they're right!
Both the phone company and CO equipment manufacturers have no
incentive to bring the loop current down. All they have to do is make a standard 2500 set
work (which has no active electronic circuitry to burn up), and the farther out it
works... the better. Until the FCC sets a new standard for high loop current (unlikely),
or CPE manufacturers take account of the high loop current problem (seems unlikely), the
Interconnect company will be left holding the repair bag for these problems. Most CPE
equipment was designed based on low loop current problems... it works well right down to
23ma. CPE manufacturers have been really slow to try to head off the high loop current
problem. In the rare case of low loop current (below 23ma), the phone company is required
to bring it up to 23ma.
The main problem created by high loop current is heat. The
components on the trunk card or telephone that connect to a CO line with high loop current
get hotter than the manufacturers planned for. When the components get hot, their specs
change, which makes the circuit work differently - usually with unpredictable results. If
the loop current is high enough, a component can get fried and the device will stop
working instead of just having problems. Eventually, the heat from the high loop current
can damage one or more components on a trunk card, shortening the life of the card.
In some cases, you may have a problem with low
loop current on an analog station port or phone line. We have a Loop
Current Booster™, which will bring the loop current up to 27ma,
and also add 24VDC to the on-hook talk battery (24V talk battery
would be 48V talk battery). We also have the Ring Voltage Booster
will add 7.5 REN of 20 cycle ringing
at 90VAC RMS to a phone line or analog station
port. It passes Caller ID, message waiting and CPC
signals. The combination of the Loop Current Booster™ and
Ring Voltage Booster™ do the same thing as the old Proctor Long Loop Adapter
46222 (which is no longer manufactured). These Boosters
are separate items, so they can be purchased separately if you
only need to cure one problem (ringing or loop current / talk
Keep in mind that neither of these units will bring up
the volume of a phone line or OPX. For that, you'd need a 2 wire amplifier
(see the Telephone Line Audio Level Primer at the end of this bulletin).
wire used to be much bigger than the 24 gauge standard today, so they could carry current
the long distances that needed to be traveled from the CO (Central Office). There is a
definite loss of electrical energy per foot of copper wire. The closer the CO got to the
subscriber's location (because there are more and more COs these days), the less there was
need for the current carrying capability of the bigger wires. Since the 1960's, phone
wires have gotten smaller, except in rural areas where the distance to the subscriber
still requires the bigger wires with larger current carrying (and lower circuit loss)
capabilities. Keep in mind that some very rural CO's operate on their own totally
different voltages and currents to get the phone service to their subscribers (like 100VDC
At the end of a long loop, the current that left the CO at
35ma might be 18ma because of wire loss (resistance), and the audio level may be well
below -8.5db, causing it to be hard to hear. The circuit loss (volume) can be given a
boost by the phone company by putting load coils into the loop, usually
on loops over 3 miles.
Glenn Witherspoon, a
retired phone man in North Huntington PA, called with an explanation of how he would
determine whether to install load coils when he was with the Phone Company. He said they
would never put load coils in the loop if it was less than
15,000 feet to the premise, in which case they would install three load coils (they
wouldn't install just one). The first would go at
3,000 feet, the next at 9,000, and the next at 15,000 feet.
I've also been told that depending on the value of
the load coil (they come in different inductance values), the last load coil should be at
least a couple of thousand feet from the premise.
The load coil uses inductance to increase the audio voltage
(sound level) at voice frequencies, while rolling off higher frequencies. If you need to
use the higher frequency capabilities of the phone line for high speed data (like a 56K
modem or DSL), you may have to get the load coil removed to allow those higher frequencies
to pass through (although the voice capabilities of the line may then be unusable due to
the distance). Data circuits have always been adjusted 13 db below voice levels (they like
lines with low volume).
In some cases, the Phone Company screws up and there's a
misplaced load coil on the pair, either too close to the premise, too close to the CO, or
not needed at all. Although everything is computerized, a lot of the plant records
indicating which pairs have had load coils put on them in the past are incorrect. That
means that when someone gives up a line that needed a load coil, and the pair is re-used,
the Phone Company might not know the load coil is on that pair, and it could create a
problem on the new phone line that pair is used for. Mis-placed load coils will change the
impedance of the line and could even make it too loud, which can distort both voice and
DTMF digits (causing voicemail DTMF recognition problems).
A special TDR (Time Domain Reflectometer) can see the load
coils on a phone line, and pinpoint the exact number of feet from the premise. If you're
having strange problems, especially with DTMF recognition, it's not a bad idea to ask the
phone company to check the line for mis-placed load coils. Most TDRs can see out to the
first load coil on a line. A special TDR (more expensive) can see out to 5 or more load
If you want to know the location of a Central Office, the
type of CO, who owns the CO, or even how far the premise is from the local CO, click
to see our Central Office (CO) Info Lookup
page to get the address of the CO, and use the Map
It! button figure out the distance between the two addresses.
Phones Companies use another piece of equipment that causes
strange problems... Pair Gain Equipment. Phone companies today have
little or no interest in putting more copper in the ground. If a premise has a six pair
going to it from the nearest pedestal, and the subscriber needs seven lines, the Phone
Company has two choices, bury more copper cable, or use an electronic gizmo to get two
lines from one of the existing pairs.
A voice T1 is simply pair gain equipment, getting 24 lines
from one or two pairs of copper. A T1 is expensive because the signal needs to be repeated
every 6,000 feet. In the case of a lack of facilities (copper) to a particular building,
the copper pairs usually aren't going very far from the pedestal or pole to the premise,
and they usually don't need to very many lines, so some fairly inexpensive electronics can
split a single pair of copper into two or more lines.
The problem with pair gain equipment is that it sometimes
does a crummy job of emulating a real phone line. This stems from the old telephone
company philosophy (excuse) that they will only guarantee that a line works with a regular
2500 set, and a 9600 baud modem. The reality of the world is that there are almost no 2500
sets connected to any Phone Company's lines, and almost nobody that wants their modem to
connect at a slow 9600 baud. But they don't care. They don't have to. It's your job to
make your system work with whatever lines they provide.
Some pair gain systems are known for putting out noise
(common mode noise that you can't hear, but that's fixed with our
having problems with Caller ID, or various other problems - especially with high speed
data (they usually won't pass anything over 24K). Reading the loop current and voltages on
the line might point out where the problems are. In some cases, you may be able to get the
phone company to switch the pair gain equipment to a different set of lines, if your
customer is using a particular line for a special application (like a fax, modem, etc.).
The Phone Company won't tell you that they're going to put pair gain on a line. You find
out when there are problems, usually after adding a new line, and the readings you take on
a couple of lines look different than the other lines.
The Phone Company may put pair gain on one or more lines
without telling the subscriber, and with no new orders for the subscriber. There may be a
new tenant moving into the building, and they're out of pairs, so they put the pair gain
equipment on an existing tenant's pairs in order to give service to the new tenant. You
have absolutely no control over that, and can't complain about it. The Phone Company can
do whatever they want to get dial tone to the NI.
CLECs make it very difficult to get problems resolved on
telephone lines. They often use the LEC's copper, so they have little or no control over
what happens, and often won't offer the same level of service as the LEC. Most won't offer
a trunk, which is conditioned better than a line, and
can fix strange problems (see the Primer on Telephone Line Audio, below).
- UNFIXABLE DATA PROBLEMS
Generally speaking, loop current above 27ma will cause problems ranging from
intermittent garbage to "no connect" on modems. It's going to be different in
every case, so you have to keep loop current in mind as a possibility when you have a
problem. I've been able to fix some of these problems by using a different brand modem,
but it's not convenient to carry around a box of modems for testing. A customer called me
about a location with an AT&T System 75 switch with single line stations, where the
users were having trouble connecting or getting garbled data intermittently with laptop
computers and modems connected to the single line station ports. The loop current wasn't
super high, I think it was in the 35 to 40ma range, but the installation of Loop
Current Attenuators at the frame for those stations using laptops solved the data
problems by reducing the loop current below 27ma. Loop current problems are
not limited to CO lines. They also can occur on PBX or Key System single
line station ports.
Since fax transmissions are just high speed data, it's common
for high loop current to cause problems with fax machines.
Keep in mind that some station ports are 24VDC, instead of
the normal 48-50VDC voltage of a CO line. The same loop current specifications apply to
the lower voltage... between 23 and 35ma. The dip switch settings on our Loop
Current Attenuator are sometimes too "coarse" at the lower voltage
(loop current would jump from 22 to 29ma). Our Loop Current Regulator,
which automatically keeps the loop current at 25ma, should work fine on a 24V line.
- COMMON SYMPTOMS OF HIGH CURRENT
Most phone systems work OK with loop current in the 23 to 35ma range,
although there are some that will cause problems above 27ma. One that comes to mind is the
Walker Poet. One of our customers reported that he had intermittent crosstalk, even after
replacing everything - KSU, cards and power supply. Walker tech support suggested that he
check the loop current, which was found to be 32ma, normal in most cases. Walker said to
get the current down to 27ma or less, and the problem was cured. In one case on another
type of phone system, crosstalk was repaired by replacing the trunk cards, but the problem
kept reappearing. Constant high current would eventually break down (damage) the
components on the trunk card, and crosstalk would return.
There have been numerous reports of memory failures in
equipment like dialers, faxes and answering machines.
I've had quite a few reports of Panasonic 2 line phones
getting damaged by loop current over 40ma. It usually effects just one line on one phone
first, but you will eventually replace all of the phones.
Many CO line (Trunk) cards are burned up on all types of
systems every day... some under maintenance and some billable. In some areas, the loop
current is running in the 80 to 105ma range. At that point there's no way you
can make money with a maintenance contract without a Loop Current Regulator on every line!
If you're going to be installing a T1 Channel Bank, verify
that the loop current is down at a reasonable level before committing to it, or you could
burn up the trunk cards in your phone system. Most modern Channel Banks have reasonable
loop current, but it's worth checking to prevent problems later.
I've had a number of calls from Long Distance dialer
providers whose dialers had all kinds of strange problems with high loop current.
Likewise, the payphone industry seems to be hit hard due to the sophisticated electronics
in their phones, and the high speed modems they use to transmit call detail data.
Symptoms of High Loop Current Include:
Burned Out Key, PBX or Data Equipment
Garbled Data and Modem Failures
Cut-offs & Squealing on Lines
Crosstalk, Echo & Hollow Sounding Lines
Numerous Intermittent Circuit Failures
Off Premise Equipment Problems
Symptoms of Low Loop Current Problems:
High or low loop
current may affect Caller ID, but the #1 reason for Caller ID
problems is high AC induced on the line (see our
Longitudinal Imbalance Tech
Bulletin), greater than .5VAC from tip to ground or ring to
Considering the problems that loop current can cause, if you
have weird unexplained problems you should probably go right out and take a reading on the
lines... it's easy! Dave Williams of Frontier Communications in
Belgrade, MT says he is checking the loop current on the CO lines on every system he
installs, which will head off customer disputes. It's not cheap to bring down the loop
current in many cases, the main dispute being who pays for the repair (everybody points at
the phone company first, but your finger will fall off before you get them to pay for
it!). If there is low loop current, below 23ma, the phone company is responsible for
getting it up to 23. I've talked to guys in rural areas who said that the phone company
told them they just couldn't get it up to 23ma. You may have some recourse through your
state's Commerce Commission, but who has the time for that? We do sell a Loop
Current Booster (above).
- MEASURING LOOP CURRENT
Measuring the loop current is easy. Our Loop Current
Tester is the easiest way to test. Just plug it into a modular jack on the line
(not in-series), push the button, and read the loop current off the LEDs.
Otherwise, you can use a good quality DIGITAL volt-ohmmeter
with a DC milliamp scale. An analog meter, except one designed for telephone work, will
usually not give you accurate readings. A Fluke, or a few of the Radio Shack meters will
do OK - within a couple of ma. Our Network Meter will display
loop current, as well as AC and DC Voltages. The connection diagram for a meter (above) shows the meter in-series with
the telephone instrument or system. That will give you the most accurate reading with a
meter, since it's going through a 600 or 900 ohm instrument (or Butt-set).
As an alternative to connecting the meter in-series with the
instrument, you can just short the pair with your meter leads in
the ma mode, and read the direct loop current, which will probably be a couple of ma
higher than the reading through a phone, but it might get you close enough to make a
decision on what to do.
If you don't use our Loop Current Regulator
or Attenuator, and try to use resistors to bring the current down, you
really need to watch the db loss as you increase the resistance, so you don't get it too
low to hear properly. Try to stay above -7db. Below that you'll have a hard time hearing
on long distance calls. Generally speaking, for every 100 ohms you put in-series with the
tip and ring (you have to keep the line balanced and put the same value resistor on both
sides of the line), you'll drop the loop current by 1ma, and the circuit loss by 1db. That
means if you want to get down to 30ma from 35ma using 500 ohm resistors, you'll lower the
volume of the line by 5db - and you probably won't be able to hear well. Both the Loop
Current Regulators and Loop Current Attenuators have special
circuitry that lowers the loop current without lowering the db level. I'll
talk about measuring db loss a little later.
As I said before, to measure loop current with
a regular meter, you need the meter in-series with the telephone
line and telephone. To measure voltage, whether the DC line voltage or the AC audio
voltage, you need the meter in parallel (across) the telephone
line. You don't want to make a mistake in your hook-up as the readings would be
meaningless (and could cause you to take an expensive improper action). Make sure you
meter is set on DC ma, not AC ma!
To hookup a regular meter to measure loop current
in-series: If you have bridging clips on a 66 block, just open one of them, and
connect one lead of the meter towards the CO, and the other towards the equipment. It
doesn't matter which color lead goes where since the meter will display a + or - that you
don't care about (except on an analog meter which will make the needle go backwards and
With the line on-hook, you should see almost no current
flowing through the meter (it will read maybe .01ma). When you go off-hook with either the
phone equipment, modem, or your butt set, you should get a reading of between 23 and 35ma
if the line is OK. Repeat and record the readings for each line, since not all of the
lines have loop current problems in many cases.
- REDUCING THE HIGH LOOP CURRENT
If you have a high loop current reading, you have three choices:
Put resistors on the line, keeping in mind that too much
resistance will lower the volume of the line too much
Put a commercially available Loop Current Regulator or
Attenuator on the line
The only two choices are obvious. Most opt for the
commercially available Loop Current Regulator. We sell modular one and
two line versions which can be plugged inline with the RJ-11/RJ-14 either for testing - or
for good, and a 66 Block version which can be plugged on to a split 66 M block (4 pins
across, split), taking the place of the bridging clips. You can also punch down the
modular versions by cutting the modular cord.
The Loop Current Regulators are all automatic.
Just plug it in, and it brings the current down to 25ma with no loss of audio level on the
line. They are the easiest to use.
The Loop Current Attenuator has a set of
four dip switches. You flip one at a time (in some combination) while watching the loop
current come down to below 27ma on your meter (but above 23ma). At that point you just
leave it, and it's fixed. You do need to buy one Attenuator per line that
has high loop current. It will not reduce the volume level of the line, like resistors
would. I've actually flipped the switches on the Loop Current Attenuator
while watching a thousand cycle tone from the CO, and the audio level actually went up a
hair all the way down to 20ma...That's pretty amazing! Don't mistake the Loop
Current Attenuator for an audio attenuator (pad). Audio
Attenuators are used to reduce the audio level of a line (db) along with the DC
loop current, usually by a fixed amount printed on the device, when the audio level of the
line is too high. I'll talk more about Audio Attenuators in a little bit.
Most phone guys don't like playing with resistors, but if
you'd like to try, here's what you've got to do - and it will work in some cases. As an
example, many phone systems will get a squeal when two lines on the system are conferenced
together if there is high loop current (and sometimes a loud line). Many years ago, TIE
suggested replacing the bridging clips with 100 ohm (or whatever works) resistors to cure
the squeal problem. It worked like a charm on every system I've tried it on, and I just
used Radio Shack 100 ohm 1/2 watt resistors in all cases (1 watt resistors would be
I figured that putting resistors in-series with the line
couldn't help the volume of the line, so I tested it with a phone at the KSU by listening
to the thousand cycle tone, and using alligator clips to jump out the resistors. I
couldn't notice a big drop, so I figured it was OK. In some cases of high loop current,
the audio level is already high, like -4db, so you have some room to fool with resistors
before the line isn't loud enough. If you're already at a -6 or -7db, you really don't
have any room to play with the audio, and you should probably go right for the Loop
Current Regulator which has some electronics to keep the level up.
At the time, I wasn't smart enough to think about what I was
doing, or why I was doing it (lowering the loop current), so I didn't draw the correlation
between that problem and others that I was having. I could kick myself now!
- A PRIMER ON TELEPHONE LINE
AUDIO LEVEL (db)
One of the more common complaints that a telephone man hears
is "Can't Hear". It may have been on only one call to Zanzibar,
but here I am on a service call and I've got to talk nicely to the customer. One of the
problems with this service call is that I'm usually pretty powerless to do anything about
it anyway, but I can't tell that to the customer.
Here's a little information that will let you actually do
some testing if you think there really is a problem. Since you talk on your Butt-set all
day, you probably have a good enough ear to be able to tell if a line isn't as loud as it
should be. The ideal level for a telephone line loop loss seems to be about -5.5db. Local
calls aren't too loud, and you can still hear OK on intercity and long distance calls,
which have a lot of loss compared to local calls.
The farther you get from the Central Office, the lower the
volume is going to be on a call. If you want to know how far the premise is from the local
CO, click to see our Central Office (CO)
Info Lookup page to get the address of the CO, and then use a free mapping
service on the Internet to figure out the distance between the two addresses.
The first thing you need is a db meter. Either use one
designed for telephone lines, in which case an analog one is OK, or get a digital unit.
The impedance of the line, be it 600 or 900 ohms seems to make a difference to some
extent. Telephone Circuit Loss Testers are made for this use, but I've found that even the
Radio Shack digital meters that have a db scale are OK, especially if you compare it to a
"real" meter to get an idea of what kind of a difference in readings you will
see. Once you use it on a few jobs, you'll get a feel for what you're looking at. We sell
the least expensive digital Circuit Loss Tester,
which is VERY accurate (under $150).
When you are measuring the db loss of a line, you're reading
only the AC component - the audio - of the phone line. At the same time, there is the DC
talk battery component of the phone line of about 48VDC. Of all measurements, the DC
voltage seems to be the one that you really shouldn't worry about (the voltage will follow
the current... according to Ohm's Law). As I stated earlier, you connect a meter in
parallel (across) with the telephone line to read db. When you're trying to attenuate loop
current by using resistors only, it's imperative that you have two meters, a meter to read
the loop current and a meter to read the resistors' effect on the db loss, so you know
whether it's gone too low to be usable (anything below -7.5 db would be hard to hear). You
could use the same meter by changing the lead configuration and range, but that gets time
consuming and frustrating. Since loop current measurements are not usually exactly the
same line to line, you should repeat the process for each line. If you assume anything
(like the loop current or circuit loss is the same on all the lines), it'll come back and
The ideal db loss to look for is
-5.5db, when calling the 1000 cycle test tone from the local CO. If
you use the tone from another Central Office, you won't be able to
tell the real loss on the line, since there is an additional
(unknown) loss from CO to CO. I believe that the phone company is
required to give you the correct phone number to use. In Chicago,
there used to be a Test Line Coordinator who would give out these numbers all day long. They got rid of him, along with the
Telephone Museum on Washington Street, years ago.
There are also numbers for silent termination, but CO
specific silent termination numbers aren't important - you just want to have a nice quiet
line to see if there is static or noise etc. on the local loop. They used to have ring
back numbers here in Chicago, but they've done away with them years ago (they sure were
handy for checking ringing!). That same ringback number had a couple of other neat
features... it would beep once if you dialed the touchtone numbers correctly from 1 to
0,*,#, and had a rotary dial speed test built in. These features aren't very important
today since electronic dials seldom go off frequency.
To see the effects of using the wrong test tone, dial up the
test tone from a CO across the city. Then dial a test tone farther away, and then long
distance to another city. I've been able to check the loss of various long distance
companies by calling the same test tone (like in New York), through AT&T (10288), MCI
(10222), Sprint (10333) etc. You usually get a couple of db loss across town, and 5 or
more db loss long distance... which is fine as long as the line started at a -5.5db. If
you start with a line at -8.5db, which is the minimum that the phone company has to
provide on a "line", that extra 3db loss makes it very hard to hear long
There are two classes of phone lines that you can order
from the telco. If you need to bring up the volume of a
real phone line,
changing it to a trunk is about the only way to do it.
A "Line" must be maintained by the phone company at
-8.5db or greater. A "Trunk" must be maintained at
-5.5db or greater (a lot louder), and can be either loop or ground start. I've had the phone company
switch over quite a few customers with lines that were below -7 or -8db, to trunks. The
phone company then re-engineers the lines (adding loading coils or whatever) to bring it
up to -5.5db. That 2 db can really make a difference if you talk primarily long distance.
Amplified handsets and headsets help, but the increased gain
from the amplifier also amplifies line noise, so it's often still hard to hear. A
"Trunk" is usually a few bucks more per month than a "Line", plus
the ordering and installation charge if you are having it changed from a
"Line". They will always install new (conditioned) pairs from the CO when
changing a line to a trunk.
If you can't get the number for the
1000 cycle tone, we sell a 1KC Tone Generator ($63), which works
exactly like the one at the phone company's CO, and allows you to do a "loop
around" to determine the circuit loss. It automatically answers a line when
it's called, and disconnects when you hang up (it detects CPC). You put it on a line at
the customer's premise, and call it from another line at the premise. You divide the
reading you get on your Circuit Loss Tester by 2 to get the average
circuit loss for the lines. If you have three or more lines, the 1KC Tone
Generator comes with a little chart you can fill out, which allows you to
determine almost the exact loss for all three (or more) lines.
Guys have asked if there's anything they can put on a line to
bring the volume up if it's too low. Unfortunately, there's a problem using an amplifier
on a 2 wire full duplex telephone line. On a 2 wire
line both transmit and receive are on the same pair, as opposed to a 4 wire circuit
where transmit and receive are on their own separate pairs.
On a 2 wire circuit, the transmit and
receive gets separated in the phone's network, where a hybrid transformer
sends receive audio up to your ear (the handset receiver), and sends the transmitted audio
back down the phone line to the other end (from the handset transmitter). The hybrid
transformer is not 100% efficient, meaning it's can't totally separate the transmit from
receive. Maybe it's 90% efficient, which means that 10% of the receive audio gets sent
back down the line - basically "feedback." When you amplify the line, that
feedback is also amplified, which ends up sounding like ringing, singing or a squealing on
the line. Sometimes you just can't amplify a 2 wire line enough to make a customer happy,
before it starts to squeal.
On a 4 wire circuit, the transmit and
receive have their own pairs. That means you can amplify the transmit and receive
separately. There's nothing in the circuit feeding back part of the audio, so there's no
squealing. Turn up the gain as much as needed. All of the trunks between COs are four wire
circuits (either analog or digital), so the audio levels are perfect on calls across the
city, or across the country.
There can also be problems with a particular phone or system
if the hybrid in it isn't designed properly. The less efficient it is, the less you can
amplify the line without feedback, and the more echo you have when the phone or system is
attached to a digital line (like VoIP).
The only 2 wire amplifier I'm aware of still on the market is
the Wilcom SB21-K2 Turbo Amp (603-524-2622, around
$280). It's an unbelievable little line powered
device, designed for installation by unskilled Phone Company installers. It's made to go
into an NI (Network Interface). The biggest drawbacks are that it has no gain control, and limited gain (maybe
9db?). It sets its level automatically when you dial the local 1KC tone (at the CO that
pair is connected to), then a quiet termination, and then flip a switch. Even without a
manual volume control, you can set it for maximum gain by dialing a 1KC tone in some other
city which fools it - but you may end up with a squeal/feedback if you do that.
You may also be able to find used 2 wire amplifiers, like the
Reliance VFR-5050. I've used it a few times on OPXs, although it's time consuming to
install. I don't think it's full duplex (it has voice switching). The VFR-5050 is on a
small card, so you need to buy a card cage with a power supply. I think it works in those
card cages that the Long Distance companies installed for their packs (with 25 pair cables
and all), so you might be able to use an abandoned card cage that's lying around (I'm not
sure it works). A nice thing about using the correct card cage is that the line gets
passed through automatically if the card is pulled, so you don't cut-off anyone if you
remove the card (an option?).
- THE VOLUME ON AN OPX (OFF
PREMISE EXTENSION) IS TOO LOW
In the old days, you could get conditioned pairs from
the phone company that would repeat the voltage, current and audio
level on an OPX (Off Premise Extension) as far as you needed to go.
It looks like those days are over.
Today, most phone
companies will give you a dry pair (unconditioned) from
the near end (where the PBX is) to the far end (where the phone or
telephone equipment is). The far end may be a branch office, an
off-site worker, an office or plant, or even the boss' house.
Spending the money
on the dry pair and having it be unusable because the volume is too
low doesn't make sense. We already talked about how to increase loop
current and ring voltage.
We also mentioned a
2 wire amplifier like the Reliance VFR-5050, which will work OK if
you find one and have a card cage for it (single line or
multi-line). The Turbo Amp won't work because it has no gain
adjustment. It's made to go on a real phone line with access to a
1KC tone at the CO (I suppose if you have enough loop current to run
the line powered Turbo Amp, and put in our 1KC tone generator on a
station at the PBX, you could probably make it work?).
A better solution
for an OPX is to use our
Long Line Volume Booster™
Set. This is a pair of line powered devices. You need
one on each end of the dry pair. It works like Load Coils,
but you don't need to space them out along the pair. You just put
one at each end of the line.
It will also work on a real phone line but you'd need to be
the phone company to use it because you'd need one at the CO or SLC,
and one at the premise.
- THE VOLUME ON CONFERENCE CALLS IS TOO LOW
Lots of guys ask if there's anything they can use to bring the volume up on
lines that are conferenced on a phone system, so the outside parties can hear each other.
Most phone systems don't have an amplified conference because it's expensive to put the
parts in the system to do that.
When you simply bridge two lines together, which is what most
phone systems do, you lose maybe 7db (the circuit loss on the lines), between each of the
outside parties. The person who created the conference in the middle (on the phone
system), can hear the outside parties just fine. The two outside parties just can't hear
each other. This is the case whether the trunks are conferenced together manually, or the
phone system is performing an external call transfer, bridging (conferencing) one trunk to
another to forward a call to another premise.
If you simply amplified each phone line the same amount in
both directions, you'd get feedback or the lines would sound too loud in one direction.
The hard part about amplifying a conference call on 2 wire lines is that you only need to
amplify the outgoing signal on each line -
directions. Since an analog hybrid transformer used to separate transmit
from receive isn't 100% efficient, you end up getting feedback and squealing when you try
to amplify in one direction.
Although there are ways to amplify conference calls using
sophisticated analog circuitry along with the most efficient hybrid transformers
available, these days it's usually done with a DSP (Digital Signal Processor). The DSP is
programmed to digitize the analog call, separate the transmit and receive digitally,
digitally amplify the signal in one direction, and then convert the digital signals back
to analog. Very few phone systems have amplified conferencing built-in, but that may
change as DSPs (and programmers in India or wherever) get less expensive.
There's NOTHING that will work to
increase the level of conferenced trunks on a phone system except a
Bridge. A Conference Bridge is a fairly expensive digital device
(with DSPs to adjust the incoming and outgoing volume) that goes on analog station ports
on a phone system. To create the conference, you dial into the conference bridge, and
manually dial codes to setup or tear-down the conference. Because the DSPs in the
conference bridge are programmed to bring the levels up to the correct levels, conferenced
calls will sound great.
I personally ALWAYS order
Calling from the CO for my customers... and they
That heads off the complaints I'd invariably get when they try to make their first
conference call using the phone system's conference feature. You just have to make sure
you order 3-Way from the phone company, and that you program a
button on every phone. That's all it takes to have
calls. Why are they perfect? Because the amplification is applied as needed
the CO on 4 wire circuits,
the line is converted to 2 wire to go to the premise. No amplification has to be done on 2
- WHAT IF THE VOLUME ON A LINE IS
Touch tones and echo are two reasons that you may have to
the volume level of a line. Reducing the level is easier than amplifying it!
An Attenuator will reduce the
audio level on a line by a set amount, and also reduce the loop current by about 1ma for
every 1db of attenuation. We make both Modular and
66 Block type
Attenuators, in 1, 2, 3, 4, 6 and 8db models. We also make a little modular
Line Loss Insertion Tester that will insert 1, 2, 4 or 4db of attenuation
in-series with a modular device. If you add a fixed 4db Attenuator
in-series with the Tester, you can get 5, 6, 7 or 8db of attenuation.
Once you know how much attenuation you need on the lines using the Tester, you can order
the correct fixed value Attenuators.
If you have DTMF recognition problems, or
there's too much echo either on a trunk or analog station port on a digital system, or a
system using VoIP lines, reducing the level of the line or port can fix or improve the
symptoms. If the phone system doesn't have an option to reduce the level through
programming, using physical Attenuators (or resistors) is the
only way to get the level down.
Two other devices that may fix DTMF recognition
and echo problems are our Half REN Box™, and our
Both of these devices
impedance of the line at voice frequencies, which may help DTMF digits get
recognized by a voice mail or IVR system, or reduce the echo on a phone line.
Just putting the Half REN Box™
on the line will tell you whether it fixes the problem. The Echo
Impedance Matcher has a pot and button that needs to be tried
to see if it fixes the problem. Since there aren't any meters that will tell you whether
there's an impedance problem, the easiest way to see if that's your problem is to just
change the impedance by putting these devices on the line.
Click HERE to see the Longitudinal
Imbalance Tech Bulletin, and HERE to see
the Troubleshooting Basics Tech
Bulletin, which have more fixes to strange problems.
Click HERE to see our
products for Fixing Phone Line Problems.