I used to do this a lot but not for last few years so am very rusty, and
need help.

My local school is televising their board meetings, live on the cable
company's Channel 45. The sound is very low, full of hum and local
radio station.
The basic pathway for the audio is:

Mics --- Alessis 12R preamp/mixer --- 100 foot cable run --- DVD
recorder (non-professional quality) ---- RCA patch cord ---- cable
company's modulator box ---- RF signal going down street to cable
company HQ

The present 100 foot cable run originates at an unbalanced RCA output on
the Alessis, uses some skinny coax, and plugs into an RCA input (there
is no balanced input) on the DVD recorder. I believe that the use of
unbalanced signal, and coax, is the source of weak signal and noise, and
want to try replacing it with microphone-style balanced cable. The main
balanced output from the Alessis is going to the powered speakers in the
auditorium and I have been asked to leave that alone. It is someone
else's baby and he will get upset if the arrangement is changed. The
remaining balanced output available from the Alessis is the Monitor
outputs, which are 1/4". Using an adapter (too lazy to solder), I can
get the Monitor output going down 100 ft of mic cable, to the DVD. But
at the DVD I need to change from balanced to unbalanced, and I believe
that I need ground isolation - I think some of the hum is occurring
because the Alessis may be on a different 120 volt phase than the DVD.

So, IF I have guessed right to change to mic-style balanced cable, all I
need is a device to make the change from bal to unbal, and lift the
ground. Am I on the right track?

Henry

Henry wrote:
I used to do this a lot but not for last few years so am very rusty, and
need help.

My local school is televising their board meetings, live on the cable
company's Channel 45. The sound is very low, full of hum and local
radio station.
The basic pathway for the audio is:

Mics --- Alessis 12R preamp/mixer --- 100 foot cable run --- DVD
recorder (non-professional quality) ---- RCA patch cord ---- cable
company's modulator box ---- RF signal going down street to cable
company HQ

The present 100 foot cable run originates at an unbalanced RCA output on
the Alessis, uses some skinny coax, and plugs into an RCA input (there
is no balanced input) on the DVD recorder. I believe that the use of
unbalanced signal, and coax, is the source of weak signal and noise, and
want to try replacing it with microphone-style balanced cable. The main
balanced output from the Alessis is going to the powered speakers in the
auditorium and I have been asked to leave that alone. It is someone
else's baby and he will get upset if the arrangement is changed. The
remaining balanced output available from the Alessis is the Monitor
outputs, which are 1/4". Using an adapter (too lazy to solder), I can
get the Monitor output going down 100 ft of mic cable, to the DVD. But
at the DVD I need to change from balanced to unbalanced, and I believe
that I need ground isolation - I think some of the hum is occurring
because the Alessis may be on a different 120 volt phase than the DVD.

So, IF I have guessed right to change to mic-style balanced cable, all I
need is a device to make the change from bal to unbal, and lift the
ground. Am I on the right track?

I had to read that a couple of times...yeah, you've pretty much got it.
You could use a TRS (Tip, Ring, Sleeve) to XLR (mic cable) adaptor at
the monitor out. Hopefully the monitor out has the same mix as the
mains, or else you can custom mix to suit the cable feed.

I'd split the signal at the other end of the mic cable with a Direct
Box, and feed one side of the split to the Cable Guy and the other to
the DVD.

Most DI's have a ground lift....

jak


Henry

"jakdedert" wrote ...
I'd split the signal at the other end of the mic cable with a Direct
Box, and feed one side of the split to the Cable Guy and the other to
the DVD.

Most DI's have a ground lift....

Yes, but most "DI"s are made for high impedance unbalanced
conversion to low-impedance balanced MIC-level. It is more
than possible to end up with a DI box that can't handle even
consumer line-level.

Henry wrote:

I used to do this a lot but not for last few years so am very rusty, and
need help.

[shake] .... [ain] ... [pfttt]

My local school is televising their board meetings, live
on the cable company's Channel 45. The sound is very low,
full of hum and local radio station.

OK, listen in along the path, just where is it it gets bad?

The basic pathway for the audio is:

Mics --- Alessis 12R preamp/mixer --- 100 foot cable run --- DVD
recorder (non-professional quality) ---- RCA patch cord ---- cable
company's modulator box ---- RF signal going down street to cable
company HQ

Mics and mic wiring an undefined variable. Specs say inputs are
balanced, but is the mic wiring proper? - first possible issue is the
use of extension cords or junction boxes with improper internal
grounding.

The present 100 foot cable run originates at an unbalanced RCA output on
the Alessis, uses some skinny coax, and plugs into an RCA input (there
is no balanced input) on the DVD recorder.

That is an imcompentent setup, it has a balanced main output as well as
a balanced monitor output, and the bal-unbal adaption should be done a
the receiving end, where also a 6 to 15 dB attenuator should be placed
to attentuate the signal to household signal levels. This of course
assumes that the main output is not used for PA, it would be a large
boardroom to require such. Specs are terse and do to say whether the
rca output is pre-attentuated 6 to 10 dB, it may so be simply by not
being (assumed) electronically balanced as the other outputs.

I believe that the use of unbalanced signal, and coax, is the
source of weak signal and noise,

No, signal strength has nothing to do with bal unbal per se, but is is
quite posible that the rca output has some 6 to 10 dB lower output
voltage.

and want to try replacing it with microphone-style balanced cable.

Yes, but it in itself may not do it, it probably has better RF
screening, but you want to use it from a balanced output and make the
bal unbal conversion at the receiving end, otherwise it is a waste of
money.

The main balanced output from the Alessis is going to the
powered speakers in the auditorium

Biiig boardroom .... O;-)

and I have been asked to leave that alone. It is someone
else's baby and he will get upset if the arrangement is
changed.

Would that person by chance be the one that made the recording wiring
too? - it does not strike me as a competent setup and it is frequently
easier to discuss solutions with the competent. Just plugging the
relevant output cable in and having a balanced running to the dvd-player
would be one very simple solution, mind you one that prevents accidental
transmission of things that should not be transmitted.

The remaining balanced output available from the Alessis
is the Monitor outputs, which are 1/4". Using an adapter
(too lazy to solder), I can get the Monitor output going
down 100 ft of mic cable, to the DVD.

Yees.

But at the DVD I need to change from balanced to
unbalanced, and I believe that I need ground isolation
- I think some of the hum is occurring because the
Alessis may be on a different 120 volt phase than the DVD.

Ah, you are in a country that belives that a lot of ground connections
give better mains safety.

So, IF I have guessed right to change to mic-style balanced
cable, all I need is a device to make the change from
bal to unbal, and lift the ground. Am I on the right
track?

Yerp, and you will have to solder no matter what, a 30 meter unbalanced
cable run may work some of the time, one of the first alternatives is to
try using the headphone output and increase the signal level in the
cable run, but it may for other very obvious reasons be impractical.

You have different ground paths on each location and you have a RF
problem, so the Scott Dorsey recipe for what you need to do is to make a
proper "symmetrisch, erdfrei" wiring, ie. have a balancing transformer
in each end of the cable, not just in the receiving end. Oh, and use a
tranny with two secondaries so as to have tranny insulation and ground
separation also between the cable company box and the dvd-recorder.

BUT: first determine where the signal actually degrades, just in case it
is something that happens in the mic wiring or in the last small wire
from the dvd-recorder to the cable company box, perhaps its input signal
is ply too weak? - just what signal level does it expect to get?

Do not assume, know! - it is likely to be less work to fix just the
problem than to fix all potential problems, of which of course a
compentent installation had left none for you to fix, but budget may
have dictated the installers available choices or that "not an
installer" was chosen to install the output to the cable company.

Henry


Kind regards

Peter Larsen

--
*******************************************
* My site is at: http://www.muyiovatki.dk *
*******************************************

On Tue, 13 Dec 2005 03:22:01 GMT, Henry
wrote:

So, IF I have guessed right to change to mic-style balanced cable, all I
need is a device to make the change from bal to unbal, and lift the
ground. Am I on the right track?

Use a DI box. They work backwards :-)

Laurence Payne wrote:
On Tue, 13 Dec 2005 03:22:01 GMT, Henry
wrote:


So, IF I have guessed right to change to mic-style balanced cable, all I
need is a device to make the change from bal to unbal, and lift the
ground. Am I on the right track?


Use a DI box. They work backwards :-)

That was my idea as well, but we both should have specified that it be a
passive DI. An active unit would not work in reverse.

There are some A/V DI's out there which take all kinds of consumer
ins/outs and supply balanced out. One of these in reverse might be the
ticket....

OTOH, Peter's plan would certainly be the 'engineer's' way to do it, and
certainly foolproof...but perhaps beyond the capability of the OP.

jak

Do they really work backwards or are you just joking? I never knew
that (or I'm completely guillible).

stern wrote:
Do they really work backwards or are you just joking? I never knew
that (or I'm completely guillible).

The passive ones are just a transformer and resistive dividing network.
The active ones will not work backwards.

jak

I used to do exactly that to feed sound to a second room. One of the
board's aux outputs (postfader, so it was basically tracking the main
mix) to an in-line TRS-to-XLR transformer, to a hundred feet of XLR
cable out one window and in another, to another in-line transformer, to
balanced input of a spare amp and speakers.

"jakdedert" wrote ...
stern wrote:
Do they really work backwards or are you just joking? I never knew
that (or I'm completely guillible).

The passive ones are just a transformer and resistive dividing network.
The active ones will not work backwards.

Yes, a transformer works in either (both) directions.
However the dividing network is likely something you
DON'T need since your signal is already too low.

And you don't really need the impedance conversion
which is the main function of a DI box. More properly,
you need a 1:1 isolation transformer. Iso transformers
are just as readily available as DI boxes. You can buy
one like this http://www.ebtechaudio.com/he-2des.html
at Guitar Center, etc.

Richard Crowley wrote:
"jakdedert" wrote ...

stern wrote:

Do they really work backwards or are you just joking? I never knew
that (or I'm completely guillible).


The passive ones are just a transformer and resistive dividing network.
The active ones will not work backwards.


Yes, a transformer works in either (both) directions.
However the dividing network is likely something you
DON'T need since your signal is already too low.

And you don't really need the impedance conversion
which is the main function of a DI box. More properly,
you need a 1:1 isolation transformer. Iso transformers
are just as readily available as DI boxes. You can buy
one like this http://www.ebtechaudio.com/he-2des.html
at Guitar Center, etc.


There's no reason why the signal level should be too low, if output from
the mixer. More likely his problem was a function of the unbalanced (or
possibly dodgey) cable used. In any case, 1:1 iso's would not address
low signal level.

IMO, he needs to split the signal because he wants to address two
destinations, the (consumer?) DVD and the cable feed. IIRC, he's now
using an output from the DVD to feed the cable; but I believe that's
adding unecessary complexity...not to mention an unessential element in
the chain, with the possiblility of *it* adding noise/distortion or
other nasty artifacts.

I was looking at a cheap, easy solution which would address his problem
with a minimum of cash outlay or engineering: slap a cheap adaptor on
the mixer out, since it's an active balanced out already; terminate that
with a transformer box (DI--reversed) with outs for each of his intended
targets (and a ground lift). Total outlay in money could be less than
$50--depending on what he already has or can borrow--total outlay in
time: less than 1/2 hour.

If it worked, great. If it didn't, then IMO the proper way would be a
an active distro amp at the far end of the chain...don't see the need
for iso' at the mixer end (although he 'could' split the sig there and
run two blanced lines--one to each intended 'in'--with appropriate
adaptors at those inputs)...should be plenty of sig to drive 100'
balanced line.

I can think of half a dozen 'right' ways to do this. Arguing over which
one is 'more' right is pointless. The way he's doing it is not
producing a good result and he wants to fix it. He's not an engineer,
although he appears to have a clue.

jak

jakdedert wrote:

IMO, he needs to split the signal because he wants
to address two destinations, the (consumer?) DVD
and the cable feed.

Tranny splitting via double secondary windings remains the most
attractive, the signal strength should be what fitst he least sensitive
input with suitable attenuation of the signal to the most sensitive
input.

I can think of half a dozen 'right' ways to do this.
Arguing over which one is 'more' right is pointless.

Looking for minimum required change of setup is not.

The way he's doing it is not producing a good result
and he wants to fix it. He's not an engineer,
although he appears to have a clue.

His instincts for this seems OK.

jak


Kind regards

Peter Larsen

--
*******************************************
* My site is at: http://www.muyiovatki.dk *
*******************************************

"jakdedert" wrote ...
There's no reason why the signal level should be too low, if output
from the mixer. More likely his problem was a function of the
unbalanced (or possibly dodgey) cable used. In any case, 1:1 iso's
would not address low signal level.

No. But suggesting a solution that includes a "dividing
network" will only make it worse.

I was looking at a cheap, easy solution which would address his problem
with a minimum of cash outlay or engineering: slap a cheap adaptor on
the mixer out, since it's an active balanced out already; terminate
that with a transformer box (DI--reversed) with outs for each of his
intended targets (and a ground lift). Total outlay in money could be
less than $50--depending on what he already has or can borrow--total
outlay in time: less than 1/2 hour.

I'm completely with you. However, might as well spend the
$50 on something more suitable (and just as available),
i.e. an iso transformer vs. a direct-box (which is a kludge
in this application). The suggested iso transformer even
includes two independent paths which can be used to feed
the cable modulator and the DVD recorder independently
(and even isolated from each other). Doesn't cost any
more to use a more suitable solution.

Richard Crowley wrote:
"jakdedert" wrote ...

There's no reason why the signal level should be too low, if output
from the mixer. More likely his problem was a function of the
unbalanced (or possibly dodgey) cable used. In any case, 1:1 iso's
would not address low signal level.


No. But suggesting a solution that includes a "dividing
network" will only make it worse.

That's assuming there 'is' a low signal issue. If that's the case, I
think the OP should look to his mixer, which 'should' be putting out
adequate gain.

I was looking at a cheap, easy solution which would address his
problem with a minimum of cash outlay or engineering: slap a cheap
adaptor on the mixer out, since it's an active balanced out already;
terminate that with a transformer box (DI--reversed) with outs for
each of his intended targets (and a ground lift). Total outlay in
money could be less than $50--depending on what he already has or can
borrow--total outlay in time: less than 1/2 hour.


I'm completely with you. However, might as well spend the
$50 on something more suitable (and just as available),
i.e. an iso transformer vs. a direct-box (which is a kludge
in this application). The suggested iso transformer even
includes two independent paths which can be used to feed
the cable modulator and the DVD recorder independently
(and even isolated from each other). Doesn't cost any
more to use a more suitable solution.

I tend to agree with you.

I was thinking more from a standpoint of being on a gig, being presented
with an issue, and wondering what might be in the workbox to address it.
Actually, what I'd probably pull out would be a "press box" if I had one.

The OP obviously doesn't have a well-stocked roadbox with a plethora of
adaptors, DI's and cables; so it makes sense to buy whatever works best
for the least amount of money. He does, however, seem to have a clue as
to what his problem might be, and I tend to agree.

I think it is germane, though, that there needs to be two isolated
outputs...no daisy-chaining the DVD and cable feeds...transformer or
otherwise.

jak

jakdedert wrote:

That's assuming there 'is' a low signal issue. If that's the case, I
think the OP should look to his mixer, which 'should' be putting out
adequate gain.
^^^^
Make that "level".

It may be that the mixer is being operated properly but its nominal output
level is -10 dBV while the cable company expects +4 dBu. Worse, the
cable company might expect the +8 dBu level that is still widely used by
the broadcast industry.

--
================================================== ======================
Michael Kesti | "And like, one and one don't make
| two, one and one make one."
mrkesti at comcast dot net | - The Who, Bargain