[[email protected]]

Hi guys,

I've recently become in-charge of one of the student societies in my
uni, and one of the things we have is a weekly gathering attended by
many people in a small hall and some nearby rooms, in a single
building.

Since we only have people giving lectures (ie no singing or music) we
have a very simple (and rather old) PA system which we set up before
the event: just a tie-clip mic, an amplifier and speakers.
This is what I know about the configuration of the system: (I had to
google a lot to glean most of it; please excuse if there is anything
blatantly incorrect in what follows)
The tie-clip microphone is a simple unpowered one (don't know what type
exactly, but it's currently broken and I have to get a new one) and
has a 3.5 mm jack plug. This is plugged into the 1/4" (6.3 mm) mic
input of the amplifier using an adapter. The output is sent to
speakers, one through 16 ohm output and the other through the 70 V
output of the amp (I believe the appropriate impendance speakers are
used in each case ).

I know next to nothing about PA systems (problem is, the others seem
to know even less than me), so I hope someone can help out with the
following queries:

1) We often experience a feedback noise problem. I have to replace
our tie-clip mic which is broken. I was wondering if there would be any
benefit of getting a mic with a XLR connector, and then use an XLR to
1/4"
jack plug adapter to connect the mic to the amplifier? Will it help
reduce/solve the
noise problem? The mic cable needs to be pretty long, because sometimes
the
amplifier has to be situated quite far from the person speaking into
the mic.

2) I would like independent volume control for both speakers, however
the amplifier has a single master volume control. Is
there any (inexpensive) gain control device I can buy and hook-up
between the amp and one of the speakers to do the trick? The speakers
themselves don't have any accessible volume control.

3) Any other suggestions/tips to help get rid of the noise?

I hope my questions made sense. I would appreciate any help in this
regard.

AK

[Richard Crowley]

akiel1 wrote ...
I've recently become in-charge of one of the student societies in my
uni, and one of the things we have is a weekly gathering attended by
many people in a small hall and some nearby rooms, in a single
building.

Note that your question is about sound reinforcement
(PA system), and this newsgroup is chartered for the
discussion of audio recording. There is another news-
group which is chartered for sound reinforcement,
namely.... news:alt.audio.pro.live-sound

The tie-clip microphone is a simple unpowered one
(don't know what type exactly, but it's currently broken
and I have to get a new one) and has a 3.5 mm jack plug.

Are you sure it is broken? If it has a 3.5mm jack, it
might be designed to be used with equipment which
provides "plug-in" power. Hint: It is extraordinarily
helpful to state specific make and model numbers so
we don't have to guess what you might have.


This is plugged into the 1/4" (6.3 mm) mic input of
the amplifier using an adapter.

Again, it would be really helpful to know what this
amplifier is. Is this input designated as a "microphone"
input? Or is this an old guitar amp or something?

The output is sent to speakers, one through 16 ohm output
and the other through the 70 V output of the amp (I believe
the appropriate impendance speakers are used in each case ).

I know next to nothing about PA systems (problem is,
the others seem to know even less than me), so I hope
someone can help out with the following queries:

1) We often experience a feedback noise problem.
I have to replace our tie-clip mic which is broken.
I was wondering if there would be any benefit of
getting a mic with a XLR connector, and then use
an XLR to 1/4" jack plug adapter to connect the
mic to the amplifier? Will it help reduce/solve the
noise problem?

Feedback is not generally refered to as "noise".
It is a specific type of electro-acoustic problem,
and calling it "noise" will tend to confuse people.

No. It doesn't matter what kind of cable/connector
you use for the purposes of trying to solve the feed-
back problem. If you were having some other kind
of "noise" problem (hum, crackling, etc.), then it
might very well be better to use low-impedance
XLR connections, etc.

The mic cable needs to be pretty long, because
sometimes the amplifier has to be situated quite
far from the person speaking into the mic.

Low-impedance, balanced connections (such as use
XLR connectors) are preferable for long distances.
Note, however, that you will likely need a transformer
type adapter from XLR to your 1/4 inch (presumably
high-impedance) amplifier input.


2) I would like independent volume control for both
speakers, however the amplifier has a single master
volume control. Is there any (inexpensive) gain control
device I can buy and hook-up between the amp and one
of the speakers to do the trick? The speakers themselves
don't have any accessible volume control.

Yes. This is generally done with "L-pad"s or "T-pad"s
these pads are avialable at places like Radio Shack or
online at places like www.partsexpress.com, etc.

Note that if the 70V speaker is properly wired, it will use
a transformer where the primary connects to the 70V line,
and the secondary goes to the speaker. These 70V transformers
very frequently have different "taps" so you can adjust the
volume of that particular speaker.

Not clear from your question whether you are just trying
to set up the system so that the volume from the two
speakers, is "normalized", or whether you need some
kind of operational control for use during the programs?
If you use a "pad" on this speaker, be sure to wire it
between the 70V transformer and the speaker. If you
connect it on the 70V side, you could deep-fry it.

3) Any other suggestions/tips to help get rid of the noise?

Assuming you mean the feedback. This is not a simple
question. It depends on many factors which you didn't
mention, including the acoustic nature of the room, where
the speaker(s) are relative to the microphone, etc. etc.

Generally speaking, the closer you can get the microphone
to the presenter's mouth the better off you will be. That
is one of the reasons headset microphones are becoming
popular.

Also keeping the presenter/microphone OUT of the field
of the speaker(s) will go a long ways towards reducing
feedback. But we have no clue what your setup is, so
we are shooting in the dark.

I hope my questions made sense. I would appreciate
any help in this regard.

I would have thought that a uni of any size, there would
be people who have experience with these kinds of things.
Even if you have to treat them to a hamburger in exchange
for 30 minutes of hands-on consulting, etc.

[[email protected]]

Sorry about posting in this group when there was a more 'relevant' one
around.

Thanks for taking the trouble to respond. Would just like to clarify
some of the points you asked about.

1) By a 'broken' microphone, I meant that there's a loose connection
near the input of the microphone (I know because squeezing it at that
point makes it come alive sometimes). And we were using the same
microphone before. I did check for some indication of part number, but
there is none. While I will check the amp make when I get a chance, I
think it's probably an obscurely-named (possibly Chinese-made) device.

2) The mic is being plugged into what has been designated as the MIC
input on the amplifier box.

3) Thanks for the info that feedback problem isn't called 'noise'
('noise' didn't sound right to me either - in fact, this seems to be
rather the opposite of noise, in terms of frequency sharpness).
However, we do experience the crackling type of noise problem too
sometimes. Will a mic with balanced connections and a transformer help?
Is the MIC input in the amplifier generally supposed to be high
impedance?

4) Yes, there is something between the 70V speaker and the 70V output
that I take to be the transformer (the 70V speaker is a permanently
fixed in the room, while the other speaker is our own). There's only
one point to which to connect the amp output to the transformer (i.e.
no taps).

5) Re the speaker setup: the +70V speaker is in the same hall (which is
around 12m x 12metre) as the program presenter and is positioned on the
wall in front of the room (behind the person presenting), at a height
of about 3.5 m about the ground... I think that might possibly be the
'culprit' speaker (assuming the feedback problem isn't due to any other
type of cause). The other speaker (actually two parallel-y connected 16
ohm speakers) are situated quite far away from the mic (maybe 12-15 m)
outside the main hall, in a narrow corridor, and there is no straight
path from the speaker to the mic.

6) Re volume adjustment, I just want to be able to independently adjust
the speaker volumes at the beginning of the program. The 16 ohm
speakers are quite a bit louder than they need to be, Would the 'pad'
you mentioned do the trick here? I would want to set the volume on the
amplifier, according to the 70V speaker, and then turn down the volume
of the 16 ohm speaker using the pads.

Thanks a lot for your help... I really appreciate it!







Richard Crowley wrote:
akiel1 wrote ...but
I've recently become in-charge of one of the student societies in my
uni, and one of the things we have is a weekly gathering attended by
many people in a small hall and some nearby rooms, in a single
building.

Note that your question is about sound reinforcement
(PA system), and this newsgroup is chartered for the
discussion of audio recording. There is another news-
group which is chartered for sound reinforcement,
namely.... news:alt.audio.pro.live-sound

The tie-clip microphone is a simple unpowered one
(don't know what type exactly, but it's currently broken
and I have to get a new one) and has a 3.5 mm jack plug.

Are you sure it is broken? If it has a 3.5mm jack, it
might be designed to be used with equipment which
provides "plug-in" power. Hint: It is extraordinarily
helpful to state specific make and model numbers so
we don't have to guess what you might have.


This is plugged into the 1/4" (6.3 mm) mic input of
the amplifier using an adapter.

Again, it would be really helpful to know what this
amplifier is. Is this input designated as a "microphone"
input? Or is this an old guitar amp or something?

The output is sent to speakers, one through 16 ohm output
and the other through the 70 V output of the amp (I believe
the appropriate impendance speakers are used in each case ).

I know next to nothing about PA systems (problem is,
the others seem to know even less than me), so I hope
someone can help out with the following queries:

1) We often experience a feedback noise problem.
I have to replace our tie-clip mic which is broken.
I was wondering if there would be any benefit of
getting a mic with a XLR connector, and then use
an XLR to 1/4" jack plug adapter to connect the
mic to the amplifier? Will it help reduce/solve the
noise problem?

Feedback is not generally refered to as "noise".
It is a specific type of electro-acoustic problem,
and calling it "noise" will tend to confuse people.

No. It doesn't matter what kind of cable/connector
you use for the purposes of trying to solve the feed-
back problem. If you were having some other kind
of "noise" problem (hum, crackling, etc.), then it
might very well be better to use low-impedance
XLR connections, etc.

The mic cable needs to be pretty long, because
sometimes the amplifier has to be situated quite
far from the person speaking into the mic.

Low-impedance, balanced connections (such as use
XLR connectors) are preferable for long distances.
Note, however, that you will likely need a transformer
type adapter from XLR to your 1/4 inch (presumably
high-impedance) amplifier input.


2) I would like independent volume control for both
speakers, however the amplifier has a single master
volume control. Is there any (inexpensive) gain control
device I can buy and hook-up between the amp and one
of the speakers to do the trick? The speakers themselves
don't have any accessible volume control.

Yes. This is generally done with "L-pad"s or "T-pad"s
these pads are avialable at places like Radio Shack or
online at places like www.partsexpress.com, etc.

Note that if the 70V speaker is properly wired, it will use
a transformer where the primary connects to the 70V line,
and the secondary goes to the speaker. These 70V transformers
very frequently have different "taps" so you can adjust the
volume of that particular speaker.

Not clear from your question whether you are just trying
to set up the system so that the volume from the two
speakers, is "normalized", or whether you need some
kind of operational control for use during the programs?
If you use a "pad" on this speaker, be sure to wire it
between the 70V transformer and the speaker. If you
connect it on the 70V side, you could deep-fry it.

3) Any other suggestions/tips to help get rid of the noise?

Assuming you mean the feedback. This is not a simple
question. It depends on many factors which you didn't
mention, including the acoustic nature of the room, where
the speaker(s) are relative to the microphone, etc. etc.

Generally speaking, the closer you can get the microphone
to the presenter's mouth the better off you will be. That
is one of the reasons headset microphones are becoming
popular.

Also keeping the presenter/microphone OUT of the field
of the speaker(s) will go a long ways towards reducing
feedback. But we have no clue what your setup is, so
we are shooting in the dark.

I hope my questions made sense. I would appreciate
any help in this regard.

I would have thought that a uni of any size, there would
be people who have experience with these kinds of things.
Even if you have to treat them to a hamburger in exchange
for 30 minutes of hands-on consulting, etc.

[Richard Crowley]

akiel1 wrote ...
Sorry about posting in this group when there was a more 'relevant' one
around.

Thanks for taking the trouble to respond. Would just like to clarify
some of the points you asked about.

1) By a 'broken' microphone, I meant that there's a loose connection
near the input of the microphone (I know because squeezing it at that
point makes it come alive sometimes). And we were using the same
microphone before.

If it were me, I would rather fix the existing microphone
than buy a new one. Any new one you buy will be an
electret condenser microphone which will require a
battery to power it. For example, here in the USA,
Radio Shack has a clip-on mic for ~$25 that sounds
equivalent to what you have. But we have no idea
where you are or what sources you have available?

2) The mic is being plugged into what has been designated as the MIC
input on the amplifier box.

We don't really know if your mic input is high-impedance
or low-impedance. Can you estimate the vintage? Does
it use tubes (valves), or is it solid-state (transistor)?

3) Thanks for the info that feedback problem isn't called 'noise'
('noise' didn't sound right to me either - in fact, this seems to be
rather the opposite of noise, in terms of frequency sharpness).
However, we do experience the crackling type of noise problem too
sometimes. Will a mic with balanced connections and a transformer
help?

What you are describing sounds like intermittent connections
in the cable/connectors, etc. This kind of problem really has
nothing to do with high or low impedance, balanced or un-
balanced. It is a mechanical problem from broken wires
caused by wear and tear (or maybe a bad solder joint?)
and simply means that whatever is causing the problem
needs to be repaired or replaced.

Is the MIC input in the amplifier generally supposed to be high
impedance?

There is no "generally supposed to be". We have no clue
what your amplifier is, so all bets are off. If I had to guess,
the 1/4-inch phone plug would make me imagine that it is
more likely to be high impedance, but that is a shot in the
dark.

4) Yes, there is something between the 70V speaker and the 70V output
that I take to be the transformer (the 70V speaker is a permanently
fixed in the room, while the other speaker is our own). There's only
one point to which to connect the amp output to the transformer (i.e.
no taps).

Since it appears that the corridor speakers are the ones that
are too loud, you likely don't need to do anything to the room
speaker. At least to adjust the loudness, its location is a very
different problem.

5) Re the speaker setup: the +70V speaker is in the same hall (which
is
around 12m x 12metre) as the program presenter and is positioned on
the
wall in front of the room (behind the person presenting), at a height
of about 3.5 m about the ground... I think that might possibly be the
'culprit' speaker (assuming the feedback problem isn't due to any
other
type of cause).

Feedback is simply caused by the sound from the speaker
going back into the microphone and being amplified again
and again and again. Puting the presenter and the microphone
directly in front of the speaker is the worst possible setup.
You have already proved that you cannot violate the laws
of physics, so if you want to reduce you feedback problem,
you must do one of these things.

1) Move the speaker so that the minimal amount of speaker
sound reaches the microphone.
2) Move the presenter and their microphone away from just
in front of the speaker. As far away as possible for purposes
of feedback reduction.
3) Get the microphone a lot closer to the presenter's mouth
and/or have them speak much louder (so you don't have to
use as much gain in the PA system).

The other speaker (actually two parallel-y connected 16
ohm speakers) are situated quite far away from the mic (maybe 12-15 m)
outside the main hall, in a narrow corridor, and there is no straight
path from the speaker to the mic.

So it is unlikely that these speakers are contributing to the
feedback problem.

6) Re volume adjustment, I just want to be able to independently
adjust
the speaker volumes at the beginning of the program. The 16 ohm
speakers are quite a bit louder than they need to be, Would the 'pad'
you mentioned do the trick here? I would want to set the volume on the
amplifier, according to the 70V speaker, and then turn down the volume
of the 16 ohm speaker using the pads.

Yes, the pad would be the most likely way to independently
adjust the volume of the corridor speakers. But first I would
try just connecting the speakers in series rather than in parallel.
That may reduce their volume just enough for your purposes.

[Laurence Payne]

On Tue, 11 Jul 2006 06:07:53 -0700, "Richard Crowley"
wrote:
You're DETERMINED to complicate this matter, aren't you? :-)

[Richard Crowley]

"Laurence Payne" wrote ...
You're DETERMINED to complicate this matter, aren't you? :-)

It looked pretty complicated when I got here.
Mostly complicated by the lack of experience
of our hero who is trying to fix the problem.