Conference napalm::guitar

RE < Note 497.0 by ILO::DOODY "Brian M. Doody" >

>    Can anyone supply a circuit ? 
>    The PA is 120w (Carlsboro)

	If all you want is a volume control, get an L-Pad.  Connect it in-line 
between the monitor input jack and the drivers.

	But your problem sounds a little bit stranger than this; are you saying 
that you have only one CHANNEL of amplification, so you want to hook up your
monitor speakers with the Mains output (i.e. mains=monitors)?  I'm not sure this
is wise or adviseable.

	It may be better for you to simply aim your mains in a bit so you can
catch some of the side dispersion of the sound they're sending to your audience.

	As far as handling feedback, E.Q. is usually the solution.  You don't 
want to turn down ALL thje frequencies, only the offending ones.

	Dan

> 497.1  If all you want is a volume control, get an L-Pad.  Connect it in-line 
>        between the monitor input jack and the drivers. 
    
Could you please elaborate on "L-PAD" - is it some sort of potentiometer
    ?
    Yes you guessed it -- 1 channel +    
    MAIN = MONITOR, This is a very small PA set up 
    1 x 120w carlsboro PA amp with vol/bass/treb per each channel (x6)
    reverb + Boss delay.  using 2 x [2x12 + tweeter] Carlsboro Speakers
    on columns. It is only used for 3 vocals + keyboards
    the monitors were driven by a now sold 90w Carlsboro amp driven
    from the slave o/p on the main amp
    We havent got a mixer, the amps just blow out into the crowd(hopefully).
    
    In fact the main problem is CASH !!! - I'm really looking for a
    quick cheap(very) solution.
        
    bd....
    
        
       

    
    I would be careful doing what you describe. It sounds like you
    want to drive two monitors with one channel of a poweramp. This
    alone can cause problems. The amp is probably designed to work 
    with a load of 4-8 ohms. If you place two 8 ohm speakers in
    series, you have a total of 16 ohms, but if you place them in 
    parellel, you will have 4 ohms, which may be Ok. Now if you
    start placing potentiometers in line with a speaker, the impedence
    will increase dramatically. If you have 8 ohms in parrelel with
    100 ohms or more, you esentially have 8 ohms load, and no signal
    will pass through the pot at all. If you place 25 ohms in parallel
    with 8 ohms you will have 10-12 ohms. It's a game. Usually, a
    manufacturer will warn that you should have the same load on both
    channels of a poweramp. Some amps are so sensitive that just using
    two differant lengths of speaker cable will effect them. They can
    heat up, and shorten the life, or even burn up on you. 
    
    You can probably get away with hooking 2 monitors to each channel
    of the power amp (use all the same gauge speaker wire and make sure
    the combined lengths on each channel are equal), just to be sure you
    have equal loads on both channels, and make sure the speakers are in 
    parellel rather than in series. If you want one to be (aparently)
    louder than the other, try angling the one you want quieter, or pull 
    it back away from the band. Another trip is to dampen one speaker
    with a piece of foam rubber, or a carpet. This may seem like a crude
    way of doing it, but it is much easier on the speaker and amplifier
    than playing with the electrical load. Remember speaker impedence
    is a complex function that varies with frequency. In line pots will 
    get you into trouble every time.
    
               +         -   +         -
 +   -----------(speaker)_____(speaker)---
    	    				 |   series = 16 ohms
 -   -------------------------------------
 
       
 +   ----------------- + ---------------   +     
                   (speaker)           (speaker)  parallel = 4 ohms        
 -   ---------------   -    ------------   -
                                                                  
    
	Looks like you better save your pennies and buy another amp.
    Then you don't need anything mickey mouse at all. In the mean time
    be careful how you hook up your speakers and if you have to try
    using some of these trips.
    
    
    Good Luck
    
    Mark Jacques
    
        

RE < Note 497.2 by ILO::DOODY "Brian M. Doody" >

>Could you please elaborate on "L-PAD" - is it some sort of potentiometer?

	Yes, an L-pad is a kind of pot - but it is used to attenuate high-power
signals.  It is placed in-line DIRECTLY before the driver(s) you want to affect.
You cannot use your average potentiometer to do this.  An L-pad can be had from
Rat Shack for as little as $5.  To install, you'll need to get inside your
monitor cabinet, drill a hole for the control knob of the l-pad, and wire it
as follows:

	input jack ----> L-pad -----> driver

	Or if there's a crossover (passive), it gets a little more complicated:

	1) to control ALL drivers by one L-pad:

		input jack ---> L-pad ---> crossover ---> driver

	2) to control only hi-frequency driver (the one most likely to feedback)

		input jack ---> crossover ---> l-pad ---> HF driver (tweeter)

	This will take care of setting a lower volume for your monitors, but 
note:  It may NOT solve feedback problems.  Feedback usually is triggered by
certain frequencies.  It is possible to have these frequencies trouble you
even if you set the whole driver volume down - you really need to attenuate
just the offending frequencies.  This requires an EQ (graphic, parametric or 
notch filter).  It seems from your description that you may have been doing
without the additional EQ already, so it may be a moot point...

	The cheapest solution is the one I suggested before; simply turning in 
the mains a bit so you can hear them.  You'll have to carefully balance how FAR 
you turn them in to avoid feedback, but I think it would work better for you.  
I went with that kind of a setup for years (I.E. no monitors, just hear what 
you can from the mains).  It's just a matter of what you get used to.

	If you are looking for a cheap amp, I have an old Dynaco power amp that
I could let go for a low price.  For more info, contact me directly 
(AKOV68::EATOND or DTN 223-6976).

	Dan

	.3 was entered while I wrote .4

	I hadn't even considered impedance...  important!  I can't say that
I understand it very much, but if some expensive equipment is at stake, its
better to be safe...

	Dan

    
    Re: .5
    
    If you use an L-pad, you don't have to worry *too* much about
    impedance.
    
    An L-pad presents a constant load to your amp, regardless  of its
    setting, unlike a simple potentiometer which presents a varying
    load depending on setting.  So an 8-ohm L-pad always looks like
    8 ohms.
    
    The paralleling problem mentioned earlier is correct, however.
    This is why most monitor speakers are 16 ohms, so you can hook up
    more of them in parallel without going below 4 or 2 ohms.
        
    BTW, while your setup is not optimal, I have used such setups for
    monitoring in the past and they can work.  The real trick with such
    setups is that the overall volume must be pretty low...at higher
    volumes you really must have separate amplification and EQ for
    the mains and monitors.                                     

                                     
    

    
    	I disagree with some of the replies here. I can do that, right?
    Getting another amplifier *is* the optimum solution. However, you
    can get "some" signal to your monitor speakers by tapping off the
    main output of your power amp. You just have to think it through
    some. Clearly, you want most of the power (which is the *square*
    of the voltage) to the two main speakers which are facing the audience.
    These remain connected in parallel to the Carlsbro, and we'll assume
    they are coming in at 4 ohms. The amp is probably rated at 4 ohms
    min, so you're stuck, right? Not yet...
    
    	The connection I'd try is this. Assuming you have 3 monitors,
    connect all three (or whatever number) in parallel. If these were
    8 ohms a piece, we'd have 8/3 or 2.7 ohms. Connect this network
    in *series* with the two main speakers, giving 4+2.7=6.7 ohms -
    within the 4-8 ohm impedance range of the carlsbro.
    
    	Since the voltage drop across Z is proportional to Z and watts
    is proportional to the square of the voltage we can say 4 esquare
    = 16 and 2.7 esquare = 7.3 and 16/7.3 = 2.2 *times as much* power
    will come out of the mains as the monitors.
    
    	If the monitors are still too loud, get a big Big BIG 1 ohm
    or 2 ohm resistor and connect it across the monitor parallel
    connection. This strategy assumes that the quality of the monitor
    speakers are up to snuff with that of the mains. Dont do it this
    way - without that big 1-2 ohm resistor - if your monitors are these
    little wimpy things whilst you mains are a pair of VOTs!
    
    	Pleas correct me if I've erred in my calculations -
    
    	Joe Jas
    

    
    	RE: .3   
    
    		A small nit.
    
    	You stated a 25 ohm speaker in parellel with an 8 ohm you'd
    	get a total impedance of 10-12 ohms. Not correct, you'd get
    	about 6.06 ohms.  Your impedance WILL ALWAYS be lower than
    	the smallest load in a parellel circuit. Figure it yourself
    
    		z = 1/ (1/R1)+(1/R2)

    
    AGAIN re "L-PAD"
    
    I've looked in one of the major electronics supply catalogues and
    can't find anything which fits the description 'L-Pad' Perhaps there
    is a European Description which is different. ?
    What is the exact technical description ???
    
    Thanks again 
    
    Bd....
     

    LPad
   
    in 
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    maintains a contant impedance (close but not exact) but varies the
    amount of the power in to that load. Look in any engineer's handbook
    for a technical description, try your local DEC library.
    
    Radio shack sells them, quality is typical rat shack....also try
    any good dealer in electronic componants. Be certain you buy one
    rated for the correct amount of power!