question about connecting PA amplifier
in [Live Sound]
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From: crevitch on 5 Sep 2008 08:31 I have a lab gruppen lab 300 PA amplifier. http://www.labgruppen.com/downloads/product/LAB300_manual_rev2.pdf It has balanced TRS phone and XLR inputs. I am trying to connect standard line level outputs to these inputs (i.e. IPOD) but find that the output level from the speakers is very low. I am using a cable that converts mini-stereo phone to 2 - 1/4" mono phone plugs. Since the 1/4" connectors only have tip/ring and not TRS, I assume I am grounding one of the balanced inputs with the ring. What is the reason for the low output? Should this work properly? Thanks
From: George's ProSound Company on 5 Sep 2008 08:39 "crevitch" <crevitch(a)none.com> wrote in message news:48c12686$1(a)news.alcatel.com... >I have a lab gruppen lab 300 PA amplifier. > http://www.labgruppen.com/downloads/product/LAB300_manual_rev2.pdf > > It has balanced TRS phone and XLR inputs. I am trying to connect > standard line level outputs to these inputs (i.e. IPOD) but find that the > output level from the speakers is very low. I am using a cable that > converts mini-stereo phone to 2 - 1/4" mono phone plugs. Since the 1/4" > connectors only have tip/ring and not TRS, I assume I am grounding one of > the balanced inputs with the ring. > > What is the reason for the low output? Should this work properly? > > Thanks start with turning the volume on your Ipod to full but even at "full" a I pod can not send enough signal to properly drive a pro sound amp you also need a preamp a simple behringer 502 mixer, about 30$ will do just fine and do not "Y" the ipod's channels into one input one the LG strange things happen to a line level stereo signal when passivly "y"ed George
From: jakdedert on 5 Sep 2008 09:28 George's ProSound Company wrote: <snip> > and do not "Y" the ipod's channels into one input one the LG > strange things happen to a line level stereo signal when passivly "y"ed > George > > That's good to know. Some stereo line-outs don't mind being tied together, although I never do it if there's an option. Lately I've been getting a lot of ipods and stereo laptop outputs. Fortunately we've got dedicated A/V DIs, with isolated stereo lines-in to a single balanced out. jak
From: Mike Dobony on 5 Sep 2008 11:05 On Fri, 05 Sep 2008 08:28:07 -0500, jakdedert wrote: > George's ProSound Company wrote: > <snip> >> and do not "Y" the ipod's channels into one input one the LG >> strange things happen to a line level stereo signal when passivly "y"ed >> George >> >> > That's good to know. Some stereo line-outs don't mind being tied > together, although I never do it if there's an option. Lately I've been > getting a lot of ipods and stereo laptop outputs. Fortunately we've got > dedicated A/V DIs, with isolated stereo lines-in to a single balanced out. > jak The device might not mind, but the signal is still messed up with a wye cable used to mix the signals. As George said, strange things happen when you use a passive "y" cable to mix signals. If you don't have a stereo input then you can use 2 mono channels to mix.
From: Rupert on 5 Sep 2008 11:30
On Sep 5, 6:28 am, jakdedert <jakded...(a)bellsouth.net> wrote: > George's ProSound Company wrote: > > <snip>> and do not "Y" the ipod's channels into one input one the LG > > strange things happen to a line level stereo signal when passivly "y"ed > > George > > That's good to know. Some stereo line-outs don't mind being tied > together, although I never do it if there's an option. Lately I've been > getting a lot of ipods and stereo laptop outputs. Fortunately we've got > dedicated A/V DIs, with isolated stereo lines-in to a single balanced out.. > jak The reason why it's not a good idea is because you're only suppose to drive high impedance inputs from low impedance outputs. When you "Y" to outputs together directly, each output tries to "drive" the other regardless of the input your connected to. Since the outputs are much lower impedance than what the circuit is really designed to drive, excessive current is sourced from each output to drive the other which can cause distortion. In extreme cases you can damage the output circuit. That said, my understanding is that in relatively modern gear there may be resistor networks built-out on the outputs to prevent overloading the outputs in the event they're "Y"ed together or shorted out. Either way, it's still bad practice in my opinion. There are simple passive resistor summing networks that you can make to ensure proper loading when tying 2 output together: http://www.rane.com/note109.html Rupert |