Some help with wiring puzzle please

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Some help with wiring puzzle please

Postby soccerman58 » Mon Jan 17, 2011 11:36 pm

Hi folks. It's been a long time.

I have a silly problem and I could really use the help.

I have two digital pianos. I have one stomp box for delays and reverbs and sub-bass filters (aha! there is the bass link, tenuous though it may be). I need to take the 1/4 inch outputs from each piano and plug them both into the single 1/4 inch input on the stomp box.

So I took two jacks, ganged them together and llooped them off to the output jack on the patch bay.

Now it gets weird. Plugging just one keyboard into either input jack works. You would expect that.
Plug them both in and only Keyboard One works. OK, so reverse the inputs and you would expect keyboard 2 to work and keyboard One to crap out wouldn't you. But it doesn't. Keyboard One still works and keyboard two still has nada. I have wired all the grounds and all the hots correctly; I have nothing crossed over. Heck it's only 6 bits of wire.
Hopefully someone smart will tell me what I have done wrong (or more correctly how to fix it) please.
Does anyone have any clue as to what is happening here please?
Thanks
Phil
soccerman58
 
Posts: 127
Joined: Fri Jan 04, 2008 12:38 am

Re: Some help with wiring puzzle please

Postby DVDdoug » Tue Jan 18, 2011 6:15 pm

I need to take the 1/4 inch outputs from each piano and plug them both into the single 1/4 inch input on the stomp box.
The bottom line is that you should NEVER connect two outputs together... The proper way to mix two outputs is with a MIXER. (It's OK to connect two inputs together... i.e. It's OK to conect two amps to one keyboard with a Y-adapter, but it's not OK to connect two keyboards to one amp with a Y-adapter.)

Basically, the signal flows out of one output into the other output. Typically, the actual source impedance (looking back into the output) is much lower than the designed load impedance, and each output puts an improper load on the other output. (Excessive current flows from one output to the other output. This reduces the voltage level, and sometimes causes distortion.)

Plug them both in and only Keyboard One works. OK, so reverse the inputs and you would expect keyboard 2 to work and keyboard One to crap out wouldn't you. But it doesn't. Keyboard One still works and keyboard two still has nada.
Keyboard 1 has a lower output impedance than keyboard two. When you connect the two together, the output from keyboard two gets "killed".

I don't know much about keyboard output levels & impedance... I assume that keyboard outputs are line level? i.e. A keyboard amp won't work with a guitar amp, right?

If you don't want to buy a mixer You can make a simple fixed-passive mixer with resistors. Try putting a 5K or 10K ohm* resistor in both signal lines. If that's not clear here is a schematic for a 6-input passive mixer. You only need two inputs, and you can (hopefully) solder the resistors inside the connector body.

Keyboard one will probably be louder than keyboard two, and the output from keyboard two should be cut in half (-6dB) rather than being killed completely. (If both outputs were equally low impedance, both signals would be cut in half... This is the downside of a passive mixer.)


* Don't worry about the tolerance or power ratings, just the ohms.
DVDdoug
 
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Re: Some help with wiring puzzle please

Postby soccerman58 » Tue Jan 18, 2011 10:48 pm

Thanks for the reply.

I might try the passive mixer thing for the price of two resistors. I can maybe compensate for the volume differences as they are both line out levels and each has a volume. Otherwise I think I am going to get a mixer/amp combo and do it properly.

Appreciate the help
Phil
soccerman58
 
Posts: 127
Joined: Fri Jan 04, 2008 12:38 am


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