Yesterday I had a question - UK Vintage Radio Repair and Restoration Discussion Forum

georgesgiralt · 8th Jun 2009, 1:49 pm

Hi !
And I was not sure about the answer.
The question was : "Why do you need to put a resistor to couple two stereo outputs into one mono input ?"
I answered that it *had* to be done this way, but I wonder why also ... (wiring the two 4.7K into the tiny 2.5 mm jack was a little difficult for my big fingers...)
So, please, help me looking less dumb ;-)

julie_m · 8th Jun 2009, 2:11 pm

Because the outputs are low-impedance voltage sources, and they may be sourcing different voltages. If you connect them together without additional resistors, then the difference between the two voltage sources is developed across no more than the sum of the effective series impedances; causing a large current to flow out of one amplifier output into the other one. This is not at all good for the output transistors.

Adding your own resistors in series with the outputs means that this current is reduced.

kalee20 · 8th Jun 2009, 5:18 pm

For stereo, the two channels will generally be carrying different signals. Connecting them directly together means the outputs will fight each other. Adding resistors stops this.

georgesgiralt · 11th Jun 2009, 2:32 pm

Thanks for your knowledge !

8th Jun 2009, 1:49 pm	#1
georgesgiralt Hexode Join Date: Jul 2008 Location: Toulouse, France Posts: 406	Yesterday I had a question Hi ! And I was not sure about the answer. The question was : "Why do you need to put a resistor to couple two stereo outputs into one mono input ?" I answered that it had to be done this way, but I wonder why also ... (wiring the two 4.7K into the tiny 2.5 mm jack was a little difficult for my big fingers...) So, please, help me looking less dumb ;-)

8th Jun 2009, 2:11 pm	#2
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Yesterday I had a question Because the outputs are low-impedance voltage sources, and they may be sourcing different voltages. If you connect them together without additional resistors, then the difference between the two voltage sources is developed across no more than the sum of the effective series impedances; causing a large current to flow out of one amplifier output into the other one. This is not at all good for the output transistors. Adding your own resistors in series with the outputs means that this current is reduced. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

8th Jun 2009, 5:18 pm	#3
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,082	Re: Yesterday I had a question For stereo, the two channels will generally be carrying different signals. Connecting them directly together means the outputs will fight each other. Adding resistors stops this.

11th Jun 2009, 2:32 pm	#4
georgesgiralt Hexode Join Date: Jul 2008 Location: Toulouse, France Posts: 406	Re: Yesterday I had a question Thanks for your knowledge !