I have a stereo input jack that is connected to the input of 2x opamp buffers (one each channel), this works fine as long as a stereo jack/signal is connected, but if a mono jack is connected, right channel is shorted to gnd by the plug, so you only get audio on the left channel.

What id like to do is somehow detect if a mono jack is inserted and connect the left output from the jack's tip connection to both channels.

Any ideas about how this can be done?

 

Yes.
Use a mono jack and wire the mono jack to a stereo plug.

 

Yes.
Use a mono jack and wire the mono jack to a stereo plug.

I know what you're saying.. could also just use a stereo/mono switch that connects the left channel to both imputs for mono, but i'm trying to make it seamless so if someone plugs in a mono source it will automatically detect it and route the mono signal to both channels.

I'm thinking this could be done with a transistor with a pull up resistor on it's base with the right channel signal also connected at this point, so when a mono jack is inserted it will pull the transistor's base down to ground, connecting the right signal to the left channel. Just not sure exactly how to make it work...

 

I know what you're saying.. could also just use a stereo/mono switch that connects the left channel to both imputs for mono, but i'm trying to make it seamless so if someone plugs in a mono source it will automatically detect it and route the mono signal to both channels.

I'm thinking this could be done with a transistor with a pull up resistor on it's base with the right channel signal also connected at this point, so when a mono jack is inserted it will pull the transistor's base down to ground, connecting the right signal to the left channel. Just not sure exactly how to make it work...

That will not work.
A stereo plug is TRS - tip, ring, and sleeve.
A mono plug is TS - tip and sleeve

The mono plug will always ground out ring to sleeve.

 

I have a stereo input jack that is connected to the input of 2x opamp buffers (one each channel), this works fine as long as a stereo jack/signal is connected, but if a mono jack is connected, right channel is shorted to gnd by the plug, so you only get audio on the left channel.

What id like to do is somehow detect if a mono jack is inserted and connect the left output from the jack's tip connection to both channels.

Any ideas about how this can be done?

Hello there,

Since normally a mono plug plugged into a stereo jack shorts out one channel as Mr Chips pointed out, you'd have to modify the jack so that it can detect that one channel is shorted, then break the connection, then connect the tip to the other channel as well. That would probably take either a relay or analog switch and a sense circuit to detect that condition and do the switching. An op amp could be used to detect the short, then an analog switch to open one connection and close the other.

Since this is a little difficult (not too bad though) some other options would include the following...

1. You could provide a "stereo/mono" switch with labels so the user can switch to mono. The user would read the labels and switch the switch as needed. (See attachment)
2. You could provide two jacks, one stereo (as it is now) and one mono, with appropriate labels. The user would read the labels and plug into the appropriate jack.

Those two are probably the simplest but #1 is the simpler of the two because #2 would still require some switching to be done or resistors added because in order to connect one channel to two channels the two channel inputs have to be connected together. An idea would be to see if you can find resistors that can do this permanently without bothering the stereo separation too much and also without bothering the mono volume level. One resistor would permanently go between the ring and the input for that channel. If that method is not possible then you probably have to also use some detection circuit and switching circuit. #1 does not need this extra circuitry just the switch and the labels.
For #1 the switch just has to open the shorted-out channel and connect it instead to the tip of the stereo jack which is now the mono plug tip. That's probably the simplest method and the wiring scheme is also simple.

In the diagram, LEFT is the left input channel to the amplifier and RIGHT is the right input channel.

 

Hello there,

Since normally a mono plug plugged into a stereo jack shorts out one channel as Mr Chips pointed out, you'd have to modify the jack so that it can detect that one channel is shorted, then break the connection, then connect the tip to the other channel as well. That would probably take either a relay or analog switch and a sense circuit to detect that condition and do the switching. An op amp could be used to detect the short, then an analog switch to open one connection and close the other.

Since this is a little difficult (not too bad though) some other options would include the following...

1. You could provide a "stereo/mono" switch with labels so the user can switch to mono. The user would read the labels and switch the switch as needed. (See attachment)
2. You could provide two jacks, one stereo (as it is now) and one mono, with appropriate labels. The user would read the labels and plug into the appropriate jack.

Those two are probably the simplest but #1 is the simpler of the two because #2 would still require some switching to be done or resistors added because in order to connect one channel to two channels the two channel inputs have to be connected together. An idea would be to see if you can find resistors that can do this permanently without bothering the stereo separation too much and also without bothering the mono volume level. One resistor would permanently go between the ring and the input for that channel. If that method is not possible then you probably have to also use some detection circuit and switching circuit. #1 does not need this extra circuitry just the switch and the labels.
For #1 the switch just has to open the shorted-out channel and connect it instead to the tip of the stereo jack which is now the mono plug tip. That's probably the simplest method and the wiring scheme is also simple.

In the diagram, LEFT is the left input channel to the amplifier and RIGHT is the right input channel.

Thanks for the detailed reply!

I think the easiest option is probably just to use a switch as you said to select stereo/mono for each input. But i still think it would be possible to do this using a transistor or JFET as a switch to route the left channel audio to the right channel input when the transistor is biased by the grounded input from the right (ring) connection. I'm just not sure how you would connect the right channel to the transistor/FET's base/gate connection with the required pull up resistor without affecting the audio on that channel when a stereo input is used.

I think im going go with the simple mono/stereo switch option, but was just curious if any of the experts here had any other ideas how this could be done without too much added complexity.. it would be pretty cool if it could automatically sense and connect a mono source if a mono jack was used.

 

Another solution is to add another jack (ignore the ring connection if TRS jack) with a built-in switch. When a plug is inserted, the switch connects the left and right channels.

 

Another solution is to add another jack (ignore the ring connection if TRS jack) with a built-in switch. When a plug is inserted, the switch connects the left and right channels.

It's a possibility, but i don't have much room for additional sockets (trying to keep the unit as small as possible), i think using a switch is probably the best option.

 

Another solution is to add another jack (ignore the ring connection if TRS jack) with a built-in switch. When a plug is inserted, the switch connects the left and right channels.

That's an interesting idea too. Do you know if they make mono jacks with a normally open contact?

 

Use a MOSFET to sense that ring is grounded to sleeve. The gate pull-up resistor can be very high, 10 MΩ.
Then use the output to switch a 2-channel analog switch as per the DPDT relay given by MrAl.

 

Use a MOSFET to sense that ring is grounded to sleeve. The gate pull-up resistor can be very high, 10 MΩ.
Then use the output to switch a 2-channel analog switch as per the DPDT relay given by MrAl.

View attachment 325699

That is the kind of thing i was thinking.. but how would you connect the ring connection so that it doesnt affect the right channel signal when there is stereo (TRS) plug connected?

I think you could probably use the output of that FET to turn on another FET or transistor that just connects the output of the left channel to the right channel ..

 

Here is an example of a 2-channel analog switch. The digital input current is very low. You don't need the MOSFET.
However, you will need a way to prevent the switch from switching back and forth with analog signals on the digital input.

 

Here is an example of a 2-channel analog switch. The digital input current is very low. You don't need the MOSFET.
However, you will need a way to prevent the switch from switching back and forth with analog signals on the digital input.

View attachment 325715

Ah! thats pretty cool, so I assume you would just connect the right (ring) output from the jack to the D1 input with a pull up resistor to keep it from switching randomly when an audio signal was present?

 

Ah! thats pretty cool, so I assume you would just connect the right (ring) output from the jack to the D1 input with a pull up resistor to keep it from switching randomly when an audio signal was present?

I believe that it is not that simple. What are the voltage levels and impedance of the input signals?

 

I believe that it is not that simple. What are the voltage levels and impedance of the input signals?

This is a guitar effect mixer that im making.. the inputs are 1M impedance and the power supply is 9v but using a 4.5v reference for the opamps, so the audio signals should never swing to gnd level (unless they are clipping), so i think it should be possible to make this work.

 

This is a guitar effect mixer that im making.. the inputs are 1M impedance and the power supply is 9v but using a 4.5v reference for the opamps, so the audio signals should never swing to gnd level (unless they are clipping), so i think it should be possible to make this work.

Even with single supply 9V stomp boxes, the signal output is often AC coupled.
I will have to take some time and think about a solution for this.

 

This is what I have. It would have to be tested with the particular analog switch.

With nothing plugged in, or with TRS source less than ±2V and above 10Hz, MUX CONTROL is at +V.
When TS is plugged in, MUX CONTROL is at 1/10 of +V.

 

This is what I have. It would have to be tested with the particular analog switch.
View attachment 325755
With nothing plugged in, or with TRS source less than ±2V and above 10Hz, MUX CONTROL is at +V.
When TS is plugged in, MUX CONTROL is at 1/10 of +V.

Thanks for this... I'll order a couple of analog switch ICs and give it a go!