Simple headphone-activated speaker bypass?

[Pat Deegan]

Greetings,

I have a simple audio circuit with an 8 ohm speaker being driven by an
op-amp. I've been trying to figure out the simplest method to add support
for headphones... the thing I can't seem to come up with is the most
effective means of (automatically) shutting down the speaker when the
headphone jack is inserted.

I'm certain there must be a relatively straightforward way of
doing this... I'd appreciate any ideas, ascii art or hints you may have.

TIA.

Regards,

 

[Jim Thompson]

Greetings,

I have a simple audio circuit with an 8 ohm speaker being driven by an
op-amp. I've been trying to figure out the simplest method to add support
for headphones... the thing I can't seem to come up with is the most
effective means of (automatically) shutting down the speaker when the
headphone jack is inserted.

I'm certain there must be a relatively straightforward way of
doing this... I'd appreciate any ideas, ascii art or hints you may have.

TIA.

Regards,

There are phone jacks specifically made for the purpose... pushing in
the plug switches connections.

...Jim Thompson

 

[Walter Harley]

Greetings,

I have a simple audio circuit with an 8 ohm speaker being driven by an
op-amp. I've been trying to figure out the simplest method to add support
for headphones... the thing I can't seem to come up with is the most
effective means of (automatically) shutting down the speaker when the
headphone jack is inserted.

Check out Mouser (http://www.mouser.com) part number 161-0084, as an example
of the sort of connector Jim's talking about. That part has a fully
isolated DPDT switch that switches when you plug in a jack, so you can do
all sorts of things with it.

 

[John Fields]

Greetings,

I have a simple audio circuit with an 8 ohm speaker being driven by an
op-amp. I've been trying to figure out the simplest method to add support
for headphones... the thing I can't seem to come up with is the most
effective means of (automatically) shutting down the speaker when the
headphone jack is inserted.

I'm certain there must be a relatively straightforward way of
doing this... I'd appreciate any ideas, ascii art or hints you may have.

---
http://www.switchcraft.com/
_____
Switchcraft 12A. Connect the output of the amp to the \/ (tip)
contact, the speaker to the contact touching it, and ground to the
frame contact (sleeve).

When you stick the phone plug into the jack, the phone plug's sleeve
will get connected to ground and the phone plug's tip contact will
break the contact between the amp and the speaker and connect the amp
across the headphones.

 

[Pat Deegan]

Hi,

_____
Switchcraft 12A. Connect the output of the amp to the \/ (tip)
contact, the speaker to the contact touching it, and ground to the
frame contact (sleeve).

Ah, a mechanical solution! I was so busy thinking of schematics... well
this is perfect. Thanks to all of you

Regards,

 

[Walter Harley]

http://www.switchcraft.com/
_____
Switchcraft 12A. Connect the output of the amp to the \/ (tip)
contact, the speaker to the contact touching it, and ground to the
frame contact (sleeve).

When you stick the phone plug into the jack, the phone plug's sleeve
will get connected to ground and the phone plug's tip contact will
break the contact between the amp and the speaker and connect the amp
across the headphones.

John, if I understand you properly, that approach will present the full
output of the amp to the headphones. While some amps do indeed do that,
it's not a good idea; the amp probably puts out enough voltage to fry the
headphones. Headphones usually max around 500mW; typical headphone
impedance is as low as 32 ohms, with some even lower; at a given voltage,
power is inversely proportional to load resistance; so an amp that generates
anything more than 2W into 8 ohms will generate more than 500mW into 32
ohms.

If the OP wants to be able to put series resistors in front of the
headphones to limit the output power, he needs to use a more elaborate
solution, like the isolated DPDT switch on the jack I suggested.

 

[John Fields]

John, if I understand you properly, that approach will present the full
output of the amp to the headphones. While some amps do indeed do that,
it's not a good idea; the amp probably puts out enough voltage to fry the
headphones. Headphones usually max around 500mW; typical headphone
impedance is as low as 32 ohms, with some even lower; at a given voltage,
power is inversely proportional to load resistance; so an amp that generates
anything more than 2W into 8 ohms will generate more than 500mW into 32
ohms.

---
Clue:

"I have a simple audio circuit with an 8 ohm speaker being driven by
an op-amp."
---

If the OP wants to be able to put series resistors in front of the
headphones to limit the output power, he needs to use a more elaborate
solution, like the isolated DPDT switch on the jack I suggested.

---
OK, or if he wants to cheat and let a little signal go into the
speaker when the headphones are plugged in he can hook the series
resistor in parallel with the tip and NC contact of the Switchcraft
jack, hook the amp's output across NC and sleeve, and the speaker
across tip and sleeve.

 

[Walter Harley]

Clue:

"I have a simple audio circuit with an 8 ohm speaker being driven by
an op-amp."

Whoops, you're absolutely right, I missed that. Sorry.

 

[Walter Harley]

Clue:

"I have a simple audio circuit with an 8 ohm speaker being driven by
an op-amp."

Speaking of which: hey, Pat, have you looked at the specs of that opamp?
Most opamps can't drive an 8 ohm load very well at all.