1. Home
  2. Electronics Design
  3. audio switch circuit, when headphones plugged in?

audio switch circuit, when headphones plugged in?

(I know about switched (lifted) jacks, but they won't work in my case because I don't want to modify the existing jack. Please read on. )

Recently, I added one of those front panel "hubs" that fits in the 5.5 in. bay and places USB, firewire, card readers so they're accessible on the front of the PC. The device I installed is from Logisys:

formatting link

This panel exports headphone audio via a mini-jack. I'd like to plug my headphones into that jack, and have it cut the output to the speakers. I have two similar systems equipped with an EPOX 8K9A2+ mother board. In one system, I'm using the on board AC97 audio, and in the other case an Audigy 2 sound card. Both sound cards behave similarly. They have a stereo line out which usually is plugged into amplified PC speakers, but also allows a nominal 30 ohm headphone.

What I'd like to do is build a widget that will connect the headphones to the line out when I plug the headphones into the multi-function front panel, and have the line out go to the amplified speakers otherwise. I can't easily use a switched mini-jack phone plug because that would require reworking the front panel.

Here's what I think this switch looks like:

--- sound card line out ---->

swtich headphone --< panel

Reply to
Gary Funck
Loading thread data ...

Gary Funck wrote: snip >

As you have already stated, how does the relais know if headphones are connected? Some phones have 600R others only 25R. They do not need amplification either. And the amp usually doesn't mind if there is any load connected. The best would be to check out if the built-in mini-jacks do have switches built-in. Most of them do, even if they are not used. You could then make a bridge from the switch O/P to the shielded cable to the speakers. What you want to do is unfortunately impossible. If there are no built in switches, forget the whole thing and do it manually with a DPDT switch.

--
ciao Ban
Bordighera, Italy
Reply to
Ban

load

Well ... when the headphones are plugged into the jack at the front panel, there will be a closed circuit between both the L and R circuits and the common. As so:

+----------+ | | V V

L C

where L = Left and C = Common (Ground) for example. If we can impose a detector circuit, that impose a very small DC current through L and C above, we can detect that the phones have been plugged in. If this DC current can be converted into a logic level voltage, we can than trigger the CMOS switch (or a relay). I'd have some questions about how we'd isolate our detector circuit, which would go wiggy once the 1 volt at 100 mA of A/C audio starts heading back towards the earphones.

Maybe we could work it the other way ... we could always allow a signal to flow to the headphone circuit. If the headphones are plugged in, a current will flow. We (somehow) convert that A/C current into a logic level, which switches out the speakers. Briefly (50 ms. or less) we'd drive both the speakers and the headphones until our switch disabled the connection to the speakers. Hopefully the amplifier circuit in the sound card won't care about briefly seeing both loads in parallel?

I'd be a little concerned that there might be a higher current/voltage sent to speakers when we remove the headphones before the sound card amplifier has had time to adjust to speakers only ...

Thanks. Will do that. It certaionly would be easier. Is some sort "de-popper" circuit needed to keep the insertion/removal of the headphone jack from causing a loud noise in the loud speakers?

Reply to
Gary Funck

Too complicated, a dc current will also degrade headphones performance.

No need to be concerned, there will be neither a "plop" when the phones are inserted. As I told you already, the soundcard output doesn't care too much what is plugged in. Maybe the overall level drops a bit, depending on the phones, but when the soundcard is able to drive the phones alone loud enough, the speakers can also be operated in parallel without any problem.

--
ciao Ban
Bordighera, Italy
Reply to
Ban

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.