[Think I've got this in the right place, admins please move or delete if wrong.]
This is something ive been thinking about for a while now as my amp has no headphone socket. Then i rencently saw this video and thought maybe this could work:
My problem is that i dont want to risk blowing my headphones up by forgetting to turn the volume down before plugging them in by using resistors but then didnt want resistors before the jack as that would affect the output to the speakers. So i came up with a practical solution (i think) where i use a 6PDT switch to go from speakers to headphones which lets me isolate the resistors to only the headphone jack.
Does this make sense? What resistors should i use and where would i place them? See dodgy diagram for what im thinking of doing.
Any feedback or help is greatly appreciated.
Amp is a Cambridge Audio A5, headphones are Bose Quietcomfort 15
This is something ive been thinking about for a while now as my amp has no headphone socket. Then i rencently saw this video and thought maybe this could work:
My problem is that i dont want to risk blowing my headphones up by forgetting to turn the volume down before plugging them in by using resistors but then didnt want resistors before the jack as that would affect the output to the speakers. So i came up with a practical solution (i think) where i use a 6PDT switch to go from speakers to headphones which lets me isolate the resistors to only the headphone jack.
Does this make sense? What resistors should i use and where would i place them? See dodgy diagram for what im thinking of doing.
Any feedback or help is greatly appreciated.
Amp is a Cambridge Audio A5, headphones are Bose Quietcomfort 15
You need a suitable DPDT switch, suitable meaning it has to handle the speaker current. For a 50 to 100 watt amp you should use something like a 10 amp switch.
Firstly, this assumes a 'normal' amp and not a Class D type or bridged amp.
The speaker grounds and headphone grounds are common and do not go through the switch at all. Only the + (plus) output of the amp is switched. 330 ohm 1 watt carbon or metal film resistors are a typical sort of value to use. You can go higher or lower as needed though.
Firstly, this assumes a 'normal' amp and not a Class D type or bridged amp.
The speaker grounds and headphone grounds are common and do not go through the switch at all. Only the + (plus) output of the amp is switched. 330 ohm 1 watt carbon or metal film resistors are a typical sort of value to use. You can go higher or lower as needed though.
@Mooly Thanks mate, i appreciate the insight. Will have a look for the bits on fleabay when I get in from work. Would you mind if I post what I find here to get a yay or nay from your good self? And AFAIK My amp isn't a class D or bridged or anything. Just a standard home stereo amp I bought new nearly 20 odd years ago
I have used jacks with insulated switches so that you can disconnect the speakers when a headphone is plugged in. The switches are insulated and not part of the headphone connection, so the headphone hot connection have an attenuator (series resistor) that the speakers do not.
https://www.sameskydevices.com/product/resource/sj-63023h.pdf
But the jack switches are not large and current limited. So, I think the most practical approach is just a DPST switch to mute the speakers and a headphone jack that is hot (attenuated) all the time. If you have a bridged amp with DC on the outputs, you need DC blocking caps and use only one side of the output for headphones. It doesn't matter if the headphones are active when you are using the speakers, and you could unplug them.
You could build a circuit that detects the headphone and drives a speaker relay, which could be the "protection" relay.
It's possible that having both speakers and phones active is useful, such when one person is hard of hearing, but you don't want to run the speakers that loud.
https://www.sameskydevices.com/product/resource/sj-63023h.pdf
But the jack switches are not large and current limited. So, I think the most practical approach is just a DPST switch to mute the speakers and a headphone jack that is hot (attenuated) all the time. If you have a bridged amp with DC on the outputs, you need DC blocking caps and use only one side of the output for headphones. It doesn't matter if the headphones are active when you are using the speakers, and you could unplug them.
You could build a circuit that detects the headphone and drives a speaker relay, which could be the "protection" relay.
It's possible that having both speakers and phones active is useful, such when one person is hard of hearing, but you don't want to run the speakers that loud.
Last edited:
@steveu Thanks matey. AFAIK my amp isn't bridged (but that's something I'll look into and double check). I'll definitely look into the part you specified with the insulated jack but agree using a switch is probably the way forward for ease. As for designing a circuit for protection is way above my skill set at the moment. Sounds like a great idea though 👍🏻
I looked at pictures of the internals and it is a standard amp.
Of course 🙂
I would suggest using 47R : 4R7 attenuator networks that are switched when the jack is inserted. Gives an 11-fold reduction and only 3W resistors are needed I think for practical headphones (you'll be deafened before they overheat, assuming standard 32 ohm 'phones). You don't want those networks in circuit with the speakers switched on as they will be capable of overheating when at high volume for speakers.
If you use a headphone jack with isolated switches like this one: https://www.buymystuff.co.uk/2-x-st...unting-jack-socket-switched-headphone-socket/, you won't need a separate switch.
If you use a headphone jack with isolated switches like this one: https://www.buymystuff.co.uk/2-x-st...unting-jack-socket-switched-headphone-socket/, you won't need a separate switch.
@Mark Tillotson thanks mate. I did a quick Google and come up with either an SMD looking thing or some kind of chip. I appreciate your help but I think this solution would be above my limited knowledge and skills to pull off, I'd have no idea where to begin
OK 🙂
If you are in the UK you can get a lot of this stuff from:
https://cpc.farnell.com/
but there are delivery charges for small orders.
You used to be able to get these kind of things already made up in a box but they seem thin on the ground these days.
If you are in the UK you can get a lot of this stuff from:
https://cpc.farnell.com/
but there are delivery charges for small orders.
You used to be able to get these kind of things already made up in a box but they seem thin on the ground these days.
Adding some large-ish DC blocking caps before the headphone output will probably keep it idiot-proof in general...
@Andersonix could you send me a link of an example of what you're suggesting please? My knowledge isn't great woth some components (hence why I'm asking on a forum! 🤣)
Note that a 47+4.7 Ohm attenuator could see almost 20Watts from a 100W+ amp, so I would use higher value resistors. In the past, I have just used 150 Ohm series resistors, which dissipates nothing when there are no phones connected.
Another thing is that if your amp is a tube amp, it may require loading; may be unstable if driven without a ~8 Ohm load. In which case, the speaker switch needs to be DT to ~8 Ohm resistor loads when the speakers are off.
Another thing is that if your amp is a tube amp, it may require loading; may be unstable if driven without a ~8 Ohm load. In which case, the speaker switch needs to be DT to ~8 Ohm resistor loads when the speakers are off.
https://sound-au.com/project100.htm
I made version 2 of the above but didn't add DC blocking caps yet, as I know the DC offset on my own amps is very low. On a big amp those resistors get HOT.
I made version 2 of the above but didn't add DC blocking caps yet, as I know the DC offset on my own amps is very low. On a big amp those resistors get HOT.
Attachments
Your ears will burn out before the resistors though as I mentioned. Higher values mean a worse damping factor. Perhaps consider fuses?
True enough, assuming the attenuator is switched out when the phones are not plugged in. As for damping factor, some believe that "current drive" is better. In the past, I used a 150 Ohm series resistor and perhaps that gave it a bass boost, which I liked just fine. The current drive people argue that there is less distortion that way. Also, you could accidently blast the phones while you are not wearing them, forgetting that they are plugged in.
@Andersonix wow!! That looks impressive but waaaaaay out of my comprehension! But I can only admire your skill and knowledge!
@steveu @Mark Tillotson i wish I understood what you guys are saying as I'm sure it's totally feasible but I'm too dumb to figure it out! Would you agree for simplicity sake doing what I plan to do with a switch and resistors? Bear in mind you are talking to basically a shaved ape when it comes to electronics?
@steveu @Mark Tillotson i wish I understood what you guys are saying as I'm sure it's totally feasible but I'm too dumb to figure it out! Would you agree for simplicity sake doing what I plan to do with a switch and resistors? Bear in mind you are talking to basically a shaved ape when it comes to electronics?