For wiring convenience and for being able to turn them off using the speaker selector of my amplifier I would like to connect my phones (of unknown impedance, Sennheiser m@b 40, not really HI-FI, but very light on my head, I love them) to my amplifier (Revox B150, 2 x 90W over 8 Ohms) speaker output.
Am I in danger to fry the phones?
Even worst, am I in danger to fry my amplifier output transistors because of lack of adequate load?
Cheers.
Sergio
Am I in danger to fry the phones?
Not if you turn up the volume carefully.
Even worst, am I in danger to fry my amplifier output transistors because of lack of adequate load?
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I don't think so, but I'm not sure. I did and nothing happened. However, the sound will be too loud and you will probably hear amplifier hiss.
For these reasons, its better to build a voltage divider with a few resistors, in order to provide adequate load to the amp and reduce the power that goes to your headphones.
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Thank-you for your advice.
I don't want to be careful with the volume setting. I want to be able to switch from phones to speaker and vice-versa mantaining roughly the same perceived acoustic level without touching the volume knob (actually volume pushbuttons on the B150).
I think that using a voltage divider as you suggested is the correct solution. Only problem is that the resistors should be quite big and lot of power will be wasted.
I'll take into account also the possibilty to drive the phones with a separate amplifier, with the signal taken from a tape-out of the amplifier.
Thanks again.
Sergio
Thank-you for your advice.
I don't want to be careful with the volume setting. I want to be able to switch from phones to speaker and vice-versa mantaining roughly the same perceived acoustic level without touching the volume knob (actually volume pushbuttons on the B150).
I think that using a voltage divider as you suggested is the correct solution. Only problem is that the resistors should be quite big and lot of power will be wasted.
I'll take into account also the possibilty to drive the phones with a separate amplifier, with the signal taken from a tape-out of the amplifier.
Thanks again.
Sergio
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You'll need to be careful with the volume control or you are very likely to cook the 'phones! Even allowing for the (likely) higher impedance of them, you're *very* likely to overload them by not altering volume when you switch over. The amplifier is unlikely to be damaged by headphones, worst-case is the open-circuit from them blowing up and it's rare an amplifier can't cope with open circuits nowadays.
Use a separate headphone amp or a resistive network as suggested. Yes, the resistors need to be high-power types and yes, you'll waste a fair bit of power but so what?
As a matter of interest, I used to hook the outputs of my old Hafler SE240 to my crappy old Sony MDR-CD270 headphones for giggles. Set the volume control on my preamp VERY low and it actually sounded kinda decent!
Yes you can fry the phones. There will always be an occasion for listening with speakers at high volume, with the other speaker output driving the headphones by mistake.
I have also used headphones directly plugged into speaker output. Unless you have an extremely rubust headphone cord, with an extremely well designed speaker terminals adapter, sooner or later, you wil eventually get a short circuit somewhere in the headphone plug or cord, which will fry the amplifier.
In fact, every time I plugged something else that a standard pair of wires in my speaker output, I ended up with the amplifier fried by an accidental short-circuit after a delay varying between several minutes and several weeks.
With the voltage divider adaptor at the amp speaker outputs, a short circuit at the headphones will do nothing, since the amp will still have a high enough resistance at the output.
Yes you can fry the phones. There will always be an occasion for listening with speakers at high volume, with the other speaker output driving the headphones by mistake.
I have also used headphones directly plugged into speaker output. Unless you have an extremely rubust headphone cord, with an extremely well designed speaker terminals adapter, sooner or later, you wil eventually get a short circuit somewhere in the headphone plug or cord, which will fry the amplifier.
In fact, every time I plugged something else that a standard pair of wires in my speaker output, I ended up with the amplifier fried by an accidental short-circuit after a delay varying between several minutes and several weeks.
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Only a poorly-designed amplifier would be damaged by a short-circuit. Almost all modern amps incorporate short-circuit protection in the form of limiters or cut-outs. I wouldn't advise relying on it though, better to avoid the short!