Adding a headphone jack to a Power Amp?

[Macdude]

I have a Dynaco QSA-300 Power Amp that I'm using along with a Hafler DH-101 pre-amp. Neither have a headphone jack. Is there any safe way of connecting a jack to the speaker terminals of the amp? I know there is an impedance mis-match, is there any way to correct this? The headphones are Koss Pro4AA's.

 

[EchoWars]

If you are bridging the Dynaco to use as a two-channel amp, you're pretty much SOL (headphones use a common signal return, which ain't gonna work with a bridged amp).

If you're using the Dynaco as a four-channel amp, you can connect the headphones to the output of two channels of the amplifier through some 330 ohm 3W resistors.

 

[Macdude]

Hmm, well the headphones and speakers would be separated by a switch and neither would be connected at the same time. The amp has been bridged to run in 2ch mode. Would the switch make any difference with that common ground issue?

 

[EchoWars]

The problem has nothing to do with running the speakers and the phones at the same time...the problem is inherent in that:

1. In a bridged amp, two channels are driven with the same signal, with one channel 180° out of phase with the other, and...

2. In a bridged amp, the load is floating (not referenced to ground). A headphone is referenced to ground, due to the shared signal return.

Sansui adds a headphone jack to the natively-bridged AU-D11 II amplifier by using a small transformer. This is probably the easiest solution for a bridged amp.

If you're willing to cough-up the cash, contact Jensen. Someone there ought to be able to help.

http://www.jensen-transformers.com/

Here's the way Sansui does it with their AU-D11 II. All you need is the right transformers.

 

[Macdude]

Good to know.. and thanks for all the information. Looks like I'll either have to just use the jack on the cassette deck, or buy a headphone amp.