True Headphone AMP?

05-05-11: Jedinite24
How would one hook up their amp so that it could power a pair of headphones? I have a Hafler DH-220 and from looking at the manual it says it can be used to power a set of headphones. From looking at the Hafler and seeing a lack of a headphone jack and just conventional speaker terminals I haven't the faintest idea as to how to hook up headphones to it. I was thinking about using the Hafler DH-220 as a headphone amplifier since it is just lying around unused right now. The preamp I plan to use in this set-up doesn't have a headphone jack. The headphones I plan to use are either the AKG 701 or Sennheiser HD580s.

Back in the old days (the 1960's and earlier, and perhaps a little more recently than that), headphones generally had much higher impedances than they do today (thousands of ohms, instead of ten's or hundreds of ohms). The higher impedance directly reduces the amount of power delivered into the phones, for a given amplifier output voltage. That was probably one of the reasons that the DH220 manual referred to using it with headphones, that amp being from the 1980's.

Also, from that manual:

Headphones are normally operated from the loudspeaker outputs, but are usually connected through a junction box which provides switching from phones to speakers. Such a box usually provides some added resistance to reduce the sensitivity of the phones, and thus minimize the likelihood of hearing component noise, because of the low setting required at the volume control. Some headphone boxes utilize a “common ground” system which makes it particularly important that you carefully observe the proper connections. While the black ground terminals can be connected together, the red ones must not be.

Some headphones, such as electrostatic types, are less sensitive and may need little or no resistance in series for normal operation. These could be easily interchanged with the speakers through the use of double banana plugs.

The AKG K701, with 105db sensitivity (presumably for 1 mW) and 62 ohm impedance, would be totally out of the question for use with that amplifier. The Sennheiser HD5801, with 97db sensitivity, 300 ohm impedance, and a "load rating" (which I would guess means maximum rated power handling) of 200 mW, might be marginally within reason if an in-line attenuator is placed between preamp and power amp, but I still would not recommend it. BTW, the 300 ohm impedance would reduce the amp's maximum rated power capability from 115 watts to around 3 watts, plus some additional amount due to dynamic headroom and margins that are provided in the specs.

FWIW, though, a way of connecting a phone plug to the output terminals of a hypothetically suitable power amp (although with less than ideal mechanical integrity), would be to use an in-line female phone jack, such as this one. Its housing would be temporarily removed, and three wires soldered onto the terminals within the housing, for the tip (left channel), ring (right channel), and shield/ground connections, respectively.

If the amp is not bridged or balanced or in monoblock form, the negative (black) speaker terminals on the amp will normally be connected to ground and hence to each other. So you would connect the L wire to the red terminal of the L channel, the R wire to the red terminal of the R channel, and the ground wire to either black terminal.

If the amp is on a single chassis but is bridged or balanced, so that the black terminals are driven with a signal rather than grounded, you would do the same except that the ground wire would go to amp chassis.

For monoblock amps, you would do the same except that the ground wire would go to preamp chassis.

Best regards,
-- Al

Are headphone junction boxes still available?

05-06-11: Almarg
If the amp is on a single chassis but is bridged or balanced, so that the black terminals are driven with a signal rather than grounded, you would do the same except that the ground wire would go to amp chassis.

Correction: The reference to the black terminals being driven with a signal applies to balanced amps, but may or may not apply to a bridged amp, depending on its design. In either case, with a bridged amp the signal wire would be connected to the output terminal that provides the non-inverting output signal, and the ground wire could go either to amp chassis as stated, or to a grounded output terminal.

Regards,
-- Al

Thanks for the replies. I'm not sure I'm interested in a DIY connection to my main amps. It would be a pain to connect and risk of shorting when pluging in and out seems to much of a risk with 300W monos.

My prime interest would be connection to my Pre amp line out to an amp specifically designed to drive headphones.

I guess to narrow things down does anyone know of a head phone amp where you can bypass the volume and drive directly off my preamp?

I'll poke around more at head-fi, but first look found nothing.

If I were you I would still connect the headphone amp onto the tape(record) outs if you have such. I would not want my headphone amp depend on the preamp volume settings. If you don't want the signal go to the main speakers just use MUTE (once again if you have it). It will not mute the record out.
In DIY area to build a descent headphone amp is a matter of within $100 that would match your headphones AND your preamp perfectly.
In consumer market area not all of them may please you for the budget. The ultimate champ in budget-no-option is Grace M903 which is also an excellent preamp and 24/192kHz USB DAC. I've also heard lots of goodies about Violectric.