j-curve
100+ Head-Fier
- Joined
- Apr 15, 2002
- Posts
- 489
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- 11
Connect two sets of headphones to a single source and gain control becomes an issue. Sticking an inline volume control on one pair of headphones can help, but often has the unwelcome side effect of causing the other person's headphones to get louder when you turn yours down. Plus, those things are notorious for channel imbalance, dropouts and crackling sounds.
This circuit uses impedance compensation to give you independent control of your listening level. Being a stepped attenuator headphone volume control, you'll need two kinds of unobtanium to make it - a fancy switch and some resistors with values less than 10 ohms. Good luck finding them.
Switch 1 (SW1) is a 4P6T shorting (make before break) rotary switch.
Switch 2 is just a regular old DPDT with centre-off position, not fancy at all.
Connecting Up
Plug your 'phones into the output of the attenuator and set SW2 to the position which best matches their impedance. The input of the attenuator is then connected to the source in parallel with the "other" headphones.
How it works
The right hand side of the circuit is a fairly standard stepped attenuator, six positions separated by approximately 3.5dB steps, for a total adjustable range of 18dB.
The left hand side is the impedance compensation part, which diverts some of the signal to a dummy load consisting of resistors R13-15. As you increase volume via SW1_P1, the amount of signal diverted to the dummy load is correspondingly reduced by SW1_P2, leaving the total input impedance of the attenuator almost constant. For this reason the other person is oblivious to your volume adjustments.
Specifications
Minimum input impedance (with 32 ohm headphones attached): 35 ohms
(with 80 ohm headphones attached): 51 ohms
(with 300 ohm headphones attached): 76 ohms
Maximum output impedance (when driven by a 32 ohm source): 34 ohms
Insertion loss: 3dB (as compared to the both-headphones-in-parallel scenario)
Of course, adjusting the volume at the source still affects both listeners, and it makes sense for the person with the more sensitive headphones to use the attenuator.
This circuit uses impedance compensation to give you independent control of your listening level. Being a stepped attenuator headphone volume control, you'll need two kinds of unobtanium to make it - a fancy switch and some resistors with values less than 10 ohms. Good luck finding them.
Switch 1 (SW1) is a 4P6T shorting (make before break) rotary switch.
Switch 2 is just a regular old DPDT with centre-off position, not fancy at all.
Connecting Up
Plug your 'phones into the output of the attenuator and set SW2 to the position which best matches their impedance. The input of the attenuator is then connected to the source in parallel with the "other" headphones.
How it works
The right hand side of the circuit is a fairly standard stepped attenuator, six positions separated by approximately 3.5dB steps, for a total adjustable range of 18dB.
The left hand side is the impedance compensation part, which diverts some of the signal to a dummy load consisting of resistors R13-15. As you increase volume via SW1_P1, the amount of signal diverted to the dummy load is correspondingly reduced by SW1_P2, leaving the total input impedance of the attenuator almost constant. For this reason the other person is oblivious to your volume adjustments.
Specifications
Minimum input impedance (with 32 ohm headphones attached): 35 ohms
(with 80 ohm headphones attached): 51 ohms
(with 300 ohm headphones attached): 76 ohms
Maximum output impedance (when driven by a 32 ohm source): 34 ohms
Insertion loss: 3dB (as compared to the both-headphones-in-parallel scenario)
Of course, adjusting the volume at the source still affects both listeners, and it makes sense for the person with the more sensitive headphones to use the attenuator.
