amb
Member of the Trade: AMB Laboratories
- Joined
- Apr 1, 2004
- Posts
- 4,933
- Likes
- 41
I present to you the ε12 ("epsilon 12"), a turn-on delay muting and DC offset protection circuit designed specifically for headphone amps. It prevents the turn-on "thump" and turn-off noise present with some amps, as well as provide protection to the headphones in case the amp's output DC offset goes awry during a malfunction.
The ε12 can be used with a traditional 2-channel amp, a three-channel amp (M³, PPA, Pimeta, etc.), or a four-channel amp (fully balanced stereo outputs), and will provide for DC offset protection on all available channels. This circuit is intended to be powered from the amp's DC rails (up to +/-30V) and has its own voltage regulation and virtual ground reference, which is adjustable.
The headphone outputs from the amp is routed through a relay, which keeps the headphones disconnected until after ~3 seconds delay after power up. The circuit will disconnect the headphones if any of the channels being monitored produces in excess of 70mV or DC offset (positive or negative), and will reconnect the headphones if the offset drops to a safe threshold. The 70mV trigger point is chosen to be sensitive enough to protect most headphones from damage under normal circumstances, but not too sensitive to get false triggering. Compare this to many DC offset protection circuits for speaker amps, which would not trigger until over 600mV. The relay as specified is a high quality Tyco/P&B RT series unit with 90-10 silver-nickle contacts rated at 8A, but you can use another 12V DC relay as long as the coil current comparable.
The adjustable virtual ground reference pot let's you set the "zero" reference upon which all channels are compared.
The circuit is designed to be simple, low in parts count and cost, yet provide all the functionality desired.
I have a working prototype on breadboard, and I invite your feedback. If you wish to build the circuit, feel free to make your own PCB or use Point-to-point wiring on a perfboard. I have no immediate plans to offer pre-made PCBs, unless there is sufficient demand to justify it.
The name "ε12" has no special significance, other than that I tend to name my DIY projects with a greek letter and a number.
The ε12 can be used with a traditional 2-channel amp, a three-channel amp (M³, PPA, Pimeta, etc.), or a four-channel amp (fully balanced stereo outputs), and will provide for DC offset protection on all available channels. This circuit is intended to be powered from the amp's DC rails (up to +/-30V) and has its own voltage regulation and virtual ground reference, which is adjustable.
The headphone outputs from the amp is routed through a relay, which keeps the headphones disconnected until after ~3 seconds delay after power up. The circuit will disconnect the headphones if any of the channels being monitored produces in excess of 70mV or DC offset (positive or negative), and will reconnect the headphones if the offset drops to a safe threshold. The 70mV trigger point is chosen to be sensitive enough to protect most headphones from damage under normal circumstances, but not too sensitive to get false triggering. Compare this to many DC offset protection circuits for speaker amps, which would not trigger until over 600mV. The relay as specified is a high quality Tyco/P&B RT series unit with 90-10 silver-nickle contacts rated at 8A, but you can use another 12V DC relay as long as the coil current comparable.
The adjustable virtual ground reference pot let's you set the "zero" reference upon which all channels are compared.
The circuit is designed to be simple, low in parts count and cost, yet provide all the functionality desired.
I have a working prototype on breadboard, and I invite your feedback. If you wish to build the circuit, feel free to make your own PCB or use Point-to-point wiring on a perfboard. I have no immediate plans to offer pre-made PCBs, unless there is sufficient demand to justify it.
The name "ε12" has no special significance, other than that I tend to name my DIY projects with a greek letter and a number.