There are many muting delay and DC offset protection circuits
published for speaker amplifiers, but I have not found one that
is sensitive enough for a headphone amplifier. Also, it would be
nice if such a circuit could support a wide range of different
amplifier designs, including those with active ground outputs and
fully-differential balanced outputs.
I often hear people complain about their amplifiers'
turn-on "thump" noise. Also, in some instances expensive
headphones have been damaged, either by the transient or by excessive
DC offset during a circuit malfunction. This calls for a proper muting delay
and protection circuit specifically for headphone amplifiers (and
applicable for pre-amplifiers, too).
In late 2004 I decided to design just such a circuit and to share it
with others in the DIY community. One goal I had was to keep the parts
count and cost reasonable. After several revisions of the schematic,
I built a prototype on breadboard for testing. A few more minor optimizations
were then made. I announced the design in early May 2005 on the
head-fi.org DIY forum.
- Version 1.0 - published May 2005
- Version 1.1 - published July 2005
- For the virtual ground splitter, replaced the BUF634 open loop
buffer chip with a OPA551 high current opamp operating as a
voltage follower. The OPA551 offers better performance in
this application and costs less than the BUF634.
- Version 1.2 - published October 2005
- Adopted parts changes from guzzler: Q5 and Q6 are now combined
into a single darlington transistor, changed the relay to a
Omron G6A-234P-ST-US-DC12, and increased R11 to 475KΩ.
- Version 1.2 addendum - October 2007
- Changed default relay recommendation to Omron
G5V-2-H1-DC12 or G6A-234P-ST15-US-DC12,
Fujitsu RY-12W-K or RA-12W-K for their
lower coil current specification. This prevents
false triggering and reduces Q1/Q2 power dissipation.
- Version 1.2 addendum - October 2009
- Added Tyco C93402 to recommended relays list.
- Version 1.2 addendum - April 2010
- Added Omron G6A-274P-ST-US-DC12 to recommended relays list.
- Version 2.0 - December 2010
- Adopted suggestions from
"agdr", with gratitude.
- Eliminated OPA551PA opamp from design and changed the circuit
to use a TL082CP dual opamp.
- Changed default transistor types to BC546B/BC556B and added
a 2N7000 small signal MOSFET.
- Use 100µF low-ESR electrolytic capacitors where 22µF
tantalum types were previously specified.
- Improved PCB layout.
- Properly support Molex KK series connectors.
- Moved "odd" mounting hole to the corner to make all mounting holes
symmertrical, and add more clearance around mounting holes to
accommodate screw heads and standoffs without risk of
- Top layer ground plane now covers entire board.
- Wider clearance between traces and ground plane to reduce
possibility of board manufacturing defects.
Main: ε12 Main |
Prev: Initial setup |