Cool Stuff

Tips, Tweaks and comments from Dr. White

1:    A trap for the unwary worth mentioning if you build the headphone amplifier into a metal box, AND bolt the phono input sockets directly to the case ( i.e. no insulation ), AND power the amp via the rail splitter, AND you use a power connector bolted to the case. In this instance the power connector must be one where both terminals are insulated from the mounting sleeve, otherwise one of the power supply rails will be shorted to ground thereby blowing either Tr3 or Tr4. Any non split rail power supply , such as a plug top, used with the headphone amplifier must be truly floating and neither side of it should be connected to a metal case.


2:    Lowering the input impedance to 10k will give ( theoretically ) lower noise and will also reduce the output offset with the LM6171, but not enough so you can dispense with the output caps. The component values that I choose for my designs are a compromise which will work satisfactorily with the widest range of attached equipment. I generally use 47k input impedance because that'll pretty much work with anything. For my personal stuff I always use 10k. That's the beauty of DIY you can tweak your gear to suit yourself.


3:   Using a fast, high gain opamp in the rail splitter will make it stiffer ( ie lower output impedance ) and more responsive to musical transients. I would guess this wouldn't affect the sound very much when using "ordinary" audio opamps but will certainly pay dividends with an opamp like the LM6171.


4:   The AD8065 is a good compromise if you don't like output caps. It a lot faster than run of the mill opamps ( 400V/us vs 20V/us ) but not anything like as fast as the LM6171. I suspect too that most high speed fet input opamps have higher hf distortion than the LM6171 - unfortunately the manufacturers don't always publish data in this respect. The data that they do publish is always very carefully chosen anyway - for instance look at the distortion data; its always tied to the output level that gives the lowest distortion with each particular opamp. The LM6171 is really a composite opamp on a single chip; a current feedback opamp with its input buffered by a voltage feedback opamp. You get all the benefits of a voltage feedback opamp without the fall off in gain at higher frequencies. All of the opamp manufacturers have composite opamps in their application databooks but only National seems to make a habit of integrating them onto a single chip.


More to follow............