Zarcon Dee Grissom's Idea Page
Updated 22DEC2010
M.C.C. HPA17r16 Intro.
The Burr Brown based Mission Control Headphone Amplifier prototype.
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First I would like to say that I have no intention's of mass producing this amplifier nor providing project kit parts, I simply do not have the time to do so. I built this amplifier for my needs exclusively for me. The design of this amplifier is based on the Burr Brown BUF634 Datasheet, and influenced by the PIMETA v1 amplifier by Tangentsoft. The primary purpose of this amplifier is for radio communications, the last thing you want with long distance DX is more noise or distortion added to the signal.
HPA17r16mb Audio amplifier stages
I spent a lot of time familiarizing myself with audio amplifier concepts at and forums befor selecting what chips to use. After looking at different hi-end audio chips and the PIMETA, I looked at the design of a lot of different microwave Low-Noise Amplifiers (LNA) before laying out the circuit boards. This lead me to build an amplifier that takes advantage of many different techniques.

HPA17r16 glamor shot
HPA17r16 front
HPA17r16 back
HPA17r16 Guts

I had few specific needs for my amplifier that no off the shelf amplifier provided. First and for most was the ability to operate directly off the on site 13.8VDC Negative-Common-Ground power buss. Second was the ability to provide superb audio quality while repelling any and all RF produced by the computers and there displays. Third was the ability to stay on 24/7 year round for the rest of time without overheating. Last was, I needed to be able to build this thing on my kitchen counter, so surface mount stuffing is not going to work for me.

This amplifier was never intended to be portable, as it is intended to be bolted to a Mission Control console. "O", with little air circulation, so there goes the idea of separate heat sinks inside the chassis. The chassis will need to be the heat sink for the amplifier.

Headphone Amplifier Overall Schematic
Quite some years ago (2005ish), I started going through many different combinations of chips. The seventeenth combination was the dual op amp [midpoint reference buffer, and amplifier stage] driving a BUF634 as the finals. The schematic went through fifteen revisions before I started laying out the circuit boards. At this point in 2009, the design shifted from built something that works cheaply, to this has to work as there is no other options left. Resulting in the final "M.C.C. HPA17r16" design.
[Full History here]

Notable post design changes.
I looked briefly for a rough guess of how much gain to use from line level input to adequately drive 45ohm headphones. My search turned up all kinds of results from +6db through +40db and higher. The BUF634 data sheet had +21dBV of gain in figure 5, so I went with a 1k and 22k ohm resistor set for +23dBV of gain. After the first power-up test, I realized that was excessive, and repeatedly reduced the 22k resistor until it was down to just 2k ohms. Then I increased the 1k resistor up to 2k ohms for +2dBV of gain. The rest of the gain of the amplifier is in the form of milliamperes delivered by the BUF634 chips.
feedback loop resistor
Yes, one 2k ohm resistor in each channel is actually two 1k resistors soldered together, I ran out of RN55D 2k resistors [Left]. I added a 220pF capacitor to each op-amp socket, as the sockets were a late temporary addition [right]. And the power filter divider/shield between the RFI filter and the Ripple filter never made it into the prototype.
220pF cap in socket
The input RFI filter was altered to keep it from reflecting the DAC frequencies back at the sound cards output stage. The output RFI filter has been temporarily replaced with an approximately -24db/octave absorptive 700kHz low-pass filter. The original output filter had an SWR spike at 15kHz, that caused a +7db increase on the output at that frequency range.

[PDF schem] T.B.D.

Geometry? Or Differential Geometry?
<NULL< - ^Ideas^ - >PG2 (operating environment)>

Home > Ideas > HPA17r16 Intro.
HPA17r16 Operating Environment.
HPA17r16 RFI filters.
The Main Amplifier Board.
HPA17r16 Vmon & Sources.
HPA17r16 Tests & Afterthoughts.

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