Zarcon Dee Grissom's Idea Page
EV9
Updated 21DEC2011
M.C.C.HPA17r16 iconNOT RoHS compliant logo 2.Home > IdeasHPA17r16 Intro.
HPA17r16 Operating Environment.
HPA17r16 RFI filters.
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HPA17r16 Tests & Afterthoughts.
M.C.C. HPA17r16 Operating Environment.
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Idea#017 Page 2 of 6 -> On this page; Operating environment below - Thermals and chassis - Iner Cabling - and Miscellaneous Thoughts.

Operating environment.

If only we lived in that perfect world. Where our audio source only sent out the intended audio with no higher frequency additives. A place where our power was always exactly the specified voltage with nothing else included. An environment with absolutely no magnetic or electric fields to induce stray voltages...
Take "The Red pill", and welcome to reality. [partly from The Matrix]
Our 5v is out of spec, and the 12v is taking the form of modern art.RFI Hell
We'll try to stand, Where no one dared to be. - Wolfsheim.

The power source is laden with all kinds of 4kHz Pulse-Width Modulation RFI with 60Hz overtones, that no amount of filters at the inverters and/or power supplies will completely remove from the power bus. During normal operations the power bus can drift from 14.2VDC to 10VDC during emergencies, if not less. The audio source is coming from a computer with all kinds of 33MHz, 66MHz, 100MHz, 400MHz, 800MHz, and 2.2GHz square wave induced noise. The 45 ohm dipole antenna headphones will provide a nice path directly into the feedback circuit from the cathode-ray-tube electron-beam-cannon monitors. The amplifier will be mounted to a console less then a foot from two 22 inch CRT monitors and there degauss windings. Aside from being inside a solar flair, I can't think of a worse RF environment for a low power analog circuit.

So the first thing I did, was grab a copy of the latest NASA standards, and use them as a foundation of how this amplifier should be built. Specifically NASA-STD-8739.1A, NASA-STD-8739.3, NASA-STD-5005C, and NASA-STD-4003, Along with the ARRL handbook chapters on Safety and station grounding.

Thermals, and longterm operations.
2M amp
It has been my experience that the last thing considered by product manufacturers if not ignored, is how well there products will keep cool when put into an industrial environment under 24/7 load. As convenient as the 8-pin dip may be in compact designs, there is no way to keep it cool enough to survive prolonged periods of abuse. To say nothing of the effects of thermal noise on a device operating near it's maximum operating temperature.

The TO-220 package is a much better choice, as it can be bolted to a massive heat sink with a copper thermal spreader, like the finals in some RF amplifiers [Example 2M-amplifier left].

The Enclosure.
Consept Art
I originally wanted to get a project box similar to the Mirage B1016G amplifier [Consept Art left], with face plates I could drill My own holes in (Without preexisting holes). Finding one is similar to locating a Harddrive/floppydrive bay project box (1.0in x 4.0in x depth), Not to be found any where. I settled with the Hammond 1455T2202BK [PDF 169,841 bytes] enclosure for my build, with the internal layout below.

PCB placement
Prefit test
All the boards are to be mounted to a single copper sheet with copper washers as standoffs. This should reduce ambient RFI effects, and serve as a heat spreader to conduct heat form all the boards directly to the chassis.

The Cabling.
Input Volume harness
SATA Cable photo
O.K so the SATA cable idea didn't fly the first time, the connections are the same for the "two conductor plus shield and single drain" cable I did use for each channel. I diced up three different SATA cables, and found that all of them were solid core wire based. Not something that I think would survive years of exposure to "launch induced vibration and ignition overpressure". I don't expect to be launching any rockets by my house any time soon, if ever, my foot dose have a tendency to thump when I'm listening to good tunes.

The other aspect of SATA cables that can be a curse, is not all SATA cables have isolated shields for each data set. A single foil shield over both channels or a shared shield drain between the channels, will not provide the best stereo separation. So dicing up a clear jacket SATA cable that you can see separate shields and drains inside may be best for making a volume harness.

The ground/common for each channel is kept separate the entire length of the harness, and is not used to ground the encasing of the volume potentiometer. This ensures good stereo separation on the input of the amplifier. I went through all that work, just to end up with the inductors on the input RFI filter measurably talking to each other above 5kHz, it only proves that there will always be room for improvements.

Test-Point and LED harnessV-mon and vol box
I decided on LMR-100A for the left and right Vref circuits, as they are somewhat critical to the performance of the amplifier. I wanted to minimize any noise or induced AC voltage fluctuations on these circuits. DC offsets are not as critical as AC fluctuations with the decoupled input and output design of this amplifier. [LMR100A 191,416 bytes PDF]

The Vreg and Op Amp Power (OAP) cables are 18AWG two conductor plus shield cables. The current draw of the V-mon board and non-critical lack of precision of the dual color LED's dose not justify using overkill gage wire here, just well shielded cables to prevent signal contamination of nearby components.

The LED harness consists of three sets of 20AWG three conductor twisted sets. The power going threw these wires is extremely low (less then 3.5mA red, 1.2mA green, at 2.1VDC), balanced, and practically DC. No shielding should be required if the wires are kept away from the terminal posts on the volume potentiometer, and at right angles to the resistive tracks in the volume potentiometer. All that considered, I am still tempted to completely enclose the volume potentiometer in a small shielded box.

The rest of the power and output wiring is 12AWG, 14AWG, and 16AWG stranded "THWN or THHN" wire [84,901 bytes PDF]. I chose the excessive gage wire to keep inductance and voltage drops to a minimum.

To mimic eons of wire degradation, I used (at least ten year old) CD-ROM audio cables to make the Test-point and Volume knob wire harnesses. These cables will be replaced shortly with the specified cables above, now that the preliminary testing is done.


Miscellaneous Thoughts.

Why not just use a Ferrite Bead; There are different types of Ferrite material, with different "Sweet spots". Some ferrite beads work better at higher frequencies then others, and others work better at lower frequencies. To block everything beyond the audio band, I would need to use a combination of all of them, with the disadvantage of overwhelming accumulative inductance affecting the audio signal. "O" and the noise would not be stopped, it would only be reduced, not an effective solution for my needs.

Can't one capacitor do it all?; No! like inductors, different types of capacitors deal with different frequencies better then others. For power filtering, Electrolytic batteries are good at DC to low frequencies. Polypropylene/Polyester capacitors are good at the realm between the electrolytic and ceramic capacitors due to there smaller farad values and better higher frequency traits. Ceramic (COG or NPO) are excellent at the upper bands. Ceramic capacitors are limited by there significantly small farad values, and the reactance and wavelength of the capacitors leads and traces connecting to the capacitor accumulatively. At these super short wavelengths, Microstrip or Distributed Element filter capacitors are the next best workable solution, even with standard circuit board FR4's horrid performance compared to Alumina substrates.

"Audiophile" Caps; If a manufacturers data-sheet(s) omitted or misrepresented any of the fundamental information needed to design the circuit boards and/or simulate the response of this amplifier to RFI, the manufacture was black-listed. I needed to be able to lay out the boards (pin diameter, spacing, footprint size, and dimension tolerance), and figure out how the design would respond not just to audio, but from DC through 10GHz. I needed to know the impedance at 100kHz if not higher, max ripple current @ 105c, ESL, Leakage Current, etc. Coincidently, this black-listing extended to ALL components selected for this amplifier (Some inductor manufacturers omitted "pin diameter" information from there data sheets). By omitting or fudging information on a part, the job of designing a project becomes more difficult. I'll go else ware to do business.

Class-A Op-Amp biasing; Considered and tossed out before the chip selection process was finished. I'm not going to waste Mission Controls battery life for a feature. The battery life was also the reason for using rather high feedback loop and input bias resistor values.

Built in EQ or "Base-boost"; Again the audio source is a computer with it's own plethora of audio adjustments. The volume knob may get adjusted once on a blue moon for some super quiet audio sources. An EQ, Base-boost, Cross-feed, or whatever sound-tazmic effect is one more circuit to fail at the worst possible moment.

Switched Capacitive Filter or DSP filter; I like the filters, however. It is powered by a digital circuit, the very kind of circuit that is creating the noise I want to filter out. "O" and ditch the LM386 output chip, please. 0.2% THD at under 100mW and quickly rising to well over 6% THD at 200mW is condemning to say the least. The Sony MDR V600 are 45 ohm, The MDR V6 and MDR 7506's are 63ohm. There all 500mW per channel headphones, The LM386 can't even drive these headphones! Perhaps in another project, if it can do more then just EQ the audio (three channel parametric EQ or Expander/compressor/limiter would be more useful), with a much better output chip.

LMH6321 or other SMT buffer; I chose not to use this and other buffers for the same reason. They require extensive external circuitry dedicated to each buffer, just to make them work with a single power rail. They limit my cooling options to what I can provide on FR-4 circuit board alone. And they require greater then 13.8VDC supply (+/-15VDC, or 30VDC with mid-rail), and/or limited output voltage swing (+/-20mV p-p), to equal the performance of the BUF634T bolted to a heat sink at 10VDC supply(+/-5VDC). To say nothing of the significantly less then 180MHz bandwidth of a lot of the SMT buffers.

Beside, how do you hand solder the tab of these TO-263 devices to a circuit board without killing the thing. The chip is thermally bonded to the tab.


Why avoid RoHS compliance?; I expect this amplifier to work 24/7 for many decades, without failure. For tin plated component leads, the tin plating must be alloyed with at least 3 percent lead (Pb) to prevent the growth of tin whiskers. RoHS bans the use of lead for any purpose in electronic parts with the exception of specific low temperature solders.

I quote, NASA-STD-5005C(5.11.3.1.4.8) Tin.
a. Tin and tin plating shall not be used in an application unless the tin is alloyed with at least 3 percent lead to prevent the growth of tin whiskers.
b. For critical GSE, lot sampling shall be used to verify the presence of at least 3 percent lead.
*GSE is Ground Support Equipment.

I quote, NASA-STD-8739-3(6.11) Solder.
6.11.1 Types and Usage. All solder used for tinning and solder connections shall conform to ANSI/J-STD-006 (Requirement). Flux-cored solder shall be either composition SN60 or SN63 containing flux types R or RMA, or equivalent (Requirement). Solid solders (no flux) for use in solder pots shall be of the same composition (Requirement).

I quote, NASA-STD-8739-2(6.12) Solder section 2.
Solder paste shall be Sn63/Pb37, Sn60/Pb40, or Sn62/Pb36/Ag2 composition.

Tin whiskers shorting out CardRailTin whiskers shorting JumperPostsTin whiskers on component leadTin whiskers growing on variable capacitor plate
What is worse for your health, inhaling Asbestos fibers or inhaling Tin whiskers? I'd rather do neither.

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