Copyright 2001 R.G. Keen. All = rights reserved.=20 No permission for local copies or serving from hosts other than=20 http://www.geofex.com.
The standard Class AB audio power amplifier allows for direct = coupling of the output of the amplifier to speakers. This is very good in that no = capacitors or transformers get in the way of the sound quality coming out. The = speakers are connected directly (more or less!) to the amplifying devices. This has = the unfortunate side effect that if an output device fails, this usually = causes the raw DC power supply to be connected to the speakers. Most speakers burn = out or are mechanically damaged by this very quickly.
Protection circuits have been designed to prevent this kind of = damage, and come in many flavors. They range from the simple - such as an added = fuse in series with the amplifier output - to the complex, with all sorts of=20 monitoring. The current favorite is a series relay circuit at the output = of the=20 amplifier, driven by some sort of DC detection circuit.
Relays have their own set of problems, though, not the least of which = is reliability. Relay contacts corrode, arc and stick over time. Worse, = each time they are tested could well be the last time they operate, so there are = some built in problems. Fuses are also problematical, as they must be = excruciatingly well sized to offer good protection, and when that is done, the fuse = resistance is modulated by the heat dissipated in the fuse near maximum power, so = there=20 are audible side effects.
Some recent advances in power MOSFETs make them attractive for = replacing or sidestepping relays and fuses on the output of the power amp. Power = MOSFETs=20 make good switches, changing from a totally nonconductive state to a=20 fractional-ohm resistor in nanoseconds with the proper drive signal. = Unlike=20 bipolar transistors, there is no offset voltage or rectification = associated with=20 a MOSFET's on-state conduction. As long as the current through the = MOSFET is low=20 enough to not cause a significant voltage drop across the MOSFET during = current=20 peaks, the MOSFET itself will not be audible.
There are some difficulties with driving MOSFETs as AC relays, = though. There is an inherent substrate diode in all MOSFETs that looks to the outside = world like a diode connected across the source-to-drain. This diode conducts = freely=20 in the reverse direction. This can actually help protect the MOSFET in = some situations, but it means that a single MOSFET can't block current in = both directions. However, putting two MOSFETs source-to-source can. The = substrate diodes then are alternately reverse biased on either signal polarity. = Suitable enhancement of gate-to-source voltage then gives us a bidirectional AC=20 switch.
The sources are always riding on the instantaneous signal level, = though, and both gates must be made about 10-12V more positive than that moving = signal level. This is very difficult to do for most floating drive = circuits.
International Rectifier has released several photovoltaic =
MOSFET=20
drivers. These units comprise an LED to produce light, and several =
photovoltaic=20
diodes in series. When the LED shines on the photodiodes, they produce a =
voltage=20
on the output pins of the unit.
In the direct replacement for an output relay, two N-channel power = MOSFETs are connected source-to-source and gate-to-gate. The drain terminals = are used=20 as the input and output of a switch. When the gate-source terminals of = the two MOSFETs have 0V between them, the devices are off, and no current can = flow, up to the breakdown voltage of the MOSFETs. When the gates of the MOSFETs = are several volts positive with respect to the sources, both MOSFETs are = enhanced, and current can flow. Modern MOSFETs can have a resistance from drain = to source in the milliohm region.
The photovoltaic isolator makes for a simple, easy, way to turn the = two units on or off because the voltage-source side of the isolator rides along = with the source terminals while the LED side may be tied to any level necessary = for operation. To turn the "relay" on, just force current through the LED. = To turn=20 it off, just interrupt the LED's current.
There are people who will instinctively distrust any active solid = state device in series with an audio signal. For those people, there are two = possible alternatives: a relay to "short" the two power MOSFETs of the circuit = above and=20 a second MOSFET protection circuit that interrupts the DC power supply, = not the=20 speaker output. The first idea is simple - relay ills are often caused = by the=20 wear and tear of interrupting an ongoing current. If we make the series = MOSFETs=20 conduct just slightly before the relay contacts close and hold for a = tiny slice=20 of time afterwards, then the MOSFETs endure all the voltage and current = stress=20 of actually closing and opening the connection. The relay serves only to = make=20 for a low-resistance path around any possible perturbation of the sound = by the=20 MOSFETs.
The second version inserts the N-channel MOSFETs into the =
DC power=20
lines going from the power supply to the amplifier itself. Each MOSFET =
gets its=20
own photovoltaic isolator. When current flows through the LEDs in the =
isolators,=20
the MOSFETs are turned on, and current flows to the amplifier.
This version of protection is most easily thought of as a very fast = relay that can turn an amplifier's power supply off in microseconds. While = the=20 amplifier circuit might have bypass capacitors that will still drain = through the=20 speakers, this will be a far smaller amount of energy than the main = power supply=20 with its large filter capacitors would feed the speakers, and so they = ought to=20 survive in good order.
In this application, the reverse substrate diode does not matter = because the voltage is always in the correct direction to reverse bias it.
This version uses two of the photovoltaic isolators because the DC = voltages on the gate/source of the two MOSFETs are widely separated.
Another variation of this approach is to use a P-channel MOSFET in = the +Vcc line and an N-channel MOSFET in the -Vcc line, and drive both gates = towards ground with the protection circuitry for "on" and release the drive for = "off".=20 This approach works, and suffers only in that P-channel MOSFET devices = usually=20 have a more limited voltage and current range than N-channel devices. = They are=20 likely to be more expensive as well.
However, if you're willing to trade more expensive power MOSFETs and = more extensive drive circuitry off against the cost and availability of the photovoltaic isolators, This circuit can work well.
The drive circuit is worth some discussion. To turn a power MOSFET = fully on, you typically need 10 to 12V of enhancement on the gate-source. = However, if you go much above this, the gate oxide can be punctured. Just to be sure = that the gate never gets outside the safe area, each MOSFET gets a 12V zener to = clamp=20 the gate voltage to no more than 12V. The resistor parallel to the zener = diode ensures that the gate is pulled to the source voltage if there is no = other=20 drive on it. 10K to 100K works well there.
The NPN/PNP transistor pair in the center have their collectors tied = to pull-down resistors to the MOSFET gates. These resistors are sized so = that the Vcc voltage minus 12V of gate enhancement voltage produces a reasonable = current, well within the zener's current and voltage rating. This needs = to be=20 sized appropriately for each different Vcc level.
The PNP transistor's base is tied to ground through a resistor, and = the NPN transistor's base is the control point. Both emitters are tied together = so that whatever current the NPN transistor conducts also goes through the PNP. = When=20 the NPN transistor's base is pulled above two diode drops, one = base-emitter for=20 each transistor, the bases of both transistors start conducting. The two = base resistors are a safety net that ensures that the bases won't be damaged = by excessive base current. Above about 1.4V, the control pin saturates the = NPN/PNP control pair. The same control signal turns on both the NPN (in common = emitter mode) and the PNP (in common base mode), and they both pull their = respective MOSFET gates into full enhancement, and therefore full = conduction.
The MOSFETs must be rated for the full voltages and currents that the = power supply can be expected to produce. For an amp with +/- 50V Vcc rails, = get a MOSFET with more than 100V, preferably more than 120V Vdsmax. Also, = since current rating is cheap in MOSFETs, get devices that ensure that there = is=20 enough current handling capability to not "choke" the output stage of = the amp.=20 If necessary, you may freely parallel MOSFETs to get more current = capability.=20 Since these devices do not handle any audio (other than that audio that = is=20 passed through the power supply) and since they are always either fully saturated or fully off, the audio performance of the MOSFETs is really = not an=20 issue. As long as the on resistance of the MOSFET is comparable to the = source=20 resistance of the power supply, this should be pretty transparent, = especially=20 for modern power amps with large power supply rejection ratios.
MOSFET current rating is an issue as well. Power MOSFETs come in some surprisingly high current ratings for the dollars you spend, and they = parallel beautifully. If you have a really high power amplifier, with maybe +/- = 100V supplies, you'll need 200V or more rated MOSFETs. You can simply = parallel as many same-type MOSFETs as you need to get a high enough current rating. = They will safely share the current they carry. An interesting point that = sets=20 MOSFETs apart from bipolar transistors as power switches is that = paralleling=20 additional devices does NOT require more drive current or power. All the = parallel MOSFETs you use will saturate as fully as a single one (within = the=20 limits of their individual transconductance rating, of course). All that = changes=20 is that the total gate-source capacitance that the voltage drive to the = gate=20 must charge and discharge goes up as you parallel more gate-source = capacitors.=20 This means that the transition between off to on and on to off again = slow down,=20 not that the devices are less fully on or off. Since the transition is = already=20 so fast that fuses look like glacial motion beside it, this isn't much = of an=20 issue. It's likely that little or no heatsinking is needed on the = MOSFETs=20 because they dissipate large amounts of heat only while they are = actually=20 switching - which they don't do very often.. Simple engineering caution = dictates=20 that you don't just assume that they dissipate no heat, but the amount = is minor=20 compared to the output devices. The sink's thermal capacitance (that is, = how=20 long it takes to heat up the sink itself) may be more important than the = thermal=20 resistance of the sink because the MOSFETs normally switch only twice = per=20 session - once on, and once off.
Devices like the IRF540 n-channel and the IRF9540 p-channel are good = types to do this with. Depending on the maker, the IRF540 is rated at up to 27 = amperes and the IRF9540 is rated at 19A. Two 9540's in the positive rail and = two 540's in the negative rail makes for a very efficient and = high-current-rated protection switch.
As to the circuitry that senses fault conditions and turns the = photovoltaic=20 isolator LEDs on and off, there are many ways to accomplish that, and in = fact,=20 any circuit that is a DC protector that drives a relay can be rearranged = to=20 drive the photovoltaic isolator LEDs as well. These circuits = usually=20 encompass a single-pole RC low pass filter with its rolloff arranged so = that the=20 highest-level and lowest-frequency bass notes will not cause the circuit = to=20 trip, but a DC level will cause a trip within a few milliseconds. For = any of the=20 circuits shown here, the detector circuit merely has to energize the = LED(s) when=20 there is no fault, and drop that current when a fault occurs.
It's worth noting that you're not limited to DC overcurrent sense as = a trip condition. Other good conditions that can be used to activate the = protection circuit are overheated heat sinks or power transformer, sensing RF=20 (oscillation) on the output, and so on. With appropriate logic (CMOS = works very=20 well here) these can help keep your amp alive.
------=_NextPart_000_0023_01D15435.1B9FC370 Content-Type: image/gif Content-Transfer-Encoding: base64 Content-Location: http://www.geofex.com/Article_Folders/ampprot/dcprot2.gif R0lGODdhgQIxAYAAAAAAAP///ywAAAAAgQIxAQAC/4yPqcvtD6OctNqLs968+w+G4kiW5omm6sq2 7gvH8kzX9o3n+s73/g8MCofEovGITCqXzKbzCY1Kp9Sq9YrNarfcrvcLDovH5LL5jE6r1+y2+w2P y+f0uv2Oz+v3/L7/DxgoOEhYaHiImKi4yNjo+AgZKTlJWWl5iZmpucnZ6fkJGio6SlpqeoqaqrrK 2ur6ChsrO0s7AXC7cAuQq7vL0IuLANx7MEwcYKz7ALyDG4xsbOub4Txde810XKwsnKzgvR1tkBxM bp1ALt3x/L37nB7BflGNXb+kPc6df64Pba0vD925BgHb8SvIa93AbdAYdlvYbgM9exSN4PP3byG7 gP/vIOaTgLBbwngeKYSc2FAgSYnuSlZ8mQOlTI8dl5UMqdLBSV8beRKr1lMkRo0tHQoNV/NhuYMf ccJ8WmNmyqkRwxG86dLoL5o+hTIDR3VpP5sCMzKb6k2eU6hsb0hNOhLpQLBDL37UyfWu3o7TuAXV C5IuwK6Al2pti7hZ0aZ98/LqOUzp2ZxXK4dtfNiw0bVlJwNuWPAY3MSkcRjWTJXy1sOq8ZJdfXkz 09ifIYzeFxmyNtSle9M4TTi1V6Ksj762PPIv2tnKORtXm7YxapS+q8cAXph47eWuVx4/2hzzcPBZ VfdDOFq39fUw3iHTOvY85OL0q+I9KN6f7Px868P/trqPSHDJ5xx7BnrQ32fwyIUPTubYd19kDPLV koR1UaiOeRJSeFGBB36oQYLCDXXVWCNOaGKBniF1WVEdKjNfYAZJBp9fGYGIoyYeErFjjj4W0mMQ Qf5IJCBD9mBXkUomciQPJi65CD9QTpnjP8VQieWHVo6zF37mfLlilmJiclZf773HEwnljcmmI1Km eaUJa7ZJJ5PonJkmnCLMWWefUZJXAp9+DjqLoIQe6oqhiC6aiqKMPkqKo5BO+omklF6qI6aarmLp pp5G0umnojIS6qimHlLqqaoKkuqqrvbR6quy4hHrrLbOUeuturqR666+HtErl7+mESyWT3JQ7LDA /yY7pZ4fMKvsEO64aiYI0Kp6rE4g3TnCtKveiGC01MRpSzy57Ckstt+EK+48M3bprXiDVcvumequ i2y7GADEpbPTVvtvuuTmO7CpEDVpr77u4olRgPMGXFi9Ahus7bgKmxRnnsL0a6+VUk5sccGjujTk tafOa9VP6cKI50br4HvvMgtfHPLLMId47rfbVmCyrLV+LNEv1Ga4M81V0DQ0xkUbPQXSOvO8NNNQ 3JQ0z1n1LPUKJFdtwdZZP3E1111X/HU25j5NzcFlN1Ee1j+WqtHa90Q9MoJAy21RuWjbDDLeQiod c71u+8332YHbnTDhQAg6eJVqNq74zHofbm3ki/9LTnGgkFtu275id8u5Do5urmUKCIf+bM2Zm476 DJaSfiDWsHPe6ezsuX1661AHvbfWuvvOO+Uq2L423J9rTbzUqSZfHezMX9zq873NnvvvIuPc+3XW S0yw8O1tHzz3dbslva65lk+a9Oj7DHr2UYE/eQjrI4Z+9X4HO39b8+f/abH8Q5W//9EJTElKnfti IsAxrSlZCXyJABtoLLpV7oCKUdwC5XQ8FtkAgku6YKAyuDK33E+CBvReCJsiLMLYzzhZ86CaQNiw FAYsXomTX/FIKD5RUY1bzkBTw5jFQccZrn0mrGFXZrixE6xQhzgs3OpylhN6scx0BKwimC5EwBL/ WXGLXOyiOB7jxTCKcYxk/KLMPkhBISyRiDfjFvZK2MYk4qqJ3TuZEh5IkDyGz4lyjOMbXMjGJxYh gU6Dourq6EY/8oqOexzfIBtYSEUCLod9q2QbAImuNFrye4PUoyE9N8FP9jEOmLShJr31vrwJbZVv pGQNr7fIIWayiD7EjViUGMRXWjJUP/OkJNlQylDa0ZNw0pgu+cgjXyYSlHBc5ijhEMxmOhKMxeTY MRvZSVb+kpG7EyUsL8nNQzKRmCm05vly+cxv8tKU3tzkGqLpyk1FspZ5WiM6nalOROozn3SAJzL7 909sImGe6eymKwn6x3Ayc5gCTdvUlFnQSQYU/6Gx7BwaafkCIGZrlpJcpzDx6U6liMGf+wSoaVBA LyVC9Jvxm+hKWXnPhoI0nppK3hqhqMJypmRHFL2mQnHYU5hhppo7jalPWVpSTxGvZ6LhGBJrKVFb tjOq2AzTUTXYMh/S0KhX7epCBcnJ4fFQq2iaIlX1M9WWlvRYc5oLCv9l1jv+dGyaRCnuxorELZ2V rUn9KoC2+cwjkhWpapyrQcHKgsbNRYop20oZq6jWwDw2i+ksB7nimgSSyrSm2kPVKTTbSsTa1U6m AK04TZrYP5XWsGdVaguox0OU4UaqVnUBBD/WILH4xEYb/Q1rI4va0f5mqBsDGD5t+kiGbMlMd/+L aIgm21vChnaaGNwgca/k1sqKkEfX1WnB1MbRvjpXvJiC1k3H9VQZSkcu0hVrMtPbsVtKtb2H3Sx9 MUfd8L4Pvrg97nalRVa9YhekvYqVaf06zheKLsDFzZh/N9hJ/oawufcFLoLTSt5L4a9+Ot2Ne2gE MjFm2CRh1FsxCTQxzxT4oy+laXlBx1UZeFSaZjDwb29MqPOFNKH2rW+NWaxN/SZYIRVWw4xdDAYb y5KdDEVkjDs73R5/QckWVTBG6fbksJ52y2MwsBWre+XvFJlYI7Zwl7XMZNFaOMsZLTOOrbA5/MGw yhiGppuX/GMoAzm4JD5jHo4c0DPrmcZ85ib/m217ZzqjwXlCLrSiWywHQCdaC7Dds2uJ9ug6SFrK XrAdlZvs5zeTYdNRLsNSCX3pTIf6DqTm8kj/y2lIXU1Rh35t2mpLVyOfdNKI8hp+Nc3MlLo6DM9r tTzJ9ut+BnvMgE1yTHh9qIPVbg+07qOxX/3sUqd6vKIWtEEFfOFRN0Pbx56pj/tZ4oE9FpzjDne5 t+NuUkIb2E6K94t33NpIz1vZSEr2pXWsh2v/IYC5PtlNa53aWAf8cvnmrJVptW9ePWjBDf8awoFH bmpDWsYVb6HGFU5vc/u227+6uHsz/nGRj1zVcjM560Aecny7Ds94czlKI85jZiN61YSzuUph/x5y SPJ8hAsHOr+9CuGNWzzlw4YV0pMe5J4z3d5OD2JQy+ZzMDfd6QONOtH/jPN6xI12U/e3dbJruaxf dOulofDXIW50AHLb438eEH/yI8SnQ6/sBc+72qfH927+veiDT58dwC25wkNc8fQLfKgZz4fzsq/o u/xus0IHeYk5De3wA1HmFypgjwkM8Z13k8pffvSEhT6lpC99I4Q96MNbGz5zd71q41sc6JqR1Sq2 pi0/b/tdM4yeqNjt8GUefES4h/WhQNpkDpx8VnXMnJ5w/m5IHn1WU0byyq8L8rMP/lsXMPzkt9v4 y49+lp8I++l/e3ibmsT1xtAvOm+/1L2bU//fT9+y9k++cZ26MvBXVt/Xf5FzfSfEH/sHVQU4Kwzk X8ulbgq4fgyoZk4UNxC4FxLIfRSoLOB1WfKFVrDHgXs3giWIHARogvAzax+mfimIebbxFezngkwz WTMYfkRhIc1mg7+Dg/xXZztIdtdzHr4EfEDYeELYUbVnhPeHckuIOgLnhC8YPkUYhTDRepNEhVXo QPV3QlqoO3YnG/7hhQYYdmOoL1BohjVXhmkYLWjIhli3hm9YcnEoh7vihnVIg3SIh7Zyh3voh38I iIEoiINIiIVoiIeIiImoiIvIiI3oiAAUXY94Q5LIg4DifZRIM/RXTkSFiWcoR8bkLJ3Yhp/pSH16 J4pNBi6ceIqryIqt6IqvCIsG4Wt5lIWxWAkiSFi1aIu3iF0qRB1AERyRuItsAn/VRFRPpYrDmGNj xTACOFjJqIx+0l/NiFfEd3zROChAUxODESBogYLYCI7hKI7jSI7laI6U0FameI5Dto6U0kNFtYkw woLtKI13YYwASI30uIwdFn/WiEr62Cf5B4r+qI4A6SPveHdwNX0GaT4MeSu66JARKZETSZEVaZEX iZEZqZEbyZEd6ZEfCZIhKZIjSZIlaZIniZIpqZIryZIt6ZIvCZMxKZMzSZM1aZM3iZM5qZM7yZOu WAAAOw== ------=_NextPart_000_0023_01D15435.1B9FC370 Content-Type: image/gif Content-Transfer-Encoding: base64 Content-Location: http://www.geofex.com/Article_Folders/ampprot/dcprot1.gif R0lGODdhgQLhAYAAAAAAAP///ywAAAAAgQLhAQAC/4yPqcvtD6OctNqLs968+w+G4kiW5omm6sq2 7gvH8kzX9o3n+s73/g8MCofEovGITCqXzKbzCY1Kp9Sq9YrNarfcrvcLDovH5LL5jE6r1+y2+w2P y+f0uv2Oz+v3/L7/DxgoOEhYaHiImKi4yNjo+AgZKTlJWWl5iZmpucnZ6fkJGio6SlpqeoqaqrrK 2ur6ChsrO0tba3trBaC7oAvA2+vLALyLMAx8YHwckNwrnKxjjEJ8EU0xPXGNq12hjNxczKwQ7v1s wEx8Hiw+bsNOko1dLjEN/7t9z/1Nrr6crd/PT189b/P4OUvwT8a/hCEGPljo8KHBg/gqQuhmDqI9 gv8I1UWMCO7iRJAr4JHUcHKdSmoTN1p8aY+eL5kNaDr42NJlzZEBu13rCU7gzJw4Q2YECNRnMH9E mxYzik7pz6FMEw6D2WoXTZs6GXKkKDJsx33fZHo8exVpToBg+5EVWi3qWGc8yXGMWjatWrhSMWI9 pfWs26Nt1a6rllGe0ZsG86IdzJaw0MQirQKljBkuZK50uU7e/Djy58+QS/8tFZgg57mHeyLe65Xk ua+Sly4jbDp1PM6r5Yb0vLaj75m4gbf1fTfl6U9ykTsEuZq1xMp6cy8d6tZsNOVQL1vHfXQq+Aip ywsu7jS5PL/LUV8fHD25y6Lkg4+XDh+7btj7dcr//8rUeP31h194+mGHHm2L5acYUu0B9h5ypiW4 0oQLMiZWYfTcZg6H932oYG2LBfhdVWvVg46HxAF4XoX5FWTfg5ps2GJkdt2ImYUhFuafcDhqhmN2 9kH0Xo6kjdaYV0E6tmSLTJaGV3cXyshcaNY1ONuOb7En25CvAbkliFtq11eZebn4y3pmjegYaaBV 1yCVnqz5YXXCmRRcOmj22FpddSpVH0PHbKURoAWmiaeCU6HlHZRcsidnpIE8F+OUkl6KqaVi7plp p5dSGmilno6KFaiVkYpqqqquymqrrr4Ka6yyzkprrbbeimuuuu7Ka6++/gpssMIOS2yxxh6LbLLK /y7LbLPOPgutNDVFS60iLYlabbZ5XIsaQuRpK2ljogSEzDzgRipuKOR2CFpQE7YZ5bmveKSuMvTa Zlu5+XrLIb7syssKveN6u6+HTxUsGrsrYgswcwaDIm6+Cx/8b78Kd4hww6ismIp3AZp48W1KapxP OiafjHLKKq/Mcssn85uYyzLPTHPNNrPMA8Mk31QSGzov9/POvPSsBsdC/xo0BklfYfTRvS5tAdRU COz000SjQXXVu0ptTRrpaq0r19h4DTPYuIpt7hncmn321WWszbataH9rxolxy+32E3PPsPfdl/TN cxRNHzG436sAPi0UxCHeM3eGQ5w3E4srMfnjWf9FTnnIRvhrecCYI5H15uV2frkKjPPtxOmkM6J6 2Xqnvrrnpk/R+uexj1L76ILLvNPNLt+uSu4Vj2G3MBwID3wjyCMPRPFDb8B88tba7oXz4hwvfSzL q5348yhlD8v2dXd/PfTgz0t9F9a7Ts35roj/NvnsR+1+6SlE78P6T5lf/+Hpc6E/3WUAf/2b1P+2 EMDhta+AHTugFhL4sAUyEDAOzAIEuUbACfYBfi+AlAsuiD0NmoKDHxQgDEDIPxGSgoQtEFiKLBYf pclvf99T4QorOAJ7YUxk8FFgDe8EtwHa8Iazi0G6JkeVF3rgWpDC4BBxh0MRHJGHCDJhCn3kPSH/ PlFdUWxIhVyoxA40hSgh3KLDiohA36FsbGY84/0okcE2zoGFjoijHONAR+XdsRN5RKPCwGSvQXnw Bnbcoxv6xjtpCIZcLpxfIdNmyEw80nSLpJjrgtiDSUaSbBakosGuYyW9aPIhm5QkCGxmAiR+kl9f o2H+SomJvaGyBKpk5OhaacUdjBKWZJjbzVIZMqmYUJTWSOQPeTkJWdbsDU5EJhwbYsxDltGZkeij HJpJzWp2kZPHzGYdt4m1aXpTj368AzbHSc436uGc6FwEC3VYOOKJs52JeOfQdtmOedLzEPY82Jl0 1Mkr7hMR76SRxXgYBnYO1BD91Fe/moHPFupz/6GDaOgOCxZRFiiUohWNXMQYFEEATpSjgLAmHkdK Uj+YFA4bTekfVspMlLp0DzCVpkBn+lLMjWxsGf1AS3FKU4/mkn42BWpJhXoYG4kGlEVyXM5kalQ7 WJRgw5vYxBAqOahGlQ5Tpcy+IGpJzmX1plsFnRrdMUNalu+gV+Vc5cS6BONQCKBlHYLqKkVHvtgo kBhrahOcuta6JuGudFNn17hHVsESgbAX6WJP3YbNxyo2rT4trLTqsK4fSnayWZSiZU+w2ea5smyZ 5WwRGEvKckp1tDTEpWmFgNrAGRYPpWVlYF8LW2A2VrUsdWVpr1qx0OI2tpTN4WoF+Nus1Ra3Of+c 5TtSBk61wYk2r2GuaNWKA+HadX9G0651b+vF7B6VLRj9LuF0S8jj3tMnkDTvw3aK3XfkwLvpNR4x P2tdjoVutp4Vb0mr616eDpW3lfXvS+MU4NQe9ITozeeBB5lgxpRFIQ2uAX0dzKfDmreRPgRthWlw YQtXWIfcvZEgqRZiYh0RsFrVYn35eMtbflVFMu5wgokLYjfOGLhXAaONA4xj1MXyTrrjcLsuFs8b f1jIXB3xPUscXA6zWLFBpjBmNfrkGM9VYiku1i6rbMTpSfCSJo4XjeWITzAzuJ4RPpVz7GTc+GL4 v/PrLHMlFswfi3HJVm6yRDaE3+Hmma0eXpL/bI1n4DlSRytt3i2eCd3gjCG6uGve1i8b/VUpF/pf XHaXhdIc1D6NrMuczaxbkaxAUK8TUZIObqPn3MhHJxG8TFZ0LhdCPlJvdV33yjMEK21rH46E0q8G Nq4dBUpa95m2LS52dAO9bHM2e7KFCmaRCizfREs7hbqOnaxrucPwZvvFwQ4pNwZs2m97clD9Hfec g/3T/Fp73aydJpfqbedam9NJ/NZSfsVzZvK2O+D5LniYV53Y76qa0+HJDG/mG+puAhniZEE1Qh9t 7oNva9rOvizDYRhupqK7gxF3ccdzXGSLt1XZRoymPBMub5MvMShSNrNAKH7u8cH8zjKfuQwz/75w DfeS43WFaLKXShURIaY3KMkTzoX+com7F2GdhvLKL55qPmsc6mKIN89V9MIq4lnWDoqgmklO1JOH muxnHruv2Xf2EqZd7ZZ+aNhvLplBT7jOPic3G+mOPjmLeO6Al527B5/zusG38E8VPMoTHz/Gg07r wP575FkUsylLHttx9nt7ewlokC9482g//OO53nWYnTrjpPd459/9+ah//Oqtl7vp9U1znKmPtGyv vUQdj3t8E9v3zwR+tPk+aeJvIu6/H/7IlZ9Mypfe+d1uumxSW/09Xvq5aySw8L/PtJEPCfq357zz +35ogwf0obWpomGuvXjys3zPu+VvcbNvff9NY539+2+1/JM/cOdHf/U3f+unZfiyd6KHcf9HgAEI gNJXb/gncWS3YiGnOQyoYK8ngNPmWl8wbAkyGYy2GXqGgSQodQV4SlAngSUoOBDIeuIGSSvIgorj gjVlSTM4IzXoWC+Ig5DAfDYIfqlnLjJoVD/4bBvUg+YHgxu4c9dUdWHnIHiXhJbngOq3hNt2dVZF aES4UEbofeW2gP7idlNYTGcFYFaYgmsHaVo4hmS4XV84Rxi3JvrxfG74dJv2N3a4WEcoCFxIfkCo h+jCh4FYKoNIiC8BiIf4IImoiEBjiI14D4wIiYUIh5NIJZJoiRaBiZmID5vIidvgiZ+IC6H/KIq2 QIqlSAuniIqyoIqrGD7Q1DKuSImyqGK06GWw+Ds0d1G2aHhitEy3xoO86EbH41zvVXYSIoww5lO/ aCT9F4zJWHzLmIuj9VZgBY3DiIcN+FZBeI3RV4nUKDLJ5ofdCDvfSI6z0IrnCEXmqI6vyI7t+D5R FH+kNI7w2Hj2J1tiU4/2CA04xERJV2YxIyL8aEDvaDdUB3JtSJA5ZZBpsmDWaIFYtZAq5Y8HuIYJ aXETSZHvuFdGxyh5NxwayZD42HMiKQmtuI8muYccqZIDw5ItCTkvCZNz8ogzWQnpaJMnWZM5qZMy yZOm5JM/mYdBKZQ3uZNFmU6uh5TdQpRL/6lNTemUj4CTUUlQR0mV/GSVV1kIU6mVhMCVXdmHWQmW BUmSYzlkUGmWDCWWaYmEaMmWHeWWb0mWSimXlvCVdblxcYmXbVmW1cOEe1mS5eeBfwmYY5aNsleH hUl0gQkGmOSSdZiSbaOXY5WYy5eYkXkrd3le3MgJKMZv12YXcaF5JKOZm/mMnWmRMzZ6DqWAwFOa omOCktRrrmZb7yWCq+dtazlYpwljsTZ7tLly/uc3ryk6mBl+bCIlujh2o6kxZuiczwmdsehb0Umd 1WmdepJJM2mc46WSSaaYrMib39lAsSme4xme5UlElYme6Ume6/mYnOmeXISC8Umf44R/2/9Zn41V fd6ZnwDEnz9QOf2ZUBJZnOcpoDQonK+0mgf6QO3Zjw7KoLRTjhE6oH9FoUJImRdaPdeJVrm3Mhpa NCP1ayBabj9ncPhJohpomMKHoil6hYTHoi6KcCcIdDKalzQ6ojbaBj+Vozq6BjxKmD6KmCs6YC0q pDRKhTF6pCVKpDZmpEvapKjnpFB6TSIqgBBGpVNjpQ/YPU8qpEDKpZNWSWwFmln6hlIHZ6fJXtnh SQRqprlFVmmKLR/IaIsDkW96poGpJHNaPnUqkV7qni21p0naQ5wmgniap392nYiKoYxqlI5ql5Aa qQDKSkQikIaBqZI6fTkzpprTNGSkqVv/x6mqAWXDFKah2pf5pEpFZnMYAahsGVrgZkWfeqqoepj9 uKqeaoxoaKsqGjUuFzhctihV5aq9anuMaaw7upjJ2qgmyqwn1YTPilhIKq3hFK3VOnTXiq3NGqXb qnPU6q1DCqPhOq3ISq7cOq7nmq3gqq5+qa3t2qDvCq9Y4HXzKlLyaq9VUK/5aoD8eqPm6q/0uqwB O6EAS7D6OrAHSzn9tmWvyq8Y5LD2uq8K+1f7NXcRi63LdbEUa4AaS5sci0Cql2sge6+lGlgYS66/ FW4fu0okS4PIRV2+BZ8uuzlwolxpSrPHGZEQmrMvi2Rh1LMSWlg9FqRBq21dym4ASGJa/4Z0R2ex Rns/ONuls7djrVWoUKtRZ5iPDMdjS4sgKLueP6O1nZFynjqb9Ia1+KopVZVlusqaFwe2kNqB3WWo b7swcWuroMqqR5cwPJu2fxtTgGutgim4g1m4UoC3aZu4ASsqSXK4PqtgWGqgj/tKY6uelNsO24e5 pilqKLK5KzmlmTqzn1tr1hMbo0u6ospaQbS40jqxqXu06Qq7ifa6s6txkWW7lKq2uftBDHtkdMW7 nie7wSu8kEe8upSwx0uXw6u8wWe8zVu8Ugq9ziu906u6zGu9x+qs2cu93eu93/tqkgu+2Tm+mYN5 e1G+hKREq9e67WpqEdm+6vq+zpi+p6d3GQtYv/mrv/vrLFgaI//Jv5qVfrwawAI8HAMya29SwPmn dxe5jQu6wJ9HgQnYpvgbwRL8diX2wAl6wZ3Vb0gCf37bwSNMwiVswrzCp/HbsyocrrfpV+EYGE97 wmkDhVgXnDMswKMnhhmJw0RVw2zIwz2cDw/5GHQowkKchkh8vUrMxE3sxE8MxVEsxVNMxVVsxVeM xVmsxVvMxV3sxV8MxmEsxqRbAAA7 ------=_NextPart_000_0023_01D15435.1B9FC370 Content-Type: image/gif Content-Transfer-Encoding: base64 Content-Location: http://www.geofex.com/Article_Folders/ampprot/dcprot3.gif R0lGODdhgQJlAYAAAAAAAP///ywAAAAAgQJlAQAC/4yPqcvtD6OctNqLs968+w+G4kiW5omm6sq2 7gvH8kzX9o3n+s73/g8MCofEovGITCqXzKbzCY1Kp9Sq9YrNarfcrvcLDovH5LL5jE6r1+y2+w2P y+f0uv2Oz+v3/L7/DxgoOEhYaHiImKi4yNjo+AgZKTlJWWl5iZmpucnZ6fkJGio6SlpqeoqaqrrK 2ur6ChsrO0tba3uLm6u7y9vr+wscLDxMXGx8jJysvMzc7PwMHY0HoEAdYA2CfaHtwL0AQA0O4T1h Td5tgH2eDt7e7X6wHuJNrt0ujnCvrm/Oz67vQZ4Ggfk6mEM3roI8gi0Y6gmX4OC8gRIESkzhsFo8 hP8bvxVMp4IeN3zXInbseM7ixBEZMyJ0GZCjDZh2IBa89++guHAkr7nD6bNnPpI8L5YEitRfRKIg 8cErubGovZM6fwbV+NGk1qJDmcLjGu+pyKUgsY4si7XBOqBHe1q96fYp3Kr2vv78ulVs0H5Ou6a9 GrZvzsF7o4LVilYu27eFbKI82Tbs46iQ6aFUB7Us5sn1OG+GOtVoYs6S0ZrOnLXa5qk+DU/WrBGz aNgea6tlwJppZqG6WQ91TZt2aJO5R1M2TvX1UdMXczf/+xy1Y6GBPoOuLD318eyIsRdnHlz7dcjg Zy+fjhy1+s64xY9v+vH7zePsAQcWq5R8e+6+wXP/T+3Ye+PJZ5REKe1222nZsWdddPuFV557fzSY GED91bXTfdTZN9xr39WnnGX0eQTRath1JZeE/xW2YHwAqXedgbYFpmCNKrIo33rxiddheDJ66J9o OFnXnV7b4XgihjPuwyRjME6Yn4MnIodeRRECSdpfPkInIGVVUijhkAlSeSWECupk5pNd6tcdYgX6 lySP84W4JXxBwlnljHR+JqV5Ip4JKJt+nBXcn/zNqWZtPb73oZ4CGmpTZyVieRpDIub4Zo76oblm l+YlaiOmcN65KaKP1rkcqRQS6V6mpB6q5Ypm9gcIg+kFSGCeMJq4nWCw1vejcft0iiClOkbgW65l /566a3KVkvepSrb5yiihqJaWap1vTjkgVXlay6udwxZbaKnsCIvohn3YChhSANIIFltLgfmWXb3h dq9f8IJGHb0ZQluOYlnVa2qT5pK710iL4fuioFdt+3CR95FIrcH/sAitkfZma19lcflar1NGXvzN vx0vZjJN0qwcg8osvwyNyzDPvIzMNN+Mc84678xzzz7/DHTQQg9NdNFGH4100kovzXTTTj8NddRS T0111VZfjXXWWm8NtbUycQ32HMlaGXbZcozd2rnwisTXxma/LcWLsok7J7uc2gx33jqcBXHd8wpH rN6CE8E3bK7+ariNg9+Cd3W/qS0Zb3jFqO7iFf/xg3nmmm/Oeeeefw566KKPrrlapJ+Oeuqqj85D 44u7TrYQKsMuaIoOu0K73rk/sPsJs6+k2oGWO9P7mEH8nk3JtoM6PDHFP3j8NsDHenvzwzzvqA8O cc7BWsZbjwz21Pew/ffSQ98m+MaIn772sY9Pgffoq18M+zfu/X77CplfPf2+2N8/HJRvfhaQX/b8 FwwAKm4HAzxg/Pi3QAT2QoHMy4HwHNY4A8JPghNcAQUNohr4ZRCCFeRgLij4we7pT4MYYOH9TFe5 CJbDhI3x4A+4R0AZJiSHJZzW1zKQQhpuAYXue6HiRshDl7FKZEwiGX4CKMSa2LB1f9uhA2eYxBb/ 2u5uO0pVpHQYxToQkYHBQxYJsehAJZLlV4L5orvCWKspWjB4MTyiCrN4PkXFCHCcgiIczybHG9RD XnhE1upA98PH0UVGBqLWH/cwxhvmp5CPLFoktTfJNFbyaJdsXeYSCcZN4qyTIbkYuD5JwiCKsoOl bAh6WAVGF0qyjqu0BCl9pyNY9lCWRexhLSNxSxQQiIf68+VMvNWttNHyl+sKpBwXVcxiqnIgGpuU G5mZiGDiMpdnVNE0gWgWaxbmm9gcojNL2bCF5M+IvDtdC8N5HlmVUxDaJKMZiblOEwySSiab5yDq ib97XjGf/vwFQOco0A0StKCsxAjhFopEhgrj/6ACvFzpNvBBQsnLXm3rqETFds5ePjB5HhzWML3G zo+agaKCzGNMShq5LgJsoCpFA0uPWcDpCTOeyWKihlJa0zHctAbI+0BGeSonlPoxqGIYKg0OCVUW iPOkK2QqHJxahqMOSDGObJgxrfoFrJKBnGB1hFiFWtZdnLWpadXF7tzJBrK2VRFydUNd53qIu8YV r4ybB+rWoFe+/jMbqlNDYAXruICsLqubQ+wraHfIrHbTsaPI3WLHOlnKhmKtXOClZknBWXPi87Og CK0WPEvazYbUsJlNLSdMmwXUutYTpHRXgDrb2tlmorYkAoNsdbsJ3s6lH5Cjwm+Bu9tz6uWLHP8z bm6RWwnhemkwh2XJc6E7CenCp2/OHS12balc4iizn92l6Xeju9o0HPe82U2vTa/LXrO69wzrjS8k YIuF+tpXvq2sIkWqm1Pv7vcR0nUdgPcn4AE3QrtrU9J4RVbcG8JXwYhgsN/8xtzbSjjBFKZreMvY nH5m+Ksz0G+HK/zhbv3omox8KIdPbAWoNtaKOy0jdYlil6YQl8QyMDGMy2tdGm9zpLh98Y+nID5L zde8UfDxkZusz4QO2a4TfvITkizlKF+1ylZuApaFrOU34LDLXvgy75bMWjKjtQRK7i9I1cxWNmdZ zmKEc5xJ0GaH1tnOSIgsmz+HZvXy2bdhxin/HQ486CmL4LCIJl+i3dyxQL+zont+dJT55AIzG/rN lpazLiGt06dKsdN4Vk6mC03UUZN60W+EgaZTXelVk5RRp6ZzS2MtaxC6SJWvFjWucx03VPv60MCO sbBLrOpiB9vWmwakspF87B4n+9kUuXWpKU1samMUode2Nqd5/Oy7QC6TQe52s50N7nADLpnMXjS3 99xo4K543cwWWDcZfdrLgMyM8TZIV2/X6n6Hb93BMvenFIrv2AaLJ7EUOJij82mJORJsIc5SvT2j rw0lPL8LVyYa2ynjZcaPVrQ2jNcYnu5yom3EXlRorfPrRbxouLVvtW46nRVh7zR3ritPplRc/96Q fHvcRR8HJTgJ+y5TRUbiOW8rVVXMZXMbe+iXKXrUmbzOJR5r6zF3rIM14+AtvnvqnvJjA18qzI3S UXlSwmuvkf0QRBKWddrGukthnYez61rBXPX5jo0a7ZZBcqGT5rtwpmqnWZf73L828pkpvOKpOgnt GOOW0V09+DlXe/ERSzrbJ1b5rEUeqUt9X8T1Puxs7zLkIo+dW3pl+meVre/kmvzei/Uvt6UtlKCO w9/7PLfkEOkwsCr90zaesMS1vOTeBunMhwCi4K8n+iSLtOjfzatr8ks3zfd94GR38sdYhjcy5X1B e97ygpu/xoee+BEuJP60wJ/rXLNZ7zCUff+fVp/RcPWtwyfNF62yRSOzfk4jM8VzgHaEbUQWVnUH RDsRgO0CgTs2gU8UYfanTgtodUXmgOejUReofC22Jm8nbQFGaB3ogTRCOaQxb61mfAyIdzDYBf8n bwSHLnsEdXLSbqm3gaKFggoRKZOzMGozb+0CdEfXfZpnTs/3gxsmdTxIePnWek2YhICngVF4BRNI hUVAgoJngmFASFvohJwHhUo4g9vDejSIXV2IeV+4ZmC2bWKIYE8Id3OIWWZ4d3IIh1ZYhY8nWXjo hnroh2RYh3QkOmcIaHPnOYLIhi+QgT1IdoN4e5fngI34cnaXcn2GhXZIiXVniUHXiS/IBKj/F4d7 iIKfKFWmCFSOODnb5xKkuHmSCGd+hmeJqGehGG+y0XGbmDBRBYja9okUJS29hHJtZzpjuIXBGGhO lmky54pWkouMGHirGItGJ3AadXB6JFLJOI2imIf843DYGEoRF4PcuIOYWI2S+H/gEkvUmHYX9YPK 2Hvfo4aUR0VHmGvyeIuud4LIWI9lpY/sN0P/aI+OVoCkFpCKxm9gCH2Z+GgJ2Y2Od2UuJocQeY5i JohcGJEOCYltQJBiaJF0iJEZ2ZAXGU2htmUkCX5pGIboiIR2NWMquVfzKGgSKZOxJWl3ho83OYM5 yZA2yZORqJCAdXVBCW00+V5AaZRHuY8e/1mUSwkF+JWFTwmVTiCVQumOVcmUAumUSqmVVumT/biT XwlkQ5lmLkmWURmWDeiVaakEV1kFzOiWXraWZUaVc2kEcFmWWYmXS6CXW3mSfQmYG3mHaCmYSfCX y2aYh/l+ddmTbcmYsuNXdLdSdxmZyEgJcnmZFAlekLmZ29g9f4Ui24WV3viZkmlUhcUmTEiXnnma 9uRvqvksSdF0eWmZrzl2GCWbHLJ8rMmZgYmbmqiIh9gmkzJOYOmawdmH7lZ0xklywrmYynmPTamO S2d70DmW0gkEidlkMamdrUmdKfmdismV3jee5GmWVHaeUIaUyqNC//iR65mcoYhz6Rg98v85ke3Z PncRdm1DhI0Wn/gJnAVpjWWHJiJokAI6ijk5SMzVeXSzc7mpoI2pnwMTghc6YtE4oX7pk/ihhYTR FxCWKCzZfxuKnen5jYrXkSlqoqgZni9JiKrIoi26nY7piDOKozSaoOUJmr8ogzp6ny86nSCHSvYJ pJhpkrApi6Z5kEfqhUKqpAVaik4apDy6o1JqpFQapYTJeJo0pVraoyI5pFj6gKQDpqAIpQGlJZp5 piWZphJqKAPapm9po5dYI2w6pzVaoc13QdmZp7/Jpa7mndL0p3FZp4oGixxZqFuapIVofV66qOz5 pm2oL0vapJF6pYGqXC1JqJiqlnuaiob/aIotQaKY46kwqqmC6lWQiqrVJ6enuqIq6kmrKkJ3ZGN+ Cqv0OYmtNHlNQqveZKsoAiK5GqwoWosIokuJGlG1SqwEKqalVjihAlHFCqzNuquNanDpwU546lkB GpncyRHQtK1F2a3Weq3PSmfRymTcmkrmSq3YanNDKGDsSknumqN8WKVkWnjMaq/7aqwSyqr+Wq39 eq/OOqbt+qUDS7CcmKolOKrvqoCLoFTRuUrgypwPm7AYxF+7xFcWi5JsZ6oZG7ESi0zjJqJ+oRTL Q0MeK6s+Col4qmoimH29hYPeyjIsi6+xyrAam03SR3LqRzDLB0c4a7C6Oq3z6X0mAnF3/yKzj0S0 52qprTqyjKAw2DJe85KhU2g9pcq1Xeu1IQdDXyu2YxuyB7uwwGCzZ6u2a8u2beu2bwu3cUtgAAe1 cktX2WhgdisJE6d/D4Nj45K2etuYt8VFI1Kziiq4oyavW8VyPxe4ieti5Jd+ENpHiAu5nDYWlJN7 h+ebl+u5nwu6oSu6Tcipo1sK5Gi6pVV1o1ebqbtgQoh43+e6JEt01tSKswtMeUF6TIq7fJC5fOJ+ vSu8w0u8xftaefG3EyM3pWu8NWFS6TIqfNm8gPW8RxK9vDu99LW726eCXvW42Stt4jRTz2m54MuQ 4pt43IW95rtmjZS8zKF27Cu/80u/9SZrv/eLv/mrv/vLv/3rv/8LwAEswANMwAVswAeMwAmswAvM wARbAAA7 ------=_NextPart_000_0023_01D15435.1B9FC370--