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Em7
09-26-2012, 12:00 PM
For those who want LED-based footswitching without the hassle of installing a battery, I am currently working on a battery-less LED-based footswitch modification for the 2-Channel amps that works with the existing TRS jack and footswitch. This solution will involve performing a completely reversible modification to the amp.

Em7
09-29-2012, 11:54 AM
I dropped my new MC58 off with the good folks at the PTC yesterday, so I took the opportunity to give Doug a copy of the schematic, the initial board layout, and a few components. I need to give him an updated board layout because I committed an engineering cardinal sin. I forgot to measure the space that is available to install the small circuit board that carries my switching circuitry. I had to revise the layout to make the board smaller.

I fabricated and populated the revised board last night. I also found a good place to cleanly tap the positive rail of the low-voltage supply that powers the relays. I need this voltage source to power my circuit. I removed all of the wires from the footswitch TRS (Tip/Ring/Sleeve) jack and soldered them to my board this morning.

The challenge that I had to overcome with the 2-Channel is that the stomp switches energize relays. The low-voltage negative supply (V-) connections on the relays are switched via the stomp switches. One cannot insert an LED in either of these paths and expect it to work reliably. As we only have three conductors to switch two circuits, one does not have luxury of being able to run positive and negative supply connections out the to pedal.

With that said, I can now say that I have in fact solved the battery-less LED footswitch problem on the 2-Channel. The photos shown below were taken as I was testing the circuit in the amp.


Here’s the eyelet board that I fabricated last night. It carries the portion of the circuit that is installed in the amp:

Front

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/AmpCircuitEyeletBoard.jpg

Back with insulator

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/AmpCircuitEyeletBoardBack.jpg


My solution requires rewiring the footswitch TRS (Tip/Ring/Sleeve) jack. Here’s the stock TRS jack wiring:

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/TRSJackBeforeMod.jpg

I had to tap the positive rail of the low-voltage supply to power my circuit. The low-voltage rail is 7.5 Volts DC after the four-diode bridge rectifier forward voltage drops.

Here’s where I tapped the low-voltage supply:

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/LowVoltagePostiveRailTap.jpg


The next step was to move the existing tip, ring, and sleeve connections from the TRS jack to my board.

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/AmpCircuitWiredIn.jpg


Here’s a shot of that shows that the channel LED will be lit on the footswitch when the lead channel LED is lit on the amp:

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/ChannelCircuit.jpg

Here’s a shot of that shows that the reverb LED will be lit on the footswitch when the reverb circuit has been activated:

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/ReverbCircuit.jpg

I still need to wire my circuit to the TRS jack, drill the holes to mount the board in the chassis, and install LEDs in my footswitch.

Dirty Bob
09-29-2012, 02:27 PM
Very cool!

Em7
09-29-2012, 10:35 PM
I mounted the board to the side of the chassis. There isn't a lot of excess real estate inside of the 2-Channel. I also took ownership of the footswitch TRS jack. All I need to do at this point is to install LEDs in the footswitch.

Here' s a shot of the board installed in the amp:

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/AmpModInstalled.jpg

themike
09-29-2012, 11:05 PM
It glows!

http://ti.arc.nasa.gov/m/profile/frank/FRANKENG.JPG

Em7
10-01-2012, 11:04 AM
I would like to add that the power MOSFETs and the resistors that I used to build the prototype are extreme overkill for this application. I built the prototype from parts that I had on hand. The board would be much easier to build with low Rds(on) resistance TO-92-packaged MOSFETs and 1/4W resistors. However, I have to say that the TO-220-packaged power MOSFETs that I used do look cool in the amp.

markie
10-01-2012, 11:22 AM
Nice!

rugerpc
10-01-2012, 11:41 AM
http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/AmpCircuitEyeletBoard.jpg

Let me guess.... LM317s ?

Em7
10-01-2012, 01:03 PM
Let me guess.... LM317s ?

Nope, they are logic-level power MOSFETs with an ultra-low Rds(on) resistance. As I mentioned above, they are extreme overkill for this application. However, they are what I had on hand.

Dirty Bob
10-01-2012, 01:17 PM
It glows!

http://ti.arc.nasa.gov/m/profile/frank/FRANKENG.JPG

In terms of technical expertise...this is about the sum total of my input as well!!!!

Em7
10-07-2012, 05:48 PM
I decided to build a new footswitch with a TRS jack to complement my battery-less LED footswitch mod. The hole through which the cord strain relief bushing passes on the stock 2-Channel footswitch is too close to bottom of the pedal to install a TRS jack. I still need to label the stomp switches, but I thought that I would share a few photos of the new footswitch with the amp.


Look ma, no batteries!

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/Em7_2-Channel_Led_Footswitch_Interior.jpg

I used a Switchcraft 12B TRS jack from my parts bin to build the pedal. The white nylon shoulder washer isolates the sleeve of the 12B from the enclosure. There is a thin flat nylon washer on the opposite side of the enclosure that prevents the jack nut and metal washer from making contact with the enclosure. I could have used a Cliff-style jack and saved myself the trouble of having to use a shoulder washer, but an all-metal Switchcraft 12B will easily outlive a Cliff-style jack in this application.

The stomp switches and the balanced TRS cable were salvaged from the stock footswitch. I covered the metal 1/4" TRS plug that I soldered onto the cable with a 4" length of heat shrink.


Clean channel selected/reverb circuit off (both LEDs off)

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/Em7_2-Channel_Led_Footswitch_Clean.jpg


Lead channel selected/reverb circuit off

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/Em7_2-Channel_Led_Footswitch_Lead.jpg


Lead channel selected/reverb circuit engaged

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/Em7_2Channel_LED_Footswitch_Lead_and_Reverb-1.jpg

Woundtight
10-07-2012, 06:09 PM
Great job!

rugerpc
10-07-2012, 08:44 PM
Very, very nice!

Em7
10-07-2012, 10:35 PM
Thanks guys! Developing this mod was a lot of fun.

To be completely honest, I hadn’t given thought to modding the amp to support LED operation in the footswitch until Experience 2012. I was surprised to discover how many people wanted to know when PRS was going to offer a 2-Channel footswitch with LEDs. I did not care for the battery-based approach, so I thought about the problem when I returned home from Experience and came up with this mod. I wanted to design a minimally-invasive solution that used the existing footswitch TRS jack.

Em7
10-12-2012, 06:25 PM
Okay, I promise that this posting will be my last installment in the battery-less LED footswitch mod saga. I know that this stuff is about as interesting as watching paint peel for most people. For those who are interested, I replaced the 4mm LEDs in the footswitch with higher mcd-rated 5mm LEDs that are mounted in less-directional holders. I also labeled the footswitch with black on clear P-Touch labels.

The customary gut shot:

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/FSW-Final-Gut-Shot_zps737f9e1f.jpg


Clean channel selected/reverb circuit off:

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/FSW-Final-Rhythm-Reverb-Off_zps19fc5c60.jpg


Clean channel selected/reverb circuit on:

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/FSW-Final-Rhythm-Reverb-On_zps8904a368.jpg


Lead channel selected/reverb circuit off:

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/FSW-Final-Lead-Reverb-Off_zpsf7f3db51.jpg


Lead channel selected/reverb circuit on:

http://i699.photobucket.com/albums/vv356/tonestack/2%20Channel%20Mod/FSW-Final-Lead-Reverb-On_zps97826c78.jpg

Em7
11-17-2012, 08:49 PM
Several people have asked me to post the schematic and board layout/wiring diagram for this mod. I have refrained from doing so thus far because modifying an amp usually voids one's warranty.

With that said, I am releasing this information for non-commercial use with the understanding that those who choose to use it accept all responsibility for their actions. I will not be held responsible for any damage to life or property that results from the use of this information.



Basic Control Circuit Schematic (there are two of these circuits in the mod)

http://i699.photobucket.com/albums/vv356/tonestack/BasicControlCircuit.jpg



I used a pair of obsolete logic-level N-Channel enhancement-mode power MOSFETs to prototype the mod. The part number is HUF76137P3. A pin-compatible substitute for the HUF76137P3 is the HUF76633P3. In reality, just about any logic-level N-Channel enhancement-mode MOSFET with a low Rds(on) resistance at 4.5V will work. For example, a 2N7000 should work; however, one will have to change the layout slightly to accommodate the different pin-out of that TO-92 device.



Board Layout/Wiring Diagram

http://i699.photobucket.com/albums/vv356/tonestack/BoardLayoutAndWiringDirections.jpg

LSchefman
11-17-2012, 09:02 PM
Just out of curiosity, what kind of battery does the un-modded circuit require?

Em7
11-18-2012, 09:45 AM
Just out of curiosity, what kind of battery does the un-modded circuit require?


The footswitch that most 2-Channel owners received does not have LEDs. Doug was offering a battery-based footswitch as an upgrade. The upgraded footswitch uses a 9V battery to power a pair LEDs. To me, it made no sense to have to use a battery to power LEDs on a footswitch that is plugged into a powered device. 


The challenge here was to find a way to power the LEDs and switch two relays using only three conductors. My solution takes advantage of the fact that the relay low-voltage supply minus one LED forward voltage drop is still high enough to turn on a logic-level MOSFET-based electronic switch. Logic-level MOSFET-based switches are used extensively in embedded systems. Embedded systems are computer systems that control external devices. The embedded computer, usually in the form of a microcontroller, turns MOSFET switches on and off, which, in turn, turn relays on and off. This type of circuit allows computers that operate at TTL voltage levels to control high voltages and large currents.

The creative and non-obvious part of this mod was piggybacking the LEDs on the control voltage. The control voltage is sent out to the pedal on the sleeve connection. It returns from the pedal minus one LED forward voltage drop on the ring and tip connections. The ring and tip connections on the TRS jack are wired to the MOSFET gates, which turn the MOSFETS on and off. The MOSFETS turn the relays on and off. The resistor (Rgs) that is wired between the gate and source pins on each MOSFET serves two purposes. The main purpose of the Rgs resistor is to limit the amount of current that the LED can draw from the supply. The secondary purpose of this resistor is to provide a discharge path for the gate when the control voltage is removed, ensuring rapid switching.

LSchefman
11-18-2012, 11:00 AM
I think your mod is actually pretty cool!

I'm sufficiently lazy that if I had one, I'd just have gotten a battery-snap adaptor for my pedal-power brick, and installed the switch box on my pedalboard.

But your solution is pretty elegant.

docbennett
11-19-2012, 07:50 AM
I have a question about this whole process that is "legal" in nature. It is no way intended to cause problems, and please do not interpret it as such. But, it is such an obvious question (to me) that I have to post it in this forum. And furthermore, if I'm not mistaken, Les as an attorney who specializes in music production so he might have some answers already in his back pocket.

Here we go.

With regard to Mark's (EM7's) mod, as described in this forum (and on the VR forum as well)....can he "patent and/or copyright" his scematic design and method of solving this "LED problem"? And, if so, would that prevent other companies (such as PRS) from being able to offer the exact same or similar mod/option on future amp footswitches without a "royalty relationship" of some sort?

I don't want to get into the pros and cons of copyright/patent law as it pertains to this mod. I was just very curious as to whether this mod is eligible for patent protection, and if doing so would prevent other amp designers from utilizing a similiarly designed "non-battery powered LED footswitch" without providing Mark with royalties.

Thanks for any clarification...again....don't want to get into the motivations behind such legalese...just curious as to whether this is something that can be protected under patent (or equivalent) law.

thanks.

Em7
11-19-2012, 11:54 AM
While I could attempt to seek patent protection for my invention, I have no plans to do so at the present time. One of the reasons why I chose to disclose my design in a public forum is to make it more difficult for a morally-challenged company to seek patent protection for my work (it becomes more difficult for another party to patent a circuit when prior art exists).

My addition of the “non-commercial use” clause in the posting is to prevent others from even thinking about claiming my work as their own. While I created this mod as a service to the PRS community, I expect anyone who wants to use it commercially to extend the professional courtesy of contacting me first. I will more than likely grant remuneration-free use of the circuit with the caveat that ownership is not claimed by the person or organization requesting use. I have already unofficially granted PRS royalty-free use of the circuit. If PRS feels the need to have a formal written agreement, all they need to do is to contact me.

With the above said, I may be forced to seek patent protection because the United States is moving from a first-to-invent to a first-to-file patent system in 2013, which will alter the definition of prior art. If there are any patent attorneys or agents on the PRS Forum who would like to help me prepare and file the patent application for this circuit pro bono, I would be eternally grateful. My goal in seeking patent protection would be to prevent an unscrupulous person or organization from preventing me and others from using my invention license-free. Sadly, patent trolls are a fact of life today, and I seriously dislike feeding trolls. :D

docbennett
11-19-2012, 12:07 PM
While I could attempt to seek patent protection for my invention, I have no plans to do so at the present time. One of the reasons why I chose to disclose my design in a public forum is to make it more difficult for a morally-challenged company to seek patent protection for my work (it becomes more difficult for another party to patent a circuit when prior art exists).

My addition of the “non-commercial use” clause in the posting is to prevent others from even thinking about claiming my work as their own. While I created this mod as a service to the PRS community, I expect anyone who wants to use it commercially to extend the professional courtesy of contacting me first. I will more than likely grant remuneration-free use of the circuit with the caveat that ownership is not claimed by the person or organization requesting use. I have already unofficially granted PRS royalty-free use of the circuit. If PRS feels the need to have a formal written agreement, all they need to do is to contact me.

With the above said, I may be forced to seek patent protection because the United States is moving from a first-to-invent to a first-to-file patent system in 2013, which will alter the definition of prior art. If there are any patent attorneys or agents on the PRS Forum who would like to help me prepare and file the patent application for this circuit pro bono, I would be eternally grateful. My goal in seeking patent protection would be to prevent an unscrupulous person or organization from preventing me and others from using my invention license-free. Sadly, patent trolls are a fact of life today, and I seriously dislike feeding trolls. :D

Based on your post above, Mark...let me add a thought. This is primarily with regard to the phrase that is bolded and italicized above.

Back around 13+ years ago, when we were pursuing patent protection for a medical patch that would deliver a formulary of medications designed to maintain relapse prevention from opiates...I was confronted by a patent attorney who shared the story of the patent for some plumbing invention...that was never pursued to completion. The guy who was my potential investment "angel" at the time had found this patent in some old file somewhere and had the application updated and filed with the US Patent office. Long story short...he ultimately received patent protection for a device that was invented by someone else, and whose patent application had been abandoned several years prior to the re-opening of the claim. And, furthermore, he made A LOT of money from said patent.

Since then, I have been curious as to patent law, and how inventors can protect themselves from "patent predators". That led to my question above concerning patent protection for the inventor...Mark (EM7).

LSchefman
11-19-2012, 01:22 PM
I have a question about this whole process that is "legal" in nature. It is no way intended to cause problems, and please do not interpret it as such. But, it is such an obvious question (to me) that I have to post it in this forum. And furthermore, if I'm not mistaken, Les as an attorney who specializes in music production so he might have some answers already in his back pocket.

Here we go.

With regard to Mark's (EM7's) mod, as described in this forum (and on the VR forum as well)....can he "patent and/or copyright" his scematic design and method of solving this "LED problem"? And, if so, would that prevent other companies (such as PRS) from being able to offer the exact same or similar mod/option on future amp footswitches without a "royalty relationship" of some sort?

I don't want to get into the pros and cons of copyright/patent law as it pertains to this mod. I was just very curious as to whether this mod is eligible for patent protection, and if doing so would prevent other amp designers from utilizing a similiarly designed "non-battery powered LED footswitch" without providing Mark with royalties.


Bennett, while I'm experienced in copyright and trademark law, I have no experience in patent law. In principle the two are related by the "Patent and Copyright Clause" of the US Constitution, which recognized these concepts in the original Constitution (not an amendment) in 1787. Article I, Section 8, Clause 8 of the United States Constitution empowers the United States Congress:

"To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries."

Just as a songwriter has exclusive right to do certain things with his/her work, so does an inventor. If Mark's footswitch is patentable, he certainly has certain exclusive rights.
I don't know to what degree making the schematics and photographs public affects the exclusivity of the rights.

Several companies have had powered LEDs on their amp footswitches. How these work would be researched, and most patent attorneys are also engineers. I'm simply not qualified to have a clue as to whether Mark's is original and patentable. I stick to the easy stuff...mostly recording and composing music!

Em7
11-19-2012, 09:53 PM
The control circuit, when viewed as a whole, is definitely original. To the best of my knowledge, one will not find it on any commercially produced guitar amp schematic. Most channel switching circuits that provide power to LEDs in a footswitch use bi-polar junction transistor driven LDRs to provide signal path switching. Randall Smith pioneered LDR-based channel switching with the Mark IIC. Mesa used relay-based channel switching on the Mark IIA and the Mark IIB, but that switching circuit has a major design flaw. Randall Smith wired the LED circuit between the relay coil V- connection and ground. The sleeve is wired to ground, and the tip is wired to the V- connection on the relay coil. With the switch contacts closed, the V- connection on the relay coil is connected to ground. With the switch contacts open, the led and the current limiting resistor are wired in series with the relay between V+ and ground. The LED imposes a voltage drop that reduces the voltage available to the relay coil. The current-limiting resistor combines with the relay coil resistance, which, in turn, prevents the coil from reaching the energy level necessary to build a magnetic field strong enough to close the contacts on the relay (this circuit is the origin of RED=RHYTHM). This circuit had poor switching reliability. The Mesa Boogie relay control circuit is shown at the bottom of the following schematic:





http://i699.photobucket.com/albums/vv356/tonestack/MarkIIB.gif



The LED current limiting resistor is the key to stability in my design because it serves as the gate discharge path when the control voltage is removed from the gate. The small charge on the gate is quickly dissipated to ground, placing the drain-to-source path in a high-impedance state. With the drain-to-source path in a high-impedance state, the connection between the V- connection on the relay coil and the supply V- connection is effectively an open circuit. The snubber diode that is wired across the V+ and V- contacts of the relay dissipates the back EMF that is created when the magnetic field in the relay coil collapses. As I mentioned above, piggybacking the indicator LED on the control voltage is the non-obvious part of the design. You will not find that part of the circuit in any amp textbook.

One last thing: Les is correct. Patent attorneys are engineers and scientists who have attended law school and passed the bar exam and the patent bar exam. Patent law is one of the few areas of law that requires attorneys to possess specialized domain knowledge outside of law. The U.S. Patent and Trademark Office also allows engineers and scientists who have passed the patent bar to practice patent law. These people are known as patent agents. Patents agents can file and defend patents. However, patent agents are not allowed to render legal opinions. It's somewhat like the British solicitor/barrister model. It's probably the only part of our English Common Law-based legal system that retains two classes of legal representation. Like the PA/MD relationship that we find in medicine today, many patent agents work with patent attorneys. I have considered preparing for the patent bar exam on several occasions, which is why I know about this stuff.

LSchefman
11-19-2012, 11:36 PM
I have considered preparing for the patent bar exam on several occasions, which is why I know about this stuff.

You should do it if you have interest the law, Mark.

You certainly have the ability to communicate your thinking clearly, you write very well, you're obviously very bright, and you are even-tempered in your approach to a problem. I have a friend who does patent work, and it's a wonderful career. He loves it.

I enjoyed my law practice for a long time, but had other goals I wanted to pursue, which is why I got into music. I always kid around and say I was a round peg in a square hole, but I got a lot of satisfaction out of the practice for quite a few years.

docbennett
11-20-2012, 06:59 AM
Reading Mark's posts, and experiencing his incredible breadth of knowledge....as well as his ability to use it to develop new technology....was the sole purpose behind my post regarding patent protection.
I would hate to see anyone benefit from Mark's creativity and knowledge other than Mark.
I am somewhat familiar with the patent process, having attempted to pursue a medical patent in the late '90's. Mark...from the tiny bit I know, and from what I've seen...you would probably be a fantastic patent agent in the field of electronics....specifically amp design and your area of expertise.

Is this a career that individuals can aspire towards...or is being a patent agent the "side effect" of working in related areas? In other words, can someone actually make a career out of having specialized knowledge and using it as a patent agent...I can envision a specialist being hired to pursue a patent on behalf of the inventor....working in collaberation with the patent attorney.

Sorry if this became a diversionary hijack...i find the topic to be extremely interesting and I find Mark's knowledge to be somewhat breathtaking...as an individual whose electrical knowledge is limited to setting up a good stereo system and using black electrical tape to join wires together :-)

Em7
11-20-2012, 11:23 AM
Les:

I will probably take the patent bar exam when I get closer to retirement. While I have downshifted career-wise, I am not quite ready to hang up my technical spurs.

Truth be told, my undergraduate and graduate degrees are in computer science, not electrical engineering. However, I attended college back when computer engineering did not exist as a separate program in most colleges and universities. Computer engineering was split between the electrical engineering (EE) and computer science (CS) departments. One either received a BSCS degree with a concentration in computer systems design or a BSEE degree with a concentration in digital design. The goal of this cross departmental program was to prepare students to become computer architects, digital logic designers, and system software designers for computer system manufacturers. In the mid-to-late nineties, a schism occurred between the two academic fields that originally combined to form the field of computer science (computer science used to be a branch electrical engineering on one end and a branch of theoretical mathematics on the other end). The electrical engineers in took their ball and ran. My undergraduate concentration now forms the core of the computer engineering program in most universities.

With that said, my goal in becoming a patent agent would be to help young software and digital device companies patent their ideas and defend against junk patents. Junk patents are becoming a major problem in the area of software engineering because large computer and software corporations are amassing huge patent portfolios. For example, LSI Corporation was awarded a fairly broad patent (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&co1=AND&d=PTXT&s1=7028023.PN.&OS=PN/7028023&RS=PN/7028023) on a commonly used data structure known as a "multiply linked list" back in 2006. The LSI patent is the poster child for junk patents. It should have never been awarded by the U.S. PTO. Prior art existed for decades before LSI applied for the patent. If this patent were to be enforced, it would bring the Internet to a halt. Linked lists are the foundation of most of the software in use today.

veysel
11-27-2012, 07:58 AM
Hope you can help lot of people Em7 and develop their knowledge in making their own unique ideas just like what you did and become a good example for those aspiring people who only just began in their careers.