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Post by leonski on Sept 4, 2024 23:33:57 GMT -5
I've seen 'tape on' accelerometers sold somewhere.
Failing that? Maybe use a phono cartridge?
Some kind of piezo electric crystal (a ceramic phono cart?)
I would expect Sub Harmonics at 1/2 and 1/4 of resonance......6db down and greater....
Phono cart thru phono PRE should give a direct amplitude / frequency output.
Other more scientific sensors will report in 'G' which would just drive me nuts.....
No substitute for measurments....
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Post by marcl on Sept 5, 2024 14:27:37 GMT -5
Indecent! I’m mounting the first speaker, the center one, on one of the boxes. I started by dropping it in with the box horizontal, aligned and pushed in the four 5mm bolts, then I used wood screws for the other four positions. They are IN Decent! And that tool was the best thing I bought in two weeks. So then, I tipped the box on its side and put flat washers on the bolt, and I started the locking nut … I planned on using the Vicegrip and that turned out real funny … I grabbed the nut, and the magnet about pulled the Vicegrip right out of my hand like “okay, I got it!” So the bolt went in fine. I can go with this approach using the wood screws to hold it and then the bolts for another level … belt and suspenders … Also note the wiring through-hole with grommets. I have to run both drive wires to hook the drive coils in parallel, and servo wires in series to the servo coils   |
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ttocs
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Post by ttocs on Sept 5, 2024 15:29:18 GMT -5
I grabbed the nut, and the magnet about pulled the Vicegrip right out of my hand like “okay, I got it!” So the bolt went in fine. So, you've got an automatic vice-grip holder! |
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Post by leonski on Sept 5, 2024 17:37:55 GMT -5
I've got a set of Magnetic Inserts for SOCKETS. That may help?
They are available in SAE or Metric. Maybe even BSW.....British Standard Whitworth
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Post by marcl on Sept 5, 2024 17:55:27 GMT -5
I've got a set of Magnetic Inserts for SOCKETS. That may help? They are available in SAE or Metric. Maybe even BSW.....British Standard Whitworth It ended up that I found the right size socket with a long shaft that was hollow enough to accommodate the bolt. So first speaker is mounted and I have the tools ready to go for the next two. All I have to do is open up four holes in each to let the 5mm bolt pass through. I'll have that done tomorrow ... then on to wiring! |
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Post by leonski on Sept 5, 2024 20:38:40 GMT -5
I've never done THIS, but if I were drilling holes thru MDF? After drilling, I'd finish in  Something on the inside of the newly-drilled.....I'd like it like a thin epoxy, which will soak into the chips. |
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Post by marcl on Sept 6, 2024 2:01:32 GMT -5
I've never done THIS, but if I were drilling holes thru MDF? After drilling, I'd finish in Something on the inside of the newly-drilled.....I'd like it like a thin epoxy, which will soak into the chips. It couldn't hurt. I think the biggest issue drilling MDF is the exit wound wants to chip and break out. You can see where it broke when the wood screw came through too. I mitigated this when I used the hole saw for the connector and wire holes (1" and 3/4") by starting from one side until the center pilot drill bit just touched the opposite side. Then using that as a guide drilled through the surface of the opposite side just 1/8". Then when I finished the hole from the original side the plug came out and hole edge was clean. For the mounting holes it would have been better to do them on a drill press before assembly. But in the end they are very secure with the flat washers.  |
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KeithL
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Post by KeithL on Sept 6, 2024 10:45:17 GMT -5
Sometimes you can also reduce break-out by the specific choice of the drill bit and drill you use. For example there are drill bits that have a relatively flat cutting surface, a point in the center, and a pair of "barbs" on the outside edges. So they at least attempt to cut through around the circumference of the hole more cleanly. (I'm sure they have a technical name but I don't know it.) The other thing I've found is that speed helps... Most modern hand drills, and even drill presses, are designed to be able to deliver a lot of torque at slow speeds. On most hand drills nowadays they spin faster as you squeeze harder on the trigger. And there is a tendency to "drill slowly and carefully". HOWEVER, you will usually get a cleaner hole in relatively soft materials if you run the drill at a higher speed. (I assume that, because it's cutting faster, it is pushing less.) And, of course, "don't push... let the drill do the cutting..." And, of course, make sure your drills are SHARP... This matters more on both hard materials and on materials prone to "break out"... Drills can be sharpened... But they're also cheap enough to just replace when they get dull... And, if you REALLY want to be obsessive... You can glue a small solid wood or hard plywood block or plate on the back side and drill through the MDF into that... (This will also enable you to use longer wood screws reliably on relatively thin MDF.) I've never done THIS, but if I were drilling holes thru MDF? After drilling, I'd finish in Something on the inside of the newly-drilled.....I'd like it like a thin epoxy, which will soak into the chips. It couldn't hurt. I think the biggest issue drilling MDF is the exit wound wants to chip and break out. You can see where it broke when the wood screw came through too. I mitigated this when I used the hole saw for the connector and wire holes (1" and 3/4") by starting from one side until the center pilot drill bit just touched the opposite side. Then using that as a guide drilled through the surface of the opposite side just 1/8". Then when I finished the hole from the original side the plug came out and hole edge was clean. For the mounting holes it would have been better to do them on a drill press before assembly. But in the end they are very secure with the flat washers. View Attachment |
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KeithL
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Post by KeithL on Sept 6, 2024 10:52:36 GMT -5
Things like room placement relative to the listening position are also going to be a lot more "interesting" with open dipole subs. For those who don't know... With normal subs, at some low frequency, the sub basically "pressurizes the room"... "It doesn't act like a wave because the wavelength of the sound doesn't fit in the room"... But, with a dipole, this NEVER happens... Unless you have a baffle that is tens of feet across the air will always be able to "blow around the sides between the front and back". Therefore what you're listening to is "the air moving past you" rather than "a pressure wave surrounding you"... So, in a really big space, it is often suggested that the best listening position for a dipole sub or big woofer is directly to the side of the baffle... And, in a smaller room, well... your guess is as good as mine...  I would also suggest that the best "damping" setting is going to depend almost entirely on the driver itself... (Since there is no cabinet, or cabinet stuffing, or port, to provide any mechanical damping for the drivers.) Test Ideas … there will undoubtedly be more!- Which settings produce the maximum bass extension (hopefully to 16Hz) and fastest impulse response? - Does distortion ever become excessive? - What (if any) are the tradeoffs? One SubMeasure frequency (amplitude) and impulse response, distortion and decay – outdoors and indoors; 3ft and 7ft; vertical and horizontal.1. Standard amplifier (no servo or EQ) 2. Standard amplifier with miniDSP EQ (no servo) – parameters TBD 3. Servo amplifier (no servo or EQ) 4. Servo amplifier, all combinations: a. Extension 14, 28, 20 b. Damping Low, High, Mid c. PEQ peak resonance correction Two Subs (Mono)Measure frequency (amplitude) and impulse response, distortion and decay – outdoors and indoors; 3ft and 7ft; vertical (5.5ft apart) and horizontal (end to end).1. Standard amplifier (no servo or EQ) 2. Standard amplifier with miniDSP EQ (no servo) – parameters TBD 3. Servo amplifier (no servo or EQ) 4. Servo amplifier, all combinations: a. Extension 14, 28, 20 b. Damping Low, High, Mid c. PEQ peak resonance correction d. Delay/Phase 0-180 Other ideas are welcome! View Attachment |
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KeithL
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Post by KeithL on Sept 6, 2024 11:13:37 GMT -5
I would be looking at some sort of modern solid state accelerometer... Most ceramic phono cartridges actually work by the pressure on the stylus FLEXING a strip of piezoelectric material. (So they rely on applying sideways force to the strip while the other end is held firmly in place... and they have very little range of motion.) Another commonly used strategy is to use a dual-voice-coil driver... And use the second voice coil as a "pickup"... (Some custom drivers have a specially designed "sensor coil".) Otherwise note that the sensor MUST be connected to the center of the driver. ALL woofer cones flex and resonate to some degree. The center is going to be the best place to actually model the position of the voice coil and so of the cone overall. (And you can glue a sensor to the center of a solid dust cap without affecting balance.) If you really want to have "fun" with modeling and programming... You can also use the back-EMF of the driver's main voice coil to "track what it's doing". But that gets really complex... Another really interesting option would be to use a tiny microphone... (The new solid state microphones used in things like smart phones and Alexa's are really tiny and very rugged.) A microphone measures the movement of air relative to the microphone... If you were to mount a tiny microphone on the cone, with the microphone "moving and detecting the air pressure in front of the cone"... IN THEORY the microphone would be giving you a really accurate indication of the actual power being coupled to the air at that point... (It's telling you the actual air pressure at that point at the front surface of the cone.) But, since this would vary across the cone, and such complications, it would be really tricky... and I've never heard of anyone doing it. But, of course, don't forget that you have to get that signal back to your electronics... (So an extra set of flex cables... ) Or... for a real science-fiction idea... How about a self-powered piezoelectric sensor... With an electronic package that digitizes the readings... And sends them back through the drive wires that power the voice coil. (You could modulate them on a high frequency carrier... like "Ethernet over power" works.) And then, of course, there's LASER interferometry... which works non-contact...  Another interesting thought is this... With an open baffle sub the actual audio signal has a very long wavelength... Therefore, having a sensor a few inches away from the surface of the driver might actually give you BETTER information about the audio wavefront itself. So how about mounting a microphone on the baffle, to the side, several inches behind the driver. Or even on a support strut, going across the cabinet, directly behind, and five or six inches away from, the center of the driver... This would show you what the actual waveform IN THE AIR looked like. (Remember that the goal is to accurately reproduce the audio waveform... and NOT specifically the movement of the driver.) And, since it's only a tiny fraction of a wavelength away from the driver... You should be able to correct the driver to compensate for variations in the air-borne wavefront... Hmmmm... that might actually work pretty well at very low frequencies. I've seen 'tape on' accelerometers sold somewhere. Failing that? Maybe use a phono cartridge? Some kind of piezo electric crystal (a ceramic phono cart?) I would expect Sub Harmonics at 1/2 and 1/4 of resonance......6db down and greater.... Phono cart thru phono PRE should give a direct amplitude / frequency output. Other more scientific sensors will report in 'G' which would just drive me nuts..... No substitute for measurments.... |
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Post by marcl on Sept 6, 2024 11:34:14 GMT -5
I would be looking at some sort of modern solid state accelerometer... Most ceramic phono cartridges actually work by the pressure on the stylus FLEXING a strip of piezoelectric material. (So they rely on applying sideways force to the strip while the other end is held firmly in place... and they have very little range of motion.) Another commonly used strategy is to use a dual-voice-coil driver... And use the second voice coil as a "pickup"... (Some custom drivers have a specially designed "sensor coil".) Otherwise note that the sensor MUST be connected to the center of the driver. ALL woofer cones flex and resonate to some degree. The center is going to be the best place to actually model the position of the voice coil and so of the cone overall. (And you can glue a sensor to the center of a solid dust cap without affecting balance.) If you really want to have "fun" with modeling and programming... You can also use the back-EMF of the driver's main voice coil to "track what it's doing". But that gets really complex... Another really interesting option would be to use a tiny microphone... (The new solid state microphones used in things like smart phones and Alexa's are really tiny and very rugged.) A microphone measures the movement of air relative to the microphone... If you were to mount a tiny microphone on the cone, with the microphone "moving and detecting the air pressure in front of the cone"... IN THEORY the microphone would be giving you a really accurate indication of the actual power being coupled to the air at that point... (It's telling you the actual air pressure at that point at the front surface of the cone.) But, since this would vary across the cone, and such complications, it would be really tricky... and I've never heard of anyone doing it. But, of course, don't forget that you have to get that signal back to your electronics... (So an extra set of flex cables... ) Or... for a real science-fiction idea... How about a self-powered piezoelectric sensor... With an electronic package that digitizes the readings... And sends them back through the drive wires that power the voice coil. (You could modulate them on a high frequency carrier... like "Ethernet over power" works.) And then, of course, there's LASER interferometry... which works non-contact... Another interesting thought is this... With an open baffle sub the actual audio signal has a very long wavelength... Therefore, having a sensor a few inches away from the surface of the driver might actually give you BETTER information about the audio wavefront itself. So how about mounting a microphone on the baffle, to the side, several inches behind the driver. Or even on a support strut, going across the cabinet, directly behind, and five or six inches away from, the center of the driver... This would show you what the actual waveform IN THE AIR looked like. (Remember that the goal is to accurately reproduce the audio waveform... and NOT specifically the movement of the driver.) And, since it's only a tiny fraction of a wavelength away from the driver... You should be able to correct the driver to compensate for variations in the air-borne wavefront... Hmmmm... that might actually work pretty well at very low frequencies. I've seen 'tape on' accelerometers sold somewhere. Failing that? Maybe use a phono cartridge? Some kind of piezo electric crystal (a ceramic phono cart?) I would expect Sub Harmonics at 1/2 and 1/4 of resonance......6db down and greater.... Phono cart thru phono PRE should give a direct amplitude / frequency output. Other more scientific sensors will report in 'G' which would just drive me nuts..... No substitute for measurments.... Wow this is quite a bit of interesting stuff (as usual). I'm going to read again in detail after I finish driving the last four bolts to mount the third driver. Just to give you some info on this specific build though .... the drivers are GRResearch 12" 16ohm servo speakers with indeed a servo coil as well as the drive coil. They will be driven by a Rythmik servo amp. Both amp and speaker are specifically made for open baffle use, with the amp incorporating the required shelf filter (internally) in addition to the user adjustable filter and damping controls. But the specific conversation about resonance and damping is not about the driver itself, it's about the cabinet and the debate over whether the recommended Norez material does anything if applied to the inner surfaces of each square cavity. So what I want to know is ... IS there a cabinet resonance? IS it significant in a range that will be excited by the drivers (16-120Hz)? And IF so ... can Norez or some other material like Dynamat significantly dampen that resonance? My hypothesis - partially confirmed by the tapping and calculation exercises earlier in this thread - is that there is no significant resonance. And if there was, the foam part of Norez would do nothing below 500Hz so it would be a matter of the Norez damping layer vs Dynamat or MLV. I think the only way to tell from here is the ground plane measurement of the three drivers playing in the cabinet and measured from 2m away. That's probably a couple days away.  |
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Post by leonski on Sept 6, 2024 13:46:28 GMT -5
I suspect a quick note to Stereophile may yield a result. They've been measuring cabinet resonance for a long time....and have a body of data which you may gain access to.....
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Post by marcl on Sept 6, 2024 15:05:29 GMT -5
I suspect a quick note to Stereophile may yield a result. They've been measuring cabinet resonance for a long time....and have a body of data which you may gain access to.....  |
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Post by sockemdog on Sept 7, 2024 2:04:07 GMT -5
I've never done THIS, but if I were drilling holes thru MDF? After drilling, I'd finish in Something on the inside of the newly-drilled.....I'd like it like a thin epoxy, which will soak into the chips. It couldn't hurt. I think the biggest issue drilling MDF is the exit wound wants to chip and break out. You can see where it broke when the wood screw came through too. I mitigated this when I used the hole saw for the connector and wire holes (1" and 3/4") by starting from one side until the center pilot drill bit just touched the opposite side. Then using that as a guide drilled through the surface of the opposite side just 1/8". Then when I finished the hole from the original side the plug came out and hole edge was clean. For the mounting holes it would have been better to do them on a drill press before assembly. But in the end they are very secure with the flat washers. View AttachmentI used these on my sub build a while back. They provide a solid machine screw type connection without needing access to the inside of the box (mine was sealed unlike yours). www.parts-express.com/10-32-T-Nuts-50-Pcs.-081-1070?quantity=1 |
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Post by marcl on Sept 7, 2024 2:26:30 GMT -5
Things like room placement relative to the listening position are also going to be a lot more "interesting" with open dipole subs. For those who don't know... With normal subs, at some low frequency, the sub basically "pressurizes the room"... "It doesn't act like a wave because the wavelength of the sound doesn't fit in the room"... But, with a dipole, this NEVER happens... Unless you have a baffle that is tens of feet across the air will always be able to "blow around the sides between the front and back". Therefore what you're listening to is "the air moving past you" rather than "a pressure wave surrounding you"... So, in a really big space, it is often suggested that the best listening position for a dipole sub or big woofer is directly to the side of the baffle... And, in a smaller room, well... your guess is as good as mine... I would also suggest that the best "damping" setting is going to depend almost entirely on the driver itself... (Since there is no cabinet, or cabinet stuffing, or port, to provide any mechanical damping for the drivers.) Test Ideas … there will undoubtedly be more!- Which settings produce the maximum bass extension (hopefully to 16Hz) and fastest impulse response? - Does distortion ever become excessive? - What (if any) are the tradeoffs? One SubMeasure frequency (amplitude) and impulse response, distortion and decay – outdoors and indoors; 3ft and 7ft; vertical and horizontal.1. Standard amplifier (no servo or EQ) 2. Standard amplifier with miniDSP EQ (no servo) – parameters TBD 3. Servo amplifier (no servo or EQ) 4. Servo amplifier, all combinations: a. Extension 14, 28, 20 b. Damping Low, High, Mid c. PEQ peak resonance correction Two Subs (Mono)Measure frequency (amplitude) and impulse response, distortion and decay – outdoors and indoors; 3ft and 7ft; vertical (5.5ft apart) and horizontal (end to end).1. Standard amplifier (no servo or EQ) 2. Standard amplifier with miniDSP EQ (no servo) – parameters TBD 3. Servo amplifier (no servo or EQ) 4. Servo amplifier, all combinations: a. Extension 14, 28, 20 b. Damping Low, High, Mid c. PEQ peak resonance correction d. Delay/Phase 0-180 Other ideas are welcome! View Attachment Speaking of placement .... I think general dipole practices apply ... away from the walls, for one thing ... My room is 12.5'x26.5' And the Magnepan 3.7s sit out few feet from the front wall but in the "Rooze" configuration where they are 45 degrees to the side wall with the edge - the dipole null - pointing at the MLP. They have an added 2'x6'x1" oak baffle screwed to the leading edge to extend the speaker baffle toward the MLP. The position in a room that excites room modes the least is the 25% position, which in this case is about 3' from the side wall and 6 1/2' from the front wall ... which is a point right at the end of that extended baffle on the 3.7. That is my starting point for the position of the subs, and by placing one side of the sub up close to that extended baffle I essentially use it to extend the baffle of the sub on that side by 2'. And BTW I can confirm that the 25% position works with respect to room modes because I tested it years ago with a regular sub ... that position had a pretty smooth room response and moving it even 6"-12" in any direction produced more resonance effect. So ... placing these subs in that position, extending the baffle, and the inherent nature of the OB dipole sub to cancel bass waves on the sides, top and bottom to further minimize engagement of room modes in those directions ... all of that should work to give me a smoother response than conventional subs. Another option which I will test is to come out to the 25% point but lay the two subs on the floor between the front speakers. Doing this essentially makes one side of the subs an infinitely long baffle. But the real baseline for all of this will happen outside. I'll get the box out away from the house with the dipole null aimed at the house and the front and rear of the cabinet pointing to substantially long distances from other buildings .... then place the mic on the ground and run through lots of variables. And this includes the hunt for cabinet resonances. I went back and read some Toole ... measuring off axis is the way to find cabinet resonances. He does it in the anechoic chamber with the "spinorama" ... I'll do it with ground plane measurements, and it should still work. Any peaks or dips in response that are in the same place irrespective of position off axis will suggest a resonance. If there is a resonance in the sub range from 16-120Hz I could try to fix it "mechanically" with damping or bracing .... or, since it will be minimum phase, it could be corrected with EQ if it's still an issue once placed in the room. Of course Dirac will have its way with these subs once they are integrated to the low end of the 3.7 fronts and also connected to the Center Sub=LFE output.   |
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Post by marcl on Sept 7, 2024 11:44:22 GMT -5
Believe me ... those markings on the Speakon connector are nearly impossible to see even with a flashlight, and I have to take my glasses off, even. And it's only an inch in diameter, mounted through a 3/4" brace. And during testing I'll be doing a lot of plugging and unplugging. So ... +1 Red Drive + -1 Black Drive - +2 Blue Servo + -2 Orange Servo - And yes I'll use wire those colors. Note that since the center speaker is reversed, it's polarity has to be reversed for both the Drive and Servo.   |
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Post by leonski on Sept 8, 2024 1:44:03 GMT -5
Marcl? Will those spades on the speakon fit a standard spade connector OR just the connector intended?
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Post by marcl on Sept 8, 2024 2:31:02 GMT -5
Marcl? Will those spades on the speakon fit a standard spade connector OR just the connector intended? Well spades are interesting, it turns out. There are the spades on the connector and the ones on the speaker terminals ... and they are two different sizes, and maybe two of them are just slightly different but one is definitely smaller. I have three sizes of terminals and they are for different wire gauges, and slightly different width. They needed to be crimped a bit to fit tight, especially on the speaker terminals. I could solder them but for now I'm using the disconnects because I need to do that for testing. I still think it makes more sense than using the spring loaded speaker posts. Also .... I thought about how to run the drive wires and decided to do it from the connector to each speaker like the right drawing, rather than the way the schematic shows (left drawing) .... because it was too complicated running wires back and forth through the holes from bottom to middle and back to front. The connector is actually very close to the middle speaker so the longest run is only 2ft. (And yes I wired the center opposite polarity ... just drew it this way for clarity)    |
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Post by marcl on Sept 8, 2024 8:42:56 GMT -5
Why reverse the center speaker? I found the thread I saw a while back about whether or not to reverse the center driver in a 3-high sub. What it comes down to is that Linkwitz and others have measured a 15db reduction in even order harmonic distortion by reversing the center speaker: “… due to the non-symmetrical construction of the woofer, cone, surround etc. they radiate distortion (non-linear properties) at different amplitudes front and rear. If both drivers are in the same orientation these sounds are reinforced. When one of the drivers is reversed, these artifacts are effectively cancelled.” Those who have measured the distortion say they still aren’t convinced it would be audible. Some propose that there is a pressure wave equalization with one reversed, but this isn’t true because you flip polarity on the center driver so they all play in phase. Danny and Davey spar (as usual) and Danny doesn’t think you can measure the distortion difference … Davey disagrees. Some say the main reason to do it is physical balance due to the weight of the magnets hanging out in the breeze. Danny points out that any difference in the acoustic center (closer to the dust cap) being asymmetrical with one reversed is not an issue at sub wavelengths. Captainhemo (the guy who sells cabinet kits) says he sells a lot with all three facing forward because people like the way it looks. In the end, people will often sacrifice theoretical benefit for aesthetics. Before doing the wiring I took an hour break for some hypnagogia via Vivaldi …. and after running the relatively simple servo wires I realized I could do the drive wires more easily like the diagram on the right rather than the left. On a schematic the daisy chain looks simple but the reality of going down and up and forward and back and up made it too complicated … and all those wires would have a tough time fitting through the grommets.   I wish the spade connectors felt more snug. But I can crimp them a bit more after I’m done testing (pulling them off and on lots of times). So … a short indoor bench test with the servo amp today. Weather the rest of the week looks good for the ground plane tests outside. |
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ttocs
Global Moderator
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Post by ttocs on Sept 8, 2024 9:40:11 GMT -5
The drawing on the right does look like a more concise wiring scheme.
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