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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
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Old 24th June 2016, 02:50 PM   #11
Lucien is offline Lucien  Singapore
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Quote:
Originally Posted by agdr View Post
Here is another brand of "9V" 7.4V nominal 600mAhr lithium ion rechargeable cell and manufacturer's fast charger:

Rechargeable 9 Volt Lithium Ion Battery: BatteryMart.com

I prefer getting lithium batteries at battery distributors like this, as opposed to generic batteries on Amazon, because they can supply a datasheet for the batteries listing the discharge characteristics. Here is the one for the battery above.:

http://www.batterymart.com/pdfs/rli9600.pdf
(opens PDF)
Interestingly, that pdf led me to look up GN Batteries & Electronics Inc., the makers of those cells. Seems GN also makes a 7.4 V 740 mAh LiPo, but it's more than twice the cost of the 600 mAh HiTech Li-Ion. And that in turn is about twice the cost of a similar Tenergy cell... I wonder how much difference there is between the HiTech and Tenergy.
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Old 24th June 2016, 06:49 PM   #12
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
Quote:
Originally Posted by Lucien View Post
Interestingly, that pdf led me to look up GN Batteries & Electronics Inc., the makers of those cells. Seems GN also makes a 7.4 V 740 mAh LiPo, but it's more than twice the cost of the 600 mAh HiTech Li-Ion. And that in turn is about twice the cost of a similar Tenergy cell... I wonder how much difference there is between the HiTech and Tenergy.
I actually have a set of those "9V" 740mAhr lithium polymers, and they work great! I bought the charger + 2 battery combo last year for the $50:

2 Bank 9 Volt Battery Charger with 2 Batteries: BatteryMart.com

datasheet:

http://www.batterymart.com/pdfs/rli-9720.pdf

Lithium polymer is supposed to be less susceptable to exploding if abused than lithium ion. Usually - at least for high power LED flashlights, which is another hobby of mine - the LiPo has a lower voltage. I was suprised to see the voltage the same as the lithium ion. Their charger works with both. You are right, the big negative there is price!
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Old 24th June 2016, 06:51 PM   #13
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
Quote:
Originally Posted by 454Casull View Post
Marked for later
Makes a good weekend assembly project!
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Old 25th June 2016, 12:23 AM   #14
johnc124 is offline johnc124  United States
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Impressive project! I'm glad to hear you enjoy the sound of the OPA1688 and I look forward to hearing other's impressions of this op amp.
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Old 25th June 2016, 03:41 AM   #15
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
johnc124 - thanks! You and TI get the credit on the excellent chip. It really does sound fantastic. If an amp doesn't sound good I'm the first to flag it, but your chip is tops. Really shows that a lot of design thought and testing went into it.

I have a question I've wanted to ask about the preferred voltage gain resistor values. I've kept your 47pF compensation cap from the datasheet, but the BOM default right now is a voltage gain of 2x using 2K for both the feedback resistor and ground return (just the standard non inverting equation of course, [[1+ 2K/2K] = 2]). So that is a 4K total load on the chip output with no headphones attached, which at the battery maximum of 11V peak ("9V" NiMH cells right off the charger) would be 11/4K = 2.75mA, not even a blip on the radar for OPA1688. But the (higher) 2K values should help reduce AC battery drain a bit, although at the cost of a bit of Johnson noise over lower values like 1K's in both positions.

Long story short... what would be your recommendation for resistors for a Vgain of 2x. given the +/-"9V" power rails? Then same question for a Vgain of 1 (purely a current buffer), would you recommend leaving off the ground return resistor (R1 in your datasheet schematic) and simply jumpering the feedback resistor (which would short the 47pF) to form a votlage follower, or would it be better to use Rf and instead use some large value resistor (say 10x Rf) for R1 (with a net result of 0.9 voltage gain rather than 1.0). Seems like the later could/would add Johnson noise from the large resistor. And finally same question for a voltage gain of 4x. Any thoughts here are appreciated!

I was looking at the datasheet today yet again and the graph below on the first page just stuck out as a thing of beauty, after a few years now of designing headphone amps! Just look at those *real world* headphone resistances: 16R, 32R & 128R. The typical historical op-amps used in CMOYs have maximum datasheet loads of 2K or more for whatever THD+N numbers are listed, and usually maximum load capacitance of 100pF. They are designed for line-level gain stages, not driving headphones. The OPA1688 is the real thing for driving real-world headphone loads, including the 500pF cable/headphone capacitance. Keep in mind the graph is done with +/-5Vdc power rails too! In this CMOY we have 6.26Vdc - 11Vdc depending on battery type and battery charge levels.
Attached Images
File Type: jpg OPA1688 datasheet page 1.JPG (87.8 KB, 406 views)

Last edited by agdr; 25th June 2016 at 04:03 AM.
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Old 25th June 2016, 06:39 AM   #16
Lucien is offline Lucien  Singapore
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Just chipping in here to say I'm interested in setting up mine with a gain of 1, I was going to ask after ordering the boards, but since it's already come up...
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Old 25th June 2016, 08:04 AM   #17
B&W_arthur is offline B&W_arthur  Hong Kong
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Looks very interesting.

Currently, I am also working on a CMOY PCB.
Have wondered whether the OPA1688 can be used in the CMOY.

agdr, thanks for your inspiration
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Old 25th June 2016, 05:42 PM   #18
adydula is offline adydula  United States
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Got parts, got boards, iron is getting hot!! LoL

Alex
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Old 25th June 2016, 09:01 PM   #19
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
Default TPS3701DDCT adapter board soldering photo-build - part 1

Here is a photo-build showing how to build up the TPS3701DDCT adapter board, if you are soldering it yourself.

Mousing over these photos will bring up the file names with the photos numbers I'm referencing below. Clicking on the arrows that show up in the lower left corner of the photos will zoom them up.

* photo 4181 shows the relative sizes of the parts vs. a US dime. Top and bottom of the adapter board, the TPS3701DDCT chip, and the 0.1uF 100V X7R 0603 MLCC chip (Mouser #81-GCM188R72A104KA4D). The blue background is a grounded anti-static mat.

Note the two pin 1 dots on the top (rightmost board in the photo) of the adapter board! There is a small one for the chip near the lower right pad in this photo, then a larger one for the pin 1 connection pin in the lower right corner of the board.

Remember that C2 on the bottom isn't used, at least for the OPA1688 CMOY project here. Those 0805 pads are in parallel with the 0603 capacitor on the top of the board, just for generality if someone wanted to add a larger decoupling cap in parallel.

* 4184 & 4185 show Adydula's tape trick in action. A piece of tape is placed over the top couple of pins, leaving the bottom 2 - 4 pins exposed. Then the chip is picked up with the tape and the tape used to center the chip over the adapter board pads. It only took me 1 additional repositioning this time to get a perfect pin to pad alignment. The first time I did this it took me about 8 un-sticks and re-sticks of the tape on the vise to get the hang of it. As with all surface mount parts havng the chip pins *perfectly* align with the pads is a key trick to making the soldering go well. Don't hesitate to re-position the chip as many times as you need to get those leads centered on the pads. I'm not using any additonal flux on the SMD pads here, as I've seen various videos do on the internet. I've never found the need for it. The flux in the solder I'm using has always been sufficient.

* 4188 shows the results of the soldering effort outside of the taped area. Yep, both sides solder bridged. Don't worry about it a bit. You will get that nearly every time due to the close spacing of the chip leads. We will easily take care of it with some solder wick later on.

* 4190 shows the results with the tape removed and the remaining 2 leads under the tape soldered. Lol, now all 3 leads on the left are bridged. Lets take care of that...

*... 4191, 4192, 4193, and 4194 show the solder wick I'm using and the results from putting the end over all 3 bridged leads and briefly heating with the soldering iron. As 4192 shows it really isn't a lot that has to come off. Then repeat with the other side and 4194 shows the now perfectly soldered-up chip.

* photo 4194 above shows something else too. I've put a blob of solder on one of the 0603 capacitor pads. 4195 shows the cap being picked up by the tweezers and 4196 is the result of heating that blob and sticking in one end of the capacitor. After I took this photo I used my finger nail (also using a grounded wrist strap here) to push down on the cap while re-heating the end to get it to lie flat on the board.
Attached Images
File Type: jpg IMG_4181.JPG (363.0 KB, 391 views)
File Type: jpg IMG_4184.JPG (240.2 KB, 380 views)
File Type: jpg IMG_4188.JPG (182.8 KB, 371 views)
File Type: jpg IMG_4190.JPG (233.9 KB, 372 views)
File Type: jpg IMG_4191.JPG (233.1 KB, 123 views)
File Type: jpg IMG_4192.JPG (186.4 KB, 118 views)
File Type: jpg IMG_4193.JPG (339.2 KB, 111 views)
File Type: jpg IMG_4194.JPG (215.8 KB, 127 views)
File Type: jpg IMG_4195.JPG (312.3 KB, 121 views)
File Type: jpg IMG_4196.JPG (226.8 KB, 113 views)

Last edited by agdr; 25th June 2016 at 09:10 PM.
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Old 25th June 2016, 09:25 PM   #20
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
Default TPS3701DDCT adapter board soldering photo-build - part 2

* 4198 shows the other end of the cap soldered, allowed to cool a bit, then back to the first end to touch it up.

* 4207 shows the connection pin strips for the adapter board placed in their holes on the CMOY board to get things lined up. They are not soldered in yet here. One end of these pins has the plastic bar closer than the other end, and the other end has the short extension piece. In this photo I've place the two strips in both ways to show the difference.

The leftmost strip in this photo is in the "wrong" way, with the plastic bar down toward the CMOY board and the extension pin area sticking up. The rightmost strip is in the "correct" way, with the extension pins down toward the CMOY board. Notice that the adaptor board just fits over the two SB140 schottky diodes used for battery polarity protection. One item in the build instructions is the importance of getting those diodes centered as much as possible over their PC outline. Solder one end of a diode first, check for alignment, then re-heat that one and and move the diode as needed before soldering the other end. The diodes should touch each other as shown to leave maximum space for the adaptor board pins.

BTW the pluggable connection pins are Mill-Max 342-10-164-00-591000 (Mouser #575-6415691 for a strip of 64 that can be cut as needed. I supply them with the adapter board though.

* 4208 now has the adapter board set down over the connection pins. Neither end of either connection pin strip is soldered yet. An important thing to note in this photo is the "pin 1" lettering on the CMOY board near the bottom right connection pin hole for IC1 (IC1 on the CMOY board is the TPS3701 adapter board). That lines up with the pin 1 dot on the adaptor board. An even easier way to check orientation is just make sure the 0603 capacitor on the top of the adapter board is toward the battery wire holes on the CMOY board.

* 4209 shows the adaptor board end of the pins soldered now.

* 4218 and 4219 show the completed adaptor board lifted off the CMOY board. The side shot 4219 shows the extension pin end of the connection pins vs. the other end a bit more clearly. I would recommend getting the entire rest of the CMOY board soldered up first before soldering in the adapator board, just to lessen any chance of damage.
Attached Images
File Type: jpg IMG_4198.JPG (239.3 KB, 126 views)
File Type: jpg IMG_4207.JPG (304.7 KB, 122 views)
File Type: jpg IMG_4208.JPG (307.5 KB, 119 views)
File Type: jpg IMG_4218.JPG (262.7 KB, 109 views)
File Type: jpg IMG_4219.JPG (275.2 KB, 101 views)

Last edited by agdr; 25th June 2016 at 09:50 PM.
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