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Old 12th July 2013, 10:21 PM   #161
agdr is offline agdr  United States
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Everyone who expressed interest previously in these extra boards has contacted me now, so all the boards are spoken for at this point. I may do another run at some time down the road.

However by using that 4 layer gerber file I posted a few posts back in this thread, anyone can do their own run at any time! 5 4-layer boards at Seeed Studio (where I in China had it fabbed) is about $100 as I recall plus shipping. Slow boat shipping (probably 1 - 2 months with no tracking) is free, DHL (3-4 days with tracking) for 5 was around $20. 10 boards was $120 plus $30 DHL. I'm still amazed at those low prices for 4-layer boards, along with the good quality of what came back.

Last edited by agdr; 12th July 2013 at 10:23 PM.
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Old 12th July 2013, 10:38 PM   #162
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Thank you very much for your detailed response! And I apologize, it seems I haven't read the thread carefully enough (or clicked an old schematic) because I completely missed that you swapped the 627s for the 49990s!
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Old 14th July 2013, 08:52 PM   #163
agdr is offline agdr  United States
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Quote:
Originally Posted by dfy View Post
Thank you very much for your detailed response! And I apologize, it seems I haven't read the thread carefully enough (or clicked an old schematic) because I completely missed that you swapped the 627s for the 49990s!
Hey I'm glad you asked the questions! I'll bet several other folks stopped reading the thread after those initial OPA627 experiments. I also came to learn that the OPA627 has been around a long time and a great many other OPA627-based headamps are out there. The main point in using the OPA627 in the initial design was to put the pot in front, due to the low input bias current of the FET amp. But with the pot back in the middle it doesn't matter anymore. Might as well use a lower-noise, lower distortion, and cheaper bipolar input chip like the LME49990. I putzed around with using the LME49720 but in the end the LME49990 has better AC specs and 2x is the same cost as the one dual LME49720.

One interesting thing, I eventually found that going to the 0.5R balancing resistors on the parallel output stage (vs. the 1R resistors in the O2) completely eliminated the 80kHz oscillation. This is further down the thread when I was using the OPA627 wrapped around the output buffers as a DC servo. I ran into that same 80kHz oscillation and the 0.5R's solved it. I believe the issue is shown in the upper left figure of page 11 on this Linear Tech data sheet:

http://cds.linear.com/docs/en/datasheet/1010fe.pdf

They have the feedback loop taken off before the balancing resistor on just one of the parallel output current buffers. The other parallel buffers are then just slaves in a "best effort" arrangement. RocketScientist / NwAvGuy warned against taking feedback off after the balancing resistors, noting that would essentially add current feedback to the voltage feedback from the IxR drop (in the 1R reseistors) caused by the output current. That is one reason he said that he chose a feedback-less design for the O2 output stage.

It seems that effect was causing the 80kHz oscillation when using the 1R balancing resistors. Dropping them to 0.5R became "close enough" to no resistor so solve the problem. It would be extremely interesting to try 0.5R balancing resistors in the O2 amplifier instead of the 1R's and run RocketScientist's suite of dScope measurements again (massless are you out there? ) to see if they are better, worse, or the same.

I did eventually find a source of 0.5R 1/8W metal film through-hole resistors for this project here on eBay from Hong Kong. I can't get a link to work for some reason, but search eBay for "0.5 ohm 1/8W". $3.99 for 100 including shipping. Neither Mouser nor Digikey has 0.5R in 1/8W. In the BOM I've had to specify 1/4W 0.5R that has to be mounted on end. These 1/8 watters fit right in and would save some money on the build.

On the resistors, I re-read what I wrote above and see several typos. Argh. For anyone else reading this with the same questions, it should read that thin film is lower noise than thick film with the SMD stuff. I used thin film for the signal path parts (usually around $0.80 each) but went back and substituted thick film (usually around $0.10) for parts in the relay circuit and clipping circuit to cut some costs.

Last edited by agdr; 14th July 2013 at 08:59 PM.
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Old 14th July 2013, 11:58 PM   #164
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In the figure you mentioned, the output are not in parallel.
Each "slave" is driving it's own, separate output.
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Old 15th July 2013, 12:42 AM   #165
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Originally Posted by Avro Arrow View Post
In the figure you mentioned, the output are not in parallel.
Each "slave" is driving it's own, separate output.
You are right! Thanks for the heads up. I went fishing on Google for the diagram I had in mind and thought I had found it. Whoops. It would be that same diagram, but with those two output balancing resistors connected together to form one channel.
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Old 15th July 2013, 09:40 PM   #166
agdr is offline agdr  United States
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Here are the latest build instructions for this project. Please let me know about any typos or anything that isn't clear and/or needs more explanation.
Attached Files
File Type: pdf ODA BUILD INSTRUCTIONS 11.pdf (203.2 KB, 77 views)
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Old 16th July 2013, 02:38 AM   #167
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Here are the latest build instructions for this project. Please let me know about any typos or anything that isn't clear and/or needs more explanation.
agdr - I'm planning to make a small run of PCB's using your gerber files for personal use. Looks like a cool project to build. Just curious if you plan on making any more revisions to the v1.4 gerbers?

Thanks for your contributions.
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Old 16th July 2013, 03:26 AM   #168
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2000impreza - Hmmm... if you are doing another run there may be a couple of minor cosmetic things that may be worth doing.

While I was doing the build I was thinking it would be nice to add "NEG" lettering on the negative size of the large electrolytics. The Eagle library has the standard "+" sign on the + lead, but the caps always have the negative side marked. I've never seen anyone do that with a PC board but it seems like a good idea that would be helpful during the build.

The board has vertical diodes in a few places and I have lettering that says "band up" or "band down". It might be nice to make the font on that one size larger too. A few more minor things like that which would be "safe" to change without doing another test board since they don't alter the functionality in any way.

So if you would be willing to wait I could probably have those done and posted by the first of next week.

One hiccup to be aware of on the build. You have to have a back panel with this project since the voltage regulators heat sink to it. I'm still in the process of sending out the panels and getting them back to verify so that could take a few more weeks, espcially if I mess something up in the CAD file and have to send them out a second time. I'll be posting measurements to hand drill it next week, so you can always drill the panels yourself that come with the B4-080 box. I hand drilled mine. I'm increasing the accuracy of the measurements though and should have those by next week to post. The back panel centers with the RCA jack mounting screw on the left and the power switch hole on the right. Then the voltage regulators mount on with mica washers and are soldered down.

Also note that the PC board goes into the second slot up from the bottom, not the bottom slot. That is because the B4-080 case has a panel screw and extrusion at the bottom of the case right in the center. The extrusion sticks up a bit. Using the second slot gives lots of clearance between the bottom of the board and the bottom of the case. All of the panel measurements I'm posting and the professional panel files for Proto Panel and Front Panel Express have the board in the second slot.

Last edited by agdr; 16th July 2013 at 03:42 AM.
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Old 16th July 2013, 05:30 AM   #169
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Thanks for the detailed response. In no rush so I will hold off and wait for the updated files. I was planning to hand drill both front and back panels for the first built just to see how everything works out.
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Old 16th July 2013, 10:17 PM   #170
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Originally Posted by 2000impreza View Post
Thanks for the detailed response. In no rush so I will hold off and wait for the updated files.
Sounds good!
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