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Old 30th May 2013, 04:06 AM   #111
agdr is offline agdr  United States
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abraxalito- thank you for the confirmation! I always worry that I've missed something in these test setups.

Well that is interesting. So it looks like 0.5R might be a better balancing resistor. Mouser doesn't have 0.5R metal film in 1/8W, but they do in the standard 6mm length 1/4W, which should fit just fine if "tombstoned" on end. The lead diameter of the bigger resistor is 0.64mm diameter and the board hole is 0.82mm, so all should still fit.

The output chips get warm at idle with no signal (and no oscillation showing up on a 100mHz scope), more so with the 17V rails, and I originally figured that was due to balancing currents. But no, it is just apparently due to the quiescent current of around 11mA per chip, from the data sheet. That would be 17Vdc * 2 rails * 11mA = 374mW, or roughly 47% of the chip's 800mW power dissipation rating taken up just with quiescent current dissipation.

I'm probably going to reduce the upper rail voltage from +/-17Vdc to +/-16Vdc. That will help with dissipation a bit. I've also been concerned about the +/-15Vdc maximum input of the NJM4556A's with supply rails over 15V, from the datasheet. I've been relying on the output swing of the gain stage LME49990s to not exceed that, from the datasheet numbers (around 2.5V below the rails maximum), but lowering the rails a volt would guarantee the parameter never gets exceeded.

Last edited by agdr; 30th May 2013 at 04:25 AM.
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Old 31st May 2013, 12:40 AM   #112
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Hey agdr, sweet project! Its clear you've put a lot of effort into it and I would love to hear it.

If I could make one suggestion, it would be to clean up your schematic in Eagle and split it up over multiple sheets. As it stands, its pretty hard to decipher everything that's going on.

Olin Lathrop has a great post on how to keep schematics tidy and legible.
Rules and guidelines for drawing good schematics - Electrical Engineering Stack Exchange
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Old 31st May 2013, 01:52 AM   #113
agdr is offline agdr  United States
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Quote:
Originally Posted by gastro54 View Post
If I could make one suggestion, it would be to clean up your schematic in Eagle and split it up over multiple sheets. As it stands, its pretty hard to decipher everything that's going on.
Excellent suggestion! Thanks. I didn't know Eagle could do that. I've only been using Eagle for about 6 months now, so definitely still in the learning mode. Its been a couple of decades since I last laid anything out.

Hey I just adopted one of your design points. I had forgotten about that 300mW surface mount version of the NJM4556A until I saw it in your design. Earlier in the thread it was suggested to add a pre-amp output jack on the front. I've finally got onboard the train and decided that is a good idea. That NJM4556AM surface mount chip makes the perfect buffer for the pre-amp out! 300mw is perfectly adequate for just driving an RCA cable and line-level pre-amp inputs. Earlier in the thread I was thinking of taking pre-amp outs directly off of the gain stage LME49990 chip, but that would be a bad idea due to cable capacitance.
Attached Images
File Type: png ODA layout.png (226.7 KB, 399 views)
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Old 31st May 2013, 04:03 AM   #114
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Quote:
Originally Posted by agdr View Post
Excellent suggestion! Thanks. ....That NJM4556AM surface mount chip makes the perfect buffer for the pre-amp out! 300mw is perfectly adequate for just driving an RCA cable and line-level pre-amp inputs. Earlier in the thread I was thinking of taking pre-amp outs directly off of the gain stage LME49990 chip, but that would be a bad idea due to cable capacitance.
IMHO distortion & noise figures will be less with NE5532 instead of NJM4556AM.
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Last edited by availlyrics; 31st May 2013 at 04:07 AM.
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Old 31st May 2013, 06:03 AM   #115
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agdr, glad to see this project. Regarding the CC license. One way to read it is that since your design is such as departure from the original design. it is no longer a derivative (in regard of copyright) and you don't need to worry about the original license.

But the wording of the CC license does seem to imply "any improvements" on the work. If this were the case, we would be still driving model Ts :-)
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Old 31st May 2013, 01:11 PM   #116
agdr is offline agdr  United States
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Quote:
Originally Posted by availlyrics View Post
IMHO distortion & noise figures will be less with NE5532 instead of NJM4556AM.
Yeah, I agree, I am already thinking that for the pre-amp out, passing along the low noise and distortion from the gain stage is important. That NE5532 or the LME49720 would fit in the same surface mount PCB footprint. Then just use an output series resistor to isolate the cable capacitance. That resistor shouldn't be a problem with the relatively high impedance line out rather and low impedance cans.

Quote:
Originally Posted by glt View Post
But the wording of the CC license does seem to imply "any improvements" on the work. If this were the case, we would be still driving model Ts :-)
Very true! That would have been the end of any innovation. The same goes for this amp. I fully expect that someone out there may eventually make their own version, and that is great. Discrete transistors could be used for the output stage and all sorts of other modifications.
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Old 2nd June 2013, 04:16 PM   #117
agdr is offline agdr  United States
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Default V 1.4 Gerbers, layout, circuit, BOM, changes

Here is the latest and hopefully last revision of this project. Gerber files, layout, circuit, BOM, and list of changes. Still coming are updated build instructions and Front Panel Express CAD files.

The change list has the details. The DC servo is gone, output balancing resistors changed to 0.5R, pre-amp jack and optional buffer/amp added, the 6th coupling cap on each channel is back for a Fc of 1.12Hz, upper power rail voltage switch choice is now +/-16Vdc isntead of +/-17Vdc, test points added for the pre-regulator voltages, and CLC resistors now 2512 size.

Feeding the pre-amp out jacks from the gain stage doesn't make any sense given that it can produce up to 10V of swing for the output stage and we need line level. Instead a twisted pair of wires from PCB holes provided can be used to hook it up in various ways:
* Connect to the input jacks for signal pass-through
* Run the input jacks through a new 1x buffer stage for pass-through with current buffer
* Same as above, but add a gain resistor for voltage gain. Default is 2.5x.
* And the option is still there to hook the pre-amp out to the out of the gain stage or even amp output, if needed.

The OPA2188 chip turned out to be useless for the DC servo in some testing I've done. I missed that the input offset current with the chip is so large that when multiplied by the servo resistors produces an offset of about the same size as what is being corrected. I tried an OPA627 instead and that did work well for the servo, dropping the DC offset to about 250mV. But the 80kHz oscillation from the start of this thread was back, due to the 90 + 60 degree lag leaving no phase margin in the loop. The new 0.5R balancing resistors further lower the output offset to about 1mV, so I'm dropping the DC servo entirely and calling that good. Still 3x lower output DC offset than the O2 amp.
Attached Files
File Type: pdf ODA circuit.pdf (938.8 KB, 85 views)
File Type: pdf ODA layout both layers.pdf (860.4 KB, 36 views)
File Type: pdf ODA layout bottom layer.pdf (807.8 KB, 22 views)
File Type: pdf ODA V1.4 BOM 6_1_2013.pdf (120.5 KB, 38 views)
File Type: pdf V 1.4 changes.pdf (50.4 KB, 29 views)
File Type: zip ODA Gerbers V1.4 double layer 1.6mm HASL 80x160mm.zip (236.0 KB, 21 views)

Last edited by agdr; 2nd June 2013 at 04:29 PM.
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Old 5th June 2013, 04:16 PM   #118
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Default Procedure to obtain PCB's

I have not ordered PCB's before so I need help. Please direct me to your
source of PCB's in China. I need there WEB Address, etc.

I also need the procedure to use to submit your Gerber files to this company.
I assume it's something like the following:

(1) Go to the PCB's Vendors WEB Site
(2) Sign up for an account
(3) Download the Gerber files
(4) etc.

I need this procedure to be filled in, I'm guessing here. I assume you pay via
PayPal but I do not know. Help!!

I believe your design is SO different from the original Objective 2 design, that
I would give it a new name, version number (1.4), etc. Objective 2 was
portable, the ODA is not. Objective 2 uses batteries, the ODA does not.
Some of the basic theory tracks the Objective 2, but can you Copyright
Theory? I don't think so. Please do not give up on this project. I learn
so much from reading your posts. I'm sure the Objective 2 creators would
above. Take care.
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Old 5th June 2013, 04:27 PM   #119
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Quote:
Originally Posted by mrsavage View Post
I have not ordered PCB's before so I need help. Please direct me to your
source of PCB's in China. I need there WEB Address, etc.
There's no need to go to a major PCB shop for small projects.

Try OSHPark. Very fast, high quality, and much cheaper. We use them for prototypes. Their site should walk you through the basics.
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Old 6th June 2013, 02:13 AM   #120
agdr is offline agdr  United States
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Quote:
Originally Posted by mrsavage View Post
I have not ordered PCB's before so I need help. Please direct me to your source of PCB's in China. I need there WEB Address, etc.
Well I may have some good news on the PC board end of things.

I just visited the various PCB vendor sites to make sure I was giving you up to date info, and I see that Seeed Studio now is offering 4 layer boards in the 10 x 20 size:

Fusion PCB Service - 4 Layers [PCB02281P] - $39.90 : Seeed Studio Bazaar, Boost ideas, extend the reach

My original intent with the desktop amp was to use a 4 layer board, but Itead Studio, the outfit I've been using, only had 4 layer up to 10 x 10. The last time I checked Seeed Studio they only offered 4 layer up to 10 x 10 so this may be something new.

So I'm going to probably order another run of boards from Seeed Studio and do it with 4 layer. Layer 2 will be a full ground plane single-point grounded (shielding and consistant impedance only, carrying no currents) and the 3rd layer used to get rid of most or all of the wire jumpers on the current board.

My main intent with the board run is to build one up and check the 4 layer design, but it looks like I get 10 boards for $120 plus $32 in DHL express costs, or about $15.20 a board. I want to keep 3 bare boards, but that leaves 6. Assuming all works properly when I build the test board up, I'll go ahead and make the remaining 6 available at cost plus shipping if anyone is interested. Just send me a PM. I'll have to PM the moderators and find out what the appropriate forum is here for such thing for the subsequent follow-ups, group buy forum, vendors forum, etc. I'll even make a $1 donation to the forum out of my pocket for each board. Anyone interested in the remaining boards just PM me and I'll parcel the 6 out in order of requests received. 1 board maximum per person and anyone can decide to "opt out" along the way if they change their mind. I'll only collect money via paypal when it comes time to ship out. Shipping will be somewhat more expensive since I insist on using something with tracking and/or delivery confirmation. Here in the US postal service priority mail plus delivery confirmation would do the job, which is probably $7 or so. I'll find out. I'm going to have to do some work to add the 3rd and 4th layers so it may be a couple of weeks before I send an order out. Then once I get the boards back probably another week or so for me to build up one and test/verify it.

mrsavage - if interested you can have first dibs on one of the extras at the $15.20 plus shipping cost so you don't have to buy a whole run of them. No pressure though, go right ahead and do an order yourself if you want. Here are the details on the board houses. Good you asked, I've forgotten to post that information.

I've been using Itead studio, which is here:

PCB prototyping

The "2 layer PCB larger than 10cm by 10cm" service is on the second page, page link at the bottom of the first page. Click "add to cart" on that "larger than 10x10" service and fill in the following parameters in the drop down boxes:
* PCB thickness = 1.6mm
* Surface Finish = HASL (hot air solder leveling)
* Etest = 100% (my understanding is there is no additional charge for this)
* PCB color = green (or any other color on the list for an additional $10 or so)
* PCB size = 10cm x 20cm (the amp is 80mm x 160mm = 8cm x 16cm)

The 10x20 size option adds $27 to the base service price of $38, for a total of $65 which will show up in the cart. That is for 5 boards. Then they offer DHL express for about $20, which is what I've been using (about 4 days ship time), or surface mail for $10 which could be a month or two ship time with no tracking. So all together $65 + $20 ship = around $85 for 5 boards, or about $17 per board.

It has only taken iTead about 8 calender days to get the boards done! This last time I submitted the design files late Sunday in the US, which is Monday AM in China, and on Friday of that same week received a ship notification email. Pretty impressive. DHL's tracking showed the boards picked up late Thursday of that week, in Hong Kong Friday, Cincinnati Saturday (US DHL hub) and to me Monday afternoon. DHL has their own in-house customs which avoids the typical 2-4 week "stall" in customs for postal mail from China.

How it works is that your shopping cart payment provides just an order number. Then you send them an email with that order number in the subject line and the Gerber files as an attachment (that exact zip file I posted). The email goes to pcb@iteadstudio.com. They then email a confirmation back. The subject line would be like: "order xxxxxxxxx (fill in your number) 8x16 green HASL1.6mm 5 pcs". They won't email again until the boards ship, and that email includes the DHL tracking number to view progress on DHL's site.

The Gerbers I posted were produced using a Eagle CAM processor job that iTead has posted on their site for use with Eagle 6.4. I haven't tried the files with other PC Board fab places like Seeed or OSH Park, which is another reason I'm kind of eager to try it with Seeed.

The processors I've heard about so far for small batches like this are iTead Studio, Seeed Studio, and like John says OSH Park. From what I read on the net most people think that iTead Studio and Seeed Studio are using the same wholesale board fabricator in China. I found a fantastic statistical analysis someone had done of all 3 services but I can't find that link now. The bottom line was that for just a few boards iTead was the cheapest with Seeed Studio close behind, but for slightly larger orders OSH Park won out. He also reviewed Sparkfun's board service and that came out most expensive of the bunch. I've seen posts saying what John is saying, that the quality of the OSH Park stuff is excellent.

Last edited by agdr; 6th June 2013 at 02:27 AM.
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