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A version of an O2 Desktop Amp (ODA)
A version of an O2 Desktop Amp (ODA)
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Old 18th July 2013, 04:15 AM   #171
agdr is offline agdr  United States
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A version of an O2 Desktop Amp (ODA)
Default Panel hole dimensions

Here is the set of panel hole dimensions I just sent out to Proto Panel for the front and back panel. I won't know if there are any mistakes until I get the panels back so it is probably best to wait, if possible, but if someone is set up with a CNC system and some stock and wants to give it a try, let me know the results!

These panels have the lettering left out to keep the costs down. Once these measurements are verified I'll post the CAD files for both panels that their "Cetina" software uses. When placing your panel order it is easy enough to fire up the software and just add any size, font, etc. of lettering wanted. Lettering raises the price and I know from past projects is pretty user-specific. Everyone has their own favorites on fonts and what the labelling is.

I'll also post a set of Front Panel Express CAD files once these measurements are verified.
Attached Files
File Type: pdf ODA panel CAD dimensions 3.pdf (64.3 KB, 59 views)

Last edited by agdr; 18th July 2013 at 04:20 AM.
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Old 18th July 2013, 11:57 PM   #172
agdr is offline agdr  United States
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A version of an O2 Desktop Amp (ODA)
Default Gain switch out of stock, use 3 position

I've discovered that Mouser is now out of stock of the 4-position 2-pole rotary gain switches for this project, Mouser #105-SR10010F-24NS. I've just had a chat with them and Mouser is being good enough to order in another 1000 which should arrive sometime in September.

In the meantime the 3 position version of the same switch can be used, Mouser #105-SR10010F-23NS with 489 in stock. Fits in the same PC holes just fine, you just get one less gain switch position.
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Old 19th July 2013, 02:38 AM   #173
agdr is offline agdr  United States
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A version of an O2 Desktop Amp (ODA)
Default V1.6 4-layer Gerber files, layout, circuit

Here is one more update with a bunch of minor cosmetic updates I noticed during the build. This is still 4-layer, with the DRC and CAM set up for Seeed Studio's 4 layer fab service.

2000impreza - I had forgotten that V1.4 was the last 2 layer set of Gerber files. The 4-layer that was fabbed was V1.5 and this 4-layer is V1.6. Given how (relatively) cheap these 4-layer boards are to fab these days, I would definitely recommend going with the 4-layer version/files. I've updated the wording in post #1 of this thread to point to these latest V1.6 files.

Some of the changes:
  • "NEG" lettering added to the negative lead of the large electrolytics, along with the standard "+" on the positive lead.
  • Increased the font one size for the letting on the vertical diodes that says band down or band up, just to make it easier to read.
  • Decreased the hole size one notch on the DC power jack.
  • Increased the hole size one notch on the 3.5mm jacks. No problem here on the existing boards, but during the build I accidentily slopped some solder in one of the holes and had to suck it out with the solder sucker (solder wick works too) to get the jack in.
  • The silk screen outline for the 4.7uF coupling caps was square from the Eagle library while the part is 8.4 x 7.2. On the existing boards I left appropriate space between the caps to compensate. I've gone back and added the exact shape to the library.
  • The two SMD 0.001uF cap pads under the 4.7uFs are removed. These are not used in the build and are left over from my past DC servo attempts. I hadn't noticed they were still there in V1.5. Just leave those pads unused with the existing boards.
  • Added a SMD pad for a 10 ohm 1206 SMD R74 in series with R31 in the power supply section. Now this is getting really perfectionist, The rail runs at 7.020Vdc in the existing builds and that 10 ohm resistor moves it to 7.000Vdc. I had space so why not. You can just put a 0 ohm jumper in there or short it with a piece of wire and solder if you don't want to mess with it.
  • Added a 1206 SMD resistor pad in series with the output on each channel of the pre-amp chip, going to the RCA pre-amp outs. For very long RCA cables some chips may be happiest with 20R or so of series resistance to decouple cable capacitance. This can easily be added with the existing boards by cutting the trace between JP15 and J5 with an xacto knife and soldering in a through-hole resistor between the pins. If not using an output resistor just short those across with a 0R 1206 jumper (in the BOM) or a wire.
  • Moved R27 in the power section up slightly from the board edge. If someone left an extremely large solder ball there it could possibly have touched the back panel. No problem on the existing boards - if you use too much solder on that end of R27, just use some solder wick to suck a bit of it off.
  • Moved C37 and C41 to better take advantage of free space on the board.
Attached Images
File Type: jpg desktop headphone amp circuit V1.6.jpg (542.5 KB, 503 views)
File Type: png desktop headphone amp layout V1.6.png (738.6 KB, 458 views)
Attached Files
File Type: zip 80mm x 160mm 4 layer 1.6mm green HASL 100pct etest.zip (206.9 KB, 31 views)
File Type: pdf desktop headphone amp circuit V1.6.pdf (945.7 KB, 92 views)
File Type: pdf desktop headphone amp layout V1.6.pdf (800.7 KB, 44 views)
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Old 19th July 2013, 04:40 PM   #174
2000impreza is offline 2000impreza  Canada
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Thanks for the update. I just sent off the updated 4 layer gerbers to seeed studio to run a batch of 10. The price difference between 5 and 10 was small enough I just went with 10.

For the 4 position rotary switch, ALPS SRBM140800 looks like it should work. Will take a closer look at the datasheet later. Mouser is out of stock but it is available from online components.

SRBM140800 Alps Electric | Rotary Switches SRBM140800 | Onlinecomponents.com

Last edited by 2000impreza; 19th July 2013 at 04:42 PM.
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Old 19th July 2013, 06:20 PM   #175
agdr is offline agdr  United States
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A version of an O2 Desktop Amp (ODA)
2000impreza - good find on that Alps switch! I had completely missed that one. From the data sheet it does look like an exact copy - or vise versa - of the Alpha switch. Mouser has 61 in stock of the version with the T18 15mm shaft (688-SRBM140700), which would match up with the 15mm shaft on the Bourns pot.

That Alps switch would actually solve a couple of problems. Right now the 20mm shaft on the Alpha switch sticks out 5mm past the shaft of the Bourns pot and may need to be cut, depending upon the knob being used. With a 15mm shaft it could use that same skinny knob from Mouser that I have in the photos on the pot. And related to that, the Alpha switch has an oddball D-shaft size. I couldn't find any of the pre-molded Mouser knobs that would fit (shaft slightly too big in diameter) and went to a knob with a setscrew. That Alps 140700 has the same T18 shaft as the Bourns pot that would fit the knob. I've attached the list of Mouser part number for that knob below in the various color combinations.

The 10 board option from Seeed is definitely the better cost-per-board. With DHL shipping included I think it works out to something like $24 a board for 5 boards vs. $15 a board for 10.
Attached Images
File Type: png knob.PNG (81.1 KB, 370 views)

Last edited by agdr; 19th July 2013 at 06:30 PM.
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Old 20th July 2013, 09:26 PM   #176
Heffa is offline Heffa  Sweden
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Quote:
Originally Posted by 2000impreza View Post
Thanks for the update. I just sent off the updated 4 layer gerbers to seeed studio to run a batch of 10. The price difference between 5 and 10 was small enough I just went with 10.

For the 4 position rotary switch, ALPS SRBM140800 looks like it should work. Will take a closer look at the datasheet later. Mouser is out of stock but it is available from online components.

SRBM140800 Alps Electric | Rotary Switches SRBM140800 | Onlinecomponents.com
2000impreza - are you going to use all your PCB'c, or would you consider selling one to me in Sweden?
I would love to compare this amp with my PPA.

I will probably order most of the the components from Mouser, and will try to find something to replace the DP4T ALPS switch with - it will probably be too expensive to order just one component from OnlineComponents.

---
Sent a long time ago from a Galaxy [Tab] far far away using Tapatalk HD
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Old 21st July 2013, 03:35 AM   #177
2000impreza is offline 2000impreza  Canada
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Quote:
Originally Posted by Heffa View Post
2000impreza - are you going to use all your PCB'c, or would you consider selling one to me in Sweden?
I would love to compare this amp with my PPA.

I will probably order most of the the components from Mouser, and will try to find something to replace the DP4T ALPS switch with - it will probably be too expensive to order just one component from OnlineComponents.

---
Sent a long time ago from a Galaxy [Tab] far far away using Tapatalk HD
Heffa - I definitely do not need all the pcb's. Realistically I will probably only use 2 or maybe 3 to experiment with. I have no problems with sending you a PCB. Before that happens I want make sure there are no obvious quality problems with the seeed boards and that I am not breaking any of the forum rules. Let me get back to you after I receive the batch of boards and get a chance to look them over.

Regarding the gain switch, agdr identified an alternative part (688-SRBM140700) that can be used. This part is currently in stock at Mouser.
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Old 21st July 2013, 02:02 PM   #178
Heffa is offline Heffa  Sweden
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Quote:
Originally Posted by 2000impreza View Post
Heffa - I definitely do not need all the pcb's. Realistically I will probably only use 2 or maybe 3 to experiment with. I have no problems with sending you a PCB. Before that happens I want make sure there are no obvious quality problems with the seeed boards and that I am not breaking any of the forum rules. Let me get back to you after I receive the batch of boards and get a chance to look them over.

Regarding the gain switch, agdr identified an alternative part (688-SRBM140700) that can be used. This part is currently in stock at Mouser.
That sounds great - I'm in no hurry, won't probably have time to build anything for some weeks/months anyway

---
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Old 23rd July 2013, 02:25 AM   #179
agdr is offline agdr  United States
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2000impreza - I'm aware of another forum member who wanted a board. I'll send him a PM.
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Old 23rd July 2013, 02:36 AM   #180
agdr is offline agdr  United States
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Default Panels are back

Well I have to say that I'm impressed with Proto Panel! The panels for this project are back already. I emailed the order just last Wednesday, so that is just 4 business days to get them in my hands. Wow. Disclaimer - I'm not affiliated with them in any way, just a happy customer at this point.

They honored the 40% off special they have running through the end of the month. For the panels with no lettering it worked out to be around $8.60 a panel with the discount, plus $6 shipping. Can't beat that. For panels with lettering it looks like it will be around $12 a panel with the discount.

The first photo below is the received panels. Packed in bubble wrap and also shrink-wrapped. The next is the back panel installed, and the third is the front panel.

The two CAD renderings are the panels with lettering. I'm going to send those in next to see how lettering works out. Proto Panel says they can only do engraved lettering for the small batches. For silk screened they would send the panels out, which would only work economically in larger batches.

The rear panel was nearly error-free (my measurements) luckily. It is hard to tell from the photo but the hole for the voltage select button needs to be moved up slightly to be centered. The hole for the RCA jack mounting screw could also be 0.2mm bigger. Everything else was right on the money and just fit right together.

The front panel needs a little more work. I had completely left off the hole for the 3.5mm input jack. Oops. I hand drilled the one in the photo below. The 1/4 inch and 3.5mm jacks need to be moved up about 0.2mm also, and the 3.5mm jack holes should be a little larger, 6.5mm instead of 6. The pot hole also needed to be 1mm larger.

So I'm going to send off another run of the panels with the adjustments and lettering this time and see how it comes out.


Here is something unrelated but interesting. In the solid state forum right now there is a posting in one of the threads that links to this slide presentation by the chairman and co-founder of Audio Precision on designing for ultra low distortion:

By Bruce Hofer, Chairman & Co-Founder, Audio Precision - Hosted by Bonis Audio - 2013 June

Skip to slide 18

Recommendations for Audio Circuits

and he concludes that 1206 sized thin film resistors are the best overall SMDs for audio use - exactly what I've used in the ODA signal path here. My main reason for using the 1206 size was to make it easy to solder, but it was in the back of my mind that the thermal change would be less with the bigger resistor (vs 0805 or 0603). RocketScientist / NwAvGuy wrote once that small SMD can actually be worse than through hole for that reason.

The presentation also suggests on that slide that very low temperature coefficient 1206 thin films are worth the money. Looking through what Mouser has it appears that the values of 5 ppm/C and 10 ppm/C he mentions are available, but largely special order and minimum 1000 quantity - and very pricey. I'm starting to see why Audio Precision analyzers cost so much. It looks like the 4.99K ground return resistor is avaliable in low tempco from Mouser at around $3.50 each. I'll add that to the BOM as an option along with the standard 25 ppm/C.

His second point on that slide under the 1206 size is one reason I stayed with larger through-hole resistors for the attenuation resistor and 1.5K feedback resistor, along with good current noise numbers. Since the gain stage can swing up to 14V peak on the output, I wanted to make sure the power dissipated was not just within the resistor's rating, but well within the rating to limit overall thermal effects in the resistor.
Attached Images
File Type: jpg IMG_1981.JPG (217.0 KB, 291 views)
File Type: jpg IMG_1982.JPG (113.6 KB, 285 views)
File Type: jpg IMG_1983.JPG (134.1 KB, 131 views)
File Type: png ODA front panel.PNG (57.3 KB, 132 views)
File Type: png ODA rear panel.PNG (54.8 KB, 87 views)

Last edited by agdr; 23rd July 2013 at 03:05 AM.
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