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Old 11th June 2013, 05:00 PM   #131
qusp is offline qusp  Australia
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you misunderstand, chips bottom, heatsink on top OF THE PCB. you will find the copper a better place to heatsink from if you do it right. much larger contact area
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Old 11th June 2013, 10:28 PM   #132
agdr is offline agdr  United States
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Quote:
Originally Posted by qusp View Post
you misunderstand, chips bottom, heatsink on top OF THE PCB. you will find the copper a better place to heatsink from if you do it right. much larger contact area
Ah, I see, hence the thermal vias. I did a design like that early on with this desktop amp, back when I was thinking about using 2 parallel LME49600s, one on top and one on the bottom for each channel. Yep, that would definitely work better than my idea of heatsinking to the case bottom.

Something interesting on the 4 layer stuff. I was just studying the information on Seeed's site and I see that the inner layers are thinner - 0.5oz as opposed to 1oz on top and bottom - so the inner traces need to be wider to handle the same current. Perfect for what I have in mind, just some additional signal routing and the ground plane. The spacings are also different. The outer-to-inner is thinner than outer to outer, which may or may not make a difference depending on what is being done with the circuit.

Last edited by agdr; 11th June 2013 at 10:50 PM.
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Old 12th June 2013, 02:06 AM   #133
agdr is offline agdr  United States
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Well it doesn't look like 4 layer is going to be difficult at all. I've left a significant amount of space between the ground plane and the traces to reduce the fringing fields. I'm leaving the jumpers on power traces since they can be top soldered as well as bottom soldered to eliminate via current problems. Saves space over just punching through multiple vias. '

The plane under the input parts will be connected to the input (clean / high impedance and low current) segment of the ground star, while the plane under the output parts is connected to the output (dirty / low impedance and high current) segment of the ground star. No plane under the power supply parts and no connection to the power supply reference section of the ground star, since there is nothing to gain and only noise to introduce.
Attached Images
File Type: png ODA 4 layer layout.png (692.9 KB, 369 views)

Last edited by agdr; 12th June 2013 at 02:35 AM.
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Old 13th June 2013, 05:23 AM   #134
agdr is offline agdr  United States
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This one just went out to fab. 4 layer Gerbers below.

I double checked the whole layout and made a few updates. The gain switch and pot are now a bit farther forward. The 3.5mm input jack now has more spacing and the PCB holes for front external input (like an ODAC or front panel input jack) are back, as well as those existing on the rear RCA input connector. In other words, an ODAC can be wired into either the front holes near the 3.5mm connector or to those near the rear RCA connector.

If wired into the rear connector then the front 3.5mm jack still remains fully functional and switch-selectable with the front panel input select switch. That method would also make the ODAC output available at the rear RCA connectors, so they would effectively become an ODAC out (for pre-amps).

I'll post again once the boards are back and I have one built up and tested.
Attached Images
File Type: png Desktop amp 4 layer layout.png (733.6 KB, 319 views)
File Type: jpg Desktop amp 4 layer circuit.jpg (543.0 KB, 307 views)
Attached Files
File Type: zip 80x160 4 layer 1.6mm green HASL 10pcs .zip (205.0 KB, 23 views)

Last edited by agdr; 13th June 2013 at 05:31 AM.
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Old 16th June 2013, 04:15 AM   #135
agdr is offline agdr  United States
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Here are a first pass at the front and back panels for this project using the Front Panel Express software. I'll also post all the hole locations and dimensions of the final result. Since all the holes are round a drill press would do the job just fine with the panels that come with the B4-080 box.

Scroll down on both-
Attached Files
File Type: pdf Desktop Headphone Amp Front Panel.pdf (14.6 KB, 57 views)
File Type: pdf Desktop Headphone Amp Rear Panel.pdf (9.9 KB, 40 views)

Last edited by agdr; 16th June 2013 at 04:18 AM.
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Old 22nd June 2013, 04:19 AM   #136
agdr is offline agdr  United States
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The boards for this project are back from fabrication already! Speedy. I have to say that I am extremely impressed with the results. Even nicer than the previous 2 layer boards I've had done. Super alignment, nice crisp printing, great surface finish. Maybe they spend more time on the pricer 4 layer boards, or maybe I just got luckier this time. The various 3rd layer traces ohm-check just fine.

I'm going to build one up and see if it flies. So far so good... The voltage on the meter for the positive rail is the correct answer, of course, for unloaded voltage off the caps with the 24Vac transformer and 121Vac on the primary from the wall socket.
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File Type: jpg IMG_1913.JPG (213.0 KB, 241 views)
File Type: jpg IMG_1914.JPG (174.5 KB, 199 views)
File Type: jpg IMG_1919.JPG (165.5 KB, 99 views)
File Type: jpg IMG_1922.JPG (182.5 KB, 89 views)

Last edited by agdr; 22nd June 2013 at 04:41 AM.
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Old 24th June 2013, 06:20 AM   #137
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Can't wait to see how this turns out.

Erik
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Old 25th June 2013, 04:23 AM   #138
agdr is offline agdr  United States
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Erik - thanks!

Well, it lives! In the photo below the new 4 layer board is in the vise at the top, with the old 2 layer board below (partially scavenged of parts). Sounds fantastic. I've been listening to a bunch of old Simon and Garfunkel songs with the new amp for the last hour or so. Almost fully built up. I can't find one of the 4.7uF caps, I need to order in LEDs or zeners for the clipping circuit reference (may try zeners this time) and need to solder in the pre-amp parts.

I'm want to do a bunch of measurements on it now, which I'll post. I want to look at full load power supply ripple on the scope and frequency response for a few waveshapes across the output load range.

The power rails were right on the nose with the meter. +7.023Vdc/-7.004Vdc and +16.092Vdc/-15.936Vdc. The output DC offset is +1.447Vdc on one channel and -1.630Vdc on the other, with the 0.5R output balancing resistors in now. The new headphone output relay circuit across the pre-regulators is also right on the mark. Exactly 6 seconds of delay, as before with the previous +24Vdc circuit.

There are no major screw ups with the 4 layer PCB! At least none I've found yet. I've also learned from another forum of an alternative place (to Front Panel Express) that has a 40% off sale, which would work out to be about half the cost of FPE. I think I'm going to test that (and my measurements) out and get the panels fabbed.

I'm still just flabergasted at the complete lack of *any* background hiss on this thing, even with the volume control fully up on the high gain setting. And that is using my uber sensitive AKG K550's. Every now and then something actually works out better than expected. During a full volume test at the highest gain setting this evening I thought that I was finally hearing just the faintest traces of hiss, right at the edge of hearing. Then I remembered that the input cable was still plugged into the laptop, which was on mute. Pulled the cable out, which grounds the inputs via the jack, and back to zero backgrouund hiss. That was the amplified hiss of the muted laptop output stage (the source) I was hearing, not the amp's internals.
Attached Images
File Type: jpg IMG_1933.JPG (232.3 KB, 114 views)
File Type: jpg IMG_1934.JPG (236.5 KB, 104 views)

Last edited by agdr; 25th June 2013 at 04:50 AM.
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Old 25th June 2013, 06:21 AM   #139
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Interested in a PCB of the final version and front panel etc... heck make it a kit
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Old 25th June 2013, 03:36 PM   #140
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Quote:
Originally Posted by decayed.cell View Post
Interested in a PCB of the final version and front panel etc... heck make it a kit
+1

agdr, what is the other site for front panels? I need some made for an O2 for a friend.

Thanks
-Erik
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