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Old 9th April 2013, 05:31 AM   #51
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Good! Glad to know things are moving in right direction on the O2 desktop front.
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Last edited by availlyrics; 9th April 2013 at 05:44 AM.
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Old 9th April 2013, 06:41 PM   #52
agdr is offline agdr  United States
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Quote:
Originally Posted by availlyrics View Post
Good! Glad to know things are moving in right direction on the O2 desktop front.
I'll be interested to hear the results myself!

The latest stuff is at the Google link in the post above in the 4/8/2013 folder, including a first pass at a BOM. Looks like about $120 for parts and another $25 for case / power adaptor / PCB. I've attached a summary below of the differences between this version of an ODA and RocketScientist/NwAvGuy's O2 headphone amp.

I just keep messing with it. Latest stuff for the 80x100mm (O2 sized) version in the B3-080 case:

* Two NJM4556AL buffers per channel now down from 3 to allow proper airflow (cooling) around the chips. Power supply re-sized to match.
* I was able to fit a DC servo using the OPA188 on one channel as a test of the circuit. The servo parts don't have to be populated - optional. The bigger version on a 80 x 160mm PCB in the B4-080 case will have servos on both channels and back to 3 NJM4556AL chips per channel.
* Input coupling caps are gone and instead the O2-style coupling capacitor in the middle is increased to 3x 4.7uF on each channel to drop the corner frequency to 1.3Hz. The O2 is 1.8Hz.
* Clipping detect LED now by the input jack and volume control.
* Optional SMD bypass caps added right at each NJM4556AL buffer chip's power pins. I haven't had any oscillation problems at all with the test PCB I'm using with the LME49990 wrapped around two NJM4556A's, but the pads are there just in case.

I'll be curious to see if I can eventually get this amp to measure at least slightly better than the O2 in a few areas. I'm up to the challenge.
Attached Images
File Type: png ODA layout both layers.png (119.5 KB, 431 views)
Attached Files
File Type: pdf ODA vs. O2 differences 3 80x100.pdf (72.1 KB, 52 views)

Last edited by agdr; 9th April 2013 at 06:47 PM.
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Old 10th April 2013, 01:24 PM   #53
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Very exciting agdr I will be very impressed if you can come close to the numbers of the O2, and probable poorer if you succeed
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Old 12th April 2013, 06:16 AM   #54
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Quote:
Originally Posted by agdr View Post
....I'll be curious to see if I can eventually get this amp to measure at least slightly better than the O2 in a few areas. I'm up to the challenge.
I'am sure you will.
Best of luck
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Old 12th April 2013, 02:31 PM   #55
agdr is offline agdr  United States
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The design just did another morph. I had actually sent it out for fab a couple of days ago but immediately had regrets over not having a 1/4" output jack and RCA input jack mounted on the PCB. Thinking about it some more those two seem like requirements for a desktop headphone amp. Plus I remember RocketScientist / NwAvGuy posting in a comment somewhere that he was going to have a 1/4" jack on his ODA. So I've cancelled the order and went back at it.

New stuff is at the Google docs link in the post above and a summary below. The PCB is now the more pricey ($14) 80mm x 160mm size that fits the more pricey ($18.50) B4-080 aluminum case that is 2.5 inches wider and 1 inch taller than the O2. 3 buffers per channel again, DC servos on both channels, pre-regulators again, and other good stuff.
Attached Images
File Type: jpg ODA 80x160mm layout both layers.jpg (1,019.8 KB, 386 views)
File Type: jpg ODA 80x160mm circuit.jpg (594.1 KB, 365 views)
Attached Files
File Type: pdf ODA changes 4-11-2013.pdf (49.3 KB, 32 views)

Last edited by agdr; 12th April 2013 at 02:33 PM.
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Old 12th April 2013, 06:51 PM   #56
Lukky is offline Lukky  United States
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I've been closely watching your progress and thought I'd my 2 cents.

I have an O2 setup as a desktop amp. I added a switch and some RCA outs to use it as a pre-amp for my Honey Badger. There's an open spot next to your 3.5mm that would be a good spot for an optional switch. Maybe by default, the 3.5mm and 1/4" are parallel. How about an option for manually switching them? I would exclude the 3.5mm and switch between RCA out and 1/4" out. With your current layout (1/4" on the board), I would have to cut the parallel traces to switch off the headphones. Yes, I am too lazy to unplug them.
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Old 12th April 2013, 09:04 PM   #57
agdr is offline agdr  United States
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Lukky - good thoughts, I've been thinking about a pre-amp out. I know a that a lot of folks have asked for that in the O2 amp thread here and over in the comments on RocketScientist / NwAvGuy's blog. So far I've been planning on handling pre-amp outs with externally mounted parts. The extra inch of height in that B4-080 box over the O2' s B2-080 case for add-ons and modificationsis is another thing that led me to go that route.

I think I have a good solution for what you are after. Those two resistors to the left of the 3.5mm output jack, R61 and R62, are new (vs. the O2). They are ostensibly for adding optional series resistance between the relay and output jacks to change the damping factor. But they can also just be jumpered with a wire and the output of the buffer chips run right to the output jacks, as in the O2.

But in your case your could mount a panel-mount DPDT switch and panel mount RCA jacks in the inch or so above those 3.5 and 1/4" output jacks. Then wire it up with a DPDT switch as the first attachment below, with R61 and R62 left off the board, and instead wires go from those holes to the DPDT switch as shown. In one switch position the relay output goes to the jacks and in the other the jacks are cut off and the output goes to your RCA pre-amp out jacks.

But... an idea just hit me. I really should add PCB holes for connecting wires to the output of the gain stage itself, right at the outputs of the gain stage LME49990 chips (the inputs to the pot in the middle). That way pre-amp out could be taken off right from the gain stage, a more appropriate place, with no need to plow through that 1x output current buffer stage. Also isolates the pre-amp outs from the headphone outs.

The LME49990 is fully specified with a 600 ohm load. In fact the AC number gets slightly better at 600R than 2K from the datasheet graphs. So if you are using the new 1K (lower noise) pot option in the ODA, then attach a 10K input to your amp at those pre-amp outs, that is still just 1K || 10K = 900 ohms, well within load range for the gain stage. You could even drop an amp input all the way down to 1.5K on those pre-amp outputs and still be at 600R total load on the gain stage with the 1K pot.

So instead you could wire things up as the second attachment below. Use a 4PDT panel mount toggle switch like #633-M204205 at Mouser

M2042LL1W01 NKK Switches | Mouser

and wire it up with using wires from those new gain stage out holes on the PCB. In one position the gain stage out is routed to your pre-amp outs and the jacks are cut off via wires from those R61 R62 holes to the other half of the switch. Then in the other position your pre-amp outs are cut off and the headphone jacks are on. Or, just connect the gain stage output wires directly to the pre-amp out RCAs to have those on all the time, but put a DPDT on the jacks via the method above with the R61 R62 holes to switch the headphones off.

Lots of options! The RCA pre-amp output jacks could be mounted on the back panel too. I like to facilitate modifications, even on my own designs!

Click on the arrows in the lower left corner of the pix below, once they are open, to blow them up to readable size.
Attached Images
File Type: png pre-amp 1.png (385.7 KB, 319 views)
File Type: png pre-amp 4PDT.png (385.7 KB, 316 views)

Last edited by agdr; 12th April 2013 at 09:13 PM.
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Old 14th April 2013, 02:19 PM   #58
uncola is offline uncola  United States
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I hope you finish your design and it gets adopted. I think a lot of people avoid the o2 amp just because all the ports are in the front. Have you thought about adding a built in dac?
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Old 14th April 2013, 07:44 PM   #59
agdr is offline agdr  United States
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uncola - I agree, I think it looks kind of bad having the power jack on the front. NwAvGuy probably did that in part to keep costs down, so that only one custom drilled panel would be needed. For the desktop unit though the power really should come in via the back, I think.

The power switch would probably be handier in front, but electrically and from a layout standpoint having the switch in back won. I wanted the switch right on the AC lines to fully cut off everything when off (and easier on switch contacts to "break" AC than DC), unlike the O2 that left the AC power supply running regardless. So if the switch wasn't in back I would have to run noisy AC lines through the PCB, or add a relay. Easiest to just have it in back.

I've left space in this one for RocketScientist / NwAvGuy's ODAC. Another plus on using the taller box. I've made sure none of the parts stick up so far as to prevent a second PCB being slid into the top slot in the box, upside down so the parts hang down and the solder joints face up. I've been thinking about whipping up an "accessory board" like that with a full Baxendall tone control, crossfeed, etc., and the space for an ODAC. That upper board doesn't necessary have to be an etched PCB for just mounting the ODAC. A piece of 160mm wide perf-board, or even copper clad board, from the local Radio Shack or other hobby store would work just fine for mounting the ODAC in the top slot. If the ODAC is butted up against the back panel a mini-USB slot could be cut into the back panel along with the other holes. The ODA PCB has holes near the input jack for hooking in the ODAC output or any other connection to the signal input lines.

There is also a B4-160 case available (twice as long) which is another way to do it. Two of these PCBs could be used end to end like I was going to do with the 80x100 PCB. Only the power supply parts can be populated on the back one, an ODAC mounted on the front 2/3, then the power cabled over to the front board that doesn't have the PS parts popluated. I've added PCB holes on the power supply lines to allow cabling like that, or to tap off power for an accessory board on the top.

I just whipped up an example layout below that has Lukky's idea for the RCA pre-amp outs (horizontal jack in this case instead of the vertical input jack on the main ODA board) and a switch. Plus the area for an ODAC and holes to let the heat from the buffer chips make it on up to the top of the aluminum case.
Attached Images
File Type: png ODA accessory PCB circuit.png (6.7 KB, 42 views)
File Type: png ODA accessory PCB layout.png (15.6 KB, 35 views)

Last edited by agdr; 14th April 2013 at 08:05 PM.
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Old 14th April 2013, 10:50 PM   #60
agdr is offline agdr  United States
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Default differential drive - moar powerz! - and gain stage bypass

Here are two more somewhat slick things that can be done with this version of an ODA:

* Differential drive to double the output swing to +/-20Vdc.

The two channels of a single ODA can be differentially driven for extremely hard-to-drive high impedance headphones by a chip like the OPA1632 that opc used in the balanced Wire amp. The two wires for one headphone channel hook to the signal out on each channel and effectively receive twice the voltage swing, about +/-20V. The OPA1632 can be mounted on a top-slot accessory board, as mentioned in the post above.

Note that two ODA's would be needed, one for each channel, since the two channels in each one form the differential drive for each headphone channel.

Doing differential drive is nothing new and has been mentioned a lot on the forum. What is new is that now the buffer chips can handle the dissipation. That has been in the back of my mind in beefing up the number of parallel output chips. The load looks like half the impedance in bridged mode and usually winds up overheating the output chips. That would be the likely outcome of the O2 run in bridged mode. The ODA here has no such problem. Since the headphones are assumed to be high impedance (600R etc) the current requirements will be relatively low and the buffer chips should be sufficient to dissipate that much voltage swing.

The other thing that is new is that with two ODAs, each is run off a separate AC wall wart, which means the power supplies are isolated from each other. The other thing that can happen in bridged mode is the common ground wire in the headphone cable winds up shorting out the amp unless the headphone cable is restrung to have two separate wires to each driver. Here that shouldn't be a problem since the two ODAs, one for each earcup, are isolated at the wall wart.

* Gain stage bypassing. In addition to PCB holes at the output of the gain stage chip I've added O ohm jumper resistors before those, between the gain stage chip and the tap off. So now if the OR resistors are left off the gain stage chips and parts can be left unpopulated and the amplifier input after the RF filter run directly to the 1K or 10K pot (and series attenuation resistors if they are installed) and on to the output current buffer stage. That arrangement is for people who don't need any voltage gain due to higher voltage sources but still need current buffering.

Last edited by agdr; 14th April 2013 at 10:57 PM.
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