I'll bet you haven't installed the jumper wire yet that goes from JP11 near the volume control to JP14 down by the 14 pin chip. That is one of the 2 inputs to the clipping circuit. Without the jumper that input would be left floating and probably do some random things. Let me know! 🙂
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Yep, definitely didn't install that jumper. I didn't have time last night to add it, but will tonight.
I did a quick search in the build instructions for JP14, and just the number 14 by itself to try to find it and the only search results were C14 and R14; no JP14. Maybe I missed it in the instructions as just "jumper" or some other verbiage...(And now that you mentioned it, I do remember a build picture from you in an earlier post that has that wire across the board)
Will post back after the fix 🙂
Entirely possible! 😱 Come to think of it, I think someone else discovered the ommission earlier in the thread. I was going to add it and I'll bet I forgot. Sorry about that! 🙂
O2 inverting vs ODA
First of all, I'm sorry I didn't post this sooner... non-fun parts of life got in the way.
But enough with that... on to the review. As always I start with the most simple conclusion I can draw: the ODA is better than the o2 inverting but for most people, I'm guessing the price difference between the two would not justify building an ODA if you are only going to build one. For me, if I had to do it all over again, I would definitely still build an ODA even if I already had regular o2 or an inverting o2.
Now with the details:
My music chain did change since I compared the O2 + booster board vs ODA. From computer to amp:
computer -> USB cord -> doodlebug (linear-regulated usb power supply and isolation) -> USB cord -> pupdac (digital-audio converter) -> RCA cords -> ODA
For the o2 inverting, everything was the same except for last 2 steps being 3.5mm cord -> o2 inverting.
The pupdac has both rca outs and 3.5mm out. I plugged in both amps and just swapped headphones quickly between the two. The headphones I tested with are HeadFi-Man HE400 and AKG K7XX. (If anyone is looking to build a DIY dac, the pupdac is pretty good.)
I found 3 audible difference between the two, and all of these difference are VERY slight. Everything that follows is really making a mountain out of a mole hill.
Bass - The o2 inverting definitely gives more bass at the same volume, but the ODA can go deeper. This was hard to really nail down cause I kept hearing "more" from the o2 inverting, but not exactly the same. It took a couple of solid hertz waves to really hear the difference at low frequencies (amps were level matched listening to music). Just listening to music only, the big difference was just more volume of bass from the o2 inverting.
Speed - I really couldn't tell this difference on the AKG headphone, but on the HE400s, this was an obvious difference. The "decay" of sounds lasts longer on the o2 inverting than on the ODA. Best way to really describe this was that there was an additional echo when listening to the o2 inverting. Or kinda like the effect when you hold the right pedal of a piano (sustaining pedal). The notes are all held longer. Now, the sounds that the o2 inverting reproduced could have been more "accurate" to what was intended with the recording. I'm just pointing out that the o2 holds notes or sounds longer than the ODA.
Distortion - At high volumes, the ODA is WAY more clear. Now this is only at the high gain for the o2 inverting, but there is distortion without clipping. If i take the ODA to highest gain (8x) and then take the o2 inverting to highest gain (i think 6.5x) and then match the volume, the o2 inverting has more distortion. If you max out the volume on the low gain of the o2 inverting, there is no distortion, only high gain, high volume and really I don't think anyone can listen long at those levels.
So that's it. In conclusion I enjoy both amps and will continue to use both. I do really like the size of the o2 inverting and the front panel power switch, but my ODA will remain my "go to" for home use. Now I need to compare the o2 + booster board to the o2 inverting to see which one will become my permanent work amp 🙂
First of all, I'm sorry I didn't post this sooner... non-fun parts of life got in the way.
But enough with that... on to the review. As always I start with the most simple conclusion I can draw: the ODA is better than the o2 inverting but for most people, I'm guessing the price difference between the two would not justify building an ODA if you are only going to build one. For me, if I had to do it all over again, I would definitely still build an ODA even if I already had regular o2 or an inverting o2.
Now with the details:
My music chain did change since I compared the O2 + booster board vs ODA. From computer to amp:
computer -> USB cord -> doodlebug (linear-regulated usb power supply and isolation) -> USB cord -> pupdac (digital-audio converter) -> RCA cords -> ODA
For the o2 inverting, everything was the same except for last 2 steps being 3.5mm cord -> o2 inverting.
The pupdac has both rca outs and 3.5mm out. I plugged in both amps and just swapped headphones quickly between the two. The headphones I tested with are HeadFi-Man HE400 and AKG K7XX. (If anyone is looking to build a DIY dac, the pupdac is pretty good.)
I found 3 audible difference between the two, and all of these difference are VERY slight. Everything that follows is really making a mountain out of a mole hill.
Bass - The o2 inverting definitely gives more bass at the same volume, but the ODA can go deeper. This was hard to really nail down cause I kept hearing "more" from the o2 inverting, but not exactly the same. It took a couple of solid hertz waves to really hear the difference at low frequencies (amps were level matched listening to music). Just listening to music only, the big difference was just more volume of bass from the o2 inverting.
Speed - I really couldn't tell this difference on the AKG headphone, but on the HE400s, this was an obvious difference. The "decay" of sounds lasts longer on the o2 inverting than on the ODA. Best way to really describe this was that there was an additional echo when listening to the o2 inverting. Or kinda like the effect when you hold the right pedal of a piano (sustaining pedal). The notes are all held longer. Now, the sounds that the o2 inverting reproduced could have been more "accurate" to what was intended with the recording. I'm just pointing out that the o2 holds notes or sounds longer than the ODA.
Distortion - At high volumes, the ODA is WAY more clear. Now this is only at the high gain for the o2 inverting, but there is distortion without clipping. If i take the ODA to highest gain (8x) and then take the o2 inverting to highest gain (i think 6.5x) and then match the volume, the o2 inverting has more distortion. If you max out the volume on the low gain of the o2 inverting, there is no distortion, only high gain, high volume and really I don't think anyone can listen long at those levels.
So that's it. In conclusion I enjoy both amps and will continue to use both. I do really like the size of the o2 inverting and the front panel power switch, but my ODA will remain my "go to" for home use. Now I need to compare the o2 + booster board to the o2 inverting to see which one will become my permanent work amp 🙂
4x4F150 - hey a big thank you for that in depth listening review and comparison!! I know that takes quite a bit of time A/B-ing with the various amps and music.
Always good to hear that the ODA is holding its own! 😀 It seems that the ODA somehow "fixed" the bass problem that the NJM4556s seem to have (subjective, to me) in the O2 by paralleling more of them. Also the fixed AC rails in the ODA probably helps, that problem in the O2 has always been most noticeable to me on batteries with the lower voltage rails. I really don't hear it when the O2 is on AC.
Another tech difference between the inverting O2 and the O2 / ODA is the BUF634 output chips. I used those over the similar LME49600s initially because they come in the upright TO-220 package which allowed vertical heat sinks. But then half way through the design TI discontinued the LME49600s, so it went from a package choice to a neccessity!
The LME49600s have the THD+N curves in the datasheet with a LME49720 wrapped around it. But the BUF634s don't. I haven't really seen good (AP, dScope) THD measurements made on a looped BUF634 pair, although I know a ton of headphone amps have been built with it over the years.
Always good to hear that the ODA is holding its own! 😀 It seems that the ODA somehow "fixed" the bass problem that the NJM4556s seem to have (subjective, to me) in the O2 by paralleling more of them. Also the fixed AC rails in the ODA probably helps, that problem in the O2 has always been most noticeable to me on batteries with the lower voltage rails. I really don't hear it when the O2 is on AC.
Another tech difference between the inverting O2 and the O2 / ODA is the BUF634 output chips. I used those over the similar LME49600s initially because they come in the upright TO-220 package which allowed vertical heat sinks. But then half way through the design TI discontinued the LME49600s, so it went from a package choice to a neccessity!
The LME49600s have the THD+N curves in the datasheet with a LME49720 wrapped around it. But the BUF634s don't. I haven't really seen good (AP, dScope) THD measurements made on a looped BUF634 pair, although I know a ton of headphone amps have been built with it over the years.
Agdr - Do you think that just that one buffer difference is responsible for the increased distortion? Or is it just that the ODA has a much more sophisticated power delivery system in general?
The reason I'm so curious about the distortion is that in casual listening, the only difference I think between the two amps is the extra distortion in the o2 inverting. But like I said in my previous post, the distortion is only high gain, high volume. You have to be actively listening for the other differences to find them. In other words, I was not really "enjoying" the music when A/B comparing... only trying really hard to focus on differences.
The reason I'm so curious about the distortion is that in casual listening, the only difference I think between the two amps is the extra distortion in the o2 inverting. But like I said in my previous post, the distortion is only high gain, high volume. You have to be actively listening for the other differences to find them. In other words, I was not really "enjoying" the music when A/B comparing... only trying really hard to focus on differences.
4x4F150 - hey good post, I'm heading out for the holiday though and will be gone all day tomorrow. I'll post a proper reply when I get back. 🙂
I have done a lot of blind AB testing or listening.....several things come to mind, trying to be objective as my brain will let me in between my moods and the number of good cold micro brews I might have sampled!! (LOL)...
One: Most amps sound better played loud.
Second: The only way I can really do a fair comparison sonically is to play a 1Khz test tone, set the level via an instrument (SPL meter, with an ingenious coupling device to get the levels as close as I can outside of a lab environment).
Them listen to a bunch of stuff I like or use for "testing" and do no touch the volume knob.
I have another person set up the stuff without me knowing which amp I am listening to ....
Thirdly...I try to come to a conclusion. More often than not I have a hard time telling one amp from another with a wide variety of headphones.
Having built the ODA, the original O2, AGDR's O2, and his new Super Cmoy.
Doing blind testing, its hard to tell them apart with efficient headphones that dont need gobs of power. But my subjective mind tells me there is!!
For the past week I was comparing differrent cmoy versions and I could swear one sounded better with a pair of Beyer T90s 250 ohms. I was only listening without any controlled spl....so after a week of this I set up and matched the levels and voila they both sound really close!
Thanks for you inputs 4x4F150, its good to read others opinions on how these amps perform with your headphones and music!
Happy 4th !!
Alex
One: Most amps sound better played loud.
Second: The only way I can really do a fair comparison sonically is to play a 1Khz test tone, set the level via an instrument (SPL meter, with an ingenious coupling device to get the levels as close as I can outside of a lab environment).
Them listen to a bunch of stuff I like or use for "testing" and do no touch the volume knob.
I have another person set up the stuff without me knowing which amp I am listening to ....
Thirdly...I try to come to a conclusion. More often than not I have a hard time telling one amp from another with a wide variety of headphones.
Having built the ODA, the original O2, AGDR's O2, and his new Super Cmoy.
Doing blind testing, its hard to tell them apart with efficient headphones that dont need gobs of power. But my subjective mind tells me there is!!
For the past week I was comparing differrent cmoy versions and I could swear one sounded better with a pair of Beyer T90s 250 ohms. I was only listening without any controlled spl....so after a week of this I set up and matched the levels and voila they both sound really close!
Thanks for you inputs 4x4F150, its good to read others opinions on how these amps perform with your headphones and music!
Happy 4th !!
Alex
I'm back in. 🙂
Interesting observations! So a few questions for you:
* So the clipping LED doesn't come on at the high-gain / high-volume levels you are trying? Pretty sure you said it didn't above, but just wanted to check.
* Also wanted to check that I'm understanding your setup. Am I reading right that for a given (large) source level (source left constant), the low gain doesn't show the issue but the high gain does?
* Are you using the BOM resistors values everywhere? For the input stage the BOM high gain resistor is 18.7K. The input resistor is two 4.99K's, for a total of 10K. I split them up to form the RF filter in the middle. That gives a high gain for the input stage of 18.7K / 10K = 1.87x.
* For the output stage the feedback resistor is 33K and the iput resitor 24.9K, so a gain of 33/24.9 = 1.33x. Multiply that input gain of 1.87 and this gain to get 1.87 * 1.33 = 2.5x total voltage gain in the high gain position. In the low gain position the input stage gain is actually attenuation, 0.75x, so the total end to end gain in the low position is 0.75 * 1.33 = 1.
* It would be really interesting to know the levels from your source. Do you have access to an oscilloscope to measure the input level? If not if you have a way to play a 400Hz tone (internet clip or burn that on a CD) of a known level you could measure the AC votlage output if you have access to a true-rms DMM.
Thanks! And thanks again for the great review.
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OPA1688 can substitute for the LME49880 and probably for the OPA2140
I can't remember if I've mentioned this in the thread yet, but the brand new OPA1688 chip from TI (what I'm using in the posted CMOY project here) can substitute just fine (may even be better) for the now-discontinued FET-input LME49880 gain stage chip in this project. It can probably also subsitute for the FET-input OPA2140 used in the output stage.
The OPA1688 wan't announced yet when I designed the inverting O2. A plus is that the OPA1688 does not have a metal power pad underneath like the LME49880 does because the idle current is so much less at just 1.6mA per section. Over half of the LME49880's package power dissipation is taken up by quiescent (idle) current dissipation at +/-15Vdc rails.
Both chips are FET-input, audio rated (low THD+N) with large drive current ability. The LME49800 is rated to drive down to 600 ohm loads, which I took advantage of with the 5K pot. Well the OPA1688 is rated down to 16 ohm loads! The OPA1688 has much lower DC offset voltage, but that doesn't matter in this case due to the coupling capacitor after the gain stage, similar to NwAvGuy's O2 headphone amp. The LME49880 was good for +/-17Vdc rails, the OPA1688 goes all the way up to +/-18Vdc. The OPA1688 has more open loop gain to provide more neagtive feedback, 130dB vs. 120dB typical.
The pinout on the two chips is the same (also for the OPA2140). You can just solder an OPA1688 right in for the LME49880 on the board.
As for the output stage OPA2140, the 2140 is a more modern chip than the gain stage LME49880 and is still in production. The one potential hiccup in replacing it with a OPA1688 is stability when wrapped around the BUF634 chip. The open & closed loop gain/phase plots of the OPA2140 and OPA1688 are remarkably similar, enough to make me believe the OPA1688 may be some sort of modern derivative of the OPA2140. 🙂 But regardless the only way to know for sure is test stability with various real world loads. I've done that with the OPA2140 + BUF634, but haven't tried yet with a OPA1688 + BUF634 pair.
I can't remember if I've mentioned this in the thread yet, but the brand new OPA1688 chip from TI (what I'm using in the posted CMOY project here) can substitute just fine (may even be better) for the now-discontinued FET-input LME49880 gain stage chip in this project. It can probably also subsitute for the FET-input OPA2140 used in the output stage.
The OPA1688 wan't announced yet when I designed the inverting O2. A plus is that the OPA1688 does not have a metal power pad underneath like the LME49880 does because the idle current is so much less at just 1.6mA per section. Over half of the LME49880's package power dissipation is taken up by quiescent (idle) current dissipation at +/-15Vdc rails.
Both chips are FET-input, audio rated (low THD+N) with large drive current ability. The LME49800 is rated to drive down to 600 ohm loads, which I took advantage of with the 5K pot. Well the OPA1688 is rated down to 16 ohm loads! The OPA1688 has much lower DC offset voltage, but that doesn't matter in this case due to the coupling capacitor after the gain stage, similar to NwAvGuy's O2 headphone amp. The LME49880 was good for +/-17Vdc rails, the OPA1688 goes all the way up to +/-18Vdc. The OPA1688 has more open loop gain to provide more neagtive feedback, 130dB vs. 120dB typical.
The pinout on the two chips is the same (also for the OPA2140). You can just solder an OPA1688 right in for the LME49880 on the board.
As for the output stage OPA2140, the 2140 is a more modern chip than the gain stage LME49880 and is still in production. The one potential hiccup in replacing it with a OPA1688 is stability when wrapped around the BUF634 chip. The open & closed loop gain/phase plots of the OPA2140 and OPA1688 are remarkably similar, enough to make me believe the OPA1688 may be some sort of modern derivative of the OPA2140. 🙂 But regardless the only way to know for sure is test stability with various real world loads. I've done that with the OPA2140 + BUF634, but haven't tried yet with a OPA1688 + BUF634 pair.
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You are right, from the looks of the datasheet! 🙂 I've never heard of the OPA172/2172/4172 before. Interesting. And I see the quad 4172 is a shipping part at Mouser, #595-OPA4172ID for $4.26. 😀
Thanks for the heads-up here, Sergey888!
Correct, clipping LED does not flash (I fixed the clipping circuit too with that jumper wire you pointed out). I don't think I could handle turning the volume up any higher to make it clip =)
Correct again, only distortion at high gain, and past maybe around 60% volume. Low gain + max volume = no distortion anywhere. All this with not changing the source level.
Yep, BOM values everywhere.
I personally do not have one but I might be able to borrow one for 1 night from work. (I haven't used an oscilloscope since college) I definitely can find some more tones for comparison. (I already used a 50 an 100 Hz to compare the 2 amps) But the pupdac is know to be a little "hot" for a dac. But then a lot of dacs output under the standard 2V, so when there's a device that actually does the std, its called "hot" 🙄
4x4F150 - thanks for the information! Lemme ponder that. I was just messing with the numbers. Even if your souce was putting out 3Vrms you still should be OK with the +/-15.3Vdc rails. 3Vrms would be 4.2V peak, x the input stage high gain of 1.87 = 7.85V peak, well under the 15.3Vdc rail. Then multiply that by the output stage with the volume control all the way up, x1.3 = 10.2Vdc, still well under 15.3Vdc.
What I want to do here is feed in a 3Vrms signal and have a look on the scope and distortion analyzer. Lets find out what is going on. 🙂
Also, in other news, I finally had a chance to try those eBay T10 torx screws for the Box Enclosures case yesterday on a different project with a B1-080 case, below. Works great! All 8 went in without snapping off. I had 2 or 3 of the box enclosures screws twist off on me a month or two ago on a project. Sure a good thing you got them to change their screws.
What I want to do here is feed in a 3Vrms signal and have a look on the scope and distortion analyzer. Lets find out what is going on. 🙂
Also, in other news, I finally had a chance to try those eBay T10 torx screws for the Box Enclosures case yesterday on a different project with a B1-080 case, below. Works great! All 8 went in without snapping off. I had 2 or 3 of the box enclosures screws twist off on me a month or two ago on a project. Sure a good thing you got them to change their screws.
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I'm very interested in this project. What's the estimate $ of all the parts? And AGDR, do you sell kits?
Thanks!
Thanks!
Hi! The BOM with all the parts totalled is with the rest of the project materials are the Google Drive link here:
https://drive.google.com/folderview?id=0B67cJELZW-i8S1VmUzZsUGZZN00&usp=sharing
then V3.6 -> BOM. The whole thing at Mouser totals up to $109 for the parts, less PC board, transformer and case. The transformer and case add another $28. But see my note under the total on the BOM:
"The 2 BUF634 current buffer ICs are $21 total, the two op-amps are $10 total, and the two voltage regulator chips are $11 total. So right there is $52 of the $103, just for the six integrated circuits."
If you happen to already have any of those chips on hand in the parts box, that would reduce the total! 🙂
No kits unfortunately, just too many parts to bag and label plus the chance of any semiconductors being bad. But if Mouser is out of stock of anything at the moment though I probably have it here. This amp uses the LME49880 chips that TI is discontinuing, but I should have those here. Their new OPA1688 should work just as well, it is another FET-input dual with great THD specs and high output current drive, but I haven't tried one in the board yet. That is the chip I'm using in my Super CMOY project in another thread.
The V3.6 is the current PC board. I have a file with a note in it on the Google Drive link explaining that the V3.61 was some minor cosmetic changes I made in the posted Gerber file, but I've never fabricated that version yet.
It looks like you are here in the US, so shippinng is just actual cost from the post office for "first class parcel", which is fine for bare PC boards and includes tracking, plus paypal's 2.9% + $0.50 of the total. The board is at-cost for anyone here on the forum. I've actually forgotten what is! I'll look it up and send you a PM. I should add the inverting board to my sales thread in the vendor forum too.
Its a fun project! I'm about to do some tests this weekend that I promissed 4x4F150 in a post above a little while back, so you might want to wait a few days if you decide to get a board. I'm going to feed in 3Vrms (4.3V peak, should be higher than just about any real world source) and and I'll post the resulting scope shots after the gain stage and the output stage. The inverting O2 has +/-15.3Vdc power rails from the 16Vac transformer. Low gain is actually attenuating - inverting amps can do that (gain less than 1) and still be stable! - while the high gain is 1.87. Then the output stage has gain of 1.33. So multiplying that out gives a low gain of 0.75 x 1.33 = 1x and high of 1.87 x 1.33 = 2.5x. I set it up that way, as a default, so the end-to-end low and high gains would match the usual gain settings of NwAvGuy's O2 Headamp.
On the high gain setting the input 4.3Vpeak should then become 8Vpeak after the first stage, and 10.7Vpeak after the output stage, all well under the 15.3V rails.
One more thing - those case screws! 4x4F150 and I both discovered the ones that come with the Box Enclosures cases tend to be weak and can twist off when screwing them into the case. I'm going to start including a set of the stronger black screws, in the post above, along with the board.
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PC board availability update - V3.61 on the way!
Here is an update on the availability of bare PC board for this project, since a few folks have inquired recently.
I took a look on Friday and discovered I was down to just one bare V3.6 board. I want to hang onto that, so I've sent a new run out for fabrication yesterday. These will be the new V3.61 with the few minor cosmetic changes to the silk screen, but no performance differences vs. V3.6. I should have the boards back in a week, probably August 8 or 9.
I've added all the pricing details just now to my sales thread in the Vendor forum here:
http://www.diyaudio.com/forums/vendors-bazaar/293309-agdr-audio-sales-thread.html#post4755643
A couple of things to note in that write-up. When doing a BOM scan this morning at Mouser I discovered that ALL the Triad AC-AC transformers are now marked "end of life"! Looks like Triad is no longer going to make them. Luckily there is a solution. 🙂 Jameco Electronics here in the US (jameco.com) has a full line of AC-AC wall transformers. Their 16Vac 1A unit this project uses is their part number 2227612 for $11.95. Just a reminder that in this project the power transformer has to be 16Vac, no more or no less, to stay within the maximum input voltage of the LT voltage regulators.
Mouser seems to have all the other project parts in stock. The 18.7K resistors that used to be a Digikey-only part are now in stock at Mouser, 270-18.7K-RC. That just leaves the LT voltage regulators as Digikey parts, and I have those as mentioned in the sales thread. I also have the LME49880 chips.
Here is an update on the availability of bare PC board for this project, since a few folks have inquired recently.
I took a look on Friday and discovered I was down to just one bare V3.6 board. I want to hang onto that, so I've sent a new run out for fabrication yesterday. These will be the new V3.61 with the few minor cosmetic changes to the silk screen, but no performance differences vs. V3.6. I should have the boards back in a week, probably August 8 or 9.
I've added all the pricing details just now to my sales thread in the Vendor forum here:
http://www.diyaudio.com/forums/vendors-bazaar/293309-agdr-audio-sales-thread.html#post4755643
A couple of things to note in that write-up. When doing a BOM scan this morning at Mouser I discovered that ALL the Triad AC-AC transformers are now marked "end of life"! Looks like Triad is no longer going to make them. Luckily there is a solution. 🙂 Jameco Electronics here in the US (jameco.com) has a full line of AC-AC wall transformers. Their 16Vac 1A unit this project uses is their part number 2227612 for $11.95. Just a reminder that in this project the power transformer has to be 16Vac, no more or no less, to stay within the maximum input voltage of the LT voltage regulators.
Mouser seems to have all the other project parts in stock. The 18.7K resistors that used to be a Digikey-only part are now in stock at Mouser, 270-18.7K-RC. That just leaves the LT voltage regulators as Digikey parts, and I have those as mentioned in the sales thread. I also have the LME49880 chips.
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Hi! Unfortunately the circuitry on this one is too densely packed for two layer. I do have bare boards but I've put a temporary "hold" on the project until I can take some promised measurements. TI discontinued the lme49880 in the gain stage. I've successfully replaced it with the opa1642, but I may make the same chip change in the output stage.
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