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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
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Old 25th June 2016, 10:22 PM   #21
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
Default New BOM revision - gain resistor choices and connection pin #

Quote:
Originally Posted by Lucien View Post
Just chipping in here to say I'm interested in setting up mine with a gain of 1, I was going to ask after ordering the boards, but since it's already come up...
Good point, in fact I've just made a new BOM revision to include some choices for the gain resistors. Attached below and also out at the project Google Drive link. I've grouped the resistor choices a bit better for 3 typical voltage gains: 1x, 2x, and 4x.

For 1x the standard thing to do is jumper the feedback resistors R11 & R12 (jumper wires instead of resistors for those pads) and then no need to install the 47pF caps C7 & C8. R8 & R9 are left unpopulated.

I'm just curous about johnc124's take on it given that he posted in the vendor forum thread that he did a bit of testing to settle on the 47pF for maximum stability with larger capacitive loads. Pretty slick being able to converse with someone who worked on the design of the chip.

In the new BOM I've also added the part number for the adapter board pluggable connection pins for completeness, although I supply those with the adapter board if anyone gets it from me.


Quote:
Originally Posted by B&W_arthur View Post
Looks very interesting.
The OPA1688 does seem like the perfect chip for a CMOY! Low battery drain yet high output current and great THD+N. As popular as CMOY headamps have been I'm surprised it has taken manufacturers so long to come up with an appropriate headphone driver chip.

An interesting thing to me is how much peripheral "stuff" is required to support an amp chip. RF filter, DC blocking caps. Without the SSR power management circuit it would be back to a virtual ground, which in turn would probably negate much of the chip's low THD+N and add crostalk. I've done a spice simulation with a OPA1688 acting as a virtual ground for another OPA1688 and the sim worked just fine. But an active VG always adds some "trash", as per NwAvGuy's famous VG analysis:

http://nwavguy.blogspot.com/2011/05/...nnel-amps.html

Then there is the problem that a VG really needs to be able to sink/source 2x the current of either channel, so both channels can be returned to it and still run at full current output. Which in turns always implies the VG needs to be a different chip, or the amplifier chip needs to settle on running at just half the rated current drive. I tried using one of the new OPA1622's as a VG in the simulation but the sim just locked right up with that one. Best to avoid the issue entirely and use the real ground if possible!

Then there is the headphone relay. I did try the OPA1688 without a relay and there was some turn-on turn-off thump. Not huge, but audible. Interestingly going from just a DPDT toggle switch for the power rails in V1.0 of the CMOY to the solid state relays in V2.0 helped reduce the thump considerably, even without the output relay. The benefits of having the rails go up & down in unison, as much as possible.


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Originally Posted by adydula View Post
Got parts, got boards, iron is getting hot!! LoL
And that is the current V4.0 here that adydula is working on now! I'll be curious to see how your case comes out. You do such a good job with the hole marking and drilling. Plus listening impressions like johnc124 said! I know you have all my amp designs there plus some others.
Attached Files
File Type: zip OPA1688 CMOY V4.0 BOM 6.zip (25.7 KB, 25 views)

Last edited by agdr; 25th June 2016 at 10:39 PM.
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Old 26th June 2016, 01:33 PM   #22
adydula is offline adydula  United States
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Hey great shots!!! I like the tape method!!! (lol).....Simple things for simple minds...
I am starting to populate this morning!

The version 3.0 that I built is still working very well and sounds very good. I compared to many amps and was pleasantly surprised how well it holds up. Actually to be brutally honest except for the gobs of power other amps might have, the actual real world hearing differences are very small....

In other words I think I could fool most people in a AB blind test most of the time! This is with most headphones that are fairly sensitive. Cans like the "HE" stuff are a different beast.

Its really great to see a company like TI develop a chip for the headphone market!!! So far its working out very, very well!..

I will post my results with V4 soon.

Alex
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Old 26th June 2016, 04:04 PM   #23
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
I'll be curious about how long it takes you to do the build! I haven't timed myself yet but I think I'm at about an hour. Half of that is spent getting the parts out of the storage bins.
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Old 26th June 2016, 04:24 PM   #24
calico88 is offline calico88  Indonesia
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I can not wait how good this cmoy is ?
Can it drive planar headphone with solid bass?
I might pushing to much ?
How much total kit cost?
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Last edited by calico88; 26th June 2016 at 04:36 PM.
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Old 26th June 2016, 04:45 PM   #25
xnor is offline xnor
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Quote:
Originally Posted by calico88 View Post
Can it drive planar headphone with solid bass?
I don't know where this question comes from because planar magnetic headphones are usually almost purely resistive loads, the easiest load for amps.

Or do you just mean low sensitivity and low impedance headphones?

It shouldn't be that hard to calculate ideal gain given your source, headphones and target peak SPL, and therefore voltage/current needed.
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Old 26th June 2016, 06:45 PM   #26
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
Quote:
Originally Posted by calico88 View Post
I can not wait how good this cmoy is ?
Can it drive planar headphone with solid bass?
I might pushing to much ?
How much total kit cost?
Please post your headphone brand and model number, or impedance & sensitivity rating, and I'll feed the numbers to a SPL spreadsheet.

The current BOM totals up to $39.75 US dollars for the parts at Mouser using the lower-cost option for the SSR (0.0625 ohms on). For the Coto CT128 SSR with the 0.0125 ohms "on" add $8 to that (those chips are $6.30 each at Mouser). Then you need the PC board and adapter board. The $39.75 assumes you are soldering your own adapter. For a fully assembled TPS3701 adapter you don't need the TSP3701 chip, adapter PC board, or 0.1uF 0603 cap in the BOM, and then add $8.50 for the assembled adapter.

With throw-away 9V batteries (primary cells) or NiMH "9V" rechargeable batteries the power management circuit would be set to cut off at +/-7Vdc for the low battery level (same as in NwAvGuy's O2 headphone amp). The OPA1688 is rail-to-rail output, so that will give a maximum swing of 7Vdc peak, or 4.95Vrms.

Lets do a double-check on current levels at that minimum-battery-level voltage swing number. Peak current into 16R headphones would be 7Vdc/16R = 44mA, rms average would be 4.95Vrms/16 = 31mA(rms). Both are just fine for the 75ma-per-channel OPA1688. In fact, multiplying that chip maximum by 2 for 2 channels, both the batteries and the solid state relays have no problems suppling the full 150mA. The SSRs are rated at 4.5A for the Coto and 2A for the Vishay.

On the other end of things, with fully charged batteries, rather remarkably everythng is still just fine! "9V" NiMH cells can go up to 11.0Vdc right off the charger. The peak current there into 16R cans would be 11.0Vdc/16R = 69mA, rms average = 7.78Vrms/16R = 49mA(rms). All still OK for the 75mA/channel OPA1688! Headphone with higher impedances would be even lighter loads of course, like 32R, and even easier for the OPA1688 to drive.

If anyone goes into the chip power dissipation calculations remember to take "music power" into account vs. sine wave testing, where the votlage peaks in actual music are typically 2x to 3x the average levels.

So in summary this little OPA1688 Super CMOY really can drive 16 ohm and up headphones with no trouble at any charge level for the two "9V" batteries. Try that with a OPA2134 chip and rail splitter in a conventional CMOY.

As for how it sounds, lets get some listening reports in from Adydula and the other folks building it up. But here is one in another thread regarding OPA1688 bass from borges where he was asking about various headphone chips vs. his existing NJM4556A. I had suggested trying the OPA1688, which he did (post 19):

Op-amp for headphones

I was commenting in there that for some reason that I have never been able to figure out the NJM4556A seems "bass challenged" to me. I never have been able to find any electrical reason for that. NwAvGuy tested the heck out of his O2 Headamp (which uses paralleled NJM4556A chips in the output stage), all posted on his blog. The frequency response is flat down to 20Hz and the distortion is low at 16R. Yet my ears have always said "bass problem", especially when running on the lower battery voltages vs. AC rails. Go figure!

Last edited by agdr; 26th June 2016 at 07:10 PM.
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Old 26th June 2016, 09:31 PM   #27
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Quote:
Originally Posted by agdr View Post
The frequency response is flat down to 20Hz and the distortion is low at 16R. Yet my ears have always said "bass problem", especially when running on the lower battery voltages vs. AC rails. Go figure!
That only makes sense if the NJMs are unhappy with inductive/capacitive loads (below and above resonant frequency of dynamic headphones) with low supply voltages.
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Old 26th June 2016, 10:59 PM   #28
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
Default Yet another BOM revision!

I have yet another BOM revision attached below and posted out on the project Google Drive link.

A big thanks to Adydula for finding two BOM omissions during his build! I left off the diodes D5 and D6 that go across the relay coil to suppress the reverse voltage surge. Those diodes are on the board and schematic, I just left them off the BOM.

I've also realized that the 9.6Vdc volt version of the "9V" NiMH rechargeable batteries (the maha energy Powerex battery in the BOM) will have a 8.0Vdc cutoff, rather than the 7.0Vdc cutoff for standard 8.4Vdc "9V" NiMH cells. I've added a set of resistors for R3, R4, and R5 to cover that case. I've re-grouped all the R3, R4 and R5 resistors to make the choices clearer. There are now 3 sets of those resistors to handle the 3 different battery types:

* A set of R3, R4 and R5 values for the 7.0Vdc low battery cutoff level of throw-away primary batteries and standard 8.4Vdc "9V" NiMH rechargeable batteries.

* A set of R3, R4 and R5 values for the 8.0Vdc low battery cutoff level of the maha Powerex 9.6Vdc "9V" NiMH rechargeable batteries.

* A set of R3, R4 and R5 values for the 6.27Vdc low battery cutoff level of 7.4Vdc "9V" lithium ion rechargeable batteries.


And a follow-up note on my post above. Where the 11Vdc comes from for a maximum power supply level is that same 9.6Vdc "9V" maha Powerex NiMH rechargeable. NiMH cells are 1.2Vdc nominal and around 1.4Vdc per cell right off the charger. The 9.6Vdc battery is an 8-cell unit, so 8 * 1.4Vdc = 11.2Vdc. The standard 8.4Vdc "9V" NiMh 7-cell battery would have a maximum voltage off the charger of around 7 * 1.4Vdc = 9.8Vdc.

I have actually seen 8.4V nominal "9V" NiMH cells charge up to around 1.45V per cell, which is around 10.15V off the charger. Apply that to the 9.6v Powerex cells gives a maximum off-the-charger voltage of 11.6Vdc, so +/-11.6Vdc power rails at that point in the Super CMOY. For a 16 ohm headphone load that gives 11.6Vdc/16 = 72.5mA, still within the OPA1688's 75mA.
Attached Files
File Type: zip OPA1688 CMOY V4.0 BOM 7.zip (27.2 KB, 11 views)

Last edited by agdr; 26th June 2016 at 11:03 PM.
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Old 26th June 2016, 11:06 PM   #29
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
Quote:
Originally Posted by xnor View Post
That only makes sense if the NJMs are unhappy with inductive/capacitive loads (below and above resonant frequency of dynamic headphones) with low supply voltages.
I don't have any objective explanation for it at all - purely a subjective thing, which I know would make NwAvGuy's head explode, wherever he is out there. And it may just be me, too. I've made the comment about the NJM4556A a couple of times over the years in posts about the O2 headamp and other folks have posted that they don't hear a bass problem at all with the O2/NJM4556A. So.. YMMV.
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Old 27th June 2016, 01:29 AM   #30
calico88 is offline calico88  Indonesia
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This is the link:
https://www.audeze.com/products/lcd-collection/lcd2

Style Open circumaural
Transducer type Planar magnetic
Magnetic structure Proprietary push-pull design
Magnet type Neodymium
Transducer size 106 mm
Maximum power handling 15W (for 200ms)
Sound pressure level >130dB with 15W
Frequency response 5Hz 20kHz extended out to 50kHz
Total harmonic distortion <1% through entire frequency range
Impedance 70 ohms
Efficiency 101dB / 1mW
Optimal power requirement 1 4W
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