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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 9th April 2019, 12:10 AM   #391
agdr is offline agdr  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
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Originally Posted by adydula View Post
As soon as I get the relay I will install and test!!
Should be there Thursday, USPS tells me.
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Old 9th April 2019, 03:01 PM   #392
eljoantonyn is offline eljoantonyn  India
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Would your amp work with 32Ohm impedance earphones ?
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Old 9th April 2019, 04:10 PM   #393
Lavalier is offline Lavalier  United States
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Definitely!
It can drive my Audio Technica M50xs that are 38ohms to ear splitting levels without breaking a sweat
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Old 9th April 2019, 04:35 PM   #394
eljoantonyn is offline eljoantonyn  India
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Thank you for the reply Lavalier !

I heard that CMoys don't work very well with low impedance phones. Thats why I asked.
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Old 9th April 2019, 04:48 PM   #395
xrk971 is online now xrk971  United States
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OPA1688 Super CMOY, 2x 9V with real ground and headphone relay - PCBs
I drive 16ohm ZS6’s no problem. But these are 114dB sensitive so not much needed to be dangerous to your ear’s health.

Is this CMOY amp good enough ?
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Old 14th April 2019, 03:57 AM   #396
FHorn is offline FHorn  United States
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Wondering why the input RC low pass filter of the CMOY of 274 ohms with 220 pF of 2.6 MHz is set so high?
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Old 15th April 2019, 01:21 AM   #397
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 FHorn View Post
Wondering why the input RC low pass filter of the CMOY of 274 ohms with 220 pF of 2.6 MHz is set so high?
Good question! They are the same values that NwAvGuy used for the input RF filter on his O2 headamp:

O2 headamp details see the "documentation package" link on the second page (scroll down) for the schematic.

I wanted to keep the design close to the O2. I think that I asked him once in PM about why that filter corner frequency was so high. Can't quite remember the answer now. Part of it might have been the corner is the 3dB point, high keeps more of the filter out of the audio band, especially if designing to pass 20kHz square waves, which takes the needed response up to at least 5x (100Khz) for the Fourier components, ideally 10x (200KHz).

Last edited by agdr; 15th April 2019 at 01:24 AM.
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Old 15th April 2019, 01:40 AM   #398
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 eljoantonyn View Post
Would your amp work with 32Ohm impedance earphones ?
Yes, we've been PM chatting on this one and I wanted to post for anyone else out there who may be interested. A few things..

Remember that it isn't just the impedance but also the headphone or IEM sensitivity that determines drive requirements. They work together. Low Z (impedance) with high sensitivity isn't hard to drive, which is the case with a lot of modern cans designed to work with cellphones and such. The same low Z with very low sensitivity (usually decades-old headphones) are harder to drive.

On sensitivity, be careful about the units, too! Older headphones and IEMs - and even some modern ones - use dB/mW. Never cans use dB/V. Makes a sizable difference in the calculations. In the SPL spreadsheet (more below), put a "0" for the one you are not using. For example, if you have 116dB/V sensitivity IEMs, put a zero (number, not letter, of course) in the input field for "per mW," then enter your 116 in the field for "per volt"

And finally, knowing all this ^^^^ you can plug the impedance and sensitivity into the headphone Sound Pressure Level spreadsheet here at the projects' Google Drive link (you might already find your headphone in the list):

headphone and IEM SPL spreadsheet

From that spreadsheet you can read off the RMS voltage and RMS current that a headphone amp would need to hit 90 dB SPL and 110dB SPL. (3rd box down, the "voltage required" and "current required" output fields WHICH ARE IN RMS) 110dB SPL is as loud as a freight train going by, intended to be the level of very short duration musical peaks (which, btw, can easily be 10x or 20x the average music loudness level, especially in classical music). Arguably you don't need more than 110dB (some do design for 120 dB).

You will find that the single chip SuperCMOY provides (often more than) enough RMS voltage and current for probably 80% of the headphones and IEMs out there. It is a rare one that would require the dual-chip SuperCMOY for more current drive, or some other headphone amplifier to provide more voltage drive. Usually those are older models.

So long story short, the SuperCMOY can power 16 ohm headphones and IEMs at low THD thanks to the OPA1688 chip, like xrk971 said. But the sensitivity will determine single or dual chip, or other headamp for more voltage drive.

* note on RMS values. The maximum amount of voltage or current (75mA maximum) per channel the Super CMOY can put out are PEAK values. To convert peak to RMS (in the case of sine waves anyway, which is audio) multiply by 0.707 (or divide by 1.414). For example, the maximum/peak current per channel of the OPA1688 is 75mA. 75mA * 0.707 = 53mA RMS. And I think I've said elsewhere if you find your requirements are even getting close to that, like say 48 mA RMS, then go to the dual chip SuperCMOY for 2x the current capability. There are small "penalties" in using the dual chip version, like double the (still extremely low) DC output offset voltage. Use the single chip if you can, based on what the SPL spreadsheet shows you need.

Last edited by agdr; 15th April 2019 at 01:54 AM.
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Old 15th April 2019, 08:55 PM   #399
adydula is offline adydula  United States
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Oscar,

The relay arrived today and I am listening with your Super Cmoy right now. Its driving my Beyer T1's and T90's very well...amp is great. Dead quiet...relay is working very well. All this on batteries!!

I hope to get this out to you tomorrow or by wed..I have a 2 day scheduled trip for tomorrow and the next day.

Thanks for being patient...you will have this soon,

Alex

Special Thanks to AGDR!!!
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Last edited by adydula; 15th April 2019 at 08:57 PM.
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Old 15th April 2019, 09:57 PM   #400
oscarmalmberg is offline oscarmalmberg  Sweden
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Great work Alex! Looks real good! A Super Cmoy is worth waiting for. Will be a joy to try it out, will keep you posted on my impression.
I guess this will make my fostex sing for sure.

And I also thank you again AGDR, both for setting me up with Alex and all the support!
Im holding my thumbs for a safe trip over the atlantic..

Oscar
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