[mgalusha]

I have the updated amp back but so far have not even opened the box. I have been incredibly busy but I'm going to try and sneak this in soon. I apologize for the delay but a large, hopefully well paying, project wins out.

[adydula]

Hey Mike!!

That's good news, I and many others thank you for your assistance in the measurements of AGDR's ODA and hopefully that one half volume measurement of the O2 amp. Many of us are really anxious for that result and the others as well.

Looks like your a really busy guy with the dScope!

Looks like your having fun running this stuff....

Thanks
Again

Alex

[agdr]

I have the updated amp back but so far have not even opened the box. I have been incredibly busy but I'm going to try and sneak this in soon. I apologize for the delay but a large, hopefully well paying, project wins out.

Hey that is OK!  Any amount of testing any any time is appreciated.   

[adydula]

Well,

The ODA has progressed to V2.1 and AGDR has made pc boards available in (2) flavors one unpopulated and one that has the small SMD parts already soldered on for those of you that are soldering challenged!! LOL 

These are available on Ebay.

Yesterday I updated my ODA V2.0 with new improved C20/C21 caps from 10uf to 22uf and added a transorb across the AC power input line.

Nothing to really change the sound etc, just to make the ODA "better" from a transient voltage protection and stability from a design perspective...

I really wanted to get out the soldering iron once again.....

Ah the smell of burning flux in the morning!!!

Alex

[jtwrace]

Will the ODA be at RMAF?

[adydula]

That's a great question that AGDR will have to answer.

I don't know if DIY stuff is there in any aspect.

Hmmmmm wonder if Mike attends?

Alex

[jtwrace]

That's a great question that AGDR will have to answer.

I don't know if DIY stuff is there in any aspect.

Hmmmmm wonder if Mike attends?

Alex

AGDR should go to CanJam....it's massive! 

[mgalusha]

That's a great question that AGDR will have to answer.

I don't know if DIY stuff is there in any aspect.

Hmmmmm wonder if Mike attends?

Alex

At the rate I'm going on testing it will still be here by then.    And of course I'm going to RMAF but not as a vendor, well not really. Hmm, that reminds me, need to make hotel reservations.

[agdr]

So far I haven't gone to any of the audio shows since my stuff is all DIY.  Hey that is an idea for next year!     That would let me show off the O2 Booster Board too.   A long time ago I was in the broadcast video/audio equipment business and used to regularly attend the national assn of broadcasters show in vegas http://www.nabshow.com/ and the SMPTE show https://www.smpte.org/ .   That was all pro-audio type equpment though.   About the closest anything got to a headphone amplifier was a headphone out jack on a mixing console.

[adydula]

Gee Mike...

I thought when you retired you should have all kinds of spare time!!!

LOL

Gee if AGDR goes to RMAF next year then maybe we will get to actually meet each other!!

Ok lets start planning!!

Alex

[mgalusha]

Still a long way from retirement but that's OK, keeps me mostly out of trouble.

Gee Mike...

I thought when you retired you should have all kinds of spare time!!!

LOL

[maty]

Hi from Spain,

Three times the output current capability at 300mA per channel

And 7.25 V:

At 16 ohms -> 3285 mW


With 300 mA:

16 ohms -> 1440 mW  (and 1700 mW you say) Limit current like O2?

32 ohms ->  ?

64 ohms ->  ?

120 ohms -> ?

300 ohms -> ?

Can you say us the real wattage at 16, 32, 64, 120, 300 and 600 ohms please?

[agdr]

Can you say us the real wattage at 16, 32, 64, 120, 300 and 600 ohms please?

Good question!     A lot of folks looking at the ODA would want to know.

The ODA will output a maximum swing of 7.25 volts rms without the attenuation resistors (or with them jumper-shorted), or half that at 3.75Vrms with the attenuation resistors, unless the output current exceeds about 60mA per output chip.  There are 6 chips in parallel, so that would be 60ma * 6 = 360mA maximum per channel. I often write 300mA for the ODA just to be conservative.  Sort of my reaction to the commercial stuff where everything is over-rated, I prefer to under-rate a little bit.   The output chips can actually do 70mA too, according to NwAvGuy, but i knock that down to 60mA to be conservative.

For output current it is necessary to switch back from rms (for power) to peak to make sure the peak currents don't exceed what the output chips can do.  So at 300R, the maximum current with a 7.25V swing would be (7.25Vrms * 1.414)/300R = 34.2 mA(peak) vs. the 360mA, well within bounds.  With the atten resistors gives even less current draw, (3.75Vrms * 1.414)/ 300 = 17.7mA.   In fact the biggest problem you may run into with some 300R  (and 600R) headphones, if they have lower sensitivity, is that even 7.25Vrms isn't enough voltage swing.  That is why the +/-15Vdc option is available, to give about 2.5Vrms more swing for 300R and up phones that need it.

The maximum power output into 300R is then (7.25Vrms)^2 / 300R = 175mW.   With the atten resistors it is (3.75Vrms)^2 / 300R = 47mW.

Using the same math on the rest:

Max current with 120R load = 85.4mA(peak), and with the atten resistors = 44.2mA(peak).  Both OK vs. the 360mA available.
The maximum power into 120R is 438mW, or 117mW with the atten resistors.

Max current with 64R load = 160.2mA(peak), and with the atten resistors = 82.9mA(peak). Both OK vs. the 360mA available.
The maximum power into 64R is 821mW, or 220mW with the atten resistors.

Max current with 32R load = 320mA(peak), and with the atten resistors = 166mA(peak). Both OK vs. the 360mA available.
The maximum power into 32R is 1.64W, or 439mW with the atten resistors.

Note that things are starting to get interesting with the current at 32R.  The maximum current with the 7.25Vrms output at 320 is getting close to that 360mA maximum.  We can expect that with a 16R load the ODA won't be able to go all the way up to 7.25Vrms without distorting.  Sure enough, that is what mgalusha's dScope tests showed.  The ODA went up to round 4.4Vrms at 16R load with THD+N levels 36% less than the O2, but then the distortion rose going up to around 6Vrms, which was the maximum before the distortion rose exponentially.

This set of numbers is for the 4.4V4ms output level, and 2.2Vrms with the atten resistors:
Max current with 16R load = 388mA(peak), and with the atten resistors = 194mA(peak).
The maximum power into 16R is 1.21W, or 302mW with the atten resistors (again, this is with 4.4Vrms and 2.2Vrms levels).

Notice the 388mA peak vs. the 360mA "max".  Remember above that I said I was being slightly conservative and using 60mA max for each output op-am vs. the 70mA that NwAvGuy used.  Looks like NwAvGuy is right once again, and Mike's dScope measurement prove it.  Using the full 70mA number the maximum output rating of the ODA goes up to 6 * 70mA = 420mA instead of 360mA.  So 388mA actually is within the OK range.

And finally, at the 6.0Vrms level (or 3.0Vrms with atten resistors) where Mike measured the maximum output before distortion zooms up, we get:
Max current with 16R load = 530mA(peak), and with the atten resistors = 265mA(peak).
The maximum power into 16R is 2.25W, or 562mW with the atten resistors (again, this is with 6Vrms and 3Vrms levels).

Notice the 530mA here vs. the "new" 420mA max.  This says that even NwAvGuy was a bit conservative with the 70mA per op-amp number.  The real maximum for the NJM4556AL chips looks like around 530/6 = 88mA or so per chip.  The current capability of the chip is taken off a graph in the datasheet with a rounded curve which is why there is some much variance in interpreting it.

[agdr]

Here is a continuation on my post above about maximum wattage of the ODA into various loads.   The post above was all done for the "standard" +/-12.5Vdc power rail option.  Here I'm going to work the same numbers with the higher voltage +/-15Vdc power rail option that is only to be used with 300R and higher headphones.  The limitation of "only 300R and up" phones is to keep the output chips from overheating while running on the +/-15Vdc power rails.  With that limitation any volume control position is OK to use.

The +/-15Vdc power rails increase the maximum voltage swing to around 9.75Vrms, 2.5V higher than the 7.25Vrms you get with the +/-12.5V rails.  The extra voltage swing is useful for some 300R and higher headphones that are lower sensitivity and need the extra voltage. 

Max current with 600R load =23mA(peak), and with the atten resistors = 11.5mA(peak).  Both OK vs. the 360mA available.
The maximum power into 600R is 158mW, or =39.6mW with the atten resistors.

Max current with 300R load = 46mA(peak), and with the atten resistors = 23mA(peak).  Both OK vs. the 360mA available.
The maximum power into 300R is 317mW, or =79mW with the atten resistors.

[maty]

Thank you very much for your long answer!!!

[adydula]

AGDR is indeed one of the trustworthy good guys when it comes to the ODA and its capabilities.... with all the hype on the O2 and many of us wanting a desktop version he is the only person that I know of that went and designed a ODA headphone for us to have and enjoy.

So far its proved to meet or exceed the O2 in several ways.

I can't wait for Mike G to complete the second round of measurements and see how it fares !!

Thanks again AGDR for the ODA!!

Alex

[adydula]

Another ODA takes flight...a fellow audio headphone DIY'er over at diyAudio just completed his "rare" ODA!!

This amp is becoming a very desirable amp, strong and clean compared to the original O2...

But due to its DIY and SMD parts its a challenge for some to actually build it, but the rewards are indeed great!

This one is in a silver case and the builder is waiting on knobs with red markings to complete.

Mine is still going strong and is a reference amp to me.

[adydula]

Its hard to believe I have had this amp for over a yeaer now, and I have used it almost everyday now for my PC sound via its preamp unity op amp outputs and for serious headphone listening.

I have built several other amps in this timeframe and have published those results as well.

Recently I was fortunate to get a set of Beyer T1's the newer 2nd gen set. I was worried with 600 ohm cans that my amps would be able to drive these well enough.

I built an OTL Bottlehead Crack and installed the Speedball mod and this amp worked well with my Beyer T90s. at 250 ohms and when I tried the T1's I was surprised how well it works with the amp as well. Even though it doesnt have the maximum power that these headphones can stand (300mw). The voltage of the Crack I am told at 600 ohms is 12-14 volts.

The presentation with the Crack is very nice and musical, but after many hours and changing over to the ODA its amazing how much clearer the T1's are....very noticeable to me. The ODA has more power at 600 ohms but a lower voltage.

I was looking at several amps and notice many do not state the power output at 600 ohms, but usually a more mundane 300 ohm level.

I am looking at a Schitt Valhalla 2 in the future that touts 330 mw RMS at 600 ohms...still debating on whether to try this amp...its a tube amp and an OTL design...like the Crack but I am not sure how similiar the two are.

Since last year a dozen or more of "AGDR's" ODA have been built and all have been working quite well.

Alex

[jtwrace]

Sounds like you need the NeuroChrome HP-1!


You're welcome.   

[adydula]

Gee Thanks!!

I have thought of that....450mw into 300 ohms, so it would be less at 600.

Schitt is touting 330mw RMS, asking jason if this is total or per channel.

The HP1 is nice, but expensive, but nice....I could build it for about $500-$600 bucks or..

Look at this Valhalla 2, or a Bottlehead Crack-a-two-a or a Bottlehead Mainline...

My birthday is coming up in June!! Time will tell if I can wait until then!!

LOL
Alex