[khaos974]

Sennheiser demo'ed their HD 800 with a Lehmann BCL which output impedance is 5 ohm though, and I seem to remember that a head-fier emailed them (or was it Beyer?) and Seenheiser recommended a near 0 output impedance.

 

[mikeaj]

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif
 
Why arent the mosfets Q1/Q2 in the feedback loops of the opamps? It would reduce the output impedance of the power supply which would probably drop the crosstalk of the amp. 60db is "good" not "excellent" if we are talking about measurements without reference of audibility.

[snip]
 
I think the 0.8db channel matching quoted in the test data may be misleading. Channel matching is primarily a function of the channel matching of the volume control, which is not generally great in those little pots. Maybe they all line up nicely at maximum volume, but at real levels Id bet on worse. Not really a fault of the designer just saying...
 
Coupling caps, UGGGGGGH. UGGGh, uggggh. Thats the kind of stuff you'd expect to see in the kind of tube amp a guy like nikongod would design. Why not DC couple it? Ugggggh. At the very least put them in front of the pot where the signal is larger. Uggh, really? And just no. I guess they will save the headphones (sort of, but no) if the gain opamp goes all funky with excessive input signal. 
 
I like the option of an output resistor outside of the feedback loop for the output op amps even though nobody will use it. I wonder if this admits the importance of subjective quality in spite of "knowing" how to do it right. heh. Id put a switch across this one so the user can switch on the fly like on the XIN amps. Its a good spot for ferrites or hand wound inductors should the need arise in actual use. 

I'm pretty sure there are comments about most (but not all) of the points you bring up, in the docs...if one were to actually read them all.  Indeed there's a comment about the output resistor option, and coupling caps, and so on.  I'm not going to do all the work for you, but I remembered a few details.
 
About the channel balance, that's easy to look up so I just did.  Yeah, that's pretty dependent on the pot you get.  But for reference the quoted value of 0.8 dB matching error is for -45 dB on the volume control, at least for the sample benched.  It's better than 0.8 dB above that level and still under 1.0 dB error at -55 dB, so these are pretty realistic levels.
 
I'm not sure where you're getting the crosstalk figure, but the first article lists -65 dB crosstalk into 15 ohms, -72 dB into 33 ohms, -91 dB into 150 ohms, and -95 dB into 600 ohms (but a few dB worse at half volume, above 1kHz).

 

[b0ck3n]

In response to Currawong, RMAA was only suggested to nikongod as he argued that dScope measurements were somehow irrelevant, seeing as only a select few (and, he implied, likely biased) people could verify them.

I don't quite see the point in arguing something that's 60 dB below the music, especially a non-issue such as crosstalk. Again, assuming that the measurements provided are in fact correct, the design has no audible weakness. As far as subjective listening reviews go, they're completely irrelevant as they'll only adress the sound of the headphones used, or possibly the source.

I don't mind constructive criticism - input on how to implement features, reducing costs or size etc, without compromising transparancy would be appreciated, undoubtebly by the designer as well. The goal is to put as good a product as possible, at a price as reasonable as possible, in as many hands as possible, and we're offered a chance to chime in and make suggestions for the amp to suit our specific needs. Afaik, it's unprecedented. So, whether the designer rubs you the wrong way or not, let's all give the design a fair shake and let it speak for itself.

 

[Armaegis]

Anyone taking bets on how long it'll be before a Chinese manufacturer swipes the design and starts selling?

 

[khaos974]

If they do it properly, I'd buy one to see how it sounds.

 

[Maxvla]

How much output power is this design capable of?

 

[c61746961]

Max Output 15 Ohms 353 mW
Max Output 33 Ohms 641 mW
Max Output 150 Ohms 355 mW

Plenty.

 

[Willakan]

Nikongod, I actually contacted NwAvGuy regarding channel imbalance (he has since then updated his article to make it clearer)
Firstly, the channel imbalance in the specs is 0.6, not 0.8db and is very much worse case. 
Regarding DC coupling caps, that's just standard audiophile BS (OMG CAPS IN THE SIGNAL PATH! THEYLL DEGRADE MY BASSES). He addresses this at length in his articles.
As for crosstalk, the 65db figure is into into 15 ohms; that's a pretty punishing load. Into 150 ohms it's 91db. Both figures are very respectable into their respective loads.
 
And @Currawong, after your annoyance at fledgling Sound Science-related conspiracies, it's a bit rich to try to start a conspiracy theory of your own about NwAvGuy ("Do you know who he is? You know what? Nobody does. Not only that, he has gone to extensive effort to hide his identity. Maybe you should wonder why.")
 
On a slightly more civil note, regarding your requests for square wave measurements, NwAvGuy has done several and they're in the original amp article, near the bottom. Square wave performance, performance into capacitive loads, slew rate and square wave rise time are all discussed. 

 

[qusp]

Quote:

I think your post is pretty good.  I was thinking something similar myself.  Rather than reading about other's opinion about reading about reading about the amp design, I would like to start reading about how the amp sounds...
 

my head hurts 

 

 

[Shike]

Quote:

RMAA isn't the be-all and end-all of measurements and wont tell you everything about how an amp behaves when playing music. If only it were that simple. Personally, I'd like to see some square wave output off a scope at least, under different loads, but I'm sure we'll get to that at some stage anyway.

But that would require hardware that many casual builders don't own!
 
See: Nikongod's post for context of what I was saying.
 
 
As for his identity: maybe he works at a larger audio company that would create issues if people knew?  There's plenty of reasons not to give your identity out online, and he certainly isn't obligated to especially when the design should speak for itself.  I can think of people who's identity we know of that are much less trustworthy.

 

[nikongod]

 
Quote:

I'm pretty sure there are comments about most (but not all) of the points you bring up, in the docs...if one were to actually read them all.  Indeed there's a comment about the output resistor option, and coupling caps, and so on.  I'm not going to do all the work for you, but I remembered a few details.
 
 

I stop reading books after the 450'th page, and articles on DIY amps after the 12'th. 
 
Quote:

In response to Currawong, RMAA was only suggested to nikongod as he argued that dScope measurements were somehow irrelevant, seeing as only a select few (and, he implied, likely biased) people could verify them.
 

Actually a proponent of the amplifier implied that RMAA measurements were somehow irrelevant by excluding them. 
 
Quote:

Where are the measurements for the PimetaV2?  Non-RMAA please.
 

 
 

 

[nikongod]

 
Quote:

Regarding DC coupling caps, that's just standard audiophile BS (OMG CAPS IN THE SIGNAL PATH! THEYLL DEGRADE MY BASSES). He addresses this at length in his articles.
 

I'm not really concerned with the bass - the caps look to be appropriately sized for good bass response. I'm really concerned with the fact that they are there and what their sonic signature will do to the amp. Ewww. 
 
With very few exceptions, every amp I have removed coupling caps from has sounded better to my ear. The one exception was adding a coupling cap to facilitate grid biasing a tube instead of cathode bias, but thats quite an apples to oranges comparison - it simply shows that cathode biasing a tube is worse than a coupling cap in that case. 

 

[svyr]

Jude has kindly agreed to lift the ban on discussion of the works of banned members to allow discussion of the O2 headphone amp. However, please pay attention to the following before posting, otherwise this thread is likely to get locked:
 
1.This thread is about the amplifier, not NwAvGuy.
2. No links to NwAvGuy's website, or resources maintained directly by him.
(The schematics and bill of materials I am mirroring (in compliance with CC license) in order that they may be linked to)
 
 
So, down to business. The O2 is a headphone amp designed purely around objective measurements and released under a non-commercial license (Creative Commons). It is cheap to build (PCB+parts run to 30 dollars) and also generally goes against much of what can be considered audiophile ideas; there are capacitors in the signal path, no less than 4 50-cent ICs and it uses an AC wall transformer as part of the power supply.
 
It also has an incredibly low noise floor and vanishingly low distortion. Other notable features include current limiting and the ability to run off two 9V (8.4 volt in reality) batteries (power management kicking in to shut down the amp when the batteries are getting too low). Finally, it also fits perfectly into a cheap ($10) enclosure and has no SMD parts.
 
Amp schematics and BoM (mirrored on my Google Docs): https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=1WGBCwFbHXt-q82Ujih8IVu2vXsgMtlNpe6oHLLg_Klrf2LFxv5e9F1zloEZK&hl=en_US
 
So, are you planning to build one? Any other thoughts?
 
 
 

where'd be those measurements ? especially at different loads ? (and power output levels )... Though I guess the first one is not necessarily necessary, since you could go off the output impedance

 

[khaos974]

where'd be those measurements ? especially at different loads ? (and power output levels )... Though I guess the first one is not necessarily necessary, since you could go off the output impedance

There are a total of 3 articles on the amp in question, the first detailing the design criteria and whether the final design satisfied them with extensive measurements of the finished product, the second explaining the design choices and the final one about details in the circuit design and construction, with links to the BOM, the PCB schematics...

 

[Satellite_6]

Quote:

where'd be those measurements

? especially at different loads

? (and power output levels

)... Though I guess the first one is not necessarily necessary, since you could go off the output impedance

 

[size=x-small]THD 1 Khz 150 Ohms[/size]	[size=x-small]0.0017% Excellent[/size]
[size=x-small]THD 1 Khz 15 Ohms[/size]	[size=x-small]0.0046% Excellent[/size]