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miceblue said:

Yeah that's what I thought. Yes I hear it even with no audio signal.
On 1.0x gain, I hear nothing, which is what I expect. On the 2.5x gain, I get that kind of background hiss that one would hear with a portable media player and IEM's. It's subtle, but I can definitely tell the difference between the 1.0x and the 2.5x gains.
 
I don't think it was my desoldering job...the circuit looked pretty clean when I took out the resistors.
 
 
This happens on multiple computers. And actually, with the O2/ODAC, I discovered that one of my USB ports on my MacBook is dying because the audio produced from that particular port is all distorted like one would find in a horror movie. Switching USB ports fixes this issue. XD

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When using 1.0x, you are not amplifying anything... That's the reason you do not hear any sort of unwanted artifacts.
I have tried to experiment and also noticed a very low level hiss when using too strong gain with sensitive phones. Like 2.5x with headphones that needs 1x in reality... You should use the lowest gain needed from several reasons mentioned on the author's blog, one being audible distortion with non-ideal gain.

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miceblue said:

Ah, mmk thanks for the feedback guys. I'm still new to the whole O2/ODAC objective stuff.
With the 2.5/1.0 gain option, I do notice a background hiss, or some high frequency noise with the 2.5 gain (while using an AC adaptor). Is this normal, because I recall seeing many people say that this amp has a very "black background" sort to speak?

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No, a hissing noise means there's definitely something wrong with the setup.

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hamilcarbarca said:

 
No, a hissing noise means there's definitely something wrong with the setup.

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Yes, this is possible as well... But I don't hear any hiss if using suitable gain even though I push the volume knob near to the max. Unfortunately every setup is going to behave a bit differently...

I use several DAC's one has a 2.25 volt output and am using the standard gain settings from JDS Labs. the 2.5 and the 6.5. I never really use the 6.5 setting. No Need with the dacs and source I use...PC USB to DAC to the Amp.
 
Both of the O2's I have, one i built and the other pre-built are dead silent on both gains settings with no input and volume all the way up.
 
As far as the dynamics and not having enough power.....well the designer did state the O2 amp will drive 98% of all the cans out there....maybe we should list those in that 2%?
 
I had several can amps and one was a 6 watt and I sold it after listening to the O2's...no real issue with dynamics here with LCD2's, AKG's and Grados...
 
Best little amp money can buy!
 
Alex

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mhoe said:

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hamilcarbarca said:

 
No, a hissing noise means there's definitely something wrong with the setup.

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Yes, this is possible as well... But I don't hear any hiss if using suitable gain even though I push the volume knob near to the max. Unfortunately every setup is going to behave a bit differently...

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It's broken if there's hiss at any gain.

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hamilcarbarca said:

 
It's broken if there's hiss at any gain.

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No, it's not necessarily true... Hiss can always be transferred from your computer via USB cable (less probably but still I would suggest to use Fidelizer to improve the computer's environment + check if the usb cable is working properly and is connected to the amp tightly) and/or there could be problems with a power supply (more probably).
 
Also, you can possibly catch noises from the outside if using SIPs as I do... I have heard it's not possible to use 1X gain ( = no gain resistors in sockets) for someone because of this. You can spot these problems even though you don't use SIPs when applying 1x (cutting one pair of resistors) I suspect. But I am lucky enough here...
 
To be even more realistic, all the noises from the outside (open vs. closed doesn't matter unless you use some noise-cancelling phones) are very possibly of much more dB than any low level hiss from the amp... I don't hear any hiss from my amp but I do hear noises from the outside (computer, kitchen, cars if my window is open etc.)

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hamilcarbarca said:

 
It's broken if there's hiss at any gain.

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Then I wonder what's wrong with my circuit... :/
 
Because it might be relevant, I've mostly been using the new V-MODA Crossfade M-100's with the O2 at home while running on the AC charger.
If it means anything, I grabbed these measurements from Innerfidelity:
Volts RMS required to reach 90dB SPL: 0.031 Vrms
Impedance @ 1kHz: 36 Ohms
Power Needed for 90d BSPL: 0.03 mW

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mhoe said:

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hamilcarbarca said:

 
It's broken if there's hiss at any gain.

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No, it's not necessarily true... Hiss can always be transferred from your computer via USB cable (less probably but still I would suggest to use Fidelizer to improve the computer's environment + check if the usb cable is working properly and is connected to the amp tightly) and/or there could be problems with a power supply (more probably).
[...]

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I don't necessarily agree with you on the details, but my blanket statement was at best an over-generalization. An amp is defective, in my opinion, only if it's the source of the noise.
 
It is possible for external noise to be injected into an amplifier or the analog stages of a DAC. How well this is rejected is a matter for the designer ... and for the purchaser to decide just how much money to spend on this type of feature.

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miceblue said:

Then I wonder what's wrong with my circuit... :/
 
Because it might be relevant, I've mostly been using the new V-MODA Crossfade M-100's with the O2 at home while running on the AC charger.
If it means anything, I grabbed these measurements from Innerfidelity:
Volts RMS required to reach 90dB SPL: 0.031 Vrms
Impedance @ 1kHz: 36 Ohms
Power Needed for 90d BSPL: 0.03 mW

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You have some very high sensitive phones, I am not surprised you are able to spot some hiss with them... You can even find a note at the author's blog that the complete "dead silent" thing is not necessarily true with high sensitive phones (just discovered this today when reading though some sections there).
 
Just checked the amp with my IEMs and I can spot a very low hiss level when using higher gain than I should (1x). As I use step-down converter right now (with WAU 16-400), I am going to purchase suitable transformer tomorrow and will test if there is any difference. It's cheap and I wanted to purchase it anyway... So now is the time to do it!
 
You should use 1x with your phones at best... What exact transformer do you use?

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mhoe said:

 
You have some very high sensitive phones, I am not surprised you are able to spot some hiss with them... You can even find a note at the author's blog that the complete "dead silent" thing is not necessarily true with high sensitive phones (just discovered this today when reading though some sections there).
 
Just checked the amp with my IEMs and I can spot a very low hiss level when using higher gain than I should (1x). As I use step-down converter right now (with WAU 16-400), I am going to purchase suitable transformer tomorrow and will test if there is any difference. It's cheap and I wanted to purchase it anyway... So now is the time to do it!
 
You should use 1x with your phones at best... What exact transformer do you use?

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Erm, I'm not completely sure. I'm using whatever parts come from the JDS Labs O2.
 
And OK, I read somewhere on that blog about sensitive headphones/earphones being able to detect a small amount of background hiss.
While on the topic of sensitive 'phones though, is there a place that I can check out regarding the sensitivity rating? I only know that more sensitive headphones can pick up background hiss more, and they're usually pretty loud in volume/can be "easily driven" by portable media players...I'm not too familiar with the technical explanation. XD

Lower-spec (lower voltage output, power rating) transformer should not make much (any?) difference for noise performance, but you can try if you want.  Consider all of the power supply filtering inside the amp, the relatively low power draw (especially when not playing at low output power level for IEMs, high PSRR of chips involved.  Thermal noise of some key resistors is a bigger contributor to noise.
 
Also, maybe this goes without saying, but confirm that it's not noise in the recording you are playing or noise from the source that's feeding the amp.  It's not like the amp won't amplify any audio-range signal that it's receiving on its input, be it music or noise.  Input some noise, and you get the noise out.  It just generates very low noise on its own.

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mikeaj said:

Lower-spec (lower voltage output, power rating) transformer should not make much (any?) difference for noise performance, but you can try if you want.  Consider all of the power supply filtering inside the amp, the relatively low power draw (especially when not playing at low output power level for IEMs, high PSRR of chips involved.  Thermal noise of some key resistors is a bigger contributor to noise.
 
Also, maybe this goes without saying, but confirm that it's not noise in the recording you are playing or noise from the source that's feeding the amp.  It's not like the amp won't amplify any audio-range signal that it's receiving on its input, be it music or noise.  Input some noise, and you get the noise out.  It just generates very low noise on its own.

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To be honest, I am not even trying to listen to music with the amp using IEMs or other very sensitive phones... Also, I can change my gain configuration anytime I want so I am just using the most suitable gain and have no audible hiss even with sensitive phones like Denon D7000. But I am curious about this and do not want to use stepdown converter anymore so I am going to try it. Not talking about the transformer here but rather about the power supply section (stepdown converter, wall sockets etc.). Also, I have ODAC included inside with USB input from my computer...

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miceblue said:

Erm, I'm not completely sure. I'm using whatever parts come from the JDS Labs O2.
 
And OK, I read somewhere on that blog about sensitive headphones/earphones being able to detect a small amount of background hiss.
While on the topic of sensitive 'phones though, is there a place that I can check out regarding the sensitivity rating? I only know that more sensitive headphones can pick up background hiss more, and they're usually pretty loud in volume/can be "easily driven" by portable media players...I'm not too familiar with the technical explanation. XD

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Yes, you can use sources like innerfidelity or just read specs of your phones... I have googled that yours are very "good" at spotting a hiss.
 
In general, I would recommend to use O2 with not very high sensitive phones as there is no real reason to do so... You are simply only adding distortion to your sound if you do not use 1x gain configuration with your phones. =/
 
Transformer from JDS Labs is WAU 12-200... It's good but not ideal especially for low impedance phones that often need more current. But it should not influence hissing or so... I can even test it, I have both transformers right here (12-200, ideal 16-400).

I can get a little hiss through my amp, and I can also hear my cell phone getting into the signal at times. Could be coming through the headphone cable or amp, I am not sure.
 
I however, am pretty darn sure its my source. I have heard hiss before with IEM and 16 Ohm phones. I am using the "line out" jack which has 100 Ohms resistance. Figured that would be "quieter" than the headphone jack-as-line-out. I haven't tried trimming from max volume to 50% to cut the THD a little though.

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Originally Posted by MHOE /img/forum/go_quote.gif
 
Transformer from JDS Labs is WAU 12-200... It's good but not ideal especially for low impedance phones that often need more current. But it should not influence hissing or so... I can even test it, I have both transformers right here (12-200, ideal 16-400).

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I think a lack of power is likely to affect the DAC first and very audibly.