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Old 3rd February 2014, 02:15 PM   #1
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Default Trick to search headphone amps by HI PSRR please?

Hello
a member at Head-FI recommended I go to these forms when I need to ask a very technical question. So I help you can help me.

At Head-FI forums I Asked the question "Can a USB powered DAC, "headphone amplifier, etc. filter out power supply noise from the USB line?" My first respondent recommended I go to these forums but one thing led to another and eventually I felt I got a satisfactory answer to my question there. But my last respondent told me powering amplifiers with the relatively cheap switching amplifier is another story. To achieve a lot of the best qualities headphone amplifiers can bring to your music and uses switching amplifier you must find an amplifier with a high PSRR.

Consequently I am looking for a commercially available amplifier (perhaps one I must enclose myself) which has a high PSRR. But it hasn't been easy to do a search for headphone amplifiers which has that attribute. So are they tricks you all could possibly think of. I mean would searching for datasheets doing a Google search with an sites be a way. I mean what would work I am clueless?
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Old 3rd February 2014, 06:34 PM   #2
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A good DAC (even usb powered) should not make any noise; if it does, make sure that your computer sound driver is set to the highest setting: 24bits, 96000hz or more. As a low noise low distorsion headamp, I recommend those based on the LME49600 buffer chip. I just built one for nightly DAC use and could not believe at first how silent it is.

Jacques

Last edited by jacques antoine; 3rd February 2014 at 06:35 PM. Reason: spelling
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Old 3rd February 2014, 06:58 PM   #3
Boscoe is offline Boscoe  United Kingdom
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Things start be hard to hear around the -60dB mark - noise/THD. Headphone amps (USB powered) are capable of easily -100dB THD+N.

Also PSRR tends to get poor quickly at higher frequencies.
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Old 3rd February 2014, 07:05 PM   #4
tvrgeek is offline tvrgeek  United States
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A few points that may help:
No need to set the sound card to anything is yu are using an external DAC. That becomes the sound card.

You don;t need to set the DAC to anything. If the file is 96K, and the DAC can handle 96K, it should detect and switch. Only some old badly designed units, like an e-mu I had would make you set it up ahead of time.

You get no higher quality setting a DAC to 96 K if the file is 44K. No magic here.

I have several Chinese DAC/Head amps. All wall-wart powered. One type is dead quiet, one type not. All are BB based. All are fixed at 44K, 16 bit standard, so they are useless with higher rate files. Someday I will get one that will switch. ( In a pinch, I can use my Focusrite 2i2 and feed my battery powered head amp)

You are not going to see any specification about PSRR on the WEB. That is something the engineers worry about. You may see a SNR, but most of those are fiction.

Be it a linear supply or a switching supply, it is not the amp PSRR, but the supply design and execution. A good engineer can design with either.
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Old 3rd February 2014, 07:49 PM   #5
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Quote:
Originally Posted by tvrgeek View Post

You don;t need to set the DAC to anything. If the file is 96K, and the DAC can handle 96K, it should detect and switch. Only some old badly designed units, like an e-mu I had would make you set it up ahead of time.

.
I own a corn-fed, all-american Schiit Modi DAC: it is noisy as hell on the default setting (16bit 44.1khz) and dead silent when manually set at 24/96k. It does not switch automatically. Several other users have noticed the same behavior.

J.
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Old 3rd February 2014, 08:35 PM   #6
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OK
I think I will have to elaborate
The responder at Head-HI forums is some sort of engineer or something and he is getting into a deeper than just noise. He admits that noise is usually not heard at human parable audio frequencies. But he says there are worse ways to headphone amplifier with a PSRR (Power Supply Rejection Ratio) that is less than high can affect your listening experience and quite deleterious ways. At this point I will quote

"Let me emphasize, however, that "noise" in the power supply is rarely something that's heard. Instead, it's a general lack of quality in sound: maybe a glare in the high-end, a general loss of dynamics, sloppy bass, etc. It simply causes effects that result in sounding blah vs. GREAT. Only extreme examples - not worthy of any consideration at all - result in noise that's directly audible in the sound signal.

Also, switching power supplies can sometimes be used with amplifiers (including headphone amplifiers) with good success. However, every amplifier circuit has a calculated property known as PSRR - Power Supply Rejection Ratio. It's a measure of how much noise in a power supply will affect the amplifier circuit itself. Good circuits have very high PSRR's. However, powering a source such as a DAC does not qualify. Once noise is introduced, it'll propagate throughout the signal stream."


Well the very last sentence in that quote did not address the question I am asking you, but the rest of it did. According to this person not finding a headphone amplifier with an adequately high PSRR quite affects your music listening it respectful of noise.

From my most recent reading on the subject operational amplifiers have families and families of curves against frequency where they show how high the PSRR is. I also know from from my per rousing of the web that do-it-yourselfers are the designers of very simply enclose products which are very high value if you can spare having all the bells and whistles and simply use these products simply. These designs are sometimes brought to market. So it is these I am seeking to find which will have a high PSRR operational amplifier. So any ideas in helping me find these would be appreciated. I could do my own enclosure by do not want to build a headphone amplifier from scratch.
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Old 3rd February 2014, 08:55 PM   #7
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Hi Bbmiller,

With all due respect, the first quoted paragraph sounds like utter gibberish and the end of the second one like plain bs. Or is it the other way around? I am not sure.
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Old 3rd February 2014, 09:16 PM   #8
sreten is online now sreten  United Kingdom
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Hi,

There is no such thing as simply high PSRR. PSSR varies with frequency
and in most cases the PSRR for the two voltage rails, assuming a dual
supply, is different dependent on the circuit topology.

Some simple circuits inherently have poor PSRR, and rely on clean
well filtered supplies. Op-amps and chip amps have high PSRR and
often you can add some mild RC filtering into one rail to make the
PSRR of both rails similar.

Personally I'm in the its more "gibberish" than talking sense camp.

PSRR tends to fall off closely following the open loop gain for both
rails, though as said, for each rail the level may be different.

As such it tends to to be low at high frequencies. Simple RC
filtering easy to use in a headphone amplifier gets better
the higher you go and it is effectively quite easy to in any
sensible respect to prevent PS noise affecting the output.

rgds, sreten.
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