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Originally Posted by appletree /img/forum/go_quote.gif
Finally I was able to do some hearing tests (but nothing serious), comparing my X-Fi vs. E-MU on my new HD 595 (50 Ohms) I am not going to discuss the overall SQ which speaks clearly for E-MU, however I discovered, that the X-Fi (which has no headphone amp) sounds like it has stronger bass..........I also discovered that when I used the backpanel 1/8" jack for speaker out with my cans, the bass was there - even superior to the X-Fi. This still keeps me wondering - afaik all of the analog outputs on E-MU 0404 USB use the same DAC but the difference between bass performance of the hedphone out vs. speaker out on my E-MU is very apparent. Can some other 0404 USB owners tell if they experienced the same effect ? Or what is the possible explanation for this ? Shouldn't amp help to the bass performance on cans instead of decreasing it ?
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Yes, there is a very good explanation for this!
Note that you state "....the X-Fi
sounds like it has stronger bass." And that is probably indeed the case........but what would it "sound like" if the 1/8" line output did not have stronger bass, but instead, it had rolled off highs? Actually, it would sound sort of the same. The lows would have a higher response versus the highs, and the impression would be that the volume was lower, but the bass was more prevalent. If you then turned up the level so the overall volume sounded "right", the bass would sound stronger.
The problem is that the line outputs on the 0404 USB are only designed to drive very high load impedances correctly--such as the 10,000 to 47,000 ohm impedances that are common on the input of power amplifiers or other gear. When you try to use such a stage to drive a low impedance like a headphone (even "high impedance" cans are relatively very low impedance compared to amp inputs) you create a low-pass filter.......the highs are rolled off so the frequency response is no longer flat....the lows are relatively higher than they should be versus the highs.
Unfortunately, the tech specs for the 0404 USB do not include the minimum impedance for which the line outs will provide flat response. If you check out specs on audio preamps, they often specify that the response is flat only into loads of 10k ohms or more.
And I know this not only "in theory": I tried to use my 0404 USB's balanced line outs to drive HD600's in balanced mode with a quick and dirty cable made from stereo microphone cable.....and found that first, the line outs don't provide enough power to drive the HD600's to normal listening volumes, and second--the highs were severely rolled off!
Think about this, too.....V = I x R, voltage = current x resistance. Line output stages are designed to be voltage amplifiers that work correctly into, let's say, 22.1 kohms, a common figure in the middle of the typical range. At a full scale digital signal level, the output should be about 2 volts into that load:
Current required to drive that line input= 2 volts/22,100 ohms = 0.0001 amps, or 0.1 mA.
When driving a 50 ohm headphone, that output stage is going to do its best to drive it at 2 volts, too:
Current required to drive the 50 ohm cans = 2 volts/50 ohms = 0.04 amps, or 40 mA.
In other words, when you plug 50 ohm cans into that line out, you are asking that line out to provide 400 times the current that it would normally put out to drive a typical power amp or headphone amp input. Some output stages can do it (the Benchmark DAC1 and Apogee Mini-DAC are said to be capable of that), but others just won't.
Moral of the story: Don't drive headphones with the line outs on the back of the 0404 USB. Either use the internal headamp, or route the line outs to an external headamp that is properly designed to drive typical headphone impedances.
