Beyer DT990 ( 250 Ohm ) amped via Grace m903: The loss of bass and sound stage

I have enjoyed my closed AH-D2000's through the Grace m903 for some time now, but wanted to investigate open cans for two reasons.  The first of which is to find a can with a nice sound stage that can image well.  The last is to find an open can that is super comfortable that compliments my AH-D2000's headphone signature.

What I found so far is that through my iPod with no EQ settings the DT990's actually sound good to great.  Sure I have to turn the volume up to 85%, but the bass is punchy and extended and the sound stage open as well as the highs being crisp and clear.  When I plug the DT990's into the Grace m903 I lose a ton of sound stage and the frequency spectrum seems damped especially the bass.

My questions to you all are:

1.)  Have you had a similar experience with amping the DT990 with various amps?

2.)  Why would the iPod sound more even and open whereas the m903 flat and bassless?

Some tracks do sound very nice, but in general the iPod sounds better per average track.

Any help would be greatly appreciated.

System:

Foobar2000

160kbps or greater mp3's ( I have tried CD's as well )

USB to m903 DAC

m903 Headphone out to DT990 ( 250 Ohms )

Quote:

Originally Posted by jupitreas 

The iPod might have high output impedance (no idea what it actually is, I dont think Apple publishes this spec). If the output impedance of the iPod is larger than about 32 Ohm, this could audibly affect the frequency range of the headphones. Bass is what is most affected by a high output impedance so what you are hearing out of the iPod could actually be a coloration introduced by high impedance. How we interpret a coloration is pretty subjective and you might actually prefer the way the iPod sounds. Meanwhile, the amp has a published headphone output impedance of 1.2 Ohms, which is a figure that in no way can affect the sound of the Beyers you have.

 

So basically, what you are hearing from the Grace is most likely the more neutral and accurate sound. The iPod might have high output impedance and it might also be introducing other artifacts through clipping and compression at such a high volume and you might be interpreting these faults as sounding better. Neutral/flat audio generally doesn't sound great to most people, its something one usually needs to get used to.
 

This I understand, but looking at the DT990's frequency response there is no doubt about the almost 8+ dB boost between 80 and 120 Hz.  There is no doubt in my mind that the m903 + DT990 ( 250 Ohm ) is not what the DT990 should actually sound like.  I will be trying a 32 Ohm version this week and see if that cures it.  I will admit that the iPod could be coloring the sound, but to hear that big of a difference was not a pleasant surprise.

Quote:

Originally Posted by BotByte 

Sounds like underamping

 

250ohms is a lot to push.

My receiver stock could hardly push that. Now it pushes 6 watts to 10ohms after my mods

That is what I was thinking.  The 32 Ohm version should be a good test, although I did hear similar results with a pair of RS-1i's ( 32 ohm ), but it was not as drastic as the DT990.  The sound goes from clear, crisp, and deep to mush and bland.  There is no way the DT990 should sound like that.

Thanks for you reply.

I should mention that I amped the DT990's with a Bithead on High gain and the clarity, bass, and soundstage were all intact.

Quote:

Originally Posted by BotByte 

Seems like it only pushed up to 50ohms HERE

 

I'm not too good with amps, but I can see this:

 

Headphone output @1kHz, 50 Ohm load

 

isn't enough. But I'm probably wrong

I must admit that spec is confusing at best.  To me it tells me that I can get a +22dB gain with a headphone load of 50 Ohms, but it does not set a trend as to what would happen if I increase or decreased the load.  The power depends upon if the amp is a constant voltage or constant current source as Power = (I^2)R + (V^2)/R.  On top of that does it mean a 22dB gain in addition to the 90 dB range the volume knob gives?

Would anyone else be able to clear up the Headphone Output @1kHz, 50 Ohms load = +22dB spec?

To me this is just a single point on the FR curve.

I think I found the issue.  The TRS connector on the headphones has one of those twist on 1/4 inch adapters.  I hate these because if they are not screwed on all they way the sound will be ok, but you loose a ton of clarity and impact.  Straight out of the box the adapter had a burr on it making it thread a little more roughly than desired.  After using my iPod as a source feeding my m903 I was able to test if it was the USB connection.  I also double checked all of the connections and did not notice anything out of place.  I removed the 1/4 inch adapter to let a friend plug the cans into his iPod.  After he was done I attached the 1/4 inch adapter again and plugged my headphones back into the m903.  I switched back to USB and tested a few other tracks.  The headphones sounded as they should.  They are now clear, spacious, and impactful.

I never liked twist on adapters nor burs for that matter.

Because the m903 is a constant voltage power source for headphones when the impedance on the headphones increases, the power draw from the amp decreases.  Since V = IR and V is constant the current draw goes down as the resistance goes up.  This gave me confidence that regardless of impedance the m903 should drive the headphones with ease.

If any of you out there have done a great amount of manufacturing you understand my woes with burrs.  Some of the greatest challenges can be caused by a simple burr.

Thanks for all of your great replies.