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Low impedence headphones more susceptible to tube-induced distortion. Why?

post #1 of 12
8/31/10 at 12:35pm
Thread Starter 

One of my power tubes is failing, causing a volume mismatch between the left and right channels.

 

My lower impedance headphones (Denon D5000 at 25 ohms) were the first to register the loss of volume and eventually produce massive distortion in the effected channel as the problem progressed.

 

 

My HD650s (300 ohm) still play distortion-free but register the volume mismatch and all the loss of sound quality that entails.

 

Can anyone explain why the lower impedance headphones would the canary in the coal mine here? (my PX100 portable, at 32 ohms, were slightly slower than the D5000 to produce distortion. Both headphones, by the way, still sound fine paired with my healthy solid-state amps).

 

Lastly, could the Denon's have caused the amp's power tube to fail prematurely? The amp is rated for use with headphones 32 ohms and up. The D5000s are 25 ohms. Does this present a problem?

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post #2 of 12
9/1/10 at 11:10am

From my very limited knowledge on the subject, the tube amp (OTL) has an easier time putting out plenty of voltage to drive high impedence HPs while it can struggle to put out enough current for the low impedence HPs.  Therefore, the low impedence HP will likely stress the tube more if its at the end of its life.

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post #3 of 12
9/1/10 at 1:41pm
Thread Starter 

^^ That actually makes a fair amount of sense. I have new power tubes on order, but perhaps I'll restrict the D5000s to my SS amps and save the LD for my higher impedance headphones.

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post #4 of 12
9/1/10 at 1:52pm

It has a lot to do with the amp's design as well. I know that hybrid amps including my SS Millett can offer more than enough current into 32 ohms. It would be best to get an opinion from someone who has experience with the little dot's topology.

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post #5 of 12
9/1/10 at 1:53pm

My A700s picked up much more of the MP-301s colorations than did the HD650s.

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post #6 of 12
9/1/10 at 1:56pm

You can use a output transformer tube amp design that will do from 8-600 ohms.


Edited by Audiogalore - 9/1/10 at 1:57pm
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post #7 of 12
9/1/10 at 2:15pm
It's because of the impedance mismatch. Despite manufacturer claims, most OTLs have an output impedance higher than 32 Ohms. When that happens, it strains the amp to deliver enough power. It can push the amp into distortion and/or clipping fairly easily. Manufacturers rarely tell you this because it would hurt sales. If you want an OTL that drives low impedance headphones, you have to use an output tube like the 6C33C or 6C41C that has really low output impedance. Otherwise, you have to put output transformers on the tubes.

Another trick used in the cheap OTL amps is heavy application of negative feedback (NFB). NFB does a good job of stabilizing circuits and can lower output impedance. NFB feeds part of the output signal back into an earlier stage of the amp to correct the signal. The problem is that there's always a small delay in that transmission, so a lot of NFB messes with the timing of the signal and strangles the sound. Really good amps use no NFB or just enough to keep the circuit stable. The cheap ones lay on lots of feedback to make it "work" with low impedance headphones a little better and make a bunch of marketing claims.

There are lots of other reasons to stay away from the cheap OTL tube amps, but that's another thread. If you want to minimize distortion with your low impedance headphones, go with solid state or use a tube amp with transformers on the outputs.
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post #8 of 12
9/2/10 at 9:12am
Thread Starter 

6C33C and  6C41C don't appear to be options for the Little Dot MK IV, so until it's time to buy a new amp, it sounds like I'd be better off keeping the low-impedance headphones away from the Little Dot. Could the D5000s actually damage the amp or cause the tubes to fail prematurely?

 

And if the D5000s are out as far as this amp goes, I assume the same would be true for the HE-5LEs?

 

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post #9 of 12
9/2/10 at 10:39am

To be fair, the Little Dot tube amps specify using phones from 32-600 ohms.  So your Denon are really pushing the limit so to speak.

 

I'd go to the Little Dot forum and ask David.  He's tell you for sure whether its a problem.


Edited by Max F - 9/2/10 at 10:40am
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post #10 of 12
9/4/10 at 9:33am


 

Quote:
Originally Posted by Uncle Erik View Post

NFB does a good job of stabilizing circuits and can lower output impedance. NFB feeds part of the output signal back into an earlier stage of the amp to correct the signal. The problem is that there's always a small delay in that transmission, so a lot of NFB messes with the timing of the signal and strangles the sound.

 

Messes with the timing of the signal and strangles the sound?

 

Say what?

 

Can you describe exactly what happens to the signal that causes it to be "strangled"?

 

se

 

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post #11 of 12
9/4/10 at 8:57pm
Thread Starter 
Quote:
Originally Posted by Max F View Post
I'd go to the Little Dot forum and ask David.  He's tell you for sure whether its a problem.


Here's what David had to say: "Oh you definitely won't cause damage or anything like that. Tube life would probably be slightly shorter than if you exclusively used high impedance headphones, but really nothing you should be worried about."

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post #12 of 12
9/6/10 at 8:05am

I have a Little DotMk6 and my D7000 is controlled with great authority. Surprised me also at 25 ohms.

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