Question about Impedances

[oguinn]

Recently I've acquired a few sets of headphones: the Sennheiser HD 6XX, the Sennheiser HD 25-13, and the Beyerdynamics DT 770 Pro. Each has a different input impedance and each, predictably, sounds totally different from one another. However, I've noticed a difference that doesn't really jive with my understanding of impedances.

With no music playing, I hear nothing when I use the HD 6XXs (300 ohms). That's been my experience with my Crack so far since it was really the only set of headphones I owned until recently.

So today, I plugged in the newly-acquired HD 25-13s (600 ohms) and was surprised to hear what kind of sounded like a tinkling or glassy jingling. After the amp warmed up a bit, it's settled into a soft, high-pitched whine that I hear when no music is playing. I also need to turn the potentiometer down a lot more when using them because they're a lot louder. This is the crux of my confusion: I'd expected there to be a blacker background with higher impedance headphones, and to need to turn the pot higher to get an equivalent level of loudness.

I was initially concerned that the HD 25-13s had their drivers replaced with a lower impedance set since the eBay seller was kind of shady (really, really bad packaging considering they were shipped from Spain, needing a new headband and screws since they were covered in cigarette tar that wasn't mentioned or shown in the listing), but measuring the resistance from tip to sleeve and ring to sleeve shows right around 600 ohms.

So this has sparked a series of quesitons:

• Is my understanding of the effects of impedance wrong?
• Is what I'm hearing indicative of a problem with the Crack? Potentially a grounding issue maybe?

[troplin]

A quick google search suggests that the HD 25-13 have much higher sensitivity than the HD 6XX (120 dB vs 103 dB @ 1mW). I'm not sure that those numbers are accurate though, I found them on random websites.

The lower impedance of the HD 600 only makes up for 3 dB (factor of 2), meaning that the HD 25-13 is still 14 dB louder at the same potentiometer setting.
This also means that it has a much lower SNR which explains the noise.

[oguinn]

Ah ok so it’s more than just impedance. It’s one stat of many that influence the sound.

[troplin]

Basically:
- The impedance determines how much power the the headphone draws from the amp at the same potentiometer setting
- The sensitivity determines how efficient the headphone is in transforming this power to an audio signal

Together they give you the actual loudness at a specific setting. If you double the impedance, you get half the power which means -3dB.
In your case the difference in sensitivity (17 dB) is more significant than the difference in impedance (-3 dB).

[tim273]

Just to add to this, I have a pair of Beyerdynamic DT-770's that I replaced with 600 ohm drivers (~630 ohms actually) and they are dead quiet on my crack.  I'm not sure what the sensitivity is on those though.

[oguinn]

Thanks, all. I was considering replacing the drivers on the DT-770s, but I'm still a little nervous about ruining them.

I might just sell the HD 25-13s after I clean them up, though...

[Doc B.]

The whine is a tube oscillating. Clean the pins and see if it goes away. If not you might need to try a different tube.

[oguinn]

I'll give that a shot. Thanks, Doc.

[Paul Birkeland]

The tinkling/glassy jingling you hear may just be the glass of the tubes getting warm and pinging a little bit, which you can sometimes hear if the tube is conducting when this happens. 

The other noise that you hear is very likely related to one of the tubes themselves.

[oguinn]

What are the things that cause oscillation besides the pins being dirty? I've cleaned the pins on a 6080 and a 6AS7G (scrubbed each pin with 0000 steel wool and then wiped with a little degreaser on a cotton swab) but am still hearing the same noise. It's worse on the 6080 than it is on the 6AS7G (which is why I switched to the 6AS7G in the first place).

The noise does not get louder with increases in volume.

[oguinn]

I should mention also that swapping the 12AU7 had no discernible impact, which is why I'm focused on the driver tube.

[Doc B.]

It is often set off by capacitance at some less than perfect connection. That is why dirty tube pins/sockets are a common cause. But a fluxy solder joint could cause it too, and even wiring dress can have some influence. You might try listening while moving the wires connected to the 6080 socket around a little bit to see if anything changes the sound.

[oguinn]

Cool, that was my next step after giving the amp some time to cool down and discharge the caps.

[oguinn]

No dice. Jammed around on the pins and anything leading into the tubes with a chopstick with some headphones attached and didn't notice any changes. I did see the lead to an LED was pretty close to another pin on the 12AU7 socket and cleaned that up but no real change.

I'll clean the 12AU7 pins and swapping in a new 6080.

[2wo]

How about trying a pair of shorting plugs, just to rule noise from upstream...John