Beyer amps with 100 ohm output impedance. Lolwhut?

 I KNOW! Sounds crazy, doesn't it?

A 100 ohm adaptor WILL NOT reduce the output impedance.

A lot of folks like the Matrix M Stage, output impedance is 5 ohms.

I use it with my DT880 600 ohm phones.  Call me happy!

Well, there's the IEC 61938 standard that has output impedance of amplifiers at 120 ohms... IEC is European, as is Beyer.  Maybe they were deviating from the IEC spec to tune their headphone sound a little bit.

Ummmmmmmm, waste 80% of the output voltage!

Very poor damping factor:

 - means looser bass and more distortion in the bass with most types of headphones.

Can lead to unwanted frequency response variations due to interaction between the amp and headphone impedance.

Also, when impedance is as high as 100 ohms and headphone is only 25 ohms, then mathematically the headphone will only see 20% of the output voltage, believe it or not, most of the amps output voltage willl be lost in the output impedance of the amp, very wasteful and very inefficeint!

But if you use a 600 ohm headphone then most of the output voltage will be seen by the headphone.

It is the proportion of headphone impedance to amp output impedance that is important.

Try reading this link, may help explain:

http://www.head-fi.org/a/headphone-impedance

Some people actually like this sound!

Vintage receivers usually have a very high output impedance (as high as 220 ohms in some cases)!

Look at how popular vintage receivers are for driving headphones!

Quote:

Originally Posted by Mad Lust Envy 

Ok, but what would that do to the sound? I'm not up with the science/math of it all.

I do not have the MMX-300, so I cannot comment on that, but for the DT880-250, the effects in practice are relatively minor, although not insignificant. Presumably the 32 Ohm MMX-300 would be worse. It slightly boosts the bass (centered at about 80 Hz) and the top of the treble response, and it adds some distortion in the bass range. The following graphs, from left to right, are the frequency response (in loopback mode; with 220 Ohm resistors in the place of the headphone it would be perfectly flat other than the roll-offs at the ends) - compare it to the InnerFidelity impedance graph , the distortion at about 80 Hz (also loopback, at about 1 Vrms signal level on the headphone, which translates to roughly 100 dB SPL), and finally the distortion recorded with a microphone at the same frequency and level (the right channel is comparable to the InnerFidelity PDF file, the left channel is lower).

The source had 100 Ohm output impedance in all tests, and the headphone is a DT880 Pro (250 Ohm). All tests were performed wearing the headphones. Although these are mostly loopback tests, I assume the changes measured in these also apply to the actual sound output. The distortion in the loopback vs. microphone tests differs by a factor of about 10 (i.e. ~0.1% vs. ~1%), so the under-damping contributes about one tenth of the total distortion (assuming that the microphone does not distort significantly, which I cannot verify other than by comparing the results to InnerFidelity). Both distortions generally decrease with frequency, but the loopback distortion stays within 10% of the microphone one; it decreases to 0.01% by about 300 Hz.

Quote:

Originally Posted by Chris J 

 

Also, when impedance is as high as 100 ohms and headphone is only 25 ohms, then mathematically the headphone will only see 20% of the output voltage, believe it or not, most of the amps output voltage willl be lost in the output impedance of the amp, very wasteful and very inefficeint!

I have even seen 330Ω serial resistors on the schematic of a receiver. That would be some really boomy bass with an HD598.

Quote:

Originally Posted by stv014 

I have even seen 330Ω serial resistors on the schematic of a receiver. That would be some really boomy bass with an HD598.

 

Crazy, isn't it?
  I understand why they do it, it limits the amount of power the receiver can drive into the 'phones.  I'm sure you have noticed that the headphones are driven by the power amp via the resistor in recievers.

Quote:

Actually, that is an advantage in some sense, since with a 100Ω output impedance, the actual power output into the headphone only varies by a factor of about 2 (~3 dB) from 16Ω to 600Ω (and less than 1.5 dB from 32Ω to 320Ω), rather than 37.5 (~15.7 dB) as with a zero output impedance. But the disadvantages related to frequency/phase response and distortion obviously still apply, as demonstrated above.

I would argue that 100 ohm o/p Z is not an advantage, even when going from 16 to 600 ohms.

However, I should keep an open mind!  Would you be interested in showing me your calculations? I would be interested in seeing your train of thought.

Obviously,

The higher impedance phones need more voltage, the lower impedance cans need more current.

 My thinking is that the high impedance limits voltage and current, which is not really necessary in a little headphone amp.   Just my opinion.

I find my Matrix M Stage works fine with both my 62 ohm AKG Q701, my 2,000 ohm Sennheisers and my 600 ohm Beyer DT880s.

But I gotta crank the volume up a bit more for the high impedance cans.  As you know, the M Stage has an O/P Z of 5 ohms.

Thanks!

Of course, if you want the highest possible maximum power output in any case, then 0Ω output impedance is ideal. Here is how the power changes with headphone impedance, from a source that outputs 1 Vrms through 100Ω:

16Ω: (16/(16+100))^2/16 W = 1.19 mW

32Ω: (32/(32+100))^2/32 W = 1.84 mW

100Ω: (100/(100+100))^2/100 W = 2.5 mW

250Ω: (250/(250+100))^2/250 W = 2.04 mW

600Ω: (600/(600+100))^2/600 W = 1.22 mW

Now the same with 0 output impedance:

16Ω: 1/16 W = 62.5 mW

32Ω: 1/32 W = 31.25 mW

100Ω: 1/100 W = 10 mW

250Ω: 1/250 W = 4 mW

600Ω: 1/600 W = 1.67 mW

The 100Ω source wastes power, but it keeps the power output into various impedances relatively constant, so the low impedance headphones do not become extremely loud (assuming that the device has enough maximum voltage in the first place to drive the 600Ω loads well). Of course, this might not be worth the degradation in sound quality.

Although, as you noted, the most common reason for implementing high impedance headphone outputs is simply to save costs.

Quote:

Originally Posted by Mad Lust Envy 

Well the Headzone clearly must be doing something right to have such a high output impedance and come bundled with the MMX300.
Why wouldn't a 100ohm adapter help in this case?

I'd have to see the 100 ohm adaptor but I think that adaptor RAISES the impedance to 100 ohms.

Only a transformer will DROP the impedance, which is what we want.

Sorry for the short answer, I gotta go out for a while!