Headphones impedance RME digiface

[DolbyNR]

A while ago I bought the RME Digiface USB, and next on the list is a good pair of headphones. I as thinking about the Beyerdynamic DT 990 Pro 250 Ohm.

But the I read the Digiface manual :

Output Analog Phones
x Dynamic range (DR): 110 dB RMS unweighted, 113 dBA
x Frequency response @ 44.1 kHz, -0.5 dB: 0 Hz – 20.8 kHz
x Frequency response @ 96 kHz, -0.5 dB: 0 Hz – 45 kHz
x Frequency response @ 192 kHz, -1 dB: 0 Hz - 89 kHz
x THD @ -1 dBFS: - 96 dB, 0.0016 %
x THD+N @ -1 dBFS: -95 dB, 0.0017 %
x Channel separation: > 100 dB
x Output: 6.3 mm TRS jack, unbalanced
x Output impedance: 2 Ohm
x Output level at 0 dBFS, 1 kOhm load: +9.5 dBu
x Max power @ 0.1% THD: 50 mW

OUTPUT IMPEDANCE 2 OHM ???????????
This means not even a DT770 32 Ohm would work.
Am I overlooking something?
Is this phones output worthless?

[MOF]

The load i.e. the headphones have to have a higher impedance than the source impedance, if the load is less then it progressively becomes a short circuit the lower it goes, the first sign of this is the loss of bass output.
The 250 ohm headphones will be quieter than the 32 ohm, the output is pretty healthy at the 1000 ohm stated I think. More electronically minded folks on the forum will tell you later hopefully.

[Hugh Robjohns]

A while ago I bought the RME Digiface USB, and next on the list is a good pair of headphones. I as thinking about the Beyerdynamic DT 990 Pro 250 Ohm.

Good choice. They'll work well with the RME.

]OUTPUT IMPEDANCE 2 OHM ???????????
This means not even a DT770 32 Ohm would work.

Does it? Really? What makes you think that?

Why would RME design a headphone output that wouldn't work with anything, and then brag about it in the specs?

Am I overlooking something?

Yes, I think so. Probably an understanding of specifications in particular, and electronic principles in general.

Is this phones output worthless?

No, not at all.

Every electronic output has a certain output impedance. It's there for a number of important reasons, including circuit stability, short-circuit protection, and improved compatibility with a wide range of headphone impedances — but as a general rule, the lower the output impedance the better.

When you connect headphones to the amplifier the headphone impedance is placed in series with the output impedance. So some of the signal voltage is developed across the output impedance and the rest across the load impedance. Obviously, the more voltage that develops across the headphone load the greater the output level, so you want the headphones to have a much higher impedance than the amplifier output impedance.

Of course, other issues affect the eadphone loudness, too, like headphone sensitivity, but that's a whole other kettle of fish...

[DolbyNR]

Ah ok. That sounds good. Because I was only looking at "Output impedance: 2 Ohm".

Because I read there's a thumb of rule that the impedance of your headphones should max. be x6 of the output impedance.
In this case 2 Ohm x 6 would mean a 12 Ohm headphone would even be the max to use, wich didn't sound plausible to me.

[DolbyNR]

Am I overlooking something?

Yes, an understanding of specifications in particular and electronic principles in general.

I must admit I only now start to understand impedance a bit. I'm a musician with very basic electronics knowledge. But that'll bring you guys into the scene

[Hugh Robjohns]

I read there's a thumb of rule that the impedance of your headphones should max. be x6 of the output impedance.

That's hilarious. And totally incorrect.

Rest assured the RME will be fine with 250 Ohm headphones... or 100 or even 32.

[DolbyNR]

Thank you Hugh

[James Perrett]

Rest assured the RME will be fine with 250 Ohm headphones... or 100 or even 32.

And certainly it is very happy here with 300 ohm headphones.

[ef37a]

Yeah, headphones and headphone amplifiers used to be pretty simple, at least for things like small mixers and later interfaces (the hi fi industry got very silly with things like 8 Ohm headphones)
You built a small 'power amp' capable of say 5-8V rms and stuck a resistor on the output of between 33 and 100 Ohms. The resistor protected the amplifier from shorts, kept it stable and tended to even out the volume for differing impedance cans.

Then, some clever bugger decided, for no valid reason I have never been convinced of, that HP amps should have a very low or even near zero output impedance. That brings in problems with stability and the fact that whilst 250 Ohm headphones might be comfortable, 32 R cans of similar sensitivity will blow your hat off. I notice the Digiface has two switched levels to cope with this. Not all AIs do. (Hugh's review June '23)

I would love SoS to do an article about this matter. Hugh does enough! Maybe Phil Ward?

Dave.

[James Perrett]

(the hi fi industry got very silly with things like 8 Ohm headphones)

Do you remember companies like Eagle or TTC with their range of headphones which seemed to be available from most TV and radio shops in the 70s? They were all 8 ohms - mainly because their headphones used a standard small transistor radio speaker in each earpiece. They were far more common than higher impedance headphones when I started out.

[ef37a]

(the hi fi industry got very silly with things like 8 Ohm headphones)

Do you remember companies like Eagle or TTC with their range of headphones which seemed to be available from most TV and radio shops in the 70s? They were all 8 ohms - mainly because their headphones used a standard small transistor radio speaker in each earpiece. They were far more common than higher impedance headphones when I started out.

Yes James I remember Eagle and another name that escapes me, part of the Radio Shack empire I think? Don't recall TTC.
This was at a time when virtually all hi fi amplifiers were transistorized and they operated with 8 Ohm speakers. So, naturally, headphones had to be 8 Ohms as well! Total nonsense of course and the headphone jacks on such amps had to include some serious attenuation to prevent customers going deaf or the cans catching fire!

My first headphones were AKG K50s, 600 Ohms and worked fine jacked across most sources.

Dave.

[Martin Walker]

Yes James I remember Eagle and another name that escapes me, part of the Radio Shack empire I think? Don't recall TTC.

Tandy?

[tacitus]

My first headphones were Eagle 8-ohm jobs. The colour scheme appeared to be multiple near shades of grey, or one shade badly executed. I leave you to work out which …

[Sam Spoons]

Yes James I remember Eagle and another name that escapes me, part of the Radio Shack empire I think? Don't recall TTC.

Tandy?

Weren't Tandy the shops in the UK and Radio Shack one of their brands? I think Realistic might have been another.

[James Perrett]

Yes James I remember Eagle and another name that escapes me, part of the Radio Shack empire I think? Don't recall TTC.

I think Eagle and TTC became the same company at some point. Realistic was Tandy/Radio Shack's own brand which sold similar stuff to Eagle.

[James Perrett]

Weren't Tandy the shops in the UK and Radio Shack one of their brands? I think Realistic might have been another.

The shops were called Tandy in the UK and Radio Shack in the USA. The Radio Shack logo was rather similar to the RS Components logo so I guess they couldn't use that branding in the UK.

Other brands they used were Archer for components and Micronta for test gear as well as a few others that I've forgotten.

[Sam Spoons]

Yes, in the UK AFAICR they sold Radio Shack branded products as well as Realistic. I remember Micronata and Archer now you mention them.

Wasn't Eagle a brand of a shop/wholesaler called Electro something or Euronics or something like that? I seem to remember they were near Strangeways but not sure.

[Wonks]

Looking round on the web, Eagle was a brand name of Laskys, as was Audiotronics, following various mergers in the early 70s. Gerry Adler was one of the chaps involved, and 'Eagle' was derived from his surname.

[Music Wolf]

Gerry Adler was one of the chaps involved, and 'Eagle' was derived from his surname.

And, since we've each seen 'Where Eagles Dare' at least 20 times, we should remember that Adler is the German for Eagle.

[tacitus]

Gerry Adler was one of the chaps involved, and 'Eagle' was derived from his surname.

And, since we've each seen 'Where Eagles Dare' at least 20 times, we should remember that Adler is the German for Eagle.

21 for me now; watched it again yesterday. How did they get all the ammo they slaughtered the Wehrmacht with into the bus?

[ef37a]

Tandy! Yes James the very chaps. They were in a large shopping precinct several miles from my home called "Weston Favell Centre" (still there) and I used to take wife and 2 kids in a 15 cwt Bedford van because there was easy, free parking.
I would drop the distaff side at Sainsburys and son and I would check out Tandy's.

Dave.

[Jimmy B]

Yeah, headphones and headphone amplifiers used to be pretty simple, at least for things like small mixers and later interfaces (the hi fi industry got very silly with things like 8 Ohm headphones)
You built a small 'power amp' capable of say 5-8V rms and stuck a resistor on the output of between 33 and 100 Ohms. The resistor protected the amplifier from shorts, kept it stable and tended to even out the volume for differing impedance cans.

Then, some clever bugger decided, for no valid reason I have never been convinced of, that HP amps should have a very low or even near zero output impedance. That brings in problems with stability and the fact that whilst 250 Ohm headphones might be comfortable, 32 R cans of similar sensitivity will blow your hat off. I notice the Digiface has two switched levels to cope with this. Not all AIs do. (Hugh's review June '23)

....

Dave.

As some headphones have impedance that varies quite significantly with frequency, is it possible that driving them from a high impedance source would create a different frequency response from what the manufacturer (presumably) intended?

[Martin Walker]

As some headphones have impedance that varies quite significantly with frequency, is it possible that driving them from a high impedance source would create a different frequency response from what the manufacturer (presumably) intended?

Yes, this can be a significant problem if you have headphones that exhibit a more than average variation of impedance over frequency, and then plug them into a heapdhone amp that has a high than average output impedance.

As always, making sure the headphone amp output Z is at least ten times smaller than that of the headphones will ensure the flattest overall response.

Here's a fascinating link if you want loads more information, complete with some frightening graphs!

https://nwavguy.blogspot.com/2011/02/he ... dance.html

[ef37a]

Yeah, headphones and headphone amplifiers used to be pretty simple, at least for things like small mixers and later interfaces (the hi fi industry got very silly with things like 8 Ohm headphones)
You built a small 'power amp' capable of say 5-8V rms and stuck a resistor on the output of between 33 and 100 Ohms. The resistor protected the amplifier from shorts, kept it stable and tended to even out the volume for differing impedance cans.

Then, some clever bugger decided, for no valid reason I have never been convinced of, that HP amps should have a very low or even near zero output impedance. That brings in problems with stability and the fact that whilst 250 Ohm headphones might be comfortable, 32 R cans of similar sensitivity will blow your hat off. I notice the Digiface has two switched levels to cope with this. Not all AIs do. (Hugh's review June '23)

....

Dave.

As some headphones have impedance that varies quite significantly with frequency, is it possible that driving them from a high impedance source would create a different frequency response from what the manufacturer (presumably) intended?

Yes Jimmy, this is one of the issues that the "zero source z" band bang on about* that and "damping" though that is soon scotched because no matter how low you make the drive resistance the headphone resistance will still be the limiting factor for EMag damping.

But, peeps have been making high quality headphones for decades, from 600 Ohms down and in all that time just about every amplifier they were connected to had an output resistance of some 30 Ohms, usually much more.

So, have AKG, Beyer, Sennies et al not known, not cared about how we drive their products? They are all strangely silent on the matter. Funny thing? Only a very few microphone manufactures market a pre amp. AFAIK, no HP maker sells an amp for their babies?

*Applies of course for passive loudspeakers but we are then talking of controlling a cone/coil mass many many times that of a headphone diaphragm more akin to a mic's and we don't feed those into a short!

Dave.

[ef37a]

"Yes, this can be a significant problem if you have headphones that exhibit a more than average variation of impedance over frequency"

But is it really Martin? Surely headphone manufacturers have always known this but none, AFAIK have every told us we must drive their products with special low source z amplifiers?

And what is a "more than average variation"? Impedance curves for cans are quite hard to find and the manner of their derivation not made plain. For instance, are the earpieces measured in 'free air' or clamped to a person's head or simulation of same. I would aver the very close coupling of lug and diaphragm will have an effect on impedance?

Does the non-zero source z not matter because the devices were developed knowing the likely nature of most headphone amplifiers?

I don't know Martin but would really like someone to find out!

Dave.