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Matching usb interface output impedance with k702 headphones

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Matching usb interface output impedance with k702 headphones

Postby forumuser915213 » Tue Apr 13, 2021 9:25 pm

I was surprised recently to discover that my Beyer DT990 Pro studio headphones (impedance 250Ohms) and AKG K702 studio headphones (65Ohms) sound quite different when plugged into my Steinberg UR22c interface (output impedance 40Ohms).

I gather they're both /fairly/ well thought of for monitoring purposes, so, all things being equal you'd hope there was some resemblance. But they're not equal!

Experienced hands will immediately recognise the problem: I read that leaving aside the question of volume inside the cans, headphone impedance should usually be 2.5x-8x the amp output impedance in order for the amp to properly control the voicecoils. This is called the 'damping factor'.

The DT990s are therefore apparently within range when used with the UR22c (and produce quite enough volume for my taste), but the k702s are not, for their damping factor is just a hair over 1.5, when it should be a minimum of 2.5.

So here's my question: is there a recommended cheap as chips low output impedance headphone amp that would sit between the interface and the k702s without screwing up the sound in some interesting new way please? Some inexpensive battery powered thingie might do it, I guess, but few sellers publish specs, that I've seen.

Or is there some other technical fix, such as an in-line impedance?

Or have I misunderstood the whole thing, or been hopelessly misled? (I only vaguely remember damping factors from studying physics 45 years ago...)

Thanks, Gavin
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Re: Matching usb interface output impedance with k702 headphones

Postby Folderol » Tue Apr 13, 2021 9:45 pm

If you're into D.I.Y a pair of LM386 chips with a handful of components would run happily off a PP3. This would cost about a fiver all told. The box to put it in would cost more!
Distortion when driving an 8 ohm load is 0.2%, and that would be a lot lower at headphone impedance.

You can also get single chip ones, but they are far more fiddly to set up.
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Re: Matching usb interface output impedance with k702 headphones

Postby James Perrett » Tue Apr 13, 2021 10:10 pm

If you look at the plots on the Sonarworks review for the DT990 you'll find that it has nothing like a flat response. The actually curve resembles a typical smiley curve. I couldn't find a review for the K702 but the K712 has a very different frequency response to the DT990. So I don't think your headphone amp is the problem - you're just listening to two pairs of headphones with very different sounds.
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Re: Matching usb interface output impedance with k702 headphones

Postby ef37a » Tue Apr 13, 2021 10:19 pm

"Or have I misunderstood the whole thing, or been hopelessly mislead? (I only vaguely remember damping factors from studying physics 45 years ago... "

A bit I think, let me refresh both our memories? "Damping factor" is the ratio of load impedance to amplifier output impedance. Thinking of amps driving speakers for a moment, a high grade transistor amp might claim a DF of 200 for an 8 Ohm speaker. That means the output impedance of the amplifier is 8/200 =0.04 Ohms. Purveyors of amplifiers make great a fuss of this but never point out that the speaker has a DC resistance of about 6 Ohms and so an output Z much lower than say 0.1 Ohms is pretty pointless AFAICS but there are other opinions about this.

"Electro-magnetic damping" is proportional to magnet strength and the 'Ampere turns' in the coil. The current through headphones is of course proportional to drive voltage and total circuit resistance. Now, it turns out that the DC resistance of dynamic headphones is always just about the same as their stated impedance (just checked my AKG K92s, 33 Ohms DC resistance, Nom Z 32 Ohms)

For decades headphone amplifiers used a physical resistor in their output. This protected the circuit from shorts but also helped even out the drive to the various headphone impedances.
There has arisen in the last few years a 'fashion' for HP amps to have a very low, near zero OPZ but I have read of no good reason for this. BTW the UR22 has a headphone output of 40 Ohms. Typo?

I do not pretend any qualifications or expertise in the matter, just a smattering of casual learning. Perhaps some one could explain it all? Backed up by physical tests of course.

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Re: Matching usb interface output impedance with k702 headphones

Postby forumuser915213 » Tue Apr 13, 2021 10:38 pm

Thanks for your thoughts folks.

I wasn't thinking of getting into diy... Maybe if things become desperate ;-)

I've seen the Sonarworks plots. The differences are striking, and I don't want to add to them!

I would point out that while the Sonarworks demo makes the two sets of headphones sound more similar, they're still far from identical and I don't know which is more nearly correct.

It may be that I've been misled by what I've read. It was online after all ;-)

It turns out there are little combination DAC-headphone amps with 3.5mm input sockets for sale out there, and a few sellers quote appropriate headphone impedances. They say nothing about linearity or distortion, however..

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Re: Matching usb interface output impedance with k702 headphones

Postby Martin Walker » Tue Apr 13, 2021 10:38 pm

Hi Gavin!

I suspect James has the correct answer - those two pairs of headphones are likely to sound rather different whatever headphone amp you feed them from. Your brain can however eventually get used to the sound of particular headphones and let you create mixes that translate, or you can use EQ software to compensate for their different sounds so that whichever headphones you use they sound similar.

And for Dave & Gavin, here are some sensible facts and figures about headphones and their amps:

QUOTE "The IEC 61938 standard goes even further in recommending an output resistance of 120 ohms, noting that "For most types of headphones, the source impedance has very little effect on the performance."

However, many headphones exhibit a wide variation of impedance across the frequency range, so read on here for a good explanation (and some graphs for Dave ;)) of the resultant effects:

https://www.stereophile.com/content/bet ... nes-page-4


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Re: Matching usb interface output impedance with k702 headphones

Postby Hugh Robjohns » Tue Apr 13, 2021 10:53 pm

I'm with James and Martin on this one; the tonal differences between the Beyer and AKG are intentional and down to the design of the specific headphones. I don't think the output impedance of the headphone amp will have a significant* impact on that aspect...

I have a couple of Benchmark DAC1 headphone amps with near zero output impedance, and my AKG k702s still sound very different both to my Beyer DT990s and my Sennheiser HD600s...

*Edited to add: I didn't realise the Behringer interface had a 90-ish Ohm output impedance which is pretty high by modern standards...

...but even then the resulting frequency response peaks/troughs due to the high source impedance will rarely amount to more than a dB or two unless the headphone's nominal impedance is particularly low and the tonal differences between different brands of headphone typically outweigh that quite comprehensively, especially at the frequency extremes!

That said, you will always get less amp/headphone interaction if the headphone's nominal impedance is much higher than the amp's output impedance -- so choosing phone's with a 250 Ohm (or higher) impedance will largely circumvent the problem of different amp output impedances... But the downside will be lower maximum volume, especially with portable devices.
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Re: Matching usb interface output impedance with k702 headphones

Postby forumuser915213 » Tue Apr 13, 2021 10:58 pm

Martin Walker wrote:Hi Gavin!
However, most headphones exhibit a wide variation of impedance across the frequency range, so read on here for a good explanation of the resultant effects:

https://www.stereophile.com/content/bet ... nes-page-4

Martin

I learned long ago that transducers such as turntable cartridges, speakers and mics are widely variable between models, even when they look similar, are made of similar materials and likely have similar design aims.

So I wasn't surprised to find them different - yet I was surprised to find them so very different. I'm not sure I could ever 'know' a pair of headphones well enough to compensate for their weaknesses. I instinctively appreciate the detail of the AKGs and the pleasing if more smeary tone of the Beyerdynamics - and daren't trust my judgement based on either.

I guess this is just another of life's questions with no very clear answer. Thanks for all your thoughts once more.

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Re: Matching usb interface output impedance with k702 headphones

Postby Martin Walker » Tue Apr 13, 2021 11:39 pm

forumuser915213 wrote:I'm not sure I could ever 'know' a pair of headphones well enough to compensate for their weaknesses. I instinctively appreciate the detail of the AKGs and the pleasing if more smeary tone of the Beyerdynamics - and daren't trust my judgement based on either.

I'm from the school that prefers to equalise different headphones using Sonarworks to make them sound more 'similar', both to each other, and to the loudspeakers in my little studio.

The Elf on the other hand prefers to get used to the (IMO) unique sound of his AKG K701/702's 'as is', doesn't like using corrective EQ, AND gets great translatable mixes.

There's no one right way!


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Re: Matching usb interface output impedance with k702 headphones

Postby n o i s e f l e ur » Wed Apr 14, 2021 2:47 am

The other thing to bear in mind when talking about Sonarworks correction and why different models / makes of phones still sound different even after correction with the demo is - those are just applying the "Average" profiles.

I would think that if you compared sets of phones across different manufacturers and models that have had them do a custom measurement, you would find a far closer match.
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Re: Matching usb interface output impedance with k702 headphones

Postby ef37a » Wed Apr 14, 2021 7:15 am

Martin Walker wrote:
forumuser915213 wrote:I'm not sure I could ever 'know' a pair of headphones well enough to compensate for their weaknesses. I instinctively appreciate the detail of the AKGs and the pleasing if more smeary tone of the Beyerdynamics - and daren't trust my judgement based on either.

I'm from the school that prefers to equalise different headphones using Sonarworks to make them sound more 'similar', both to each other, and to the loudspeakers in my little studio.

The Elf on the other hand prefers to get used to the (IMO) unique sound of his AKG K701/702's 'as is', doesn't like using corrective EQ, AND gets great translatable mixes.

There's no one right way!


Martin

Could yourself Martin and the other highly experienced chaps at SOS comment on whether the differences between high end monitor speakers are as great as they seem to be for 'so called reference' headphones? After all, you and the likes of Hugh, Paul et al get to audition vastly more high end kit that us mere mortals could in several lifetimes.

My guess is that monitors at the £2000+ per pr are more consistent brand to brand than similarly 'high end' headphones. That, IMHO would go some way to answer the increasingly posed question "can I make good mixes on headphones?" My suspicion is "yes" but top end monitors (in a good room) will always be better?

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Re: Matching usb interface output impedance with k702 headphones

Postby CS70 » Wed Apr 14, 2021 8:33 am

n o i s e f l e ur wrote:The other thing to bear in mind when talking about Sonarworks correction and why different models / makes of phones still sound different even after correction with the demo is - those are just applying the "Average" profiles.

I would think that if you compared sets of phones across different manufacturers and models that have had them do a custom measurement, you would find a far closer match.

Hmm... the definition of "good manufacturer" is that the variance between different items of the same model will be small. A custom measurement will go the extra mile, but all the previous miles are the ones that matter most! I would be very surprised if two different DT990Pro would sound _very_ different, in an obvious way.

I have a number of AT M50x for tracking here, and one AT M40x. The former all sound pretty much the same to me and the clients, they're literally interchangeable. The M40x sounds a little different though - not much, but for example you need to adjust the listening volume level as they are noticeably harder to drive.

Also by definition, headphone correcting software can take into account only the headphones, and there's more in the chain that the headphone. Try to put the same headphones and a correction software on a laptop's phones out vs. a good monitor controller and you will need to adjust.

It's not like these softwares aren't useful - they obviously are, as many people here attest; and what works, works. But they can do only so much without having data on the full chain at every single desk.

I guess the reason for which they are useful is because - for mixing - absolute sound is (within reason) less important than the ability to discern details and not mix up the relative relationships between the various frequency bands.

With the usual color analogy, all glasses will alter the whites a little bit, but what's important is that yellows stay yellowish and the reds stay reddish and if there's a hue change between two reds, the relative difference is preserved. In these conditions, the precise yellow and reds become less important (so long they don't become blues and greens!), because human perception is based much more on relative differences than absolute values.

If Sonarworks and similar brings different headphones a bit closer in that respect, it can become easier for many not to have to re-adapt as much as otherwise.
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Re: Matching usb interface output impedance with k702 headphones

Postby n o i s e f l e ur » Wed Apr 14, 2021 8:54 am

Do you have a custom profile for any phones from Sonarworks and have you made that comparison?
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Re: Matching usb interface output impedance with k702 headphones

Postby CS70 » Wed Apr 14, 2021 9:21 am

n o i s e f l e ur wrote:Do you have a custom profile for any phones from Sonarworks and have you made that comparison?

No, still have my ears :D If several different phones aren't the different, any customization can't find any difference, right? Or do you mean something else?
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Re: Matching usb interface output impedance with k702 headphones

Postby Hugh Robjohns » Wed Apr 14, 2021 9:47 am

ef37a wrote:Could yourself Martin and the other highly experienced chaps at SOS comment on whether the differences between high end monitor speakers are as great as they seem to be for 'so called reference' headphones?

I don't think they are quite as great, but there are certainly significant differences both in the on-axis response and, possibly more importantly, in their off-axis response which affects the way they integrate into different rooms.

My guess is that monitors at the £2000+ per pr are more consistent brand to brand than similarly 'high end' headphones.

Yes, I'd agree to that.
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