Review and Measurements of Topping DX7s DAC and Headphone Amp

[Mad_Economist]

Yes I have seen this. But which amps that "don't break the bank" are available with 0 ohm output impedance?
I see O2, Magni3... Basically outside of that there aren't many 0 ohm output impedance amps period that are under $500... and even at $500 there aren't many.
What I have found is most are $800 or $1000+ often with no measurements at all....
I really want an Amp that is just like the A30 but with a proper output impedance.

JDS EL Amp - and most amps derived from the lme49600 reference design (AN-1768 http://www.ti.com/lit/ug/snaa052a/snaa052a.pdf) - should meet the near-0Ω criterion, as for that matter would most generic implementations of IC opamps intended to drive headphones. Oddly few of them around for a low price, granted, but they do exist, and when you expand tolerance to an ohm or two (which is unlikely to cause substantial frequency response deviation outside of multi-driver IEMs with very low impedance armatures) then many of the cheaper amps and DAC-amps become viable (E10K, for example, although due to its voltage restrictions that's not much of an amp if you really need high output).

At the cheapest, even a simple single-stage "C.Moy"-style DIY project will result in a low output impedance with stable operation if the right opamp was used - heck, the 4556 from the O2 would work (even when sold from dubious Chinese eBay storefronts with even more dubious design premises http://nwavguy.blogspot.com/2011/07/cmoy-with-gain.html ) , although it's a bit nonlinear for my taste (even if you are unlikely to hear the difference). Really, it's more confusing that so many "high-end" designs have such high output impedance...

If you want to know what really confuses me, though, it's the fact that nobody seems to have made some of the cheap options for making a simple, linear headamp into a very cheap box - the OPA1622, for example, is an essentially ideal single-chip solution for a simple headphone amplifier, but I don't know of any cheap boxes I can buy it in with preconfigured gain, some input filtering, and a power supply on the cheap, even though AFAIK it costs similarly to the TPA6120A2 in volume. Perhaps it's just too new and some Chinese company (perhaps Topping ) will make me a happy man by making a $50 box with a 1622 in it that I can recommend people.

What I have used before is the UE Buffer jack, and I have wondered if that really does "lower" the output impedance or if it doesn't, if its possible to build such a device that is simple and can lower the output impedance by say presenting the amp with a 100 ohm or more load and then having its own output of 1 or less.

The buffer jack presents itself to simply be a resistor voltage divider, to my understanding? If so, it would work - with the obvious caveat of sacrificing some voltage sensitivity, although I suspect in the case of sensitive IEMs that's more a feature than a bug. Sony did something similar on their old open-air designs using a pair of 22 ohm resistors, I recall, directly soldered onto the driver terminals, and for the same purpose: decreasing the change in frequency response from a high source resistance.

Edit: Wow, they want $280 for the EL Amp? That's...a bit steep, to be honest. I'd assumed it'd be closer to half the Element's price, given that it's close to half the Element. Shame FiiO never used the LME49600 combo in a desktop amp to my knowledge - the mobile amp based on it, the E12, was quite good - and wonderfully cheap - but for some reason their desktop stuff still uses the TPA6120A2 last I checked.

2 Edit 2 Furious: Actually, the E12 and the A5, its direct successor, are probably a quite reasonable options if the A5 still performs like the E12 did when Tyll Hertsens measured it (it is identified as "Fiio Mont Blanc here https://www.innerfidelity.com/images/AllAmpsJuly2013.pdf in a PDF that can only be found via this news post https://www.innerfidelity.com/content/innerfidelity-july-2013-update - I really love Innerfidelity, but I reeeaaaaally hate its organization). It has a very long battery life and can to my knowledge be used while charging, and the E12 measured quite well. I'm a little skeptical of the boutique opamps in the A5, but FiiO seem to be pretty competent, and I hope that they didn't mess with a good thing...

 

[Jimster480]

JDS EL Amp - and most amps derived from the lme49600 reference design (AN-1768 http://www.ti.com/lit/ug/snaa052a/snaa052a.pdf) - should meet the near-0Ω criterion, as for that matter would most generic implementations of IC opamps intended to drive headphones. Oddly few of them around for a low price, granted, but they do exist, and when you expand tolerance to an ohm or two (which is unlikely to cause substantial frequency response deviation outside of multi-driver IEMs with very low impedance armatures) then many of the cheaper amps and DAC-amps become viable (E10K, for example, although due to its voltage restrictions that's not much of an amp if you really need high output).

At the cheapest, even a simple single-stage "C.Moy"-style DIY project will result in a low output impedance with stable operation if the right opamp was used - heck, the 4556 from the O2 would work (even when sold from dubious Chinese eBay storefronts with even more dubious design premises http://nwavguy.blogspot.com/2011/07/cmoy-with-gain.html ) , although it's a bit nonlinear for my taste (even if you are unlikely to hear the difference). Really, it's more confusing that so many "high-end" designs have such high output impedance...

If you want to know what really confuses me, though, it's the fact that nobody seems to have made some of the cheap options for making a simple, linear headamp into a very cheap box - the OPA1622, for example, is an essentially ideal single-chip solution for a simple headphone amplifier, but I don't know of any cheap boxes I can buy it in with preconfigured gain, some input filtering, and a power supply on the cheap, even though AFAIK it costs similarly to the TPA6120A2 in volume. Perhaps it's just too new and some Chinese company (perhaps Topping ) will make me a happy man by making a $50 box with a 1622 in it that I can recommend people.



The buffer jack presents itself to simply be a resistor voltage divider, to my understanding? If so, it would work - with the obvious caveat of sacrificing some voltage sensitivity, although I suspect in the case of sensitive IEMs that's more a feature than a bug. Sony did something similar on their old open-air designs using a pair of 22 ohm resistors, I recall, directly soldered onto the driver terminals, and for the same purpose: decreasing the change in frequency response from a high source resistance.

Edit: Wow, they want $280 for the EL Amp? That's...a bit steep, to be honest. I'd assumed it'd be closer to half the Element's price, given that it's close to half the Element. Shame FiiO never used the LME49600 combo in a desktop amp to my knowledge - the mobile amp based on it, the E12, was quite good - and wonderfully cheap - but for some reason their desktop stuff still uses the TPA6120A2 last I checked.

2 Edit 2 Furious: Actually, the E12 and the A5, its direct successor, are probably a quite reasonable options if the A5 still performs like the E12 did when Tyll Hertsens measured it (it is identified as "Fiio Mont Blanc here https://www.innerfidelity.com/images/AllAmpsJuly2013.pdf in a PDF that can only be found via this news post https://www.innerfidelity.com/content/innerfidelity-july-2013-update - I really love Innerfidelity, but I reeeaaaaally hate its organization). It has a very long battery life and can to my knowledge be used while charging, and the E12 measured quite well. I'm a little skeptical of the boutique opamps in the A5, but FiiO seem to be pretty competent, and I hope that they didn't mess with a good thing...

The EL amp has a design that won't fit my audio stack and while its on the "cheaper" side its still $279 + shipping.
Otherwise there are really not many cheap options... Gilmore Lite MK2, also $500... Someone talked about the Arcam rHead here and that being available for around $200 currently (internationally shipped).
Otherwise you really don't have any options, I have spent quite alot of time looking... many high end Amps have no specs and high output impedance... making them worse than the A30!

I would love to design an amp that has both balanced and single ended using a cheap but effective solution. My friend bought the $800 Cayin Amp and in balanced mode it is slightly better than my O2 when volume matched. But once again its hard to tell the difference with most songs using my low impedance headphones.

In terms of the buffer Jack.... I think it does work because the my PM-2 always sounds "veiled" when plugged into my A30's 10ohm jack and the "veil" goes away when I plug it into my O2 at the same volume. With the Buffer Jack it doesn't sound "veiled" but it does require quite a bit more power as I have to be around 2-4 oclock on 9DB gain on the A30 vs around 9-10 oclock otherwise.
The buffer Jack describes itself as a device to reduce sensitivity and fix output impedance mismatch.... I do wonder how it works though as there are no specs outside of that.

Would be interesting if there was a way to test one.

Edit: Yes the element is expensive, and yes there are more mobile options with low output impedance (even from topping itself) for a lower price.... I wish one of those designs would end up in a nice desktop amp. Especially one that has balanced or so.

Looking at component costs I believe it would be fully possible to build such an Amp with both single ended and balanced with near 0 ohm output impedance for both outputs at a price under $300 (including chassis, making a $50 profit or more at that price if it was built in mass quantity). And that would be with using all the best audiophile type components like Nicheon, WIMA, Dale, etc

 

[Mad_Economist]

The EL amp has a design that won't fit my audio stack and while its on the "cheaper" side its still $279 + shipping.
Otherwise there are really not many cheap options... Gilmore Lite MK2, also $500... Someone talked about the Arcam rHead here and that being available for around $200 currently (internationally shipped).
Otherwise you really don't have any options, I have spent quite alot of time looking... many high end Amps have no specs and high output impedance... making them worse than the A30!

I suspect it is less a case of a lack of options out there and more a lack of characterization of what's out there - you wouldn't necessarily assume that a generic Chinese headamp would have fairly low distortion and low output impedance, but often this does prove to be the case (if not often enough to take it for granted). The relative dearth of sites doing technical reviews and the sparseness of a lot of manufacturer specifications is, I think, a more likely culprit than a genuine lack of low-impedance amplifiers out there for your troubles in finding a good headamp. And, of course, there are always the tried and true options like the O2 and Magni, or the wonderful world of DIY.

In terms of the buffer Jack.... I think it does work because the my PM-2 always sounds "veiled" when plugged into my A30's 10ohm jack and the "veil" goes away when I plug it into my O2 at the same volume. With the Buffer Jack it doesn't sound "veiled" but it does require quite a bit more power as I have to be around 2-4 oclock on 9DB gain on the A30 vs around 9-10 oclock otherwise.

It would be atypical for the PM2, specifically, to make you particularly aware of output impedance. Its impedance is low, but per Tyll's measurements (https://www.innerfidelity.com/images/OppoPM22014StockPads.pdf) - and is is typical of a planar magnetic - it is quite uniform with regard to frequency other than a single, relatively small rise around 300hz. Certainly this rise could cause an audible frequency response feature in theory with a very high output impedance, but for 10Ω the change should be quite small.

The buffer Jack describes itself as a device to reduce sensitivity and fix output impedance mismatch.... I do wonder how it works though as there are no specs outside of that.

Would be interesting if there was a way to test one.

The classic solution here would be, depending on which end of the electroacoustic cow you tend to start from, either measuring the deviation in frequency response of a headphone which is known to have a frequency-varying impedance with a high-output-impedance source both with and without the buffer jack, or if you are more sensible simply measuring the voltage drop across a resistor with a known value with the jack and without it. They serve the same purpose but the former justifies owning a lot more test equipment

Edit: Ah, yes, voltage divider, right, knew there was something I was forgetting. Forgive me for being so scatter-brained, I swear I'm not normally like this. Since the divider will shift the voltage output in and of itself, go with two resistors. Since you have a known value for Zout, I'd suggest one equal to that and one that's a much higher value. If the buffer drops the OI, you'll see the difference between the voltage across the resistor that matches the specified output impedance and the much higher value one drop.

 

[Jimster480]

I suspect it is less a case of a lack of options out there and more a lack of characterization of what's out there - you wouldn't necessarily assume that a generic Chinese headamp would have fairly low distortion and low output impedance, but often this does prove to be the case (if not often enough to take it for granted). The relative dearth of sites doing technical reviews and the sparseness of a lot of manufacturer specifications is, I think, a more likely culprit than a genuine lack of low-impedance amplifiers out there for your troubles in finding a good headamp. And, of course, there are always the tried and true options like the O2 and Magni, or the wonderful world of DIY.



It would be atypical for the PM2, specifically, to make you particularly aware of output impedance. Its impedance is low, but per Tyll's measurements (https://www.innerfidelity.com/images/OppoPM22014StockPads.pdf) - and is is typical of a planar magnetic - it is quite uniform with regard to frequency other than a single, relatively small rise around 300hz. Certainly this rise could cause an audible frequency response feature in theory with a very high output impedance, but for 10Ω the change should be quite small.



The classic solution here would be, depending on which end of the electroacoustic cow you tend to start from, either measuring the deviation in frequency response of a headphone which is known to have a frequency-varying impedance with a high-output-impedance source both with and without the buffer jack, or if you are more sensible simply measuring the voltage drop across a resistor with a known value with the jack and without it. They serve the same purpose but the former justifies owning a lot more test equipment

Edit: Ah, yes, voltage divider, right, knew there was something I was forgetting. Forgive me for being so scatter-brained, I swear I'm not normally like this. Since the divider will shift the voltage output in and of itself, go with two resistors. Since you have a known value for Zout, I'd suggest one equal to that and one that's a much higher value. If the buffer drops the OI, you'll see the difference between the voltage across the resistor that matches the specified output impedance and the much higher value one drop.

Despite the measurements suggesting that it doesn't have much of an impedance response.. NwAvGuy did a writeup on this talking about despite the response that it would still have other sonic effects on the headphone since the voltages themselves would be different. I have tested with my Aeon also and its the same also plugging it into the DX7 directly (essentially the same kind of Amp as the A30) vs plugging it into the O2.
My friends Cayin is the same with its 0-ohm balanced output, when we test it in balanced the output is cleaner with the Aeon and PM-2 vs in its single ended output which has 10 and 100 ohm output impedance IIRC).

It is the same also with the AH-MM400 Denon headphones, although those have some other impedance response issues too IIRC... 0-Ohm output impedance sources just sound cleaner vs other sources with higher impedance. Ofc if the headphone has 200+ ohm then it sounds just as clean on any of the sources. I tested my friends Eikon on my DX7 via balanced and it was just as clean as single ended via the DX7,A30(6.35mm jack) and O2 aswell.

I have no problem buying random chinese brands as after all I bought Topping products when they were considered a random Chinese brand, because the specs and quality looked good and they offered the features I was looking for in a price I could justify spending.
I have checked through most available chinese Amps and other than boutique chinese brands like Audio-GD most of the headphone amps are using the same TPA6120/A2 meaning that they will also have 10 ohm+ output impedance aswell...

I looked at SMSL SAP line and SMSL VA1 but neither talks about the output impedance.... and also ideally I need an amp that has RCA passthrough

 

[stalepie]

Really, it's more confusing that so many "high-end" designs have such high output impedance...

beyerdynamic gave reasons for why their A20 amp has a high output impedance:
https://support.beyerdynamic.com/hc...he-A-20-have-an-output-impedance-of-100-ohms-

1. It protects the A 20 from damage in the event of a short circuit on the headphone side.
2. When using headphones that have different impedances one after the other, the jumps in volume level are much lower than if a very low output impedance was selected.

I also saw an article in Sound and Vision give similar reasons:
https://www.soundandvision.com/content/output-impedance-show-and-tell

Why do some headphone amps have such high impedance? Two reasons. First, people tend to connect and disconnect headphones often and with the amplifier running, which creates a greater possibility for accidental short circuits. The extra impedance means there's always enough load to prevent a total short that might fry the headphone amp. Second, headphones vary a lot in sensitivity - i.e., some play loud with a given signal level, some don't - and the extra impedance evens out level changes and reduces the possibility of blasting your ears out if you switch from a low-sensitivity headphone to a high-sensitivity model.​

If you want to know what really confuses me, though, it's the fact that nobody seems to have made some of the cheap options for making a simple, linear headamp into a very cheap box - the OPA1622, for example, is an essentially ideal single-chip solution for a simple headphone amplifier, but I don't know of any cheap boxes I can buy it in with preconfigured gain, some input filtering, and a power supply on the cheap, even though AFAIK it costs similarly to the TPA6120A2 in volume. Perhaps it's just too new and some Chinese company (perhaps Topping ) will make me a happy man by making a $50 box with a 1622 in it that I can recommend people.

Is this using the OPA1622?
https://www.kadenken.com/shopdetail/000000000558/ct103/page1/order/

Petit rose 3 (kit version)

I just happened to be browsing there recently. I think it is a small store in Tokyo that sells DIY odds-n-ends. They reply by email though quickly because I was asking them something.

 

[Jimster480]

beyerdynamic gave reasons for why their A20 amp has a high output impedance:
https://support.beyerdynamic.com/hc...he-A-20-have-an-output-impedance-of-100-ohms-

1. It protects the A 20 from damage in the event of a short circuit on the headphone side.
2. When using headphones that have different impedances one after the other, the jumps in volume level are much lower than if a very low output impedance was selected.

I also saw an article in Sound and Vision give similar reasons:
https://www.soundandvision.com/content/output-impedance-show-and-tell

Why do some headphone amps have such high impedance? Two reasons. First, people tend to connect and disconnect headphones often and with the amplifier running, which creates a greater possibility for accidental short circuits. The extra impedance means there's always enough load to prevent a total short that might fry the headphone amp. Second, headphones vary a lot in sensitivity - i.e., some play loud with a given signal level, some don't - and the extra impedance evens out level changes and reduces the possibility of blasting your ears out if you switch from a low-sensitivity headphone to a high-sensitivity model.​

Is this using the OPA1622?
https://www.kadenken.com/shopdetail/000000000558/ct103/page1/order/

Petit rose 3 (kit version)

I just happened to be browsing there recently. I think it is a small store in Tokyo that sells DIY odds-n-ends. They reply by email though quickly because I was asking them something.

sounds like a poor excuse for a bad design. If people just plug their headphones in without checking their volume levels... idk what to say... but that isn't really an excuse for making expensive amps with poor performance

 

[Wombat]

Beyerdynamic head phones will work just fine on their amps.

If individuals choose to buy low impedance headphones I don't think Beyerdynamic should bear the burden of their choice and the limited options.

Beware self-centrism: https://www.urbandictionary.com/define.php?term=self-centric

 

[Zilfallion]

Edit: Wow, they want $280 for the EL Amp? That's...a bit steep, to be honest. I'd assumed it'd be closer to half the Element's price, given that it's close to half the Element.

If you actually look at their own lineup, and the Element. The DAC in the Element is about comparable to the USB-only OL DAC, it's not really spectacular. Slightly different, but similar enough for this comparison. If you subtract their market-price of the O2 enclosure+front/backplate(30.97), you're left with about a 70-dollar market price DAC. 350-70=280. The Amp and it's enclosure is the bulk of the product that is the Element. I'd call it 50 for the knob, and 230 for the amp itself. It's still a bit expensive, but the price of Element vs. EL Amp is reasonable relative to the rest of their product line.

 

[Jimster480]

Beyerdynamic head phones will work just fine on their amps.

If individuals choose to buy low impedance headphones I don't think Beyerdynamic should bear the burden of their choice and the limited options.

Sure, but their headphones IMHO are designed using ancient and outdated techniques and their super high input impedance works fine with literally any cheap amp of today... if they are claiming to sell a premium product then it should support other headphones aswell.

 

[Wombat]

Really? As a DT 88oPRO owner I would like you to tell me what Beyerdynamic are getting wrong re ancient and outdated techniques. Beyer can choose to design a headphone amp to suit their own products.
Should you prefer low impedance headphones, go find an amp to suit and ask yourself why your headphone mfr doesn't offer one or otherwise are thin on the ground.

 

[Jimster480]

Really? As a DT 88oPRO owner I would like you to tell me what Beyerdynamic are getting wrong.

I'm not looking to start a subjective arguement about headphones. Personally I haven't heard a DT880 but Beyers are known to be mostly bassy Vshaped sound headphones. There are some models which aren't but many indeed are. Their super high impedance is indeed old technology, but it doesn't necessarily mean that its bad.

Edit:
There are also random amps like this one:
https://crimsonaudio.com/products/

Never heard of it and it doesn't have any output impedance spec

 

[Wombat]

I

I'm not looking to start a subjective arguement about headphones. Personally I haven't heard a DT880 but Beyers are known to be mostly bassy Vshaped sound headphones. There are some models which aren't but many indeed are. Their super high impedance is indeed old technology, but it doesn't necessarily mean that its bad.

Edit:
There are also random amps like this one:
https://crimsonaudio.com/products/

Never heard of it and it doesn't have any output impedance spec

Unsubstantiated hearsay. C'mon.

I use an O2 amp with the Beyer's and have no issues. I do believe that Beyer are more authoritative than your personal opinion. You are welcome, of course, to prove otherwise.

PS. I am not looking for a subjective argument but an objective input regarding your "out-dated and ancient techniques" statement re Beyer product designs.

 

[Jimster480]

I



Unsubstantiated hearsay. C'mon.

I use an O2 amp with the Beyer's and have no issues. I do believe that Beyer are more authoritative than your personal opinion. You are welcome, of course, to prove otherwise.

PS. I am not looking for a subjective argument but an objective input regarding your "out-dated and ancient techniques" statement re Beyer product designs.

Sure I will be glad to provide one at another time, its the middle of the night and frankly I don't have the energy to find the articles discussing their coil winding and how its been the same style of magnets for 30+ years and their need for the high impedance to maintain accuracy.
If you remind me (especially in PM) I will be glad to find said article(s) for you at a later period in time.

PS: Your O2 should be able to drive the beyer anyway, since in reality most headphones don't require that much power to achieve 90db SPL or greater. If you had the 600 ohm model, it might be a different story depending on which model of O2 you have.

 

[Wombat]

Sure I will be glad to provide one at another time, its the middle of the night and frankly I don't have the energy to find the articles discussing their coil winding and how its been the same style of magnets for 30+ years and their need for the high impedance to maintain accuracy.
If you remind me (especially in PM) I will be glad to find said article(s) for you at a later period in time.

PS: Your O2 should be able to drive the beyer anyway, since in reality most headphones don't require that much power to achieve 90db SPL or greater. If you had the 600 ohm model, it might be a different story depending on which model of O2 you have.

Looking forward to your information. No hurry.

Cheers.

 

[stalepie]

Some of their early headphones were low ohm, I believe, like DT 48 (25 ohms).

In this article, Tyll says one of them is specified to be used with a source impedance of 120 ohms...

https://www.innerfidelity.com/content/historic-beyerdynamic-dt-48-e-25-ohm-sound-and-summary

... then goes on to say that he figures it will make very little difference, and it doesn't.

I have wondered if older headphones were expected to be used with sources of fairly high output impedance. There is an article by Benchmark showing the effects of high output impedance on the MDR-V6:

https://cdn.shopify.com/s/files/1/0321/7609/files/Headphone_Amplifier_Performance_-_Part_2.pdf?1361

which is interesting to me because I thought they sounded better from an older source I got (but I don't know its output impedance) - warmer, and generally more balanced/neutral. But the bass boost they're showing here is too grand for what I'm talking about.
It would only make sense that manufacturers would test their headphones on equipment of the time (in the case of the Sony, the mid 80s), where there may not have been many or any low output impedance sources in a studio environment (?), and thus such distortions are factored into the design of the headphones, on the expectation of how customers would hear them.

 

[Jimster480]

Some of their early headphones were low ohm, I believe, like DT 48 (25 ohms).

In this article, Tyll says one of them is specified to be used with a source impedance of 120 ohms...

https://www.innerfidelity.com/content/historic-beyerdynamic-dt-48-e-25-ohm-sound-and-summary

... then goes on to say that he figures it will make very little difference, and it doesn't.

I have wondered if older headphones were expected to be used with sources of fairly high output impedance. There is an article by Benchmark showing the effects of high output impedance on the MDR-V6:

https://cdn.shopify.com/s/files/1/0321/7609/files/Headphone_Amplifier_Performance_-_Part_2.pdf?1361

which is interesting to me because I thought they sounded better from an older source I got (but I don't know its output impedance) - warmer, and generally more balanced/neutral. But the bass boost they're showing here is too grand for what I'm talking about.
It would only make sense that manufacturers would test their headphones on equipment of the time (in the case of the Sony, the mid 80s), where there may not have been many or any low output impedance sources in a studio environment (?), and thus such distortions are factored into the design of the headphones, on the expectation of how customers would hear them.

Yep that makes perfect sense, would be hard to design headphones for Amps that basically didn't exist yet.... Most things had high output impedance back then, from 100-600 ohms.... this was probably due to the need to stabilize and protect the amp itself. Also measurements of how they affected sound quality weren't as easy to create I would bet. I wasn't around then so I can't say for certain, hopefully someone with more experience (and age) can chime in.

 

[Johnny2Bad]

Headphones would normally be used either plugged directly into the console, directly into a tape deck, or via a distribution amplifier to multiple pairs of 'phones in the live studio space. Using headphones with televisions was probably more common than using them with preamplifiers/preamp sections of integrated amplifiers.

It was common as well to wire them directly to the outputs of power amps (fabbing up an inline phone jack), where again a high impedance was not a problem and was actually desirable to limit output power.

Koss Pro4A's ... the studio standard in the 1960's and in some cases through the 1980's, for example with location recording and live venues, and dealing with certain deaf musicians because they had excellent isolation so you could turn the monitor mix in the phones way up and it wouldn't bleed into the microphone ... were 250 ohms.

Inside where the console was, by the late 1970's everybody switched to Sennheiser 414's as they were about a hundred times more comfortable than having that Koss Vice around your skull. But 414's were 2000 ohms. Sennheiser in the late 1970's made the HD400W @ 32 ohms while the standard HD400 was 600 ohms and all the other models in the catalog were either 600 or 2000 ohms.

Others from the usual suspects were no different ... the contemporary AKG 240, also a Studio standard in the late 70's, were 600 ohms (not to be confused with the 240Pro or 240II, both different beasts of recent vintage).

Headphones were very much almost exclusively a Pro Audio market up until the 21st century. Even "audiophiles" didn't own a pair in the majority of cases. The iPod is only 13 years old, and nobody bought them until the Windows-friendly USB models arrived. Headphones as mass market consumer devices is a phenomena less than a decade old.

 

[DonH56]

Yup, some old receivers, integrated amps, and probably some power amps connected the headphone jack in-line with the speaker outputs. Saw more than one blown output from bad headphones or even someone who didn't get them plugged in all the way and shorted the amp's outputs. Bad idea, yes, but it was common years ago. Hopefully today separate and isolated headphone jacks are all that's around but I do not know.

On the flip side, my old Sennheisers had very high impedance (~2 k ohms IIRC) and some amps would oscillate with such a light load. Have not seen that problem in years, but thanks for bringing back a bunch of bad old memories...

 

[Dimitri]

Headphones as mass market consumer devices is a phenomena less than a decade old.

Not sure how you came up with that.
Earbuds "exploded" since the iPod. True.
Even during the cassette walkman days Sony had a pair that was more earbud like" than a "can".
But "regular/old-fashioned "cans" "for consumers" have been around available for anyone to buy and try for "a long time".
In almost every Hi-Fi shop (back in the days of "Hi-Fi" shops) there were alway 5-10 different pairs of headphones on display ready to be plugged in and listened to. Circuit City, The good guys had a large number on display much like Best Buy does now.

What seems to have changed "in the 21st century" is a flood of reviews and postings aka seems almost everyone has or wants a pair of H600.
There is a lot less talk about Koss or Audio Technica or AKG.

But a 21st century "phenomenon" it's not.

 

[Don Hills]

Over the years I have examined the headphone circuits of literally dozens of amplifiers dating from the 60s to the present. Of those where the headphones were driven from the PA stages, I have yet to find one that did not have series resistors in the headphone outputs, apart from one I modified myself...
The advantage of such resistors is that they evened out the level differences caused by differences in headphone impedance.