Topic: Speaker amps and headphones (Read 6723 times)

Solderdude · « **Reply #10 on:** February 06, 2013, 08:01:52 AM »

The reason you cannot grab any power amp and hook up any headphone directly to the speaker terminals has to do with impedance, power rating of the HP and output VOLTAGE.
A purely technical reason so to speak.

For this reason in the old days (when amplifying parts where relatively more expensive to resistors) it was common practice to drive headphones from the power amp (and with an intarnal switch in the HP socket that switched off the speakers) via a single resistor or a resistor divider network (2 resistors).
Nowadays, as already mentioned and opamps are cheap mostly a 'dedicated' HP amp section is used.
Inessence a 'C'Moy' is inside.
I prefer to call it a 'standard application' as a C'Moy is nothing more than the basic schematic found in almost every datasheet/application note and is not an invention of Chu Moy, he just used that standard application and mounted it in a tin can with 1 or 2 batteries.
Brought him even more fame...

Adding 1 (or 2) resistor(s) is done for several reasons.
The biggest one is to protect headphones from blowing up (respecting the power rating of most headphones being around 0.2W)

a power amplifier delivers an output VOLTAGE and must be able to provide the CURRENT the load 'dictates/asks/draws/demands'.
With amps being able to deliver current to 4/8 Ohm speakers it is easy to deliver currents to resistances higher than that as the current drawn will be much less as when speakers are used.

I will humor everyone and start throwing POWER figures for headphones for 2 amps.
a 15W/8 Ohm amp and a 60W/8 Ohm amp.
Since this is a factor 4 in power you can do the math for a 240W/8Ohm amp.

On ear headphones in general have impedances between 32 Ohm and 300 Ohm so these will be in the example.

15W amp ... 32 Ohm ..... 300 Ohm (headphone impedance, not output resistance)
power: ...... 3.8 W ......... 0.4 W
max output voltage: 11V

a 32 Ohm HP with 0.2W rating runs a risk of being fried

!

60W amp ... 32 Ohm ..... 300 Ohm
power: ...... 15 W ......... 1.6 W
max output voltage: 22V

So that is the main reason power amps are not suited.
Also they in general have more noise (as already stated) and thus also limited dynamic range for small signals.
Also as mentioned the gain is relatively high and thus turning up the volume knob results in screamingly loud levels when barely touched (true for low impedance headphones, somewhat less for higher impedance HP's).
But the main reason it should not be done is the... rated power of the HP.

There is a cheap and easy fix though and that is using a series resistance.
Common values were between 100 and 600 Ohm and this also explains why those oldie amps often sound so good out of their HP sockets.
I have touched on that subject in my articles about power and impedance.

same amps but with a 100 Ohm output R added in the 15W and 330 Ohm in the 60W

15W amp ... 32 Ohm ..... 300 Ohm
power: ...... 0.22 W ......... 0.22 W
max output voltage: depends on voltage division

60W amp ... 32 Ohm ..... 300 Ohm
power: ...... 0.12 W ......... 0.35 W
max output voltage: depends on voltage division

This could be 'better' figures when a voltage division network is used (which was often the case)
It also shows on some HP's (due to the higher output R) some headphones can sound considerable more 'mellow'.
Also this voltage division adresses the gain issue (volume control is more usable) and noise is lowered, dynamic range increased.

So there you have it, the real reason it cannot (should not) be done with low Impedance HP's, unless rated for several watts such as orthos and some monitoring headphones that can 'double' as small speakers on the console.
High impedance headpones 300 /600 Ohms CAN be connected directly when power ratings allow.
Certainly to the lower wattage amps.

Since orthos have a lower efficiency and high power rating there is little to no objection powering them direct from speaker terminals.
Watch out with the power though (amps above 40W)

Maxvla · « **Reply #11 on:** February 06, 2013, 08:18:19 AM »

Great post. Thanks for going into detail. Karma +1

Sounds like something to play with down the road. I have LCD-2s coming soon and HD800 is rated 300ohm but is above that, sometimes significantly so, throughout it's frequency response.

ultrabike · « **Reply #12 on:** February 06, 2013, 10:41:42 AM »

Regarding the HD800, from Solderdude's discussion using a 60W/8ohm amp with a 330 ohm resistor in the divider network would provide from 0.32W to 0.365W across the frequency range, given the HD800 impedance range is about 300 to 650 ohms (see attached output power vs Rload plot.)

However, the sensitivity vs frequency might also play a role here, and doing this with the HD800 could result in quite a bit of warmth. I think Marv discussed that here using a more benign 120 ohm load.

The LCD2 might be a better candidate.

mkubota1 · « **Reply #13 on:** February 07, 2013, 08:55:40 AM »

I've read that some speaker amps will oscillate if they see high impedances like those on most headphones. You can apparently ruin your cans if that happens; but I've never heard of anyone who has. I've had pretty good luck with most things I've hooked my K1000s (120 ohms) to. For grins, I even connected them to one of my old Carver amps (PM-350... 350wpc/8, 450wpc/4) and they sounded fine.

hoshiyomi · « **Reply #14 on:** February 09, 2013, 02:44:00 AM »

Quote from: mkubota1 on February 07, 2013, 08:55:40 AM

I've read that some speaker amps will oscillate if they see high impedances like those on most headphones. You can apparently ruin your cans if that happens

A properly designed/built power amp shouldn't do that, the zobel network loading probably wouldn't be ideal but it really should not oscillate. However I would watch for possible bridging/BTL output when using headphones, shorting output "ground" together on both channels as with TRS headphone plugs may let the magic smoke out of your amp.

One thing you can possibly do is build a resistive divider on the output in dealing with the excessive voltage swing.
Something like 10 to 20 ohms in series with a 1 ohm resistor should bring it down to a manageable level, and if the output is taken between the resistors you get a nice low output impedance.

Solderdude · « **Reply #15 on:** February 09, 2013, 08:02:13 AM »

Quote from: hoshiyomi on February 09, 2013, 02:44:00 AM

One thing you can possibly do is build a resistive divider on the output in dealing with the excessive voltage swing.
Something like 10 to 20 ohms in series with a 1 ohm resistor should bring it down to a manageable level, and if the output is taken between the resistors you get a nice low output impedance.

That is technically correct and will give a very low output R of just under 1 Ohm BUT when this divider is used on say a 60W or more amp and the HP is driven loud those resistors should have a power rating of at least 20W.
To keep heat dissipation low manufacturers use higher values.
For a 60W amp for instance and a 1:10 divider when using a 100 Ohm and 10 Ohm you still end up with a 9 Ohm output R the maximum dissipated power in the resistors is still 4W.
While acc. to Voldemort this is already unacceptably high, many of the normal headphones will not care.
MA drivers will likely have screwy FR but why would one (they only can handle a few mW) want to use those on a 100W amplifier.
It's the reason why older amps have resistor divider networks with values between 100 and 600 Ohm in it.

Indeed some (very few !) exotic designs may become unstable when loaded with a capacitance and a higher impedance in parallel (say above 120 Ohm) and the Zobel network cannot help.
A Zobel is there to compensate for inductive loads, not capacitive loads.
The inductor (with a resistor in parallel) that is found in some amps is active against capacitive loads.
It could be seen as part of a Zobel network though.

A HP cable + high impedance HP in essence is a capacitor with a high 'resistive loss' inductor in parallel which forms a resonance network that could in some extremely rare cases cause instability in a feedback loop.
For speakers that is less of a problem as that resonance network is severely damped by the low impedance of the speaker, a HP has a much higher impedance.

RexAeterna · « **Reply #16 on:** February 17, 2013, 09:13:32 PM »

i personally think you can use any headphone you want off of speaker outs if your careful. personally i always used speaker outs besides separate preamp headphone outs on headphones ranging 600ohms and low as 8ohms with speaker outs with good results.

also noise is only an issue if the headphone is ultra sensitive and depends on the amp itself. also if you have a power amp with input gains for left and right channels you can easily modify the gain and background with those or you can use a different preamp with that power amp or vice versa. also sometimes some receivers and amps have a -20db gain switch which can solve lot of noise issues.

i always used headphones straight off of power amp speaker outs and never blew a single headphone or experienced anything i couldn't easily fix or so.

Quote from: Analixus on February 06, 2013, 05:13:52 AM

Most speaker amps are not built w/ a focus on low level detail and uber resolution that some ToTL headphones can mange fyi. So choose wisely. You may very well get trapped into choosing a best compromise.

i don't know if that is true are not. i think it all depends on how well an amp is designed.

i also never used any totl headphones like hd800's with speaker outs so i have no idea on how it would sound so i can be completely wrong on my part.

opera-kid · « **Reply #17 on:** February 20, 2013, 06:52:00 PM »

I was thrilled when a google search pulled up this thread, I entered a similar question on another forum a few weeks ago and received no substantive responses. So, "I logiced" my way into a 10 ohm 10 watt non-inductive wire wound Vishay across + and - of my tube amp (100 watts) and a 100 ohm similar resistor to go in line with the HD800's.

I should be hooked up next week, I'll share the results.

Maxvla · « **Reply #18 on:** February 23, 2013, 04:34:45 AM »

Hey Opera, your thread on HF was the straw the broke the camels back in getting me to finally post this thread.

Going to copy your post from there:

Quote (selected)

HD800s came in. Using 2 well respected, for the HD800s, headphone amps from 2 very well repsected headphone amp companies, they performed well. A bit coarse in the highs, and a bit prominent up there, and a little lean through lower mids and upper bass, but very clear, very transparent, and about what I would expect from reading hundreds of comments about the 800s. Problem is, not as easy to listen to as most of my speakers. Granted, speakers, amps, preamps, all that upstream stuff totals several hundred thousand dollars in my systems, so it hardly seems fair to compare when I'm substituting a few thousand dollars of headphone electronics for over $100k of preamp and amp.

So the itch remained to try my normal preamps (the headphone amps do not have an input avoiding the preamp, so I cannot avoid the headphone pre and use my great preamps) and my favorite speaker amp. Interestingly, many folks (the majority) said headphone amps are better, speaker amps have poor signal to noise ratios that headphones would uncover, not enough detail. I decided to try the idea I talked about earlier of loading the output of the amp at 10 ohms (could not find a high quality 8 ohm non-inductive wire wound resistor) and a dropping resistor after that and before the phones, starting with 100 ohms.

Well, the sound is phenomenal. A huge difference in width (not my most important thing), a major increase in refinement (really I'm saying major decrease in grain/distortion), beautiful sweet sweet, delicate highs with tremendous extension, deeper bass (the HD800s go amazingly low in the bass if the amp is stiff enough). The transparency is really amazing. I wondered if a bunch of parallelled tubes (8 per side) would be a bit crude or coarse at extremely low power levels, but it is the opposite. I will be trying several other speaker amps and will report. Certainly, information I received about speaker amps being too noisy, volume control will be at the bottom, headphone amps being much more detailed were simply not true. At least not universally true.

What amazes me is the sparse information about running headphones this way. To use dual mono amps I will need to make a "balanced" headphone cable as it would not be a good idea to have the speaker ground terminals of 2 different chassis tied togethere. Hope this information answers some questions for folks. I also would like to caution folks to not try this unless you understand exactly what you are doing, and have confirmed that the grounds of your left and right output lugs on the amp are connected. Further, no bridged amps, no dual mono amps. Dual mono amps should not be used unless the headpone wiring is balanced (different grounds on L and R channels, NO headpone plugs!)

This is really encouraging to hear. I need to figure out how to make the necessary connections to give this a try.

pp312 · « **Reply #19 on:** March 09, 2013, 09:04:22 AM »

Quote from: purrin on February 06, 2013, 05:01:03 AM

Modern integrated amps take the internal pre-amp output and throw it to an op-amp for the headout.

I'd dispute that. It's come up before in other forums and one member even went to the trouble of emailing various manufacturers, who for the most part replied that their headphone jacks were connected to the power amp through resistors. I currently have two integrateds, a NAD C326BEE and Cambridge C340A which both use that approach.

Author Topic: Speaker amps and headphones (Read 6723 times)

Solderdude

Re: Speaker amps and headphones

Maxvla

Re: Speaker amps and headphones

ultrabike

Re: Speaker amps and headphones

mkubota1

Re: Speaker amps and headphones

hoshiyomi

Re: Speaker amps and headphones

Solderdude

Re: Speaker amps and headphones

RexAeterna

Re: Speaker amps and headphones

opera-kid

Re: Speaker amps and headphones

Maxvla

Re: Speaker amps and headphones

pp312

Re: Speaker amps and headphones

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Author Topic: Speaker amps and headphones (Read 6723 times)

opera-kid