I'm new to this whole DIY thing, and am in the process of building my first tube amp. Because I have one speaker that needs refurbing, I wanted to be able to use headphones with the amp for a while.
I searched here and googled, and found a link at Elliot Sound Products in Australia (http://sound.westhost.com/project100.htm) for a Headphone Adapter for Power Amplifiers. The one-channel schematic is shown on the link above.
The resistance values quoted are for matching a 120 ohm impedance headphone, and a maximum 5v. My headphones are AKG K271's, with the following specs:
91 dB SPL/mW Sensitivity
200 mW Max Input Power
55 ohms Rated Impedance
The amp I am building will produce on watt or less (more on that later).
So, in trying to understand the schematic, I need to know if the following is correct:
1. To match the impedance, the R2 resistor needs to be 55 ohms? Or was R2 just coincidentally set to 120?
2. R1 and R2 together are a voltage divider.
a. If I want 200mW max, at 55 ohms, the max voltage would
be E = Sqrt(P x R) or E = Sqrt(.2 * 55)=3.3v
b. But if I produce 1W at 8 ohms, the amp voltage is E =
Sqrt(1*8) = 2.8v, so I don't need to do anything with the
voltage. Does this mean both R1 and R2 are out of the
circuit?
c. R3 seemed to be used to bring R1 and R3 back up to the 120
ohm level in the example. So if no R1 (and R2) are out, then
R3 is 55 ohms?
So, being a slightly confused newbie, I get to the 55 ohm resistor answer, I'm just a little unsure if I need to put it across the outputs, or in series, or both?
Thanks in advance for putting up with what is likely to be a stupid question.
-----------
Bill
I searched here and googled, and found a link at Elliot Sound Products in Australia (http://sound.westhost.com/project100.htm) for a Headphone Adapter for Power Amplifiers. The one-channel schematic is shown on the link above.
The resistance values quoted are for matching a 120 ohm impedance headphone, and a maximum 5v. My headphones are AKG K271's, with the following specs:
91 dB SPL/mW Sensitivity
200 mW Max Input Power
55 ohms Rated Impedance
The amp I am building will produce on watt or less (more on that later).
So, in trying to understand the schematic, I need to know if the following is correct:
1. To match the impedance, the R2 resistor needs to be 55 ohms? Or was R2 just coincidentally set to 120?
2. R1 and R2 together are a voltage divider.
a. If I want 200mW max, at 55 ohms, the max voltage would
be E = Sqrt(P x R) or E = Sqrt(.2 * 55)=3.3v
b. But if I produce 1W at 8 ohms, the amp voltage is E =
Sqrt(1*8) = 2.8v, so I don't need to do anything with the
voltage. Does this mean both R1 and R2 are out of the
circuit?
c. R3 seemed to be used to bring R1 and R3 back up to the 120
ohm level in the example. So if no R1 (and R2) are out, then
R3 is 55 ohms?
So, being a slightly confused newbie, I get to the 55 ohm resistor answer, I'm just a little unsure if I need to put it across the outputs, or in series, or both?
Thanks in advance for putting up with what is likely to be a stupid question.
-----------
Bill
To start I would just add some parallel resistance across the secondary of the output transformer to assure it never operates with no load attached. Something on the order of 12 - 16 ohms ought to be ok as a first approximation.
91dB/mW really means that you aren't going to need more than say 300mVrms (16mW) across the headphones to be as loud as you will ever need - this will get you in the vicinity of 102dBspl. In all likelihood you should attenuate the signal to your headphones by as much as 20dB to protect your hearing and in the event that the noise floor is not as low as you would like this will buy you the same improvement in noise levels. I'd try a ~470 resistor in series with the phones.
One other alternative would be a 10 ohm/1 ohm resistive divider driving the headphones, this will give you a low driving source impedance at the phones (1 ohm) which might or might not have any effect on damping in the headphone transducers and still provide a moderate load (~11 ohms) to the amplifier. (The 12 - 16 ohm resistor previously mentioned is not required as long as this network is hardwired across the output.)
(I have done both above and could not decide which was better, although the 11:1 attenuator was very definitely quieter on the LF noise front. My Sony HP are 64 ohms.)
Lots of experimentation may be required to get to the point where it sounds right. (Resistor type and value.)
Given the impedance and sensitivity of these phones your ears could be in jeopardy so be very cautious when testing.
This is neither a stupid nor trivial question, driving headphones well with small (se) tube amplifiers is not as simple as it seems. Hopefully I have adequately addressed the 3 major issues I've encountered doing this. (Amplifier protection, excessive spls, and noise)
91dB/mW really means that you aren't going to need more than say 300mVrms (16mW) across the headphones to be as loud as you will ever need - this will get you in the vicinity of 102dBspl. In all likelihood you should attenuate the signal to your headphones by as much as 20dB to protect your hearing and in the event that the noise floor is not as low as you would like this will buy you the same improvement in noise levels. I'd try a ~470 resistor in series with the phones.
One other alternative would be a 10 ohm/1 ohm resistive divider driving the headphones, this will give you a low driving source impedance at the phones (1 ohm) which might or might not have any effect on damping in the headphone transducers and still provide a moderate load (~11 ohms) to the amplifier. (The 12 - 16 ohm resistor previously mentioned is not required as long as this network is hardwired across the output.)
(I have done both above and could not decide which was better, although the 11:1 attenuator was very definitely quieter on the LF noise front. My Sony HP are 64 ohms.)
Lots of experimentation may be required to get to the point where it sounds right. (Resistor type and value.)
Given the impedance and sensitivity of these phones your ears could be in jeopardy so be very cautious when testing.
This is neither a stupid nor trivial question, driving headphones well with small (se) tube amplifiers is not as simple as it seems. Hopefully I have adequately addressed the 3 major issues I've encountered doing this. (Amplifier protection, excessive spls, and noise)
Kevin, thanks for the help. I tried what I guess is both at the same time - 10:1 divider, and the 470 ohm resister in series. It works, but the volume is low, and cannot reach loud even at full tilt.
So, in rereading your post, it occurred to me that maybe you didn't mean to both divide the voltage and attenuate 20 dB with the 470. Was I incorrect doing both at the same time?
Second, given the very good idea of protecting my ears, which do I fiddle to up the level a bit (your 'experimentation' comment)? The divider or the attenuation resistor, or both?
Thanks again for the help.
And also thanks to the others in this thread How to connect 62 ohm headphone to 8 ohm output working out a similar problem.
So, in rereading your post, it occurred to me that maybe you didn't mean to both divide the voltage and attenuate 20 dB with the 470. Was I incorrect doing both at the same time?
Second, given the very good idea of protecting my ears, which do I fiddle to up the level a bit (your 'experimentation' comment)? The divider or the attenuation resistor, or both?
Thanks again for the help.
And also thanks to the others in this thread How to connect 62 ohm headphone to 8 ohm output working out a similar problem.
bwarden said:
You'd use one or the other, not both. (Using both would result in extremely low output levels.) Personally I think the divider is probably the better way to go as it drives the headphone with a source impedance of somewhat less than 1 ohm. Both approaches incidentally will provide attenuation, but the divider should result in a flatter overall frequency response depending on driver inductance and capacitance.
This is for a tube amp? If so I think it's best to always ensure the load impedance matches the transformer. OK you might get away with a big mis-match but just don't turn it up.
I think best to place an 8R power resistor accros the speaker terminals and then wire the headphone and a variable resistor in parallel tot he 8R load.
According to my calculator, 16mW into 55 ohms meeds 940mV:
0.94*0.94/55 = 0.016W = 16mW
Cheers
Ian
Last edited:
Hello All,
This post is about tube amplifiers and headphones.
It seems that headphones are most often a bolt on afterthought added to a power amplifier. Then the headphone bolt on is a one size fits all network of resistors and voltage dividers.
When thinking of a power amplifier driving a cone speaker we think of output impedance and dampening factor to control the Q of the speaker cone. It is the exception rather than the rule to discuss those thoughts in terms of headphones.
When a resistor network is placed between the power amplifier and the headphones the small load of the headphones is lost compared to the connected resistive load. There is little or no interaction between the headphone driver and the amplifier as you would see with the typical power amplifier and driven cone speaker.
It seems to maximize the performance of a tube headphone amplifier and headphone pair the amplifier and specific headphones should be designed to play together. The output transformer should match the load impedance to the plate impedance of the output tube(s) without resistors in between IMHO!
If you do not want to let out the magic smoke do not unplug the headphones when the amplifier is turned on!
Any thoughts? Are headphone connections to power amplifiers one size fits all after thoughts?
DT
All just for fun!
This post is about tube amplifiers and headphones.
It seems that headphones are most often a bolt on afterthought added to a power amplifier. Then the headphone bolt on is a one size fits all network of resistors and voltage dividers.
When thinking of a power amplifier driving a cone speaker we think of output impedance and dampening factor to control the Q of the speaker cone. It is the exception rather than the rule to discuss those thoughts in terms of headphones.
When a resistor network is placed between the power amplifier and the headphones the small load of the headphones is lost compared to the connected resistive load. There is little or no interaction between the headphone driver and the amplifier as you would see with the typical power amplifier and driven cone speaker.
It seems to maximize the performance of a tube headphone amplifier and headphone pair the amplifier and specific headphones should be designed to play together. The output transformer should match the load impedance to the plate impedance of the output tube(s) without resistors in between IMHO!
If you do not want to let out the magic smoke do not unplug the headphones when the amplifier is turned on!
Any thoughts? Are headphone connections to power amplifiers one size fits all after thoughts?
DT
All just for fun!
I asked a similar question elsewhere about a year ago. It was prompted by coming across a standard that said that all headphones should be driven via a 120 ohm resistance. I asked the question that, since this clearly completely lost any damping factor, did it mean damping factor is not important for headphones? In short, the answer was yes though I forget the reasons why. I'll see if I can find the original thread and post a link to it.
Cheers
Ian
The reason for this is that we want to maximize power transfer to the speakers and because speakers are hard to drive. We don't think about this with headphones because headphones don't have this problem
A pair of head phones with a resistor in series is about as easy to drive as the resistor. The problem is that some headphones might be 32 ohms and others 600 ohms. So we place a variable resistor in series with the headphone
In such case we would want to make equal impedances of output stage and a speaker.
Yes we do this with a transformer. We carefully select the right transformer that will have a good match.
But with headphone we have power to burn so we can afford to waste it with a resistor network.
Ian,
I have been studying and playing with this topic on and off over the last couple of years. I still have more questions than answers.
I have followed that 120 ohm thing. There is an ISO standard that calls for an 120 ohm amplifier output impedance for headphones. It is a current output thing vs. a voltage output.
Not all of the headphone manufactures follow the standard however. This is my impression; manufacturers that do follow the 120 ohm standard produce headphones with very flat impedance curves or low Q drivers.
Other headphones like the 300 ohm Sennheiser HD 600, HD 650 and HD 800 all have impedance curves with huge peaks, easily above double the nominal impedance. Bass response of these headphones is very amplifier output impedance dependant. Low output impedance and related dampening factor is critical for tight bass for these headphones.
Tom,
Pete’s latest contraption has 10 ohm 2 watt resistors in parallel with the headphone outputs. Not much dampening factor going on there either.
ChrisA,
Yes the resistor is easy to drive however any dampening factor is lost.
DT
All just for fun!
I have been studying and playing with this topic on and off over the last couple of years. I still have more questions than answers.
I have followed that 120 ohm thing. There is an ISO standard that calls for an 120 ohm amplifier output impedance for headphones. It is a current output thing vs. a voltage output.
Not all of the headphone manufactures follow the standard however. This is my impression; manufacturers that do follow the 120 ohm standard produce headphones with very flat impedance curves or low Q drivers.
Other headphones like the 300 ohm Sennheiser HD 600, HD 650 and HD 800 all have impedance curves with huge peaks, easily above double the nominal impedance. Bass response of these headphones is very amplifier output impedance dependant. Low output impedance and related dampening factor is critical for tight bass for these headphones.
Tom,
Pete’s latest contraption has 10 ohm 2 watt resistors in parallel with the headphone outputs. Not much dampening factor going on there either.
ChrisA,
Yes the resistor is easy to drive however any dampening factor is lost.
DT
All just for fun!
Yeah. He basically plays the resistors in parallel game. 10 ohm || 300 ohm = reasonable load for an 8-ohm secondary OPT.
Got the job done. Kept output tube happy.
~Tom
DT,
Thanks for the reply. I think it was an ISO standard I found too that had the 120 ohms. I was never happy about that as a universal method of driving headphones and your comments about the different types and their impedance curves serves only to reinforce that view.
The only thing I am not clear on is just what level is really required to drive them to 'normal' listening levels? I am getting replies from people saying that 80 to 90dB SPL is more than enough. If that is the case, since just about every headphone has a sensitivity of over 90dB SPL per mW, then 1mW should be sufficient for any headphone. This would made driving headphones with tubes trivial. But I suspect 90dB SPL is a typical average listening level and that peaks could easily be 20dB higher meaning we need to design for 100mW which is a different proposition altogether.
Cheers
Ian
Thanks for the reply. I think it was an ISO standard I found too that had the 120 ohms. I was never happy about that as a universal method of driving headphones and your comments about the different types and their impedance curves serves only to reinforce that view.
The only thing I am not clear on is just what level is really required to drive them to 'normal' listening levels? I am getting replies from people saying that 80 to 90dB SPL is more than enough. If that is the case, since just about every headphone has a sensitivity of over 90dB SPL per mW, then 1mW should be sufficient for any headphone. This would made driving headphones with tubes trivial. But I suspect 90dB SPL is a typical average listening level and that peaks could easily be 20dB higher meaning we need to design for 100mW which is a different proposition altogether.
Cheers
Ian
I suspect 100dBspl would be a relatively reasonable upper limit technically speaking, and is too loud for any sort comfortable of long term listening. (IMHO, YMMV) My average listening levels are in the high 70s through headphones. There is also some EC directive that limits the achievable Spls delivered to the user, I seem to recall this as being a relatively modest 85dBSpl??
The most power I have ever used with common 32 & 64 ohm headphones (Sony, Koss, JVC, Sennheiser) was a couple of mW and it was pretty loud (too loud for comfort, probably well over 90dB) but unfortunately I did not even attempt a rough measurement of the Spl level at the ear cuff.
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