Sennheiser HD 650 Question

[nuhor68]

Hello!

I am building an headphone amplifier for HD 650. In their specs it is written:

Sound pressure level at 1 kHz 103 dB (1 Vrms)

Is maximum 1 Vrms delivered to the headphones enough to drive them well with enough sound volume?

Thank you,

Igor

[unsolved]

I suppose you will have some headroom, (if you not have any headroom, you can fry the headphones). So, no, i would not build with only 1 V.

[Osprey]

Here's a simple headphone amp I built to use with the Sennheiser HD 650. You can use any triode you want as long as it has reasonable gm. I wouldn't use a high mu tube; you're building a buffer not a voltage amp. You can use any of the small signal transformers with a primary around 15k and the secondary 600 or 300 and 600.

[nuhor68]

HD 650 specs say that Pmax = 500 mW. If I consider a power a bit less than Pmax, let's say 400 mW as a maximum power to be delivered to HD 650 by the amplifier then:

Umax = sqrt(P x Rh) = sqrt(0.4 x 300) = 11 Vrms

Thus, if I consider input signal of the amplifier as of 0.1 Vrms than the amplification of my amplifier should be 11/0.1 = 110.

Are my assumptions above correct?

Thank you,

Igor

[nuhor68]

Also, if U = 11 Vmrs then SPL in the headphones is 103 dB + 20log(11/1) = 124 dB. Is this OK or too loud or not enough?

Thank you,

Igor

[pmillett]

Originally posted by nuhor68
Also, if U = 11 Vmrs then SPL in the headphones is 103 dB + 20log(11/1) = 124 dB. Is this OK or too loud or not enough?

120dBSPL is generally considered the threshold of pain. So unless you are quite the masochist, I'd say it's enough

For headphones like the HD6xx I usually shoot for good performance at 1V RMS (i.e., I measure THD and spectra there) and a maximum output at clipping of at least 5V RMS... 10V RMS is better and provides more headroom.

That said, if you look at RMS voltage at "average" listening levels it's usually well under 1V RMS...

BTW, most headphone amps (commercial) have gains around 10x, or 20dB.

Pete

[Robert McLean]

I know I am going to sound like an old party pooper , but I am always amazed when I hear people talk about sound levels they are considering listening to. I have measured what I listen to, and it is generally about 75dB. When I am feeling a bit wild and daring I will crank it up to 85dB. My background level seems to vary between 25 and 30 dB. May be it is just personal taste.

But hear, I mean, here, is something else to consider. The legal limit for workplace exposure to noise in Ontario where I live is 85DBA averaged over 8 hours. I believe this very simialr to most other jurisdictions. Here is a link http://www.e-laws.gov.on.ca/html/reg...851_e.htm#BK19 that defines what is meant by averaged over 8 hours.

Rearranging the equation given gives the following formula for the time of exposure, in hours, to a single sound level that will be equivalent to 8 hours of 85 dB :
t = 8 * 10 ^ (8.5 - .1*SPL)

so for 85 dB the allowable time is 8 hours

95 dB : 48 minutes, or time for 1 vinyl LP

103 dB quoted in the headphone spec above : 7.6 minutes, or time for 1 or 2 tracks

120 dB mentioned above as the threshold of pain : 9.1 seconds

and

124 dB calculated above as resulted from 11 volt input : 3.6 seconds.

I just think it is bizzarre that people will go willingly to some 105 dB rock concert, and maybe even complain it wasnt loud enough, or build 100W amps and hook them up to 95dB sensitivity speakers, but if their employer was to tell them they had to work in some noisy room at 90 or 95 dB they would be within their rights to refuse or demand hearing protection.

I think I will stick with my 75 or 85 dB levels.

[Osprey]

nuhor68,

I'd have to agree with the other posters on this topic that I believe your assumption as to the amount of power needed to drive the headphones is way too high. Further, the drive voltage assumptions are off by a factor of 10.

In other words I assume the headphone amp will be driven by a CD player or i-pod type device. Both can easily output 2 VRMS; that is the spec. for the line level output. That said, I design for an input of 1V at full output to give me headroom. 1V will drive the headphones to full output for most people, so that is why I stated in my original post that you are not building a voltage amp, you are building a buffer. You are building a impedance matching device in the case of the CD player and headroom for the i-pod.

[rdf]

Originally posted by Robert McLean
I know I am going to sound like an old party pooper..

I've been spending a lot of time with the 24-bit/96 kHz live recordings available on the Internet Archive and constantly catch myself listening way too loud. A perfectly natural and clean level is often audible two rooms away when I put the headphones down to grab something from the fridge.

[dsavitsk]

Originally posted by rdf


I've been spending a lot of time with the 24-bit/96 kHz live recordings available on the Internet Archive and constantly catch myself listening way too loud. A perfectly natural and clean level is often audible two rooms away when I put the headphones down to grab something from the fridge.

There is a real tendency, especially with phones, to keep turning it up a little at a time. It is a good justification for a stepped attenuator in a headphone amp with a consistent setting. Mine only has three steps that I use, which keeps reasonable http://www.ecp.cc/less-pressivo-plus-plus.html

BTW, most headphone amps (commercial) have gains around 10x, or 20dB.

Most don't get turned past 9:00, either. They are almost always working as attenuators. There seems to be a psychological bias that we want to attenuate, so most amps are built so that they are never used anywhere near their may output.