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Old 22nd December 2012, 07:03 PM   #31
Blues is offline Blues  United States
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Quote:
Originally Posted by EUVL View Post
According to the website of AKG :
K 701 - AKG
the K701 is specified at 200mW, 62 ohm, 105dBSPL/V.

It is not specified whether the V is rms or absolute.
Let's assume best case, that 1V means +/-1V sine wave. I think it is not an unreasonable assumption.
+/-1V sine into 62 ohm = 1*1/62/2 = 8.06mW.
It is not the correct assumption. In EE speak voltage, V (or I or P) not stated as peak (Vpk), peak-to-peak (Vpk-pk or Vp-p), or DC (Vdc) is always rms. Rms is sine wave and is the DC equivalent value. I'll always remember this now as I made the same mistake as a young student more than 2 decades ago.

P=(1Vrms)^2/62 ohms = 16.13 mW

Quote:
Originally Posted by EUVL View Post
So 105 dB/V => 105dB/8.06mW = 105-20*log(8.06) dB/mW
= 86.9 dBSPL/mW_rms
Again your formula above in converting is not correct. For one thing your power ratio should be x10 not x20.

It is our reference value plus the dB/mW that we account for in the conversion:

dB/mW = 105 + 10*log(16.13mW/1mW) = 117.08 dB/mW

You can verify this by googling for a conversion calculator.


Quote:
Originally Posted by EUVL View Post
If one wants to use peak power instead of RMS then it will be 6dB less.
i.e. 80.9 dBSPL/mW_pk

Assuming max. power rating of 200mW, constant impedance, and sinusoidal signals,
v_pk = sqrt (200mW x 2 x 62) = 4.98V
i_pk = 4.98/62 = 80.3mA
Again for Prms its V^2= P*62 = 3.52V
Irms = 3.52/62 = 56.8mA

Again these are maximum allowable input into the K701 and should never be specified for more. It is much safer to assume though that at these values or just over, the cans will naturally distort badly or even self-destruct -that's around 128dB/mW SPL (116dB/V). But why try going over the cliff?

Btw, the US will if the politicians in Wa DC do not get their act together.

Quote:
Originally Posted by EUVL View Post
Allow a factor of 1.5~2 for transient current in case of unfavorable back emf phase.
It would not be unreasonable, IMHO, to specify a maximum output current of 150mA for the headphone amplifier.
The amplifier would then in any case not be the limiting factor in this audio chain.

Patrick
If you are trying to specify for the headamp's power supply it is good design to go for at least twice the maximum circuit requirement.

My point in raising this awareness is why do we attenuate a source/dac's output voltage only to amplify it again thru a headamp? All the music info is already contained in the 2Vrms and we only need to attenuate it to suit the particular headphones' which are mostly crossoverless, more resistive than reactive (again check impedance/phase plots).

My findings with regard to issues with loading is, thru an effective attenuator it is not enough to negatively offset the requirements of these high sensitivity headphones -either low or high impedance. Sources/DACs today have enough juice (V and I) to spare that we only need to attenuate to taste. Sound quality and enjoyment are of course still factors that are on the particular headphones and the recorded material that you choose.

Peace on earth and goodwill to all this Christmas and 2013!

Last edited by Blues; 22nd December 2012 at 07:06 PM.
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Old 22nd December 2012, 07:49 PM   #32
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What is your taste ?
To be honest, I do not like either. I have an old HD565 ovation. Bass is not its strenght, but everything else is great. It is old and I'm looking for replacement for long time. I had a HD600 and K701 also for a few days, I did not like. I've tested HD650 and HD800 also for longer tests. I like the HD800 very much, that has (and much more) what my HD565 has, it is not a step back. It is just a price tag I do not like and the size. The HD700 is perfect ergonomics and size for me, but far away from the 800.
What taste is important. On a test I had a friend who found the HD700 much better. He told he do not like the HD800 wide soundstage. I said what ? You are joking? But he did not, he expect a different sound from a headphone. I respect that and realize it is very much depend on the taste. Also, for some friends my HD565 is nowhere, because its weak bass. My wife has a new sennheiser. Great for pop music, has great bass, but I do not like to listen to acoustic instruments on that at all. It has a soundstage like a dot.

Impedance is also very important. You can make an OTL SE tube amp for a high impedance driver much easier. I do not think it matter if it changes some over the freq range.

Another one I like, the higher series Grados. That is very much my taste, except one thing, the ergonomics is terrible for me.

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JG
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Old 22nd December 2012, 08:05 PM   #33
EUVL is offline EUVL  Europe
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You are right, my mistake.
It should have been 10*log(16.12), since it is dB power and not dB voltage, and 1V is rms.

So 105 dB/V => 105 - 10 x log(16.12) dB/mW = 93 dB/mW.

And since I am such a idiot with headphones, I took extra care to consult some other specialist sites, just to be sure :
dB per milliWHAT? Efficiency vs. Sensitivity vs. "How loud do they really go?"

AKG K701 93 dB/mW 105 dB/V

Also :
HD525: 94 dB/mW
HD535: 97 dB/mW
HD545, HD565: 96 dB/mW
(all 150 ohm)
HD455: 94 +/-2 dB/mW, 52 ohm
HD465 (old), HD475: 94 +/- 2 dB/mW, 60 ohm
HD560: 94 dB/mW, 300 ohm

and :
http://stereo.50webs.com/

And 117dB/mW would have been 24dB more sensitive with similar size cans / magnets.
Quite some achievement.


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Last edited by EUVL; 22nd December 2012 at 08:09 PM.
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Old 22nd December 2012, 10:04 PM   #34
Blues is offline Blues  United States
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Not clear enough? Here's a direct conversion...I'll let you decide:

Given: 105 dB/V AKG K701 sensitivity and 62 ohms impedance

dB/1mW = 10*log(Prms/1mW) --> Equation 1

Prms = (Vrms)^2/62 ohms -->Eq. 2

dB/V = 20*log(Vrms/1 Vrms) or;
Vrms = (antilog(dBV/20))*1Vrms --> Eq. 3

Now substitute Eq. 3 to Eq. 2:
Prms = ((antilog(dBV/20)*1Vrms)^2)/62 -->Eq. 4

Substitute Eq. 4 in our original Eq. 1, fill in the given values then compute:
dB/1mW = 10*log(Prms/1mW) becomes;

dB/1mW = 10*log((((antilog(dBV/20)*1Vrms)^2)/62)/1mW)
dB/1mW = 10*log((((antilog(105/20)*1)^2)/62 )/0.001)
dB/1mW = QED?
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Old 23rd December 2012, 05:35 AM   #35
EUVL is offline EUVL  Europe
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For me it is clear enough.
And I also understand why I arrived at 93dB/mW, the same figure as the others before me also seem to come to.


Cheers,
Patrick
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Last edited by EUVL; 23rd December 2012 at 06:02 AM.
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Old 23rd December 2012, 05:26 PM   #36
Blues is offline Blues  United States
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Even if AKG overstated their specs we still can't conveniently escape the math...
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Old 23rd December 2012, 11:16 PM   #37
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This will make some people's "eyes water" as they say, but yesterday I plugged my k701s into the speaker terminals of an F5 amp and just ... TURNED IT UP!

Not full power naturally, but it's not a bad combination! [If you don't mind running a 140W headamp!]
Who said you have to be conservative all the time!

This was to check the sound coming from the CD Card reader -> Ian's reclocker stage -> 1541A dac in NOS -> Cen O/P stage -> passive china SMD vol pot (not the best!) - Caddock resistor based F5 amp (tech diy store kit).

It's a fairly normal procedure here to leave the k701s plugged into the F3 amp (with the Warpspeed Vol control!) - they have about 5 metres, balaced wire system - it's not quite as 'clean' as the DAO and nowhere as fast, but a rather good combination, indeed.

The Sen 650's also function extremely well, but are far too bass heavy in the same F3 system - a rather different sound.
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Old 24th December 2012, 04:06 AM   #38
EUVL is offline EUVL  Europe
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Try it with a LM1876 in balanced mode.
It is much more environmentally friendly.


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Old 24th December 2012, 05:10 AM   #39
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Your usual neat and tidy design, Patrick. Nice work around the 634 buffer.
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Old 24th December 2012, 05:21 AM   #40
EUVL is offline EUVL  Europe
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The 634 is used as a virtual Gnd so that we can use a laptop SMPS as PSU.
For a PC active speaker.
Very high end, as you can see.




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