# How much DC offset would you consider harmful to HP and after which point would you throw the commercial amp out?



## svyr

http://www.head-fi.org/forum/thread/500181/musiland-md11-to-be-released/60#post_7424276 

I recently remeasured the DC offsets on Line out and HP out of Musilol MD11 and it's not pleasing. 

Line out: 70mV, HP out: 150mV (both constant regardless of the volume level). 
I think tangent recommended an abs cut-off of 100mv or something (and most people set 15-25 as their personal limit), but seeing this on a commercial amp is just nothing short of horrifying. 

How much is actually enough to damage low impedance HP (or does it just displace the diaphragm and distort the sound)? (say for 16 ohm 105 spl ones, or 32 ohm 105 spl ones)

Would anyone else having and MD11 like to measure their unit to deny or corroborate?


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## High_Q

Hey, how are you measure it?
	

	
	
		
		

		
			





  I'm interested in measuring all my sources.  I need a accurate measuring tool, probably need a O-scope.  Don't have one, too expensive.  
   
  I would think offset would be harmful since it causes peak of the wave to span out from 0 point since its V=Offset +ASin(wt).  In which increasing the volume would hopefully with a most linear amp, would increase A only not create distortion.
   
  Depends on how much peak power the headphone can handle until it blows.  Care to experiement?
	

	
	
		
		

		
		
	


	



   
  No experiment necessary for ibuds, it blows from maximum volume of the source(iphone) it comes with. Which makes no sense.
	

	
	
		
		

		
		
	


	




  You should feel ashamed Apple.


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## svyr

high_q said:


> Hey, how are you measure it?
> 
> 
> 
> ...




errr, with a multimeter in mV-DC mode using a 3.5mm to RCA cable or a torn off earbud cable  ..Scope would sure be a nicer bet, and an basic digital one might be relatively inexpensive nowadays but probably still a few hundred. Something tells me for the MD11, it's best not to look at the scope measurements  considering the problems I already found, I have a feeling it won't do too well.


> Depends on how much peak power the headphone can handle until it blows.

I thought people were saying something about it overheating the voice coil or something...From DC offset that is, not tearing them up with <-> excessive AC where the DC offset size would probably be insignificant...



> Care to experiement?

don't own any earphones cheap enough to do that


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## High_Q

I don't see voice coil heating(probably would  be insignificant) causing damage, if you think about how much current travels through audio system.


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## jr41

I'm interested in an answer to this question also.
   
  I too understand that you can measure DC offset using a multimeter, as svyr describes. I use a 1/4" to 1/4" or 3.5mm to 3.5mm cable as necessary (or a 1/4" to 3.5 adapter ( - or visa-versa)). The tip is left, the center ring is right and the "rear-most" ring is ground.
   
  Both of those measurements from the MD11 sound very high. I don't know of any definitive guidelines, but for low impedance headphones, I recall reading anything above a few 10s of mV is dangerous. Low impedance headphones seem to be more susceptible to DC offset damage.
   
  Apparently DC offset at best degrades sound quality and at worst damages a headphone's delegate wiring. Also, I believe that unless an appropriate capacitor and resistor combination is used in the signal path (to filter the DC offset), any DC offset from the source is multiplied by the amplifier, depending on gain and volume setting.


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## FallenAngel

Purely rule of thumb for me is 10mV after amplification is at which point I start to look into debugging the build.


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## svyr

mmm few mv or even few tens of mv doesn't seem that dangerous. (what High_Q said)

It seems like you need the DC offset voltage (or signal + dc offset voltage) to be > max rated input 

 E.g. for my really cheap woolies in-ears with impedance of 32ohm, and rated max power of 10mw - the dc offset voltage alone would have to be: http://www.ajdesigner.com/phpohmslaw/ohms_law_voltage_power_resistance.php about 0.56v (So even for these flimsy in-ears the 0.15v musiland outputs doesn't seem to be nearly enough, at least alone)


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## jr41

Quote:  





> [...]
> 
> E.g. for my really cheap woolies in-ears with impedance of 32ohm, and rated max power of 10mw - the dc offset voltage alone would have to be: http://www.ajdesigner.com/phpohmslaw/ohms_law_voltage_power_resistance.php about 0.56v (So even for these flimsy in-ears the 0.15v musiland outputs doesn't seem to be nearly enough, at least alone)


 
   
  That sounds like a reasonable theory, I've never thought to work it out that way.


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## High_Q

Check what the max power the phones are rated for
   
  P=IV or IVrms
   
  Check for Impedance, usually they will till to the max impedance at a frequency, you want to use the lower bound at a given output frequency range.
   
  Pmax=Vmax^2/Impedance_low
   
  Vmax = sqrt(Pmax*Impdedance_low), just a rough estimate
   
  Once you find Vmax
   
  Solve for Offset:
   
  Vmax = Voffset +ASin(wt)
   
  Vmax -ASin(wt) = Voffset
   
  This is only for a tone, but ASin(wt) is the peak voltage deviation from the DC offset.  I would use an O-scope to look for peak voltage deviation of the signal.
   
  ^Actually you can't really solve for that, but O-scope is your friend to find the offset.


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## qusp

need a scope for measuring dc offset? er.......no, a dmm is usually fine depending on the signal you are using. if you wanna get crazy you can use a voltmeter and play a known voltage sine wave, whatever the heating value over what is expected after gain and converted to dc is your offset. of course as scope can be used to good effect, but isnt needed imo. 
   
  as far as i know, you dont need to go over max rated input to cause issues, that is not what its about except in the extreme case where you will overheat the coils, its more about excursion of the driver and therefor dynamic range being effected. your 110mv amount is pretty high in this regard


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## High_Q

ignoring qusp... As, I should be on his ignore list
	

	
	
		
		

		
		
	


	



   
  I mentioned headphone power rating because we cannot know hen the headphone will blow, all we have is the manufacturer's power rating to go by or our worse case scenario because we don't really know what the maximum worse case is unless we find out for ourselves
	

	
	
		
		

		
		
	


	




.  Also, I don't know how accurate a DC meter would be vs a O-scope, and O-scope is easier to spot and more accurate.  I would rely on O-scope.  Anyway, offset cannot amplify beyond the saturation point or Opamp supply voltage, and the amplified signal will get clipped if saturation is reached.  When a signal goes into a amplifier there is a decoupling to separate DC(or offset) and AC. The output offset voltage cannot be larger than Opamp supply.
   

   
  I found an interesting video from MIT that goes over the math of the amps(it great if you're an extreme amp geek, here is some school for you qusp, you may need it)
http://videolectures.net/mit6002s07_agarwal_lec10/
   
  For example, my CMOY is supplied by 9v(+_ 4.5) and offset cannot be greater than 4.5.  If I increase the gain of the small signal, it will start clipping at a certain point.  So, if the supplie is much greater, and we know it will headphones will blow at a certain point(voltage), it would be an issue if gain is increased to max out the swing of the signal to reach the high supply voltage.  So, knowing the max power, thus max voltage is helpful.
  Quote: 





jr41 said:


> I'm interested in an answer to this question also.
> 
> I too understand that you can measure DC offset using a multimeter, as svyr describes. I use a 1/4" to 1/4" or 3.5mm to 3.5mm cable as necessary (or a 1/4" to 3.5 adapter ( - or visa-versa)). The tip is left, the center ring is right and the "rear-most" ring is ground.
> 
> ...


 
  wire can be damaged if there is enough current to create enough heat to melt the insulation etc... Typically for high current application, larger cross section is used.
   
  R=pL/A  
   
  Larger the area, less R, which relates to heat given off by power equation:
   
  P=IV=I^2R=V^2/R
   
   
  but again, its audio signal(not enough to create heat issues, look at unnecessary thick ALO cables made out of silver or gold LOL)


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## qusp

yeah well i'm ignoring you already. i forgot you know more than the rest of the forum combined
   
  making people think they need an oscope for measuring offset on their amps is irresponsible and needlessly overcomplicated for the basic measurement needed to know if dc is higher than it should be. dc not only damages headphones before you go past the power rating, but causes lower audio quality WELL before that. an oscope is the perfect tool for it, but it is not needed in the slightest, stop flexing your brain already, its just not that complicated


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## nattonrice

Quote:


> its just not that complicated


 

 This.
  You don't need elementary algebra to figure it out.
  Take a digital multimeter and measure DCmV from the output to ground.
  If it is higher than 10mV and it is a diy job, fix it.
   
  If it is a commercial product... wait this is the diy section! Go build something!


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## svyr

> You don't need elementary algebra to figure it out.

relax. The ohms law re-arrangement was to see how much DC is deadly to dodgy in-ears. 

As for an oscilloscope, sure you don't need one, it's also probably easier to measure the DC offset using a DMM (however looking at the O-scope plots is potentially a lot more informative than noticing a DC offset shift). Still, if there are cheapie $200-ish oscilloscopes around (the PC only types, with a low-ish hz-khz range) it's probably not a bad buy compared to an average DMM. 
Speaking of that, any recommendations and do those exist, or did I just imagine them  ?


> If it is a commercial product... wait this is the diy section! Go build something!

It's a DIY related question . Plus I'm a bit over DIY audio after fixing my SRM-1/mk2 with the help of the big two  ... (yes, you can build what you want, how you want it, with what you want...No it takes a lot of time to find/order parts, not burn down the house if you're a nub like me, and then solder it up and debug. Case work connectors, panels and knobs are also potentially a pain. )


>Also, I don't know how accurate a DC meter would be vs a O-scope, and O-scope is easier to spot and more accurate. I would rely on O-scope. 

The DC accuracy is quoted in the manual for any DMM you can buy. Generally it's higher than AC and sufficient to know whether you're below 10mv ... 


>If I increase the gain of the small signal, it will start clipping at a certain point. So, if the supplie is much greater, and we know it will headphones will blow at a certain point(voltage), it would be an issue if gain is increased to max out the swing of the signal to reach the high supply voltage. So, knowing the max power, thus max voltage is helpful.

I see, thanks. That looks to be the upper bound for DCoff + Ac signal, since above that it'd be clipped?


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## High_Q

True.  I said DC, but actually it is DC + Audio signal Max Peak
  
   
  If DC + Audio Signal Max Peak > Vcc(Amp supply voltage), there will be clipping.
  Quote: 





svyr said:


> I see, thanks. That looks to be the upper bound for *DCoff + Ac signal*, since above that it'd be clipped?


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## jr41

Can I ask how 10mV was decided as the safe limit? I'm following some of what is being discussed here, but I'm quite new to the concepts.
   
  I understand that an increase in current results in an increase in heat and that there is a point where the conductor will break down. I've seen shots of Grado drivers, and the wiring from the cable to the magnet is extremely thin - not much beyond human hair thickness perhaps.
   
  Though one thing which is confusing me about this (and this is really getting down to basics), is that I've read higher impedance headphones are less susceptible to damage from DC offset. I understand higher impedance headphones need higher voltage to drive - to overcome their higher resistance (impedance) - but since (I think) we're saying too much current through a very fine conductor is the ultimate cause of DC offset damage, I'm struggling to understand, therefore, where voltage values come into the equation? Surely it would be better to specify dangerous current levels, thus negating the fact headphones of different impedances have differing levels of susceptibility to DC offset voltages. I hope this makes some sense!
   
  To take a real world example, I measured the DC offset from my iphone 4 line out recently and it was 10mV. I've got it hooked up to my AMB Mini³, which has no decoupling caps, and at listening levels the DC offset is amplified to 50mV by the mini³. I'm driving Sennheiser HD 25-1 II, which are 70ohm. (By the way, I couldn't measure the DC offset from the iphone with music playing as it confused my DMM - I've read this is a common limitation, but for a few seconds after pausing the music my DMM read 10mV.) Apologies if this last paragraph is getting a bit off topic.


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## FallenAngel

I think 10mV was the rule of thumb after amplification, not before.


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## wdahm519

I think staying under 25mV is good.  Under 10mV is better.  Keep in mind, when you measure DC offset with no headphones plugged in, its going to be a a lot higher, and it may start oscillating from 20mV to 80mV.  
   
  I was measuring my Bijou's offset and it was doing that.  I plugged the headphones in and it dropped to 1.XX mV.


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## JoetheArachnid

It's 00:42 here. Why did I just spend the last 40 minutes obsessively tweaking my DC offset trimpots? Because of this F&*^ing thread.
  I checked on impulse, and though one channel was 0.6mV, the other had drifted to nearly 26mV. It doesn't help that it changes by +/-5mV or so depending on the temperature, whether the lid is off the case, whether the screwdriver is touching the trimpot...
  Broadly: Grach. Damn you, thread.


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## DouglasQuaid

Anything over 20mV offset will not touch my Grados.


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## svyr

I'm not really happy with what I got off the D100 either. The XLR/RCA outs were fine and measure near 0-10mv DC, but HP out was between 30-60mv on one channel and 80-160 on another. (unlike MD11 where it was constant, here it was sort of oscillating between the two values. Unfortunately the HP terminals are sealed, so I can't measure the offset on HP plugged in. (edit: hmmm, actually, I can on the driver terminals on DT48e and it's about 0mv on those...for D100. For MD11 however, even with DT48e connected, it's still 140mv on one channel and 130mv on the other))

I'm confused...are we supposed to measure with a load connected, or just the DMM? (load connected seems to make sense, but then you need dodgy HP and an unsealed terminal to measure or an open unit (not a problem for DIY but I'd need to void the warranty on the MD11 if I didn't have DT48e where the driver terminals are exposed  )


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## wdahm519

Quote: 





svyr said:


> I'm not really happy with what I got off the D100 either. The XLR/RCA outs were fine and measure near 0-10mv DC, but HP out was between 30-60mv on one channel and 80-160 on another. (unlike MD11 where it was constant, here it was sort of oscillating between the two values. Unfortunately the HP terminals are sealed, so I can't measure the offset on HP plugged in. (edit: hmmm, actually, I can on the driver terminals on DT48e and it's about 0mv on those...for D100. For MD11 however, even with DT48e connected, it's still 140mv on one channel and 130mv on the other))
> 
> I'm confused...are we supposed to measure with a load connected, or just the DMM? (load connected seems to make sense, but then you need dodgy HP and an unsealed terminal to measure or an open unit (not a problem for DIY but I'd need to void the warranty on the MD11 if I didn't have DT48e where the driver terminals are exposed
> 
> ...


 
   
  I did both.  I measured without a load, and it started oscillating from 20mV to 80mV slowly, each channel.  I feel like this method is useless because it doesn't tell you anything.  I plugged in some cheapo AD-700's and offset dropped to 1.?? mV.  
   
  The only way to get a proper reading is to have a load connected.


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## svyr

wdahm519 said:


> I did both.  I measured without a load, and it started oscillating from 20mV to 80mV slowly, each channel.  I feel like this method is useless because it doesn't tell you anything.  I plugged in some cheapo AD-700's and offset dropped to 1.?? mV.
> 
> The only way to get a proper reading is to have a load connected.




in that case D100 is fine, MD11 is most certainly not


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## jr41

I don't really want to de-case my mini³ to measure DC offset from my iphone with a load connected (I'm measuring the output from the iphone lineout - but this is still subject to a small load, i.e. the mini³). I don't want to hook up my Grado Labs SR225i to my Mini³  and iphone 4 until I know the DC offset is less than 10mV.
   
  I'm building a M³ at the moment, so I'll measure DC offset from the iphone 4 lineout during testing.
   
  I've yet to come across any definitive guidelines on dangerous DC offset levels and all the influencing factors.


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## svyr

jr41 said:


> I don't really want to de-case my mini³ to measure DC offset from my iphone with a load connected (I'm measuring the output from the iphone lineout - but this is still subject to a small load, i.e. the mini³). I don't want to hook up my Grado Labs SR225i to my Mini³  and iphone 4 until I know the DC offset is less than 10mV.
> 
> I'm building a M³ at the moment, so I'll measure DC offset from the iphone 4 lineout during testing.
> 
> I've yet to come across any definitive guidelines on dangerous DC offset levels and all the influencing factors.




relative max voltages DC+Signal above derived from the impedance and max rated input power sounded fairly definitive re: will this burn out my HP.
I think you're a bit hung up on the 10mv thing...


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## scottiebabie

cant speak much for dedicated headamps nor porty headamps but im a big vintage gearhead so checking & adjusting Bias current & DC offset is part & parcel of every amp after i bring it home. most if not all (havent read it all so i cant say) service manuals states static measurements with unplug source at AUX, zero/min vol with all ancilliary controls off & amp should be warmed up for atleast 10mins prior.
   
  DC offset should be as close to 0mv as possible - not very probable unless the amp is output cap coupled.
   
  Bias current settings according to each mfgrs specs as indicated in service manual - theres quite a bit of play room with Bias if overheating &increased power consumption isnt an issue with possible substantial sonic benefits eg. improved dynamics, bass, clarity, quickness, soundstage, coherance, decreased distortion, etc...etc but this will be specific to each individual amp of which only our own ears can be the final arbiter.
   
  once adjustment is made, i usually let it run for a few minutes to have the final stabilized reading. FWIW, i use alligator clip lead cables to attach the DMM to the amps appropriate test lead points if for nothing else, i can read & adjust on the fly. more importantly, i dont accidentaly short any parts nor (MOST IMPORTANTLY) electrocute myself as readings can only be done with a 'live' amp so USE APPROPRIATE CAUTION!
   
  some of the vintage stuff i bring home shows ori DC readings up over 100mv-200+mv at amp output (usually read at speaker taps but some amps have internal test leads too) with no detrimental effect on speakers or headphones even at fairly high output - usually btwn 1-3o'clock. just saying this as IMO i think DC offset is highly overrated plus i havent personally nor read of any transducer blowing as a direct result of DC. theres a fairly high built-in tolerance of DC in transducers & more likely than not, most blown drivers are caused by amp clipping thru inappropriate over driving IMHO.
   
  ps:though it doesnt apply on single transducer speakers nor headphones but wont the inline caps at transducer crossover circuits also serve as DC blockers too? i think its more appropriate to worry about symetric readings on both channels to preserve channel/stereo coherence rather than worry about the lowest possible setting - again just my 2cents ofcos after playing with a fair number of vintage amps/receivers. 
   
  most research i've done says any DC offset upto 50mv is considered normal & 'safe' under practical real world conditions. alot of amps deviate upwards to a few hundred mv with no apparent detriment to speakers (atleast structurely). 0mv is best ofcos!


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## High_Q

I've tried measuring audio signal out of my DMM one time, and thought occurred to me:  Wait a minute, I'm measuring a audio signal(it was fluxuating), maybe I should use a O-Scope!!!   One thing I don't understand is, how is offset of 10mV(thats 100th of a volt) dangerous if the signal swings way beyond that if you raise the gain?   
   
  All these concepts may help with understanding headphone impedance.
   
  From wiki:
   
   
 Complex impedance  Impedance is represented as a complex quantity 
	

	
	
		
		

		
		
	


	




 and the term _complex impedance_ may be used interchangeably; the polar form conveniently captures both magnitude and phase characteristics,






   where the magnitude 
	

	
	
		
		

		
		
	


	




 represents the ratio of the voltage difference amplitude to the current amplitude, while the argument 
	

	
	
		
		

		
		
	


	




 gives the phase difference between voltage and current and 
	

	
	
		
		

		
		
	


	




 is the imaginary unit. In Cartesian form,






   where the real part of impedance is the resistance 
	

	
	
		
		

		
		
	


	




 and the imaginary part is the reactance 
	

	
	
		
		

		
		
	


	




.

  Where it is required to add or subtract impedances the cartesian form is more convenient, but when quantities are multiplied or divided the calculation becomes simpler if the polar form is used. A circuit calculation, such as finding the total impedance of two impedances in parallel, may require conversion between forms several times during the calculation. Conversion between the forms follows the normal conversion rules of complex numbers.

   
   
 Inductor   

  For the inductor, we have the relation:






   This time, considering the current signal to be






   it follows that






   And thus






   This tells us that the ratio of AC voltage amplitude to AC current amplitude across an inductor is 
	

	
	
		
		

		
		
	


	




, and that the AC voltage leads the AC current across an inductor by 90 degrees.

  This result is commonly expressed in polar form, as






   Or, more simply, using Euler's formula, as


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## High_Q

Impdance Z is dependant on frequency w, and what is listed below.
   
 Inductance formulae  The table below lists some common simplified formulas for calculating the approximate inductance of several inductor constructions.


Construction​Formula​Dimensions​ NotesCylindrical air-core coil[6]​



 
 _L_ = inductance in henries(H)
 _μ0_ = permeability of free space = 4_π_ × 10−7 H/m
 _K_ = Nagaoka coefficient[6]
 _N_ = number of turns
 _A_ = area of cross-section of the coil in square metres(m2)
 _l_ = length of coil in metres (m)
​   Straight wire conductor [7]​









 
 _L_ = inductance
 _l_ = cylinder length
 _c_ = cylinder radius
 _μ_0 = vacuum permeability =4π nH/cm
 _μ_ = conductor permeability
 _p_ = resistivity
 _ω_ = phase rate
​  exact if ω = 0 or ω = ∞





 -0+3% 
 _L_ = inductance (µH)
 _l_ = length of conductor (mm)
 _d_ = diameter of conductor (mm)
 _f_ = frequency
​  
 Cu or Al
 _l_ > 100_d_
 _d_2 _f_ > 1 mm2MHz
​ 





 +0-3% 
 _L_ = inductance (µH)
 _l_ = length of conductor (mm)
 _d_ = diameter of conductor (mm)
 _f_ = frequency
​  
 Cu or Al
 _l_ > 100_d_
 _d_2 _f_ < 1 mm2MHz
​ Short air-core cylindrical coil[8]​



 
 _L_ = inductance (µH)
 _r_ = outer radius of coil (in)
 _l_ = length of coil (in)
 _N_ = number of turns
​ Multilayer air-core coil[_citation needed_]​



 
 _L_ = inductance (µH)
 _r_ = mean radius of coil (in)
 _l_ = physical length of coil winding (in)
 _N_ = number of turns
 _d_ = depth of coil (outer radius minus inner radius) (in)
​ Flat spiral air-core coil[_citation needed_]​



 
 _L_ = inductance (µH)
 _r_ = mean radius of coil (cm)
 _N_ = number of turns
 _d_ = depth of coil (outer radius minus inner radius) (cm)
​ 



 
 _L_ = inductance (µH)
 _r_ = mean radius of coil (in)
 _N_ = number of turns
 _d_ = depth of coil (outer radius minus inner radius) (in)
​ Toroidal core (circular cross-section)[_citation needed_]​



 
 _L_ = inductance (H)
 _μ0_ = permeability of free space = 4_π_ × 10−7 H/m
 _μr_ = relative permeability of core material
 _r_ = radius of coil winding (m)
 _N_ = number of turns
 _D_ = overall diameter of toroid (m)
  
​


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## High_Q

Quote: 





wdahm519 said:


> I think staying under 25mV is good.  Under 10mV is better.  Keep in mind, when you measure DC offset with no headphones plugged in, its going to be a a lot higher, and it may start oscillating from 20mV to 80mV.
> 
> I was measuring my Bijou's offset and it was doing that.  I plugged the headphones in and it dropped to 1.XX mV.


 
  Output voltage(without load) is total output from the amp, and the lower value is when loaded with headphone(voltage drop accross it) depends on how much output impedance there is.  But offset or DC voltage drop is real as it deals with Real impedance, which also depends on real part of output impdeance, and heaphonephone impedance.  If output voltage is much greater than headphone impdance, there will not be significant offset on the headphones.
  
  Quote: 





joethearachnid said:


> It's 00:42 here. Why did I just spend the last 40 minutes obsessively tweaking my DC offset trimpots? Because of this F&*^ing thread.
> I checked on impulse, and though one channel was 0.6mV, the other had drifted to nearly 26mV. It doesn't help that it changes by +/-5mV or so depending on the temperature, whether the lid is off the case, whether the screwdriver is touching the trimpot...
> Broadly: Grach. Damn you, thread.


 
   
 Large-signal models  [edit]Ebers–Moll model 

 Ebers–Moll Model for an NPN transistor.[21] * _I_B, _I_C,_I_E: base, collector and emitter currents * _I_CD, _I_ED: collector and emitter diode currents * _α_F, _α_R: forward and reverse common-base current gains​​




 Ebers–Moll Model for a PNP transistor.​​


  The DC emitter and collector currents in active mode are well modeled by an approximation to the Ebers–Moll model:









   The base internal current is mainly by diffusion (see Fick's law) and






   where


 _V_T is the thermal voltage _k__T_ / _q_ (approximately 26 mV at 300 K ≈ room temperature).
 _I_E is the emitter current
 _I_C is the collector current
 α_T_ is the common base forward short circuit current gain (0.98 to 0.998)
 _I_ES is the reverse saturation current of the base–emitter diode (on the order of 10−15 to 10−12amperes)
 _V_BE is the base–emitter voltage
 _D__n_ is the diffusion constant for electrons in the p-type base
 _W_ is the base width

  The α and forward β parameters are as described previously. A reverse β is sometimes included in the model.

  The unapproximated Ebers–Moll equations used to describe the three currents in any operating region are given below. These equations are based on the transport model for a bipolar junction transistor.[22]


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## High_Q

Quote: 





svyr said:


> I'm confused...are we supposed to measure with a load connected, or just the DMM? (load connected seems to make sense, but then you need dodgy HP and an unsealed terminal to measure or an open unit (not a problem for DIY but I'd need to void the warranty on the MD11 if I didn't have DT48e where the driver terminals are exposed
> 
> 
> 
> ...


 
  You can deduce from amp's output voltage reading and output impedance of the amp and headphone impedance.  Investigate the output impedance of the amp and headphone impedance and you can figure out what will be outputted to the headphones.


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## High_Q

ipad O-scope
	

	
	
		
		

		
			





  The only problem with ipad O-scope is it cannot go beyond the audio range, but were testing out audio.  You guys know of anything for PC?


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## High_Q

We should be able to use the microphone input on the PC as a O-scope to check source's output right?
  Just need a software for it.


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## svyr

high_q said:


> We should be able to use the microphone input on the PC as a O-scope to check source's output right?
> Just need a software for it.




or we could get one of these  and not use an i-poop (plus, considering the quality of built-in codec inputs, I doubt the mic in is a measurement tool ) : http://cgi.ebay.com.au/DSO5200-200MS-s-PC-USB-Digital-Storage-Oscilloscope-2CH-/280575179679?pt=BI_Oscilloscopes&hash=item4153957b9f
http://cgi.ebay.com.au/2x100MHz-PC-USB-Oscilloscope-Spectrum-Analyzer-Meter-/120673812295?pt=LH_DefaultDomain_0&hash=item1c18b83f47
http://cgi.ebay.com.au/DSO2090-USB-PC-Virtual-Storage-Oscilloscope-40MHz-100MS-/280456846465?pt=BI_Oscilloscopes&hash=item414c87dc81
http://cgi.ebay.com.au/Hantek-DSO5200A-Virtual-USB-Oscilloscope-200Mhz-250MS-s-/120586423894?pt=AU_B_I_Electrical_Test_Equipment&hash=item1c1382ce56
http://cgi.ebay.com.au/ADS1022-DIGITAL-STORAGE-OSCILLOSCOPE-25MHz-500MS-s-USB-/330558379920?pt=BI_Oscilloscopes&hash=item4cf6d09790
http://cgi.ebay.com.au/5-Channel-Portable-PC-USB-2-0-Oscilloscope-Probe-CD-/260760514721?pt=BI_Oscilloscopes&hash=item3cb689b0a1

also, what's with the copy pastes from wiki ? just use links to the appropriate paragraphs and at least annotate the links.



>most research i've done says any DC offset upto 50mv is considered normal & 'safe' under practical real world conditions. alot of amps deviate upwards to a few hundred mv with no apparent detriment to speakers (atleast structurely). 0mv is best ofcos!

I would've thought speakers accept a lot higher signal voltage, so absolute DC offset voltages of a 1-few hundred mV wouldn't be of much concern as opposed to HP outs, where that same 100mv would be 10-20-30% of the total input...


>I've tried measuring audio signal out of my DMM one time, and thought occurred to me: Wait a minute, I'm measuring a audio signal(it was fluxuating), maybe I should use a O-Scope!!! One thing I don't understand is, how is offset of 10mV(thats 100th of a volt) dangerous if the signal swings way beyond that if you raise the gain? 

When you raise the gain e.g. 1000x for 60db stax amps the 100mv becomes 0.1x1000=100v (although that's at the max vol setting, about 70% on the listenable). 100v is pretty audible on the stax headphones  (a hum, at least it was on the SRM-1/mk2, I don't think you'd want to put an extra 50-100v DC through your amp output stages anyway...)


>Output voltage(without load) is total output from the amp, and the lower value is when loaded with headphone(voltage drop accross it) depends on how much output impedance there is. But offset or DC voltage drop is real as it deals with Real impedance, which also depends on real part of output impdeance, and heaphonephone impedance. If output voltage is much greater than headphone impdance, there will not be significant offset on the headphones.

I'm still interested why D100 had a large drop to near 0dc offset when I connected a 25 oh load, and MD11 stayed at around 120/140V.


----------



## scottiebabie

i dont know if full range home speakers have a much higher DC tolerance but even assuming that they do, dont forget that the ave home amplifier has a gain of 30+db & output wattages of 50w rms while the ave gain of dedicated headamps is round 6 db at 150mv. so the normal headphone has only a fraction of the amplified DC to deal with as opposed to speakers.
   
  personally i think its a wash as to which particular transducer has more DC tolerance in actual real world application. the standard home amp has as much as 100x more power than the ave headamp so deviations of a few hundred mv's isnt rare & built in trim pots are there to solve this.
   
  OTOH an 100x less powerful amp with similar DC offset issues signifies a much bigger problem & is prolly inherently a bad design. however its still relatively easy to solve it with a simple mod-a-trim-pot at the appropriate transistor output to adjust DC or cap coupled it at the output.
   
  its still my opinion that its a storm in a teacup to worry about DC offsets destroying cans esp when dealing with the micro voltages inherent in dedicated headamps & headfi.
  
  Quote: 





svyr said:


>





> I would've thought speakers accept a lot higher signal voltage, so absolute DC offset voltages of a 1-few hundred mV wouldn't be of much concern as opposed to HP outs, where that same 100mv would be 10-20-30% of the total input...


----------



## svyr

>i dont know if full range home speakers have a much higher DC tolerance but even assuming that they do, dont forget that the ave home amplifier has a gain of 30+db & output wattages of 50w rms while the ave gain of dedicated headamps is round 6 db at 150mv. so the normal headphone has only a fraction of the amplified DC to deal with as opposed to speakers.

I though we were talking about DC out on the actual voltages post gain - i.e. speaker amp outs, not your source going into the speaker amp. If we're talking about source->amp DC offset, I think it's more of a relative thing anyway. I.e. Even if it's a 50w rms system, the proportion of the DC offset to the signal would hold post gain, so as long as we aren't talking about hundreds of mv DC (from the 2v line out signal max) it should be fine despite the higher gain. (i.e. it'll be within the speakers max rated power, if DC + signal amplified is less than the rated voltage)


Although I have confused people by providing the figures for MD11 line out along with the HP out and then talking about the 1000x gain for the stax amps connected the it.


----------



## High_Q

I had an inclination Mic input O-scope can be done..  mic input is analog in to pc.  It takes in voltage, and there is a ADC in the sound card on the pc, and you need to find the right software to analyze the data.  Here is a blog of someone attempted that.
   
http://my.opera.com/ceez/blog/pc-oscilloscope
   
   
  And also, this:
   
http://my.opera.com/ceez/blog/pc-oscilloscope


----------



## scottiebabie

Ahhh ic what u mean. Folks here's been getting all OCD'd bout even as little as 10-20mv of DC offset & I naturally assumed tat it was at the source since offset readings are done with both source unhook'd & unloaded.

What I meant was tat it's fairly common for home amps to have as high as a coupla hundred mv of DC offset considering the Voltage/wattage involved but it's darn ridiculous to have similar amts of offset from microamps ie. headamps! Fo sho there's something rotten in the state of Denmark & it's a faulty design or a faulty circuit atleast.

Since most headamps don't have DC (or Bias current for tat matter) trimpots, I think a lot of them are output cap coupled - I know both of my ex iBasso porty amps are. Regardless I wouldn't lose any sleep as long as DC is <50mv - at output tat is. If it does destroy your cans, I'd be hoofing it to the nearest lotto outlet....pronto


----------



## svyr

It's magical musilol time:



			
				svyr said:
			
		

> just to clarify what I want support tell me about MD11:
> 
> a) Will the replacement unit have a working power button? (currently you can turn the unit on, and sometimes it shows a white screen and nothing happens)
> b/c)Just to clarify what I mean by DC offset on HP and line out.
> ...





[quote="Customer]
Back upstairs, this is normal.
 OCL output, then there will be a DC offset. Line out the current, to a small point. Headphone output as current, to a large point.

Back upstairs, this is normal.
 OCL output words (no additional capacitive coupling), there will be a DC offset. Line out the current, to a small point. Headphone output current is high due ...
[/quote]





			
				svyr said:
			
		

> You're supposed to trim DC offset on both line out and HP out. (it remains constant at any volume level). 150mv DC offset on HP out is not acceptable. Neither is 70mv on line out.
> 150mv DC is nearly 1/3 of what I listen to (ac out of 450mv on some of my IEMs). That reduces dynamic range for headphones.
> 
> For line out, when you connect a directly coupled amp (e.g. stax driver units) the high DC offset translates to extraneous audible noise.
> ...




[quote="Customer]
Line output is less than 10mv, headphone output is less than 50mv, no capacitive coupling for the circuit is difficult to do. Need to do a lot of auxiliary circuits, such equipment is astronomical, do not appear in MD11, MD30 price of such devices.
 Op-amp output will be more than 50mv DC offset, not to mention the power transistor output. Unless it is coupled with a capacitor.
[/quote]





			
				svyr said:
			
		

> >Line output is less than 10mv, headphone output is less than 50mv, no capacitive coupling for the circuit is difficult to do.
> 
> yes, but MD11 is 7 times that on line out and 3 times that on hp out. Surely Musiland could have at least made an effort to get closer to 10 and 50?
> 
> ...





So um, musiland basically said - too hard, this is normal (by design for both HP/line out), trimming DC offset is hard for them/expensive (woot?), cptn obvious reports the line out an hp out are not capped, to get capped outputs you need to buy a $600 MD30 lol  ... uuuum 'cool story?'
again, it doesn't mention anywhere that the line out and the hp out are not capped, and What would normal people do, buy cables with caps for hp and line lol ? Yea, those are really sold in your corner shops...My DIY line out one looked like something out of horror films


----------



## svyr

Tried to measure DC offset on XLR out for another source the other day... Got

IF I remember correctly I had about 300mv DC offset between 2 and 3, but not between 1/3 or 2/1 

The pins are: 
http://upload.wikimedia.org/wikipedia/commons/8/81/XLR_pinouts.svg
1 Chassis ground (cable shield)
2 Positive polarity terminal (hot)
3 Return terminal[4] (cold)

Crickey, bin it? or supposed to measure to ground?


----------



## wink

Quote: 





> IF I remember correctly I had about 300mv DC offset between 2 and 3, but not between 1/3 or 2/1


 
  The DC offset is between pins 2 & 3.
   
  300mV is excessive.


----------



## svyr

wink said:


> The DC offset is between pins 2 & 3.
> 
> 300mV is excessive.



thanks!
eeeh my bad, memory did not serve me right. It doesn't have a positive to neutral offset, it has the 300mv offset when I measure either neutral or positive *to ground*... nooo idea what that means though.


----------



## nikongod

Quote: 





svyr said:


> Quote:
> 
> 
> 
> ...


 

 For headphones the output could be quite a few volts above ground, as long as pins 2&3 measure ~0V between each other. 
   
  For a source to an amp there are cases where having the output a bit off from ground does nothing, and others where it screws everything up. Best practice is to use floating outputs but very few people want to pay extra for the circuitry or transformers transformers to do that. Mega kudos to the people who do. Second best is to reference the output to ground. A LARGE majority of consumer balanced gear has ground-referenced balanced outputs and very few people have problems.


----------



## High_Q

Why I stated to use O-scope?  The Digital Mulitmeter (DMM) only measures RMS.  Explain to me how the dmm measure the offset.
	

	
	
		
		

		
		
	


	



  
  Quote: 





qusp said:


> need a scope for measuring dc offset? er.......no, a dmm is usually fine depending on the signal you are using. if you wanna get crazy you can use a voltmeter and play a known voltage sine wave, whatever the heating value over what is expected after gain and converted to dc is your offset. of course as scope can be used to good effect, but isnt needed imo.
> 
> as far as i know, you dont need to go over max rated input to cause issues, that is not what its about except in the extreme case where you will overheat the coils, its more about excursion of the driver and therefor dynamic range being effected. your 110mv amount is pretty high in this regard


----------



## nikongod

Quote: 





high_q said:


> Why I stated to use O-scope?  The Digital Mulitmeter (DMM) only measures RMS.  Explain to me how the dmm measure the offset.


 
   
  I have no idea why you said to use an oscilloscope to do something that is easily accomplished with a very affordable and accessible tool - a simple volt meter. Re-read your first post in this thread.
   
  As an engineer is it morally acceptable to you to recommend a tool that is harder to use and generally more expensive when a tool which is much easier to use and FAR less expensive to achieve the same end result?
   
  Now, for how to measure voltage offset with a DMM we must start with the assumption (going FAR out on a limb here because it could be anything with you) that the DMM you are reffering to is a voltage, current, and resistance type - perhaps with a few other features thrown in (such as AC volts) but I think this is a reasonable assumption. People with LCR meters , digital oscilloscopes, or other more specialized meters typically refer to them as such despite the fact that they are equally or more digital compared to most voltage/current/resistance devices.
   
  Now, assuming your DMM has a DC VOLTAGE setting one would set the DMM for "DC volts" on the appropriate range (or if it autoranges just let it do its own thing) and measure the offset from output to ground in an SE amp or inphase output to outofphase output in a balanced or bridged amp. 
   
   
   
  You left the units off of RMS in this sentience:
  "DMMs measure only RMS" 
  RMS what? My DMM Measures RMS current amongst other things.


----------



## High_Q

No, we recommend for accuracy and precision(simply for the right values) as an EE.  So, we don't say measure offset using dmm if it's not measuring offset.  We recommend a tool that measures offset.
	

	
	
		
		

		
		
	


	



   
  Wrong, DC measurments, when input is DC.  Audio signal is AC+ offset(there will be some offset).  explain to me how offset is being measure by the dmm with the DC setting.
	

	
	
		
		

		
		
	


	



  
  RMS what? Duhh. Voltage.. what is this thread about again, you should know by now..
	

	
	
		
		

		
		
	


	



   
  FYI
   
   
  The RMS value of a set of values (or a continuous-time waveform) is the square root of the arithmetic mean (average) of the squares of the original values (or the square of the function that defines the continuous waveform).

  In the case of a set of _n_ values 
	

	
	
		
		

		
		
	


	




, the RMS value is given by:






   The corresponding formula for a continuous function (or waveform) _f_(_t_) defined over the interval 
	

	
	
		
		

		
		
	


	




 is






   and the RMS for a function over all time is






   The RMS over all time of a periodic function is equal to the RMS of one period of the function. The RMS value of a continuous function or signal can be approximated by taking the RMS of a series of equally spaced samples. Additionally, the RMS value of various waveforms can also be determined without calculus, as shown by Cartwright.[1]

  In the case of the RMS statistic of a random process, the expected value is used instead of the mean.

  Quote: 





nikongod said:


> I have no idea why you said to use an oscilloscope to do something that is easily accomplished with a very affordable and accessible tool - a simple volt meter. Re-read your first post in this thread.
> 
> *As an engineer is it morally acceptable to you to recommend a tool that is harder to use and generally more expensive when a tool which is much easier to use and FAR less expensive to achieve the same end result?*
> 
> ...


----------



## svyr

high_q said:


> No, we recommend for accuracy and precision(simply for the right values) as an EE.  So, we don't say measure offset using dmm if it's not measuring offset.  We recommend a tool that measures offset.
> 
> 
> 
> ...




all of the above aside, if the DAC does not mute the output with no signal and the offset is constant at any volume (it is for the two devices I measured), then you're just measuring vanilla DC voltage on the outputs and a DMM is more than adequate


----------



## nattonrice

Quote: 





high_q said:


> Wrong, DC measurments, when input is DC.  Audio signal is AC+ offset(there will be some offset).  explain to me how offset is being measure by the dmm with the DC setting.


 

 I had a reply typed but came to the conclusion that this is better: LOLWUT?


----------



## High_Q

There a bit you have to take into consideration.  It's not a easy as, get a dmm, find DC(if it does measure DC and do you know how?)
   
  If you are concerned about offset damaging your phones.  
   
  First look up mtf power rating of lowest rated headphones, it could think of a good one, ibuds.
   
  If you are absolutely sure your offset is 150mV?  You have to take into consideration offset across the headphones(when load is applied).
   
  Since there is output impedance, the load offset will be lower.
   
  Find the lowest rating
   
  apply the formula:
   
  P=V^2/Z  , where Z=effective impedance of the headphone, and V=offset voltage across the heaphones
   
  If the offset value alone is not at damaging level, to find out with audio signal applied:
   
  1. Measure the peak at max voltage(O-scope required)
   
  Find the output impedance of the headphone out, and minimum impedance of the headphones.
   
  Either figure out the via voltage divider, headphone load voltage at max voltage swing outputted by the headphone out.
   
  P=V^2/Z
   
  2.  Measure load(across the headphones) voltage
   
  P=V^2/Z
   
  Z=lowest effective headphone impedance
   
  measurement tools must have necessary precision.
   
   
  Finally, if is way beyond mft recommendation, dig lower if you are curious where it would tear apart your headphones.
	

	
	
		
		

		
		
	


	



   
  It makes no sense to measure offset and think it would damage the headphones without taking into consideration max voltage deviation from the offset(worst cas scenario)


----------



## qusp

hahahahahahaha. how do you ever get ANYTHING done??


----------



## High_Q

It's not about getting it done, it's about getting it done precisely(so you have the crucial measurements) so you don't damage anything(or make it sound like crap) in the process.
	

	
	
		
		

		
		
	


	




  Understanding what is going on helps exponentially.  Stuff I mentioned is pretty elementary and should be quick, from the your reply it seems like it would take you a long time to figure it out?
  
  Quote: 





qusp said:


> hahahahahahaha. how do you ever get ANYTHING done??


----------



## qusp

is that so, and your method is* precisely* how useful for someone who doesnt have a scope? and i speak of the vast majority of people on this forum that you are 'advising'. its also bull, measuring how much dc offset at the output to a safe level is easily within the usefulness of a regular 2 dollar dmm


----------



## qusp

Quote: 





svyr said:


> all of the above aside, if the DAC does not mute the output with no signal and the offset is constant at any volume (it is for the two devices I measured), then you're just measuring vanilla DC voltage on the outputs and a DMM is more than adequate


 


  this


----------



## svyr

>If you are absolutely sure your offset is 150mV? You have to take into consideration offset across the headphones(when load is applied).

Surprisingly, at the driver terminals of DT48e it's still the same lol - about 145mV. (we've been over it about a page or two ago  )


>http://www.head-fi.org/forum/thread/550811/how-much-dc-offset-would-you-consider-harmful-to-hp-and-after-which-point-would-you-throw-the-commercial-amp-out/45#post_7517967

I don't understand the purpose of that post at all. We've been discussing DC offset on balanced line out, not for driving HP, and a very specific situation where the grnd to + and neutral and an offset and but not + to neutral.
The discussion about how much DC is harmful to your HP has been concluded on the prev page where it seemed to reach a consensus of - a) DC offset = bad, if enough may overheat the driver for low impedance/high sens IEMs b) the driver will possibly die from the quoted max power stated on the box, if the signal + dc offset manage to exceed that rating.


>1. Measure the peak at max voltage(O-scope required)

Not really, have a look at the manual for a good RMS meter. Most have have a min/max mode (don't really need it if applying a pure tone anyway), and you can get the V by applying a pure tone? Then adding the DC offset. (If I understand correctly AC is measured as a change so whatever the DMM will display will just be the AC amplitude....

any regular meter will measure the DC offset just fine lol. For AC signal measurement (accurate ones), maybe you'd want a true RMS meter, but IDK, a ballpark figure/accuracy of the meter for AC should all be in the DMM manual


----------



## High_Q

Quote: 





svyr said:


> >If you are absolutely sure your offset is 150mV? You have to take into consideration offset across the headphones(when load is applied).
> 
> Surprisingly, at the driver terminals of DT48e it's still the same lol - about 145mV.





> *I question the value because you have said you measure it with dmm*





> >http://www.head-fi.org/forum/thread/550811/how-much-dc-offset-would-you-consider-harmful-to-hp-and-after-which-point-would-you-throw-the-commercial-amp-out/45#post_7517967
> 
> I don't understand the purpose of that post at all. We've been discussing DC offset on balanced line out, not for driving HP, and a very specific situation where the grnd to + and neutral and an offset and but not + to neutral.
> The discussion about how much DC is harmful to your HP has been concluded on the prev page where it seemed to reach a consensus of - a) DC offset = bad, if enough may overheat the driver for low impedance/high sens IEMs b) the driver will possibly die from the quoted max power stated on the box, if the signal + dc offset manage to exceed that rating.
> ...





> *My point is that dmm does not meaure offset, it measure RMS.  It all depends on what dmm is used, some dmm have more functionality and precision vs the $2 dmm qusp mentioned.*





>





>


----------



## qusp

i say the words in the previous post also because i dont see how you can find time to be an engineering student and still spend sooo much time in all areas of this forum carrying on utterly pointless arguments that are of no consequence whatsoever
   
  so i'll leave you to it again. someone linked me to the particularly humorous content in the last couple of pages, only reason i'm here.


----------



## High_Q

And your last few posts have any points?
	

	
	
		
		

		
		
	


	




  I see that you have much greater amount of time to burn than I do base off of your 6,564 post since 6/2008
	

	
	
		
		

		
		
	


	




  I can see that forum count is not proportional to how much expertise you have in the area.  And you DIY. And you are on other forums with more forum counts.
	

	
	
		
		

		
		
	


	




  Where do you find time to do... stuff... ?
	

	
	
		
		

		
		
	


	




 I don't DIY(unless is worth my time), I have a job.  I don't sit on my ass looking for places to swap out for V-caps.  Is your job posting in forums all day?
   
  I'm actually on the forum looking for good equipment, and try to contribute to posts by adding feedbacks based on what I know.
   
  Quote: 





qusp said:


> i say the words in the previous post also because i dont see how you can find time to be an engineering student and still spend sooo much time in all areas of this forum carrying on utterly pointless arguments that are of no consequence whatsoever
> 
> so i'll leave you to it again. someone linked me to the particularly humorous content in the last couple of pages, only reason i'm here.


----------



## svyr

>You can send a tone signal to the DAC, but must be digital. It's a DAC? You will know the real V analog at the output. You will need to measure that. And figure out the offset?

Tone to DAC obviously. and obviously you need to measure it (signal and offset). and That's what the post above says lol...


>It all depends on what dmm is used, some dmm have more functionality and precision vs the $2 dmm qusp mentioned.

You'd be pretty hard pressed to find an electronics multimeter without DC/mV mode. Sure the cheaper ones will have less precision and accuracy, but for a ballpark figure it'll do. qusp was meaning to say $5  (could be $2 at production  )


>My point is that dmm does not meaure offset, it measure RMS. 

I have no idea what you're trying to say there, RMS and DC in the same sentence don't make sense to me...For measuring just the offset, http://www.head-fi.org/forum/thread/550811/how-much-dc-offset-would-you-consider-harmful-to-hp-and-after-which-point-would-you-throw-the-commercial-amp-out/45#post_7517798 that seems like an adequate way.


PS, guys please remain civil and don't flame each other. I've had enough flaming in the burson thread for one day . High_Q please explain yourself clearly, so people who are currently going 'lolwut' can actually solidify where they think you're wrong and correct you, or change their mind.


----------



## nattonrice

Since this is bound to be locked soon I may as well get in...
   
  All you seem to be doing is copy and pasting whatever you read on wikipedia in a thinly veiled attempt to appear intelligent.
  Why do you feel the need to drag these threads into "I'm smarter at this than you are" pissing contests?


----------



## High_Q

Quote: 





svyr said:


> >You can send a tone signal to the DAC, but must be digital. It's a DAC? You will know the real V analog at the output. You will need to measure that. And figure out the offset?
> 
> Tone to DAC obviously. and obviously you need to measure it (signal and offset). and That's what the post above says lol...
> 
> ...





> *I aways try, thats why I include equations.  It doesn't get more clear than equations, they tell the truth.  Also, qusp has been very rude in other posts, so I really don't have great history with him or nikon.  Also nikon has responded to my post in a very rude manner, don't have much respect for him eithe*r.


----------



## High_Q

No, I copy and paste because I'm too lazy to type in latex??
	

	
	
		
		

		
		
	


	




  I would not paste it, if I didn't understand it in the first place because they go with my explainations.
	

	
	
		
		

		
		
	


	




  Want me to derive Maxwell's equations infront of you?  I can get to the point to find the c constant.
  
  Quote: 





nattonrice said:


> Since this is bound to be locked soon I may as well get in...
> 
> All you seem to be doing is copy and pasting whatever you read on wikipedia in a thinly veiled attempt to appear intelligent.
> Why do you feel the need to drag these threads into "I'm smarter at this than you are" pissing contests?


----------



## nattonrice

You don't need latex (or wikipedia) to tell someone how to measure an amplifier's dc offset with a dmm...


----------



## qusp

you seem to have 2 rather vital types of voltage switched around. AC measures rms heating value, DC does not, not even a little bit. otherwise for instance, measuring a fullscale AC signal at the output of an amp, or a dac, or a signal generator with a low leakage cap at the output would still read DC, guess what? it doesnt, it measures 0vdc


----------



## High_Q

What I'm saying is audio signal is fed in to the dmm.  you can set the dmm to DC, but it's pointless as the signal is ac, its an audio signal.  it fluctuates.  Put it in AC, you get the RMS of AC, as the dmm gives you a number, not respresentations of the audio signal fluctuations.  My question is, if you set it to DC, how do you measure the offset?  One idea would be to send in a tone signal of known value. measure rms, do some math and you will get the offset.


----------



## qusp

many analogue Rms responding voltmeters are AC coupled even. my hp3400A is, so how it could possibly read a few mv of dc i dont know. a large amount would screw with it due to leakage, so you cant have it set to amplify the input too much or you'll kill the input stage, but a small amount wont even make it through
   
  DC causes AC heating reading errors sometimes, but i have never encountered AC causing DC errors


----------



## JoetheArachnid

Yet for most humans who do not dwell in the vaunted halls of Wikipedia copypasta, measuring DC from signal to ground with a DMM with no music playing is adequate, as once you have <10mV any DC contained in the actual signal is negligible and would probably be hard to get rid of anyway. In fact, measuring DC on an amp with a source attached with music playing is a really stupid idea to beging with, as any DC from the source will be passed on through the amp and give you innacurate measurements.
   
  I understand what you're trying to say with your talk of scopes, but for audio it really isn't necessary to get that accurate. Our ears and transducers are only so good, after all.


----------



## High_Q

So when the music is not playing, offset is measureable?, and it is infact the same offset that the voltage swing will be centered around when playing?  I'm doing this not because I'm anal about offsets, but to get a better understanding of measuring.
	

	
	
		
		

		
			





  Every little bit helps.
  Quote: 





joethearachnid said:


> Yet for most humans who do not dwell in the vaunted halls of Wikipedia copypasta, *measuring DC from signal to ground with a DMM with no music playing is adequate*, as once you have <10mV any DC contained in the actual signal is negligible and would probably be hard to get rid of anyway. In fact, measuring DC on an amp with a source attached with music playing is a really stupid idea to beging with, as any DC from the source will be passed on through the amp and give you innacurate measurements.
> 
> I understand what you're trying to say with your talk of scopes, but for audio it really isn't necessary to get that accurate. Our ears and transducers are only so good, after all.


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## digger945

Quote: 





high_q said:


> So when the music is not playing, offset is measureable?, and it is infact the same offset that the voltage swing will be centered around when playing?  I'm doing this not because I'm anal about offsets, but to get a better understanding of measuring.
> 
> 
> 
> ...


 


   
  Yes. Measure the DC with the amp not playing anything at all. Make sure the amp is turned on and warmed up. 
   
  You should either have your source hooked up or ground the input signal line to ground. 
   
  If you show offset with your source connected then you should first check the source to see if it has offset. Again without music or test tone playing, just idle and turned on.


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## kasimoa

Hello,
   
  Can Planar Magnetic headphones like the LCD's or Hifiman HE 500 also get damaged by the DC current since they dont have a voice coil? All answers are highly appreciated. Thanks in advance


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## DingoSmuggler

Quote: 





kasimoa said:


> Hello,
> 
> Can Planar Magnetic headphones like the LCD's or Hifiman HE 500 also get damaged by the DC current since they dont have a voice coil? All answers are highly appreciated. Thanks in advance


 
  Yes, the exact same principal applies, they still have a conductor, equivalent to a voice coil, but in a single plane. It will heat up and burn out if there's enough dc across it.
  I would expect them to tolerate dc a bit better than the average headphone though (higher power handling, and large surface area for cooling)


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## kasimoa

That makes sense thanks for making that clear to me.


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## wakibaki

There have been a lot of formulas bandied around here to no great effect.
   
  What can we actually do with formulas?
   
  Here are a couple of very basic ones...
   
  1) V = I x R
   
  2) P = I x V
   
  ...rearranging (1)...
   
  I = V / R
   
  ...and substituting into (2)...
   
  3) P = (V x V) / R
   
  This tells us the voltage required to produce a given power in a given resistance.
   
  Harmful is one thing, desirable is another.
   
  So let's think about this issue in a different way. How much voltage can you hear?
   
  Modern music systems are, for the large part, 16-bit. In the mastering process dither is applied to the least significant bit. Presumably this means that you can hear the effect, otherwise why do it?
   
  I have a pair of Shure e2c IEMs here. They're considered reference-quality by people like Seigfried Linkwitz, and, more importantly, they're representative of the typical impedance and sensitivity readily obtainable at a reasonable price. They have an impedance of 16 ohms and a sensitivity of 100dB/mW.
   
  Taking 100dB as a reasonable listening level, from (3), the voltage to give 1mW in 16 ohms is 0.126mV.
   
  16 bits means 65536 levels.
   
  So the lowest perceptible level in these phones is 0.126 / 65536 = 1.9uV. Yes, that's right, ~2 _microvolts_.
   
  So, IMO, we would ideally like to see a DC offset <2uV. Then we could reasonably claim it was insignificant.
   
  This is difficult to achieve in a mass produced item. It really requires a DC servo and individual adjustment of each unit. It could be automated using an OTP preset digipot, although measurement at such levels would require reliable contact in the test jig. A low-leakage cap is not really a viable solution. The typical leakage current in a 220uF cap with 1V across it will produce ~35uV in 16 ohms, you need a big cap to maintain the FR, and anyway, nobody wants a cap like that in their system. You probably can't hear it, but if it's not there, then you definitely can't hear it.
   
  Of course this is all nit-picking to the n-th degree, but I don't really consider an offset >1mV acceptable from an aesthetic POV, considering what is achievable, although a few mV of DC is probably of no consequence in these Shure IEMs. I mean, they should tolerate an output of 120dB, which is 1.26 (rms) volts in.
   
  Obviously less sensitive, high-impedance phones can tolerate bigger offsets and systems intended to drive speakers have historically been built to higher tolerances.
   
  But 300mV? For a headphone amp, knowing what is likely to be plugged into it? No way.
   
  w


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## duncan1

After all the maths I was waiting to see if somebody would say apart from burning out the voice coil if very high another problem that is basic is that  in Audio Power amps when connected to moving coil loudspeakers --The voice coil is -as long as the amp is switched ON-OFFSET from its NORMAL NEUTRAL position.
  There bye causing a DISTORTED reproduction as the loudspeaker designer made the high spec./low distortion taken into   consideration the NEUTRAL position of the voice coil. And yes input which has a DC content when applied to an input that is DC connected  ALL DC connected signal circuit[ NO output capacitor]. will  be amplified in proportion to the gain of the amp.
  So the same would apply to MC head-phones.
   I too have a HP bench voltmeter -HP 3455A with a no leakage isolating cap at the input.


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## UnknownAX

I thought that I should measure the DC offset of my phone before plugging any amp to it. Here are the results:




Everything was measured between the R/L and ground, all values were absolutely constant, even the ones with music playing.

I think the left channel looks a bit weird, right? What could cause the DC offset when music isn't playing? Is it safe to hook up an amp to it? I'd really appreciate some help


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## duncan1

The figures you quote aren't bad and in hi-fi company engineering terms --"are within the specified range".
          But that depends on what your amp is -If a twin amp audio chip then those figures could  apply.
             A single channel audio chip can have its off-set reduced to zero by adjustments to the off-set tags on the chip.
                Some don't. If it is a power amp that is discrete [no chips] then that can be adjusted from the input balance.
                    If a power audio chip then it depends on the chip. It is also possible  to create a circuit at the output to balance it.
                                 Specialist companies usually try to get it to zero to enhance the engineering qualities of their product.
                                      So yes it probably will be able to achieve -zero -It depends if you think that small amount is bothering you?


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## duncan1

Just noticed you are talking about a pre-amp-so it stands a good chance it is a twin audio chip which doesnt have off-set .
           But if it is discrete then again can be zeroed by adjusting the input balance. Can you post a link to a circuit diagram?


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## UnknownAX

Well, it's the DC offset measured on the 3.5mm jack of my *mobile phone*.
  I was asking because I want to connect it (the *mobile phone*) to a Grado RA-1 clone, which will be connected to my *headphones*.


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## wakibaki

If it's a true RA-1 clone and has DC blocking (AC coupled, a series cap.) then the DC offset of the source is irrelevant.
   
  w


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## duncan1

The audio chip is a 2134 which I have used in the past .Quite a good chip but it is a dual amp chip no means of adjusting the offset that  leaves a resistance network at the output to bring it down to zero. There have been some  posted on Head-Fi in the past. I am sure somebody quicker than me can tell you where although I could dig one up at home.


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## duncan1

Wakibaki-The one of the clones I looked up is direct DC connected by a 38 OHM resistor


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## UnknownAX

This exactly how mine is built (as soon as I finish it).: http://www.williamneo.blogspot.fi/2008/03/grado-ra1-headphone-amplifier.html
  I might add a sijosae discrete rail splitter, though...


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## duncan1

It is not a big job to provide several resistors connected to the  output and supply connections to zero down the offset. 
        Just remember to do it while your mobile phone is connected to make sure it is zeroed in working condition.
              I also have to say UNLESS you have  an expensive version of the chip[military grade] you will find the chip itself has some offset as it is a dual version. 
                  This is covered by the chip manufactures under the heading---The----chip/ic/ etc is within the MANUFACTURERS SPECIFICATION it is a "coverall" for legal statement to cover the use of their components as used by the -GENERAL PUBLIC.


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## wakibaki

The 4u7s provide DC blocking in your case, so the phone offset is blocked from the amplifier output.
   
  w


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## UnknownAX

Thanks! I almost forgot about the input caps. They're so huge that they should propably block most of the dc-offset...


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## metalgear

still going through the thread to get my head around the science behind this (and the dangers!) but keep going guys... really ground break stuff.
  
 just one quick question - would you think that it would be possible to create a "magic box" that gives users control over some of the key variables (resistance?) to allow quick matchups between a variety of headphones and amps?


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## nikongod

metalgear said:


> still going through the thread to get my head around the science behind this (and the dangers!) but keep going guys... really ground break stuff.


 
  
 This thread has nothing to do with what you are asking.
  


metalgear said:


> just one quick question - would you think that it would be possible to create a "magic box" that gives users control over some of the key variables (resistance?) to allow quick matchups between a variety of headphones and amps?


 
  
 Very easy. Take an amp with 0.0ohm output impedance and add resistors to suit.
  
 If you have an amp with high output impedance things get harder, but are still very easy. Install a transformer at the output of the amp to reduce the output impedance for headphones that benefit from that. 
  
 As you said, it requires magic (actual magic, Give Gandalf or Dumbledor a call) to design a one-size-fits-all solution.


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## nikongod

nikongod said:


> As you said, it requires magic (actual magic, Give Gandalf or Dumbledor a call) to design a one-size-fits-all solution.


 
  
 On second thought:
 Do not meddle in the affairs of wizards, for they are subtle and quick to anger.


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