Adding impedance to headphones

[grndslm]

Soo... the resistance should equal (150-60)/2=90ohms [or sqrt(150*60)=95ohms]???

Which means I'll want to add a 35ohm resistor in series??

This really shouldn't be that hard to figure out, right?

 

[Clutz]

The reason behind adding resistors in series with the headphones is to increase the load seen by the headphone amplifier, thus reducing the current demands placed on the headphones. Taking the mid point of the maximum and minumum impedance of the headphones, and trying to raise that value to a specified target is no more (and probably less) effective than trying to simply raise the minimum impedance value of the headphones to the desired point.

The argument that by adding 94 ohms resistance (or sqrt(150*60)) of resistance to the headphones will equal out the power distributed to the headphones across all the frequencies doesn't make any sense at all. You're just adding a constant linear amount of resistance to the headphone, it's not going to respond dynamically as described. Or at least I can't see how it will have the described effects. If it was really that easy (or desirable) to do that, all headphone manufacturers would be doing it, as it doesn't cost very much money, but you could gain a lot in terms of marketing - for something like an extra $0.02 per headphone.

 

[grndslm]

You're not gonna gimme a straight answer are you?

How much more resistance do I need to add to this PITA?

 

[grndslm]

Forget it... I just found out that SuperMini has built-in 75ohm resistor that seems to be common enough around here for most cans that need more resistance.

 

[ericj]

Quote:

Originally Posted by Clutz /img/forum/go_quote.gif The reason behind adding resistors in series with the headphones is to increase the load seen by the headphone amplifier, thus reducing the current demands placed on the headphones.

Uhh . . .

I think you meant to say "increase the impedance seen by the headphone amplifier, thus reducing the current demands placed on the amplifier"

 

[majkel]

My logic is my starting point logic, you have to experiment to find the sweet spot. Actually there are two schools of output impedance for headphones:
1) As low as you can - giving maximum damping factor (read: drive accuracy)
2) optimal power distribution. When you search through some webpages, you'll find that it had been advised to use 120 ohm resistors for old type headphones. 75 ohms came probably from Ety's ER-4P->S adapter, but yes, it works very well for Sennheiser HD 595.
I know other types of headphones that respond better to zero output impedance, like Senns PX100, Senns HD580, various types of IEMs (X3, CX300), but some from Beyerdynamic also improve with some resistors in series, bu again - for high impedance headphones 75 ohms is almost nothing. jung provided you with all the theory that is used for output power balancing. You cannot use arythmetic mean, but geometric mean to calculate optimum reesistance, because power is a square function of voltage, not linear, while the resistor acts like a voltage divider. The division ratio changes accordingly to impedance swing over the spectrum.
Last thing - resistors do not lower battery life because you work on the same current going thru the headphones and there is still the same supply voltage on the output buffer, but it's partly wasted in resistors instead of output transistors. However, depending on output buffer solution and biasing, YMMV.

 

[holland]

Quote:

Originally Posted by grndslm /img/forum/go_quote.gif How much more resistance do I need to add to this PITA?

To do what? Why are you trying to add resistance? The OP wanted to reduce hiss. The best way is via a voltage divider using a parallel and series resistor, but you need to work the math for voltage and current variations...which is how a potentiometer is modeled. It's not clear why you want to do it. If you want to change the sound quality, you need a starting point and you're going to have to do trial and error. As the resistance increases the current load decreases and the power demands decrease. With such, you will usually get less bass as you won't get much as much movement from the diaphragm at low freq.

There may be some threads about the KSC75 and sound quality with various resistors. Buy a whole bunch, try and see. You can start with 94, you can start with 30, you can start with 10, it's up to you and what you perceive to be better. All the math won't do you any good because it's a linear resistor that's being added and the driver doesn't respond in a linear fashion because it's not just a resistive load. It's only purely resistive at Fs (IIRC from basic speaker design) which is the peak that you see in your chart.

What's being intimated is that 94 ohms could be a starting point, how much you deviate from that is up to your personal preference. If you want to see that for real look at the impedence curves for the ER4P and the ER4S. Try to fit this math into that curve and ask yourself why 75?

The 75 Ohm in the supermacro, supermini, etc. is for the ETY P->S cable. It's not hard and fast applicable to all headphones as a golden switch.

 

[grndslm]

I feel soo ridiculously st00pit right now...

At least I learned what P->S stands for!

 

[Clutz]

Thank you Holland for saying more clearly what I was trying to.

 

[grndslm]

Quote:

Originally Posted by holland /img/forum/go_quote.gif To do what? Why are you trying to add resistance? The OP wanted to reduce hiss. The best way is via a voltage divider using a parallel and series resistor, but you need to work the math for voltage and current variations...which is how a potentiometer is modeled. It's not clear why you want to do it. If you want to change the sound quality, you need a starting point and you're going to have to do trial and error. As the resistance increases the current load decreases and the power demands decrease. With such, you will usually get less bass as you won't get much as much movement from the diaphragm at low freq. There may be some threads about the KSC75 and sound quality with various resistors. Buy a whole bunch, try and see. You can start with 94, you can start with 30, you can start with 10, it's up to you and what you perceive to be better. All the math won't do you any good because it's a linear resistor that's being added and the driver doesn't respond in a linear fashion because it's not just a resistive load. It's only purely resistive at Fs (IIRC from basic speaker design) which is the peak that you see in your chart. What's being intimated is that 94 ohms could be a starting point, how much you deviate from that is up to your personal preference. If you want to see that for real look at the impedence curves for the ER4P and the ER4S. Try to fit this math into that curve and ask yourself why 75? The 75 Ohm in the supermacro, supermini, etc. is for the ETY P->S cable. It's not hard and fast applicable to all headphones as a golden switch.

I'm trying to clean up the muddy bass in the KSC75 (there are at least a thousand posts about KSC75 and adding impedance adapter, but none of them explained why they chose 75ohms)... plus I'm considering buying IM716 which also benefits from the P->S adapter as they are essentially Etys.

The Supermini's extra 75ohm resistor suits my situation perfectly! Sure I'd prefer more resistance if possible, but now I don't have to add anything inbetween cables or have an adapter that can be lost.

Perhaps the KSC75 would benefit from an additional 20ohms, but the tradeoffs for SQ prolly aren't even worth me thinking about this for another second.

 

[Clutz]

I don't think there was a specific electronic reason why they chose 75 ohms. They didn't want to increase the impedance too much and make the headphones too hard for the headphone amplifier to drive, so adding multiple thousands of ohms would be a bad thing. At the same time, adding only another 5 or 10 ohms probably isn't going to change things much. Someone tried out 75 ohs, found it worked- and everyone followed.

 

[grndslm]

Quote:

Originally Posted by holland All the math won't do you any good because it's a linear resistor that's being added and the driver doesn't respond in a linear fashion because it's not just a resistive load. It's only purely resistive at Fs (IIRC from basic speaker design) which is the peak that you see in your chart.

I don't understand why the math won't do me any good & the driver not responding in a linear fashion bit, tho...

 

[Clutz]

Quote:

Originally Posted by grndslm /img/forum/go_quote.gif I don't understand why the math won't do me any good & the driver not responding in a linear fashion bit, tho...

The impedance of the driver is a function of the frequency - that means that the impedance of the driver changes with the specific frequency of the tone being played.

The impedance of a resistor is constant, across all frequencies.

Therefore the impedance of the headphone + the resistor at a given frequency is just the impedance of the headphone + the constant amount of the resistor. If the impedance of the headphone is 60ohms at 1Hz, and 100 ohms at 5khz and you choose to add a resistor of 60 ohms, then the new observed impedance with the resistor added in-line, from the perspective of the amplifier is 60 ohms at 1Hz, and 160ohms at 5khz.

Short answer: Headphone impedance affected by frequency, resistors are not. The old impedance of the headphone is going to be something like f(Hz), and the new impedance of the headphone with the resistor is f(Hz)+resistor_value. Note: It's just a simple linear change.

 

[holland]

Quote:

Originally Posted by Clutz /img/forum/go_quote.gif I don't think there was a specific electronic reason why they chose 75 ohms. They didn't want to increase the impedance too much and make the headphones too hard for the headphone amplifier to drive, so adding multiple thousands of ohms would be a bad thing. At the same time, adding only another 5 or 10 ohms probably isn't going to change things much. Someone tried out 75 ohs, found it worked- and everyone followed.

The 75 was actually from ETY itself. Most likely they did it in reverse. They designed the ER4S and to try and broaden the market, they release a derivative product...the ER4P with more emphasized bass and reduced detail. Knowing what they know of their drivers it was chosen through detailed modeling and measurements. If I were in their shoes, that's what I would do. Design for the high end, and trickle down for derivatives and mass markets.

For head-fi'ers, that was as you say, just jumping on the 75 to apply everywhere else, as far as I can tell. Some people do experiment though.

 

[holland]

To add to this, drivers are, in the most basic sense, modelled as an inductive, capacitive, and resistive load. There's no easy explanation. People have literally written books on this subject. If it was as easy to model a driver as just 3 discrete elements, everybody would be designing drivers.