Low-Z / High Z

[intlplby]

what is the proper and best way to implement both low z and high-z ability into a headphone amp?

2 jacks? selector switch?

whats the best way to wire it?

 

[DCameronMauch]

A series resistor for the low-z phones would technically work. But your damping factor would be trashed. Thus your bass would sound terrible.

I would say selector switch. To change overall gain. Make sure your design takes into consideration the voltage levels required for high-z phones and the current levels required for low-z phones. And don't forget thermal management. According to Nelson Pass, audio god, semiconductors should never be allowed to go above 50 deg C.

 

[drewd]

Quote:

Originally Posted by DCameronMauch And don't forget thermal management. According to Nelson Pass, audio god, semiconductors should never be allowed to go above 50 deg C.

Apples and oranges, I know, but just for comparison, when we test our memory module designs prior to manufactur, we use 100C as the maximum. Not the absolute max (they'll work up until the solder melts), but as a safe operating maximum. I thought that was an interesting bit of trivia.

But I won't argue with Nelson - I don't want a bolt of lightning out of the audio sky to fry me!

-Drew

 

[PeterR]

I've never found it necessary to adjust the gain for different headphones. I've got an amp with switchable gain, have yet to use that feature...

Or are you talking about an output for phones designed for 120 Ohm output impedance? There isn't really a superior way, depends whether you prefer 1 jack and one switch (120 Ohm in series with the output, bypassed by the switch for the low output impedance setting) to two 2 jacks (connect the 2nd jack via 120 Ohm resistors to the first one).

 

[drewd]

Quote:

Originally Posted by DCameronMauch A series resistor for the low-z phones would technically work. But your damping factor would be trashed. Thus your bass would sound terrible. I would say selector switch. To change overall gain. Make sure your design takes into consideration the voltage levels required for high-z phones and the current levels required for low-z phones. And don't forget thermal management. According to Nelson Pass, audio god, semiconductors should never be allowed to go above 50 deg C.

I remember what I was going to say before I headed off on a tangent.

Is the damping factor all that important for headphones? We're not talking about all that much mass, especially compared to, say, a moderately sized woofer. I agree that a selector switch to adjust the system gain would be the best solution (short of two separate amps), but I wonder if a resistor wouldn't be a viable (read: cheap and not too intolerable) alternative.

-Drew

 

[PeterR]

Quote:

Originally Posted by drewd Is the damping factor all that important for headphones? .

Depends on the heaphones. Some (like the Sennheisers, Sony 7506) benefit from a high damping factor, others (Grado) don't care too much. Then there are phones designed for the 120 Ohm standard (Beyer) that need a high output impedance (i.e. a low damping factor).

 

[DCameronMauch]

I have read that Grados do seem sensitive to damping factor. Not so much in bass quantity as quality. I can not say myself. At least with Senn HD600s, I can tell the difference between 6 ohms and near 0 ohms output impedance. That's only 2%. But that same impedance with any Grados is 25%. That has to make a big impact, I would think. A few people have mentioned that the TPA6120A2 headphone driver doesn't have good bass with Grados. And it requires a 10 ohm output impedance for stability. Add 2 and 2...

The driver itself is low mass. So by itself damping factor shouldn't make a big difference. But when attached to your head, there is air somewhat trapped between the driver and your head/ear. That creates what appears as a much heavier driver. So even with headphones, damping factor does matter. But to what extent seems to depend on the headphones.

 

[drewd]

Quote:

Originally Posted by DCameronMauch The driver itself is low mass. So by itself damping factor shouldn't make a big difference. But when attached to your head, there is air somewhat trapped between the driver and your head/ear. That creates what appears as a much heavier driver. So even with headphones, damping factor does matter. But to what extent seems to depend on the headphones.

Excellent point! I was thinking in terms of free air, but you're right, if at least one side of the driver is sealed (as in against your head), the air is going to act like a balloon pushing back against the driver.

-Drew

 

[amb]

Having low output impedance (and hence high damping factor) is not just good for diaphragm control. More importantly, the amp's output impedance and the headphone (or speaker) load impedance forms a voltage divider. The load impedance is not purely resistive and varies wildly across the frequency spectrum. Thus, a high output impedance would cause frequency response aberrations that can be quite audible. A low output impedance minimizes this effect, especially for low impedance loads.

-Ti

 

[PeterR]

Quote:

Originally Posted by amb Having low output impedance (and hence high damping factor) is not just good for diaphragm control. More importantly, the amp's output impedance and the headphone (or speaker) load impedance forms a voltage divider.

That's just a different way to look at the same effect.

Quote:

The load impedance is not purely resistive and varies wildly across the frequency spectrum.

Again, this depends on the headphone. Unlike speakers, many headphones employ a lot of mechanical damping. The impedance plot for e.g. Grados therefore looks very, very flat.

 

[amb]

Quote:

Originally Posted by PeterR Again, this depends on the headphone. Unlike speakers, many headphones employ a lot of mechanical damping. The impedance plot for e.g. Grados therefore looks very, very flat.

I was making a general comment, and admit not having seen the impedance v. frequency plot of Grados. However, mechanical damping only tames the impedance peak of a transducer at the resonance frequency. It does not do anything for impedance rise at higher frequencies due to voice coil inductance. Here again, I don't know how inductive Grados are. But my point is that having an amp with a low output impedance is a good thing, and eliminates a potential source of sonic degradation regardless of what headphones you use.

-Ti
-Ti

 

[PeterR]

Quote:

Originally Posted by amb It does not do anything for impedance rise at higher frequencies due to voice coil inductance. Here again, I don't know how inductive Grados are.

Not very, but you're correct basically.

Quote:

But my point is that having an amp with a low output impedance is a good thing, and eliminates a potential source of sonic degradation regardless of what headphones you use. -Ti -Ti

Generally speaking that's true, however there's still that 120 Ohm IEC standard. If your phones are designed with a source resistance of 120 ohms in mind it's a low damping factor that will give you sonic degradation.