[jcx]

why the limitation to series R?

the HissBuster thread shows a divider on the amp output which lowers drive Z - the higher the attenuation you need the lower you can make the output Z of the divider

http://www.head-fi.org/forums/f4/his...8/#post2387308

of course the real High End option would be a (custom - nobody currently makes them for this app) output step-down transformer

 

[poo]

Quote:

Originally Posted by EliasGwinn /img/forum/go_quote.gif I apologize if I mis-directed the conversation. I simply wanted to explain the science behind this type of configuration.

Not at all Elias, I was very happy to see your involvement in this thread, and it's always a great thing to have the technical explanation you provide! I would hope for nothing less

Quote:

Originally Posted by EliasGwinn /img/forum/go_quote.gif Poo, are you using the Airport Express to drive the DAC1? If so, just attenuate the volume in iTunes. The digital volume control in iTunes performs very well, and, although digital volume control will reduce dynamic range, I think it is the least-problematic (and cheapest) way to solve your problem.

I am using the AE in this case. I was trying to avoid using iTunes attenuation due to the distortion it imposes, but also because I use the same machine in another setup and am sure to forget I have set iTunes quieter and spend hours trying to figure out "what's wrong!" I agree though that of all the potential 'fixes' this is the easiest and cheapest solution by far!

Thanks to everyone for their input, always great to have a few ideas to play with to solve a problem!

 

[marozie]

Quote:

Originally Posted by jcx /img/forum/go_quote.gif the HissBuster thread shows a divider on the amp output which lowers drive Z - the higher the attenuation you need the lower you can make the output Z of the divider

I'd just like to point out, going back to what I said a while ago, this HissBuster circuit is the exact same circuit used to regulate the gain of an amp. So, again, I fail to see the difference between using resistors to attenuate the signal just before the output of the amp vs. using them just after it. Just sayin'.

 

[Punnisher]

.

 

[EliasGwinn]

Quote:

Originally Posted by marozie /img/forum/go_quote.gif I'd just like to point out, going back to what I said a while ago, this HissBuster circuit is the exact same circuit used to regulate the gain of an amp. So, again, I fail to see the difference between using resistors to attenuate the signal just before the output of the amp vs. using them just after it. Just sayin'.

If you put the resistors between the amp and the headphones, the amp will have less control over the physical motion of the drivers in the headphones.

See this article (Damping factor - Wikipedia, the free encyclopedia).

All the best,
Elias

 

[jcx]

the HissBuster output divider has lower output Z, better damping than just a series R

its not clear that most headphones rely on high damping ratio - there is a IEC/DIN? standard for 120 Ohm series R in headphone amp outputs - although probably not generally used – 5-100 Ohms are often used to “isolate” SS amp outputs from the destabilizing effects of cable C, tube amps can have lots higher output Z

so, unlike the multidriver loudspeaker amplifier world, headphone amps don't generally even try for super low output Z, High damping factor

if someone would post the impedance curve of a few of these multi-driver iems we could see if there is enough impedance variation to matter and with how much output Z - the measurement should be provided by the manufacturer's but with a soundcard and series R you could do a good enough job


someone who has spent ~$1K on high end iem should ask the manufacturer for the info - a impedance/frequency curve and "official" recommendation on amplifier output Z - post the result so we can kill the aimless speculation


if you can lower the gain of the amp circuit by altering the feedback network resistors then do that 1st, if you've got a commercial portable amp/player most are hard to open and few people want to mess with smt rework given how closely packed their insides are

even with just unity gain amplifier after the DAC the signal level can be too high for sensitive iem - output attenuation can deliver more S/N by allowing the DAC/amp to run closer to full scale and then attenuating both DAC/amp noise and amplified signal - attenuating between the source and amp means that the amp is still amplifying its own input noise and adding it to the lower signal level

the "best" in principle output attenuation is a transformer that preserves the full S/N and power capability of the amp while translating to the lower signal level - provided the transformer is designed for the purpose with low winding R - but quality transforemrs for low distortion over the full audio range are expensive

 

[rds]

Quote:

Originally Posted by EliasGwinn /img/forum/go_quote.gif Subjectively speaking, you may be right. That is, a lower damping ratio may give the headphones a certain coloration that one may enjoy. But objectively speaking, the lower the damping ratio, the less accurate the headphones are.

The choice of damping ratio is a little more complicated than that. Generally you want critical damping. Whether a headphone was designed for critical damping when plugged into a zero ohm amp is really hard to say unless the designer tells you. Also how much an extra 100 ohms is going to affect the damping ratio of a pair of headphones is also hard to say. In most cases the affect is probably negligible since (as far as I know) most headphones are close to a purely resistive load in the first place.

 

[EliasGwinn]

Quote:

Originally Posted by jcx /img/forum/go_quote.gif the "best" in principle output attenuation is a transformer that preserves the full S/N and power capability of the amp while translating to the lower signal level - provided the transformer is designed for the purpose with low winding R - but quality transforemrs for low distortion over the full audio range are expensive

Almost every transformer will add significant distortion, especially in the low-frequency phase and amplitude domains.

Best,
Elias

 

[EliasGwinn]

Quote:

Originally Posted by rds /img/forum/go_quote.gif Also how much an extra 100 ohms is going to affect the damping ratio of a pair of headphones is also hard to say.

An extra 100 ohms will decrease the damping ratio by:

[(Z_load / Z_source) - (Z_load / [Z_source + 100])]

For a set of headphones with Z_load = 75 ohms, and the HPA2, which has a Z_source = 0.1, this would change the damping ratio from 750 to 0.75. In other words, it would reduce it by a factor of 1000.

Best,
Elias

 

[marozie]

Elias, what the hell is your profile pic???

 

[rds]

Quote:

Originally Posted by EliasGwinn /img/forum/go_quote.gif An extra 100 ohms will decrease the damping ratio by: [(Z_load / Z_source) - (Z_load / [Z_source + 100])] For a set of headphones with Z_load = 75 ohms, and the HPA2, which has a Z_source = 0.1, this would change the damping ratio from 750 to 0.75. In other words, it would reduce it by a factor of 1000. Best, Elias

Admittedly I'm pretty far from an expert in control systems, but don't you need to know the transfer function of your system before you can calculate the damping ratio? Also if you're assuming a purely resistive system how would it even have a damping ratio?

I've always calculated damping ratio as follows:
given T(s) = A/(s^2 + qs + b), then q = 2Cw, where C is the damping ratio and w is the natural frequency.

 

[EliasGwinn]

Quote:

Originally Posted by marozie /img/forum/go_quote.gif Elias, what the hell is your profile pic???

Thats my dog, Frank Dogg!

I love that it is so ambiguous...yet, once you know, its so obvious!

-Elias

 

[EliasGwinn]

Quote:

Originally Posted by rds /img/forum/go_quote.gif Admittedly I'm pretty far from an expert in control systems, but don't you need to know the transfer function of your system before you can calculate the damping ratio? Also if you're assuming a purely resistive system how would it even have a damping ratio? I've always calculated damping ratio as follows: given T(s) = A/(s^2 + qs + b), then q = 2Cw, where C is the damping ratio and w is the natural frequency.

Well, first of all, I apologize for using incorrect nomenclature... I was referring to 'Damping Factor'...which is a ratio, but 'Damping Ratio' is used to describe something similar but not loudspeakers.

Anyway, here is the wiki article on 'Damping Factor' :
Damping factor - Wikipedia, the free encyclopedia

By the way, Z is reactive and resistive, as physical loudspeakers are reactive and resistive, including IEM loudspeakers.

Best,
Elias