[Haidar]

Is it possible to design a headphone amp with continuously adjustable output impedance?

 

[goodvibes]

Easy enough. Put a variable resistor or L-pad in series with the output.  

 

[castleofargh]

 
done! ^_^

 

[SilverEars]

This is what i need when I fly with the old planes, so I don't damage my hearing from PA announcements.  I wonder what the quality difference between something like above and one of those Alps pots used on portable amps?

 

[castleofargh]

I think channel balance precision is a good start.

 

[SilverEars]

  I think channel balance precision is a good start.

My Alps were pretty bad on my O2, but that was with 119dB sensitive iems which is understandable with the amound of gain that was with it.  Even with unity those iems has imbalance at 45 degree turn.
 
I asked JDS labs and they told me you get lucky with balanced pots.  Luck of the draw.

 

[cjl]

  My Alps were pretty bad on my O2, but that was with 119dB sensitive iems which is understandable with the amound of gain that was with it.  Even with unity those iems has imbalance at 45 degree turn.
 
I asked JDS labs and they told me you get lucky with balanced pots.  Luck of the draw.

If you set your PC output to 24 bit, you should be able to turn down the volume on the PC by quite a lot (without hurting the music quality), allowing you to use much more of the range of your volume pot (even with sensitive IEMs). That should fix the problem.

 

[SilverEars]

  If you set your PC output to 24 bit, you should be able to turn down the volume on the PC by quite a lot (without hurting the music quality), allowing you to use much more of the range of your volume pot (even with sensitive IEMs). That should fix the problem.

Right, I've run into this concept just recently, but I'm not completely understood on the concept.  Anybody have good literature that explains it well?  I'm wondering if that impedance adjuster can be used as variable impedance adapter if you are looking to play around with various impedance at the output of the amp.  It should be easy to measure the resistance to know how much you are adding to the output.  

 

[castleofargh]

Anybody have good literature that explains it well?

damn I fail 2 out of 2 requierements.
 
still I'll try. when you lower the volume on foobar, you change all the values of the song by a number. say you lower by 40db, the sample that was at 0db is sent to the dac as being -40db. the sample that was -60db is sent as being -100db. obviously if you're in 16bit, that can't happen(minimum being -96db) so that value simply cannot be used by the dac.
 
but if you send the signal as 24bit, then -100db is a value just like another and the dac knows what to do with it so no data is lost.  and as 8bits allow for 6*8=48db of dynamic, you can lower the foobar volume from as much with no loss.

 

[cjl]

  Right, I've run into this concept just recently, but I'm not completely understood on the concept.  Anybody have good literature that explains it well?  I'm wondering if that impedance adjuster can be used as variable impedance adapter if you are looking to play around with various impedance at the output of the amp.  It should be easy to measure the resistance to know how much you are adding to the output.  

If you're playing 16 bit audio but have your soundcard set to 24 bit, it means you're basically padding the lower 8 bits of the 24 bit word with zeros. This means that you can turn down the volume by 8 bits worth (48dB) before the reduction in volume starts to impact the sound quality. In reality, no 24 bit device has 24 real bits of resolution (due to noise), but even if it has 20 bits, that still allows you to lower the volume by 24dB digitally without impacting sound quality (and 24dB is quite a bit). This means that if you're running sensitive IEMs with an O2/ODAC, you'd probably get the best results by turning the gain to the low setting, turning the volume knob up to somewhere around the 10/11 o-clock position, and setting the software volume control (preferably the OS volume control) to a level that provides a comfortable output.
 
With my Denon D5ks and a stock O2/ODAC, I usually end up with the Windows volume control around 75%, and with my Senn IE80s, I usually have it down around 15-30% or so, depending on the level of the music/sounds I'm playing (and I have the volume knob on the O2 anywhere from 9 o-clock to 1 o-clock on the dial). This gives excellent quality without the channel imbalance issues you get by running the volume knob right at the bottom of its range. It also makes it so you actually have a usable amount of adjustment on the volume knob, instead of fiddling with a tiny range right near zero.

 

[castleofargh]

I set foobar around -20db, it gets me well out of the my O2's imbalance for most usages. and OFC I still use some more if needed as I fine tune the volume level only with foobar and don't touch the O2.
 
oh and ofc -48db is the theory cjl is right about it not being factual. but even 3 more bits give around 18db of margin so I feel pretty safe with -20db. plus it's not like a lot of our albums actually have music recorded at -80 or -90db. so even eating up the bottom of the 16bit should be ok with most music.
in any case nwavguy recommended using 24bit from the start as output value to feed the odac. it costs nothing, and can help so why not?

 

[Haidar]

  Easy enough. Put a variable resistor or L-pad in series with the output.  

Given an output transformer with multiple taps, what is the difference between using its lowest impedance tap followed by a resistor (to increase output impedance to some number of ohms)
vs. using another tap whose output impedance is that same number of ohms?

 

[ab initio]

Given an output transformer with multiple taps, what is the difference between using its lowest impedance tap followed by a resistor (to increase output impedance to some number of ohms)
vs. using another tap whose output impedance is that same number of ohms?

The voltage swing delivered to the speaker will be completely different

Cheers

 

[SilverEars]

Hi guys, I have a question for you.  As you know, the Astell&Kern series have a volume adjustment knob so it has an attenuator.  My question is, how does it attenuate the signal and keep the output impedance the same?  Is there a buffer after to lower it?  I thought it would be a pot for adjusting the gain.  What do you guys think?  Doesn't the pot change the resistance value to change the gain?  Or is the volume just an attenuation of the line signal? Either way, the output impedance has to be dropped or kept constant, how is that done?
 

 

 

[ClieOS]

  Hi guys, I have a question for you.  As you know, the Astell&Kern series have a volume adjustment knob so it has an attenuator.  My question is, how does it attenuate the signal and keep the output impedance the same?  Is there a buffer after to lower it?  I thought it would be a pot for adjusting the gain.  What do you guys think?  Doesn't the pot change the resistance value to change the gain?  Or is the volume just an attenuation of the line signal? Either way, the output impedance has to be dropped or kept constant, how is that done?
 

 
Because conventionally, volume pot is implemented after the line-out of the DAC section and before the amp section, thus it will not 'see' the load directly and therefore has no influence on output impedance (*which is determined by the amp section).
 
However, A&K's implementation might not be conventional. It could very well be a pot feeding an ADC, which then send the signal to the SoC to adjust the volume digitally (as I understand that's how AK100 is, don't know about the other models). But again, since the pot isn't place after the amp section, it still doesn't 'see' the load directly and therefore still doesn't affect the OI.