High linearty low-power BJTs and Class A headphone amp

[mutter]

This link is in Korean, but the schematic is pretty clear:

http://www.headphoneamp.co.kr/bbs/zboard.php?id=diy_sijosae&page=1&sn1=&divpage=1&sn=off&ss=on&sc=on&keyword=zen&select_arrange=headnum&desc=asc&no=166

 

[AndrewT]

my new Sennheiser HD465 have just been delivered.
They are 32ohm and specified as 1Vac max for 110dB max SPL.

That is equivalent to 30mAac into 32r0 load.

I would ensure that a headphone amp to drive these can do at least 3Vac AND 100mApk

 

[Fanuc]

simple and very linear is just using iPod's output for 1V, 30mA

"High linearity" isn't going to happen with a single device

if you expect a different sound from a "simple" circuit it will most likely be because the amplification is less accurate/linear


the best linearity from a single device would come from heavy bias so you use a smaller fraction of its nonlinear response curve

then you could argue over whether degeneration helps or if more low order distortion is less noticeable than the feedback introduced higher order low level distortion components

which also depends on the music you listen to - its commonly pointed out that "audiophile" demos often use "simple" music such that the IMD products with the higher levels of low order distortion doesn't muddy the sound as it would with a complex symphonic work

Hi John,

Your talking about multi modals to replace a single op amp. VFB op amps with CFB op amps to extend the AVL and so on hence loop gain, psrr and THD to higher frequencys etc etc.

Do you have experience replacing a high bandwidth VFB op amp with this compound arrangement. ?

The theory is fine (and yours) and all that, but I am not seeing practical solutions. I suspect IC houses could do this a lot better than by us guys trying discrete op amps.

Kevin

 

[Bigun]

my new Sennheiser HD465 have just been delivered.
They are 32ohm and specified as 1Vac max for 110dB max SPL.

That is equivalent to 30mAac into 32r0 load.

I would ensure that a headphone amp to drive these can do at least 3Vac AND 100mApk

If you don't need to go all the way to full power and can live with the voltage levels output from the source (CD player) then all you need is a current buffer for the headphones ?

Then, if the original poster wants to try a single device approach he can build an emitter follower as a current buffer. Of course this still loads the source down. With two devices though, the world opens up (Darlington, CFP....) ????

 

[jcx]

Hi John,

Your talking about multi modals to replace a single op amp. VFB op amps with CFB op amps to extend the AVL and so on hence loop gain, psrr and THD to higher frequencys etc etc.

Do you have experience replacing a high bandwidth VFB op amp with this compound arrangement. ?

The theory is fine (and yours) and all that, but I am not seeing practical solutions. I suspect IC houses could do this a lot better than by us guys trying discrete op amps.

Kevin

crossing threads?

thermal feedback/cross coupling on a single chip and common power supply pin impedance coupling can limit the perfromance of dual op amps

if you want to optimize input and output characteristics for the application it is easier to choose different op amps having better specs for each function and combine them in a composite amplifier circuit with improved performance over either of the component amps

http://waltjung.org/PDFs/ADI_2002_Seminar_Ch6_Audio_Drivers_I.pdf


some of my composite amp sims:

http://www.diyaudio.com/forums/showthread.php?postid=512806#post512806

with Walt Jung and Jerald Graeme's books/articles you can design composite/multiloop amps which better the performance of single op amp implementations - whether the improvement in predicted or measurable specs is required over the better single chip implementations depends on the application and is for the user to decide

I have used Graeme's composite op amp ideas in strain gage amps where I work in products that have been shipping for a decade


I have prototyped several composite amps of my design for headphone amplifier use and tried measuring output distortion with indirect methods that should have given a ~160dB sensitivity
I wasn't able to see any 2nd order distortion from an IMD measurement after replacing npo caps with polystyrene - because I did see ~-110dB distortion with the npo caps


I have built a fairly complex headphone amp with composite topology and Class A >400mA output entirely with op amps

http://www.diyaudio.com/forums/showthread.php?postid=1392458#post1392458

more pics:

http://www.head-fi.org/forums/f6/ad8397-class-188758/#post2263321

so I would say I have "reduced to practice" these ideas - however "practicality" in audiophile land is rather harder to establish


its even hard to "sell" the idea of using a single transistor buffer inside an op amp feedback loop to reduce Curl's "deadly 7th" by 20X:
http://www.diyaudio.com/forums/showthread.php?postid=1813250#post1813250