I decided to bite the bullet and build an O2 headphone amp.
I don't currently have any headphones and I figured i'd better have an amp if i'm going to go auditioning headphones.
6L6 convinced me to build an O2 and i'm glad I did, it really seems like a solid product and I whipped it out in about 10 hours, chassis build and all.
I picked up all Dale RN50 resistors where available and since I knew I would be building my own chassis I chose to buy Panasonic FM caps which are a bit taller than the specc'd ones in the BOM.
No real reason other than I don't trust Lelon caps and the Panny FM caps are very low ESR and much higher ripple rating.
I will be running on batteries only and I picked up some 9V Li-Ion 600mAh and a charger for them.
I did build the complete version in the event I do want to go to NiMH and a wall wart but for now i'm happy using an external charger as long as I can get 20 hours or so of run time form the Li-Ion batteries.
I should be able to get about a weeks worth of evening listening for use while i'm out of town for work.
This little guy worked without a hitch from the first power up and i'm really excited to go down to the Headroom and audition my short list of headphones again.
I don't currently have any headphones and I figured i'd better have an amp if i'm going to go auditioning headphones.
6L6 convinced me to build an O2 and i'm glad I did, it really seems like a solid product and I whipped it out in about 10 hours, chassis build and all.
I picked up all Dale RN50 resistors where available and since I knew I would be building my own chassis I chose to buy Panasonic FM caps which are a bit taller than the specc'd ones in the BOM.
No real reason other than I don't trust Lelon caps and the Panny FM caps are very low ESR and much higher ripple rating.
I will be running on batteries only and I picked up some 9V Li-Ion 600mAh and a charger for them.
I did build the complete version in the event I do want to go to NiMH and a wall wart but for now i'm happy using an external charger as long as I can get 20 hours or so of run time form the Li-Ion batteries.
I should be able to get about a weeks worth of evening listening for use while i'm out of town for work.
This little guy worked without a hitch from the first power up and i'm really excited to go down to the Headroom and audition my short list of headphones again.
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Sounds like you should be good for >=25 hours of battery life. I've been burning through the ~10 hours of a FiiO E11 in 2 or 3 days lately, but then again I'm mostly in bed right now with two hurting feet after a bit of an accident while on vacation.
Which gains did you end up picking? Note that you could increase maximum voltage swing at higher gains (and reduce distortion at the same time) by easing up on feedback resistor values a bit (plus a bit of an input series resistor tweak if needed).
Which gains did you end up picking? Note that you could increase maximum voltage swing at higher gains (and reduce distortion at the same time) by easing up on feedback resistor values a bit (plus a bit of an input series resistor tweak if needed).
Note that with a nominal +/-9 V supply (have you measured actual voltages?), the gain stage may clip when subjected to nominal CD player 0 dBFS level (2 Vrms) even at 2.5x gain (and definitely would do so at 6.5x). Just something to be aware of, and one of the few major gotchas of this design - this is the price you pay for the extremely low noise. If you have a variable-output source, not a problem.
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The purpose for this HPA for me is portable use only.
It will never be hooked up to a Redbook Compliant piece of equipment.
I will be driven from my Android or my iPod exclusively.
There is an outside chance I may drive this from my phono stage so I can listen to my records while the kids are in bed.
I originally thought about gain settings of 1.5 & 4.5 since all of the headphones on my short list were driven to very loud levels with 3/4 volume on my iPod.
I will revisit the gain resistors at some point in the future if I feel there is a need.
But, for right now i'm just excited to go audition headphones with this great little amp.
It will never be hooked up to a Redbook Compliant piece of equipment.
I will be driven from my Android or my iPod exclusively.
There is an outside chance I may drive this from my phono stage so I can listen to my records while the kids are in bed.
I originally thought about gain settings of 1.5 & 4.5 since all of the headphones on my short list were driven to very loud levels with 3/4 volume on my iPod.
I will revisit the gain resistors at some point in the future if I feel there is a need.
But, for right now i'm just excited to go audition headphones with this great little amp.
Momentums are supposed to be decent but hardly spectacular (something that you could say about many of Sennheiser's closed efforts, actually), so no worries about the amp being the limit for the time being. 
Judging from measurements, they seem to have a rather... polite high end. I can understand your decision though, as right now Headroom does not seem to be stocking any HD558/598 (huh?!), K242HD or HD239, and options in that price range aren't so plentiful. Did you try, say, the Shure SRH840 (or the 940), or Beyer's COP? Or heck, one of the cheap-n-cheerful Superlux cans?
BTW, a modern-day iPod should be quite adequate for running Momentums "barefoot" (while low in impedance, they are quite sensitive), so you may want to do some comparisons there. Might save you some "juice".
My full-size headphone "workhorses" here are HD580s, the very similar forerunner to the HD600, and the super comfy HD590s, both long discontinued. Once you get to something in the HD600 / DT880 / K701 league (often referred to as the "Big Three" over here) that suits your tastes, the upgrade bug may not bite again for a loooooong time, possibly never. While I do not think that it is impossible to make a closed headphone of truly equivalent quality, I have not yet encountered any (and a Stax SR-4070 would seem a tad too heavy on the budget).
Judging from measurements, they seem to have a rather... polite high end. I can understand your decision though, as right now Headroom does not seem to be stocking any HD558/598 (huh?!), K242HD or HD239, and options in that price range aren't so plentiful. Did you try, say, the Shure SRH840 (or the 940), or Beyer's COP? Or heck, one of the cheap-n-cheerful Superlux cans?BTW, a modern-day iPod should be quite adequate for running Momentums "barefoot" (while low in impedance, they are quite sensitive), so you may want to do some comparisons there. Might save you some "juice".
My full-size headphone "workhorses" here are HD580s, the very similar forerunner to the HD600, and the super comfy HD590s, both long discontinued. Once you get to something in the HD600 / DT880 / K701 league (often referred to as the "Big Three" over here) that suits your tastes, the upgrade bug may not bite again for a loooooong time, possibly never. While I do not think that it is impossible to make a closed headphone of truly equivalent quality, I have not yet encountered any (and a Stax SR-4070 would seem a tad too heavy on the budget).
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