Selling my custom inverting O2 amp described here:
Details and pictures.
https://www.audiocircle.com/index.php?topic=140425.msg1496995#msg1496995Benefits and additions vs stock O2:
* power jack in the back
* voltage regulators heat sinked to the rear panel, which means that both front and rear panels have to be drilled (which is why NwAvGuy put his jack in front, to reduce panel costs).
* toggle power switch rather than slide to help the two power rails disconnect at the same time
* Includes a headphone relay for zero turn-on and turn-off thumps. NwAvGuy's power management circuit was trying to do several things at once, including thump elimination, and IMHO it didn't do any of them especially well.
* power LED on each rail to help diagnose when a power rail is lost. This comes from helping a ton of people fix their O2's over on another forum over the last few years. One of the first things I often have people do is check their power rails.
* Input select switch and separate PCB holes for switching between the front panel 3.5mm jack and the ODAC board.
* 1/4" output jack, along with the 3.5mm jack
* heat sinked 250mA buffer chips. There is a known problem with the O2 where 16 ohm headphones that are low sensitivity, combined with high volume and music with high peaks can overheat the NJM4556A chips in the O2 and cause them to crack in half. The O2 has 140mA maximum, this one has 250mA.
* +/-15.3Vdc power rails vs. the +/-11.6Vdc in the O2 (12V minus the Shottky diodes minus the PM mosfet voltage drop). That is 3 extra volts of voltage swing for high impedance headphones that need more voltage swing. Higher chip voltage also helps reduce chip THD by a tiny amount.
* Low-dropout low(er) noise adjustable voltage regulators vs. the fixed 12V vregs in NwAvGuys O2 headamp. Note - these are only lower noise if the set resistors are noise bypassed with a capacitor, per the data sheets, which I've done. The very low noise figure given for both chips (LT3015 and LT3080) in the data sheets is actually for the minimum adjustable voltage, about 1.8V or so, and that noise "gains up" with increasing voltage unless bypassed. This is something that isn't terribly clear from the datasheets (the given noise is for the lowest possible output voltage) but I've confirmed it with the LT applications engineering folks.
* has power rail clamp diodes so a lost power supply on one rail won't cause a high reverse voltage on the other, something that has wiped out U6 for folks in the O2 headamp on occasion.
* includes a clipping indicator circuit and LED so you know when the input stage (or output stage, I sense both) are being overdriven. That is another one of the things is asked/suggested to NwAvGuy a couple of days after he released the O2. His reply was simply no space left on the PC board to add any more parts.
* 5K pot vs the 10K in the O2, combined with a lower 24.9K ground return resistor (5x the pot to prevent loading) vs. 40.2K in the O2, to lower Johnson noise. A 5K works here because the LME49880 gain chip is THD+N specified down to a 600 ohm load. The NJM2068 in the O2 is only good for a 2K load. The coupling cap here is bumped up to 4.7uf vs. 2.2uF in the O2 to keep the frequency response the same with the lower value of ground return resistors.
* But... no batteries! This one isn't "transportable". And this one uses the B5-080 case which is about 1 inch wider than the B2-080 used with the O2 headamp. That allowed more front panel room for thngs. No batteries means no power management circuit, as in NwAvGuy's O2, I actually had one designed an on the board at one point, back when I was considering having batteries, but it wound up as all surface mount to fit. Without the PM circuit 95% of this board is now through-hole parts, making it easier to solder, like the O2.
Selling for $150, cost me $200 to build + wallwort....which is TRIAD 16VAC - 1000ma
Shipping $10...so total sel price is $160
One of a kind. works wonderfully. This amp sounds like a "tube" version of the O2.
Selling just because I have tooooo many amps !!
Alex
