The Objective2 (O2) Headphone Amp DIY Project

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billyk

billyk

Member
Joined 2005
McQuaide said:
So is there a good way to attenuate it so I can have the best of both worlds?:D
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I am fuzzy on the details, but the O2 has a gain switch, I would think that you may be able to mod it in such a way that one of the positions would be suitable for the phone pre and the other for CD, etc.
I use my O2 every day, but would need to look it up regarding just what values to use to set the gain.

Edit: From NavGuys' blog:
THE GAIN RESISTORS: Before you solder in the four gain resistors by the gain switch, you might want to consider different gains than the approximately 2.5X and 6.5X default values. You want just enough gain so typical music plays loudly enough with your headphones and source and not much more. Extra gain means using less of the volume control’s range, more noise, more distortion, and makes accidental headphone damage more likely. Here’s what you need to know about calculating gain:

Lower Is Safer - Lower gain settings make the amp less likely to damage headphones by limiting the maximum output voltage to only approximately what’s needed.
Lower is Cleaner – As shown in the first article, there’s a slight increase in distortion, especially at high frequencies at higher gain settings. Lower gains also result in lower noise.
Resistor Values – The O2’s gain (for one channel) is:
Low Gain Ratio = 1 + R16/R17
High Gain Ratio = 1 + R16/R19
Voltage Gain in dB = 20 * Log(Gain Ratio)
Example – The standard amp has R16=1500 and R17= 1K so 1 + 1500/1000 = 2.5X. And 20*Log(2.5) = ~ 8 dB.
Feedback Impedance – For many reasons it’s generally best to leave R16 and R22 at 1500 ohms unless U1 is replaced with a weak op amp that can’t handle a 1.5K load.
See The BOM - Gain resistors are pre-calculated in the BOM parts list for gains of 2X – 12X. For 1X gain just leave out the gain resistors (or clip one end if they’re already installed).
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Hopefully a setting of 0 gain can get you close, looks easy enough to try...
 
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M

McQuaide

Member
Joined 2011
I wasn't getting testy with you, just curious. It's true that you shouldn't "throw away gain" as Nelson says, by turning the volume way down in subsequent stages. At 65db with a cartridge that puts out 2.5mv, I know I'm overdoing it. But it sure sounds good. I'm listening to some Brubeck right now with the F5 volume control at 12 o'clock.

Thanks again for the great advice and for actually reading the documentation on the O2 gain, which I obviously didn't do!
 
availlyrics

availlyrics

Member
Joined 2011
grandpatzer said:
Seems like I have only Mono sound(both earphones have same sound).

Any suggestions?
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Did you build it? Do you have a multimeter? If yes then use it to -

1. chk. continuity(amp OFF, batt. removed) of your 3.5mm connector(end to end) ,
2. between 3.5mm connector & input socket's pins protruding underneath the PCB.
3. Same goes for 3.5mm connector & output socket's pins. Chk for any "solder bridges" at vol. pot which might have shorted up L & R ch.

In case you have "phasey mono" o/p sound or don't have a multimeter then try different 3.5mm connector cable and/or h/p combination. There might be contact problem between 3.5mm connector & common gnd of the socket. Avoid using 1/4" to 3.5mm converter.
 
D

digitfan

Member
Joined 2006
I'm sure you'll get plenty of suggestions - I've just taken the O2 headphone output straight into the input of an integrated amp, with no circuit modifications to the O2 at all. I figured that the very low output impedance of the O2 would mean that it wouldn't be affected by any power amp input.

It's not a particularly high quality power amp (an old Videologic Sirocco), and there are now two volume controls (I just turn the Sirocco one up to max), but I've been more than happy with it for the last year or so.

I built the O2 into a larger case that I described earlier in this thread (#2647)
 
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