The Objective2 (O2) Headphone Amp DIY Project

[generalguy]

Don't want to discourage your DIY skills but if you are new to electronics stick with the original RS design as 5V from USB involves SMPS ckts using SMD components.

I'd use it as a learning opportunity to get more familiar with SMD soldering--I'm more of a software guy though (do fpga's count?).

 

[6L6]

65dB gain with your HOMC cartridge is severe overkill... 🙂 🙂 🙂

 

[McQuaide]

65dB gain with your HOMC cartridge is severe overkill... 🙂 🙂 🙂

But it works so well with the DCB1 and the F5! No overloading or distortion with that rig.

 

[billyk]

But it works so well with the DCB1 and the F5! No overloading or distortion with that rig.

But that's just it. The DCB1 has no gain and the F5 under normal conditions could use a bit, so that combination would work nicely. The O2 that already has a good bit shows how much the phono pre is putting out. (too much!)

 

[McQuaide]

But that's just it. The DCB1 has no gain and the F5 under normal conditions could use a bit, so that combination would work nicely. The O2 that already has a good bit shows how much the phono pre is putting out. (too much!)

So is there a good way to attenuate it so I can have the best of both worlds?😀

 

[McQuaide]

Idea: Put the DCB1 in line between the Pearl and the O2?

I don't have the amp wired with a pre-out, but that could be done. Unfortunately it would mean a complete tear-down to drill for more jacks, but it may be worth it.

 

[6L6]

Nah. Put the B1-F5 in your second system...

The solution is simple -- It's time to build a new amp!! 😀 😀 😀

BA-2 or-3, complimentary output.

 

[billyk]

So is there a good way to attenuate it so I can have the best of both worlds?😀

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).

Hopefully a setting of 0 gain can get you close, looks easy enough to try...

 

[McQuaide]

That worked! I snipped all four gain resistors, and the distortion is gone. I'm still running it on the low gain setting with the knob around 10-11 o'clock. Thanks for the tip.

 

[billyk]

That worked! I snipped all four gain resistors, and the distortion is gone. I'm still running it on the low gain setting with the knob around 10-11 o'clock. Thanks for the tip.

That's good news!! 10-11 on the volume may not be ideal but no distortion is great!! Glad to hear it worked for you.

 

[McQuaide]

What volume setting would be ideal? Even with the Pearl 2 at 55 db, the O2 had way too much gain on the low setting.

 

[billyk]

Sorry man, did not mean to imply... As a matter of fact I just looked at my O2 and the knob is exactly at 11:00! Just my subjective opinion based on nothing... The ideal volume setting is where you can listen to it and it sounds good.

 

[McQuaide]

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!

 

[billyk]

I wasn't getting testy with you, just curious.

No worries, just did not want you to think I was saying that it was not right in any way. I am truly glad it worked out and I was able to be of some help. Doesn't often happen around here, usually it's the other way around, folks are helping me out!!!

 

[McQuaide]

Doesn't often happen around here, usually it's the other way around, folks are helping me out!!!

Same here. I doesn't happen often, but when I actually have something to offer based on past experience it feels good!

 

[grandpatzer]

Seems like I have only Mono sound(both earphones have same sound).

Any suggestions?

 

[McQuaide]

65dB gain with your HOMC cartridge is severe overkill... 🙂 🙂 🙂

Well it was still clipping the O2 with the gain resistors removed, so I put the Pearl 2 gain somewhere in between 55 and 65db by using 680r resistors in R14. All good now.

 

[availlyrics]

Seems like I have only Mono sound(both earphones have same sound).

Any suggestions?

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.

 

[bircoe]

I've built one o2 and love it, I've got a PCB and enough components to build another but want to configure it for preamp duties instead, has anyone got any tips on converting an o2 for preamp duties? I'd imagine it's just resistor changes but not sure where to start?

 

[digitfan]

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)