The Objective2 (O2) Headphone Amp DIY Project

[Willakan]

Originally Posted by DJNUBZ

Is there any reason I couldn't short C13 and C14 to remove all caps from the signal path? I know that I would have to be concerned with DC offset but is there any other reason I couldn't do this? Has anyone else tried this and noticed any difference in anything.

Is there any reason you would want to do this?

[DJNUBZ]

Originally Posted by Willakan

Is there any reason you would want to do this?

yes there is a reason I would want to do this but not most people.

[agdr]

Originally Posted by DJNUBZ

Is there any reason I couldn't short C13 and C14 to remove all caps from the signal path? .

That would work, but then you have a DC coupled amp with all the inherent fun - mainly the DC offset issue (possibility of frying the headphones). An alternative modification suggestion would be to actually add another C13 and C14 sized cap in series with R3 and R7 on the input to the amp, to keep any input DC offset voltage (from the source) from clipping the first stage. Like C13 and C14 those should make an inaudible difference on the low end, but prevent clipping.

Longer answer... if you do try shorting C13 and C14, some things to keep in mind:

* Before connecting any headphones to the O2, unplug the input jack, so it shorts the inputs to ground, and measure the DC offset voltage on both channels of the O2. With those caps shorted the DC offset of the O2 first stage, caused by U1 input bias current through the input resistors, will reflect to the second stage and may add or subtract from the nominal 3mV. 5mV or less offset voltage on each O2 output would probably be OK.
* The gain switch setting may affect the O2 output offset voltage now too, since any DC offset voltage in the first stage also gets multiplied by the stage gain, then added/subtracted from the output stage offset voltage. Try measuring the O2 output offset voltage on both channels with the gain switch in both positions.
* With the headphones still unconnected, plug in the source and re-measure the O2 DC offset voltage. Any DC on the source will now reflect back to the output.
* The O2 offset voltage may now vary a bit with the volume control. The two caps kept the second stage input bias current out of the pot (RocketScientist says that also causes a rustling sound when turned, too, which you now may get). Now the pot is in parallel (DC circuit model) with R12 and R13, so the voltage drop caused by the U3 & U4 op amp input bias current will vary slightly with pot position.
* If the input stage starts clipping against one rail due to too large of an input signal, then even if there is no DC on the input signal a net DC offset voltage (additional) may be generated by the first stage due to the asymmetric signal and fed to the second stage. Try to make sure your input signal is going to be well within the O2 input specs.

In summary.... not recommended to short C13 and C14. They are doing many good things and really should have zero effect on the sound. For some further proof that they really don't make any audible difference in the signal, buy 8 more of them or so. Solder 4 in parallel to give a total of 8.8uF, then attach leads and solder that right over C13 for a total of 11uF, 5 times the capacitance which would drop the 3bB point of the highpass filter being created down to just a few Hz. Then do the same on the other channel with C14 and give it a listen. You could even do an A/B test by using a DPST switch to run one lead to C13 and C14 through, switching that extra 8.8uF in parallel on each channel in and out while you listen.

Hope this helps!

[DJNUBZ]

That helps a lot. I know that they are doing more good then bad and should be inaudible but every piece of equipment I have heard, without caps in the signal path, I have noticed a difference. I wasn't sure if DC offset would be so terrible it wasn't worth trying but I may play around with it. I just finished my first amp and love it so far, now it is time to build a second one and play around a bit.

As a tinkerer and skeptic I am always curious if things couldn't be made even better, even if it is more for academic purposes and not useable real-world improvement. Because of this I am curious if there are improvements that could me made to this amp. I may try some other opamps with the only goal being to improve the slew rate (or make it worse) so I know how that affects the sound of devices. I fully admit there may be nothing to gain by this but knowledge.

[ethanolson]

The O2 mods thread has great improvements. On my list are DC coupling caps on the input and DC offset correction (tricky but doable). The reason I'm not interested in changing out the OpAmps is because the other ones I've found that look like they could be a drop-in without having changes to the feedback circuit also have a nasty ringing in positive feedback designs... if I remember correctly. Anyway, I disqualified everything out there for this circuit, so if I want something different, I'm not even going to be using the O2 board to do it. That's how integrated this is. There is room for improvements but they come with cost and complexity.

I do hope the ODA has the following improvements:
-three or more gain positions, one potentially a voltage divider circuit for <1 gain
-preamp output option
-DC coupling caps
-on/off relays for pop-free transients
-tunable DC offset correction (because ultra-sensitive IEMs are very affected)
-choice of three input sources constantly connected

[necplusultra]

greetings,

completed my build,

started testing phase.

tried odac with dcb1 and f5. very nice sounding with foobar.



regards.

[jtktam]

Originally Posted by necplusultra

greetings,

completed my build,

started testing phase.

tried odac with dcb1 and f5. very nice sounding with foobar.



regards.

did you solve your previous U4 voltage problem?

-joe

[DJNUBZ]

Originally Posted by necplusultra

greetings,

completed my build,

started testing phase.

tried odac with dcb1 and f5. very nice sounding with foobar.



regards.

ditto except I tried the O2 as my preamp and it sound amazing as well. I am going to A/B it against my aleph 1.7 when I get some more time.

[necplusultra]

all good.

tried odac o2 combo. The bass sounds heavy.
need more gain for my headphones type which were the ones that came with a squire/fender practice kit.

pretty nice desktop setup for work...

[Stealth3si]

hi diy forum peeps hope to get a useful tip here....

help!

i need to ground the input jack to the case...i looked a t a couple of pictures


..

can i use a thin wire from a resistor cap thingy to ground the case from the white 'ground' that is labeled in the second picture?