The Objective2 (O2) Headphone Amp DIY Project

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#1,343
Going thru the BOM I've found a potential issue with the 2.2uf coupling caps C13 and C14.

Both the Wima the Kermet are rated fairly low for Voltage with higher frequency Ac if you look at the charts in the datasheet.

The 50V Kermet isn't even good for 1v rms over 100 hz looking at the datasheet, the Wima is a little better. Even the 63V Wima caps that fit the pcb don't quite match the amp's headroom over the audio-band. I think it will be difficult to find a 2.2uf film cap that fits the board and can handle the treble region.

I am just going to put a pair of polypropene caps under the board.
 
#1,345
@regal: the coupling caps don't get any significant AC voltage across then over the entire audio band. So even when they aren't good for more than 1 V AC at 100 Hz, this is not a problem. But I think you missed something...

The 50 Volt MMK series are good at a tad below 1V for frequencies up to 100000 Hz, the 63V MMK (which is the cap in the BOM) go higher. The little "k" on the horizontal axis makes all the difference.
 
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#1,346
@regal, that's the voltage across the capacitor. And in the O2, the voltage across the capacitor is less than 0.001 volts. You have to remember the capacitors are not connected or referenced to ground so they don't see the audio signal with respect to ground. They have a very low impedance at audio frequencies--especially at high frequencies--so the voltage drop across them is essentially just from their effective series resistance (ESR) which is really tiny.

Anyone with an O2 is free to put a DMM or scope across their caps with the amp operating to verify the above if you have any doubts. As described in the design article, I even evaluated the signal across several brands/types of caps using the dScope to verify the caps are indeed transparent.

The only way to get a significant voltage across them would be at frequencies below 2 hz or at DC and at those frequencies, they're rated for far more than they will ever see in the O2.

EDIT: +1 for what Timpert said (we cross posted)
 
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#1,347
RocketScientist said:
@regal, that's the voltage across the capacitor. And in the O2, the voltage across the capacitor is less than 0.001 volts. You have to remember the capacitors are not connected or referenced to ground so they don't see the audio signal with respect to ground. They have a very low impedance at audio frequencies--especially at high frequencies--so the voltage drop across them is essentially just from their effective series resistance (ESR) which is really tiny.

Anyone with an O2 is free to put a DMM or scope across their caps with the amp operating to verify the above if you have any doubts. As described in the design article, I even evaluated the signal across several brands/types of caps using the dScope to verify the caps are indeed transparent.

The only way to get a significant voltage across them would be at frequencies below 2 hz or at DC and at those frequencies, they're rated for far more than they will ever see in the O2.

EDIT: +1 for what Timpert said (we cross posted)
Click to expand...

Ok this is good thanks for the info.
 
#1,348
In regards to the volume pot used on the O2, is the shaft the standard 1/4" shaft used in most audio applications? I have a vintage Pioneer SX-727 receiver that doesn't look like I'll be able to get back to a good life, so I'm thinking that I should have about 5 knobs available, if they fit. If so, I'll take a picture with the new front panel and post it to see how it appears.

I'll reserve one knob to set aside for Questhate, as I owe him tremendously for his graciousness. We can then talk about the others that remain based on appearance and fit.
 
#1,349
@T2T, the standard pots are the Asian industry standard 6mm "D" shaft which is different than USA 1/4" (6.35mm) pots. Some Japanese receivers use the 6mm D shaft while others use the 6mm "star" shaft (that looks like an asterisk when viewed from the end). There are a couple of Bourns alternate pots that fit the O2 PCB and use the star shaft.
 
#1,350
Thanks, RocketScientist,

It still appears there might be some glimmer of hope - if one were to add a few layers of tape on the shaft to get it up 6.35mm. I'll have to see when the amp arrives. Possibly, a few wraps of black electical tape could be one solution to "cheaply" integrate a metal knob onto a pot.
 
#1,352
Finally got motivated and built the Objective2 headphone amplifier in one session. It took 4 hours total, because I was careful in assembly. I don't have the case yet. I measured the DC offset and it is about 3.6 mV for both channels. I will now try it the first time with my headphones. Should be fun experience 😀

Here are pics:
http://dl.dropbox.com/u/7337908/IMG_9242resized.jpg
http://dl.dropbox.com/u/7337908/IMG_9240resized.jpg

Ok now plugged headphones in. Total silence 😀
Now some music ->
 
#1,355
RocketScientist,

I can't thank you enough for coming up with such a beautiful portable amp -- and sharing the fabulous design. I have a few soldering irons in my collection, but I'm not really into doing serious board and component work, so I was able to get my O2 from another builder.

I only have a Walkman MP3 player as my source, with a LOD cable at the moment. However, I'll visit Radio Shack later today and get an RCA to mini plug adapter so I can connect my Squeezebox (which has Burr-Brown DACs) and play some FLAC files through the amp pushed out to my modified Grado headphones with Magnum drivers.

At this point, I want to set my sights on some good, quality portable sources. I know from readings, the Walkman NWZ can be quite good, yet the versions that were manufactured for other parts of the world, cost a bit more and have a higher build quality. I also have a LOD cable for my Sansa player and try that too. After all, with amps, it's basically the same as with computers: "Garbage it, Garbage out."

Thanks again !! :wave:
 
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