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Building the Objective2 headphone amp for desktop use

March 2, 2012

The Objective2 (O2) headphone amp was designed as a portable amp first with a few options to be used as a headphone desktop amp. So most of the documented builds out there deal with the portable version and never/barely touch on the desktop options.

In this post I document my desktop build so that other people can benefit from it. Note that as I write this NwAvGuy (the creator of the O2) is busy with the Objective Desktop Amp (ODA) design which will render the use of the O2 as desktop headphone amp obsolete.

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A big kudos to NwAvGuy for sharing his design and for the wealth of knowledge (and objectivity) available at his blog.

Vanilla O2 .vs. Desktop O2

There are a few things that are mentioned in the O2 blog (and are included in the BOM as options) to customize the vanilla O2 build for desktop use:

– Use RCA connectors for the amp input instead of the 3.5mm jack

– Use a 1/4” output jack instead of the 3.5mm jack

– Use the B3-080 enclosure, which is taller, to accommodate the larger connectors

– Do not solder anything battery-related. These includes the battery terminals and a few components that are clearly marked in the schematic as optional when only using AC.

Another option that is not mentioned there but that is useful in a desktop build is:

– Use a panel mount power jack and place it in the back of the enclosure.

There are probable other options but those above are the ones that I cared about.

Enclosure height

The assembled vanilla O2 board fits nicely in the standard B2-080 enclosure but if you want to use the RCA connectors, the 1/4” jack and move the power jack to the back you have to look carefully at the space available since all of them must be panel mounted.

The easiest thing to do is just to buy the B3-080 enclosure but if you are not going to use batteries and you don’t plan to solder the battery connectors then there’s probably enough space in the battery area to accommodate everything.

At the end I used the taller enclosure because I had it already.

Front panel holes

I had never tried to drill holes through anodized aluminum before and I don’t own any fancy power tools so I spent several hours digging for info on how to achieve decent results without a drill press and fancy bits.

Turned out that a simple cordless drill and a set of cheap step bits did the job nicely.

I took the vanilla front panel express file and made the necessary size and hole adjustments for the B3 enclosure for both the front and back panels. The new panes are here.

After that I just printed the designs in wire mode, taped them to the panels using magic tape, placed the panel in a vise and then drilled the holes.

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A few notes about this process

– Make sure the paper is perfectly aligned and doesn’t move or you might end up with unaligned holes.

– One or two steps before reaching the expected hole size flip the panel around and continue drilling on the other side. This will smooth both sides. The results will not be as good as a machined panel but they are good enough.

– I didn’t use any drill lubricant but it might be a good idea to extend the life of the bits

The power jack

The DC jack in the BOM is not designed to be panel mounted so a different one is needed. I already had the PCB-mounted DC jack so I ended up not moving the power jack to the back.

For future builds I found the 163-MJ21-EX which is designed for panel mounting.

Wiring the jacks

To wire the RCA and 1/4” jacks I used a couple of connectors I took from an old PC case similar to this one and these type of headers. This is really not necessary but is useful.

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Remember to ground only one of the RCA jacks to the enclosure but do wire the ground terminals of both jacks together. Note how the red jack on the right was installed without the plastic washer and the anodizing was removed in that area.

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And here’s the other side

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Things to do different

Here’s a list of things that I learned along the way

– Use bigger gauge wire between the panel-mounted jacks and the board. I don’t have any issues with what I’m using now but wouldn’t hurt to use CAT5 wire.

– Do not use a large hoof iron tip. Is nice to heat things up fast but if you are not extra careful you are going to spread solder on unpopulated pads and it will be a pain to remove it later… even with solder wick.

– Use a panel-mount DC jack like 163-MJ21-EX

– Start the panel holes with a nail since the step bits tend to walk a little… not much but they do.

– Use a rotary tool to smooth the holes’ inner circumference. The step bits do a decent job but they are far from perfectly smooth.

– Also use a rotary tool or sand paper to remove the anodizing from the grounding areas. It’s easier than doing it with a screwdriver.

Build done!

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From → Audio, Objective2

4 Comments
  1. MHOE permalink

    Hi there!

    I do very like your modded O2 and am planning to have something similar built as well… The builder would like to know if you have any measurements available for your version?

    Thanks in advance,
    Tomas

    • Hi Tomas
      You can find all the necessary info in the post. The front panel template link is in the ‘Front panel holes’ post section.

      Cheers

  2. Lauri permalink

    Hello
    How should the panel-mounted jacks (RCA, DC and 1/4″ headphone) be soldered? I have the parts, but I’m a bit lost since I can’t find any how-to’s… Links or direct instructions would be appreciated!

    Thanks

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