The Objective2 (O2) Headphone Amp DIY Project

[sebash]

Originally Posted by MrSlim

RocketScientist, do you think there would be any issue with socketing the gain resistors? I was thinking of doing that for the people I am building for, so that they could change the gain at a later date depending on the headphones they are using. Using individual pin sockets would only raise the resistors up 2-3 mm..

Sockets like this? 510-AG90D-10 TE Connectivity / AMP IC & Component Sockets

looks like you could break the strip apart and use the pins where you need them. Its also nice that if the resisters ever do fall out, the O2 will fail to a safe multiplier.

[RocketScientist]

@sebash I can't seem to find a data sheet for that one. The mouser link goes to a product page and there are no drawings, etc. listed (but I might be blind).

There is a data sheet to Mouser part 535-10-0518-10 and it's specifically designed to be cuttable and says it accepts leads down to 0.015 in. The resistors are 0.016 so that should just work. I'll order one with my next order and check it out.

The no-solder solution is especially a good thing if the board is pre-assembled, as MrSlim is offering. Many of his buyers may not even own a soldering iron. But, if it works well, it makes experimenting with various gains much easier for anyone.

@MrSlim, were you planning on including a few extra gain resistors?

In my testing I can't say the expensive Mil Spec RN50 resistors have any real advantage to justify their higher cost. Even at 7X gain there's less than 1 dB difference in the noise performance (they are about 0.8 dB quieter but it's debatable if I can trust the dScope and my test set up to that sort of resolution when you're trying to measure a 3 uV signal to sub 1 dB accuracy.). At 12X they might be worth it but I haven't checked.

So I'm reworking the BOM to make the mil spec resistors optional to save everyone some money. They're also not RoHS compliant (lead free).

[sebash]

@RocketScientist I can't find a data sheet for that one either but I dug a bit and found a page listing for the product line. It includes more information than whats on the direct link from mouser http://www.te.com/

[Kibble Fat]

subscribed

[RocketScientist]

Thanks sebash. That is more info. I can't find the actual pin diameter. But they do say 35 mil holes. The board only has 28 mil holes. The Aries part I linked has 20 mil pins. I could upsize the holes still on the PCB but the 16 mil resistor leads will be lost in a 35 mil hole.

[MrSlim]

Originally Posted by RocketScientist

@MrSlim, were you planning on including a few extra gain resistors?

Yes, after b0ck3n asked me if I would do custom gains, I figured why not put sockets in for them and provide some extra resistors to give them the flexibility.

[RocketScientist]

Thanks MrSlim. I'll check it out the Aries part, and if all is well, I think it's a great idea (and I have to admit not something I thought of even though I've seen it done elsewhere). It's also popular for some phono preamps to set the termination impedance/capacitance.

[MrSlim]

Great, hopefully you can sort that out before we do any parts orders..

[RocketScientist]

The O2 Documentation Package has been revised. There's a new PDF, spreadsheet and the PC board files have been revised as well. This is hopefully the final version and what 200-some people will be receiving from the group buy and also the MrSlim builds.

The latest changes:

• The Parts List (BOM) has been substantially revised showing new alternate part numbers, correcting a few minor errors, and updating various costs.
• One hole on each of the positive battery terminals was made slightly larger.
• The MOSFET and IC regulator holes were made larger to allow all the specified part numbers to drop fully into the holes to stay within the maximum height for the enclosure.
• The top ground fill areas were sightly revised.
• The RF filter capacitor at the AC power jack has been removed
• R10 was moved slightly to provide more clearance for S1.

I don't think any of the above changes to the PCB are risky enough to delay things with a fourth prototype PCB. If anyone wants to look over the files, however, another set of eyes would be welcome

Hopefully the O2 is pretty much ready to leave the nest! The files, as always, are available here:

O2 Resources

[Jelle Schrijver]

I do not know if this is already mentioned. (not enough time to go through all the 22 pages). But is it possible to replace the input and output 3 pin (TRS) jacks with a 4 pin (TRRS) jacks? The 2e Ring should then go straight from output to input. This way you can keep using an iPod remote equiped headphone with the amp and just make yourself an 4 wire kabel from the iPod to the amp input. The remote signals and microphone would then be passed through by the amp.