The Objective2 (O2) Headphone Amp DIY Project

i am only using an amp in the first place b/c i am using the odac, and it requires an amp.. and im only using the odac to improve the signal, mainly ruducing noise, from my computer.

anyway, i'd prefer to be able to just clip more resistors if thats possible.

thanks, i'll look into your suggestions as well.
 
Clipping the resistors is to change the gain to unity (no gain). You can't really reduce it from that without some type of transformer or voltage divider circuit somewhere along the line. The simple hiss buster circuit linked would be one of those voltage dividers.

Just turn the volume down in software. You have an ODAC after all, with 24-bit playback. That's what it's for.
 
My amp was working for about a week, until last night it just stopped instantly. The light still works, just no output.


This is the measurements I get, with the pictures from the nwavguys's site.

o2 amp testing - Imgur

When I built it, it had a loud turn on pop, if that's any help.

I had a hunch and charged the batteries and it works of those only. So does this point to a problem in the way the AC adapter charges the batteries?
 
I only measured resistances when I first built it, because it worked straight away with junk headphones and batteries. They are the measurements from now.

It works with batteries. And when I test ground+the power jack I get the right voltage. So according to the O2 docs, that means the problem is in between the power jack and U5, right?
 
Yes, if your O2 works on batteries but not AC power, the problem must be from the wall outlet to D1 & D5 inclusive. With the O2 switched off, make voltage measurements along those paths (AC, and DC positive & negative). If you've verified the AC voltage is correct at J1, you can start at the DC side of D3 & D4. When you get the regulator outputs fixed, continue with the voltage checks from the O2 docs.
 
Yes, if your O2 works on batteries but not AC power, the problem must be from the wall outlet to D1 & D5 inclusive. With the O2 switched off, make voltage measurements along those paths (AC, and DC positive & negative). If you've verified the AC voltage is correct at J1, you can start at the DC side of D3 & D4. When you get the regulator outputs fixed, continue with the voltage checks from the O2 docs.

Okay, at J1 I get 16V AC. +- 20.85V at D4 and D3. Then when I measure U6, starting from the pin the pin closest to J1 is 0V, 20V, then 0. U5 is 20v, 0, then 0.4V.

Should the last one be 8V?
 
I replaced Q2 and solved the DC output problem I was having (thanks Swalter). Now it seems I'm getting a loud turn on transient, it makes a big pop when I turn it on (AC power, batteries not in use). It sounds very loud, unlike the earlier O2 amps I have built. I double checked component values, and reflowed the solder several times around the board. All my voltages seem within acceptable range of the troubleshooting values. The power rails are not quite balanced at +11.8V and -11.6V on U2 pins 8 & 4, but this should be ok?

There is something I can't get my head around, when I look at the "Power Management" section of O2 Circuit Description on the nwavguy blog, it says that the output of U2B goes high in order to turn on Q2. I'm seeing something different to this behaviour. When I removed U2 (and U1,U3,U4) from the board I am still seeing 8.4V on the Q2 gate, which causes Q2 to be operational and pass voltage to the drain pin. At the same time, Q1 is not sending voltage to the drain pin, because it requires a gate pull down from U2. When I look at the circuit diagram, it seems R8, R25 and R9 would provide about 8.3V on Q2 gate without even needing U2 to be present? This would seem to go against the purpose of U2, which should turn on both Q1 and Q2 at the same time. I must be missing something, or is Q2 not under the control of U2 when running on AC power?
 
When I look at the circuit diagram, it seems R8, R25 and R9 would provide about 8.3V on Q2 gate without even needing U2 to be present?
I'm not sure how you arrived at that voltage. My rough calculations show it to be a little less than that. Either way, I'm not sure how relevant voltage readings are for an un-stuffed board. You're measuring a voltage divider made up of the series pair R8 and R25 in parallel with R5. This in series with R9. Remember also that the power management circuitry is not referenced to circuit GND.
 
Hi All,

Successfully build an O2 + Odac. Lows and mids sound fine, but there is a strong emphasize on high frequencies (sibilance). No obvious distortion. Both line in and Odac show similar sound in the highs. Can this be caused by some high frequency (<20khz) oscilation? If so, how could I measure this without an oscilloscoop? Or are the other things to check out first?

Thanks for any advice.
 
Question for the designer; he mentioned PCB layout is critical to the DAC performance and especially noise.

Of course, but this is something I would merely be fooling around with(no official releasing or claims of good design), but if I do feel okay about the finished product, I may post the designs just for critiquing and for anybody to do whatever they wish with them.
 
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Of course, but this is something I would merely be fooling around with(no official releasing or claims of good design), but if I do feel okay about the finished product, I may post the designs just for critiquing and for anybody to do whatever they wish with them.

Thanks, I follow you reasoning. Best wishes and do share if all works out well.