The Objective2 (O2) Headphone Amp DIY Project

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#4,901
Davey said:
Simplify. I think I would use the O2 as your preamp AND headphone amplifier, and set your other unit aside. If you need some sort of input selector, you could find a different unit or maybe utilize just that portion of your NE5532 unit.

Your "hybrid" powering scheme seems to have created a loop, or difference of potential, or some sort of asymmetry in the power supply circuitry. I suppose you could continue to investigate and possibly solve that issue, but it seems more straightforward (IMHO) to use the O2 for both applications.

Cheers,

Dave.
Click to expand...

Thanks. Yes, getting rid of the other amp seems the right approach (the input switch will feed the O2 directly). It would seem handy to have separate volume controls for headphone and preamp though. What I am wondering about is where to place the second pot: in parallel with the O2 volume control, or picking up signal from the O2 output ?

Also, this is probably a stupid question, but, if I use the normal O2 output, and no headphone is connected, the O2 will see a very high impedance load (I guess much higher than any headphone). Might this be a problem (not enough current) ? I understand that it's low impedance loads which are usually a problem, but is too high possible too, not in the sense of damaging the O2 but in generating higher distorsion ?
 
#4,902
No, I would not incorporate an extra pot within the O2 architecture. That's a compromised position that would affect O2 normal operation and also create a high output resistance source as your "preamp" output. Forget about that idea, please. 🙂

There is no issue using the O2 output as a preamp output. The output buffers will easily tolerate any load from low-impedance headphones to infinite.

Cheers,

Dave.
 
#4,905
I don't know. You're asking the wrong guy. 🙂

Apparently he attempted to attach the O2 power input across one half of the secondary of an existing bipolar supply transformer.

The O2 uses a standard voltage-doubler scheme so a simple AC wall-wart can be used to create a bipolar supply. Attaching that to a portion of another power-supply unit brings all sorts of variables and possible pitfalls to the table.

Dave.
 
#4,907
medoc92 said:
The other circuit uses a full wave rectifier
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The center tap provides ground for the preamp, correct? If so does the CT also provide ground for the O2? If the O2 is randomly connected, there is a 50-50 chance that CT would provide ground for the O2 also, but also a 50-50 chance the CT feeds the two diodes on the O2, which might maybe possibly could cause some issue.

My guess is you've wired it correctly, just asking the obvious, is it plugged in, is it turned on, etc.
 
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#4,908
mlackey said:
medoc92 said:
The other circuit uses a full wave rectifier]./QUOTE]

The center tap provides ground for the preamp, correct? If so does the CT also provide ground for the O2? If the O2 is randomly connected, there is a 50-50 chance that CT would provide ground for the O2 also, but also a 50-50 chance the CT feeds the two diodes on the O2, which might maybe possibly could cause some issue.

My guess is you've wired it correctly, just asking the obvious, is it plugged in, is it turned on, etc.
Click to expand...

Yes, the center tap provides the O2 ground, and thank you for menaging my susceptibility, but yes, it was by pure chance...

It's not a case of things not working at all, I just have a slight hum, which only becomes a real problem if the source is lower level and I have to push the volume on the O2 (or the preamp for that matter, this also causes a hum from the preamp).

My guess is that powering the 2 boards in this way creates an intractable (by me) ground loop or similar issue, which is why I was then wondering about providing the 2 outputs from the O2 only.
Click to expand...
 
#4,909
medoc92 said:
mlackey said:
My guess is that powering the 2 boards in this way creates an intractable (by me) ground loop or similar issue, which is why I was then wondering about providing the 2 outputs from the O2 only.
Click to expand...

One other random thought... push the volume on both units, then using a DVM in AC mode measure across the grounds of each unit and see if there is AC present. Since the hum is small, the AC is likely small, in the 10mV to 100mV region. You might also try DC while you're at it. Might provide another clue.
Click to expand...
 
#4,910
I have around 5mv AC across the input ground and the O2 power ground, and 17mV DC on one channel, none on the other curiously.

I do think that this power issue is intractable as is. Maybe I'll have a look at disabling the preamp power entirely and power it from the O2 DC power rails. Or use the O2 only.

Thanks to all who responded !
 
#4,912
medoc92 said:
I have around 5mv AC across the input ground and the O2 power ground, and 17mV DC on one channel, none on the other curiously.

I do think that this power issue is intractable as is. Maybe I'll have a look at disabling the preamp power entirely and power it from the O2 DC power rails. Or use the O2 only.

Thanks to all who responded !
Click to expand...

Finally: I got rid of the regulators on the preamp and power it from the O2 ones. Hum is gone, problem appears to be solved.

The O2 regs may have gone from 40 to 41 Celsius (not even sure), the power consumed by the preamp and its high impedance load is quite probably negligible. Now just need to stuff everything back in the box 🙂

Thanks again !
 
#4,913
lights are on

but no ones home.

Hi there,
building an o2 for the first time. Usually i build guitarstompboxes on veroboard.
This time i needed an headphoneamp. did the desktopversion from headnhifi.
(no batteryclips no d6 and d2/ r1 and r2). Led is working but no sound.
resistor testing gave me 275 on r14/r20 only with plugged J2 (without around 10k) - looking good so far.

wired rca input to j2 and did the cuts - 1/4 Jack to rca also checked.

voltages are a mess:
P2 : 50,2 (down)
42,1 (upper)
d3: 24,3 (ok)
d4 -24,4 (ok)
IC pin4 : -0,38
IC pin8: -12,4
d1: -0,45
d5: -12,4

resistors checked, right ic´s in the right spots
anyone an idea what went wrong?
thx in advance
 
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#4,916
desoldering from this pcb is a pain..

got right voltages on d1 d5 d3 d4 pin4 /8 (-11,8/+11,8) on U2 but only on pin4 on the remaining sockets (-11.8) and -2,8 on pin 8 of U3 U4 and U1
edit: pin 8 on the others is constantly changing from -4 to -2.00
 
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#4,918
....and then fit just U2 in its socket making sure it is the right way around.

which, of course, I did NOT.
guess its time for a big break 😉

... with the ic in it, all the pin8 of the other show beautiful +11,8

very sorry for waisting your time and thank you very much for looking into my postings.
 
#4,920
O2 AMP no sound

Hello, i just DIYed o2 amp. Parts i bought from headnhifi. When i turn it on, red diode lights but when i plug headphones there is no sound coming. Can it be that i am missing 1 solder pad- (i soldered gain switch wrongly had to resolder and one pad gone missing). Do you have any suggestions? Am i screwed or can it be repaired on low budget? Also can it be by wrong inserted chips in sockets and if yes how do i recognize good inserted chip? Also can it be that i touched some of the ESD sensible parts?
I am sorry for my english and for your eyes and also i thank everyone who gives me any suggestions.
 
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