The Objective2 (O2) Headphone Amp DIY Project

Quick question - Can I use this 12V toroidal with an O2 - simply wire up one of the secondaries to the J1 inputs?

That would work as long as you have fairly high mains voltage and a light headphone load. One half of a 15Vac-0-15 would be better.

The problem is the dropout voltage across the voltage regulators. If the mains voltage is low (here in the States 105Vac - 110Vac) that reduces the voltage going into the regulators. If the headphone current draw is high, the increased regulator current increases the dropout voltage requirement. 14Vac or above takes it all into account just fine regardless of mains voltage or headphone load. But at 12Vac you are more restricted.

For what it is worth, NwAvGuy who designed the amp posted about the issue once that in his view it didn't matter if the regulators go into dropout on some 12Vac setups since the power supply rejection ratio (PSRR) of the op amp chips is so high. I never quite bought into that, especially since it is a half wave power supply with a relatively rough ripple voltage waveform. So YMMV. :)
 
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I used a small toroid on my own, one 15v tap directly to the O2, the other 15v goes though some linear regs down to 5v to feed the ODAC.

I worked on the basis the input voltage to the regulators should ideally be at least 2v higher than the output (12v). So 12v + 2v + diode drop = ~ 15v. I think mine measured ~18v unloaded which is also below the advised maximum which was something like 20-22v. Hence 15v was the magic number i came up with.
 
Hi guys, I also have a big problem with my O2 since I shorted one of the gain resistors to ground (amp was in perfect working order until I shorted it), my technical skills are very limited and would be eternally grateful if someone could help me fix my amp, I emailed HeadnHifi asking for advice just now but thought I might get a bigger audience on here so if anyone could spare a little time to help me then that would be great. Here is a little background: The LED does NOT light any more and amp doesnt sound healthy at all with headphones plugged in, a deep BURRRR sound, I only checked with cheap IEM's luckily. I am pretty sure I shorted R19 to ground and now the amp is sending 11VDC out from the two inner most pins on 'P2' instead of the very small voltage that should be present. My circuit tracing skills are very limited but I have checked the voltage across C15 and this seems to be perfect (23.5VDC) so I think the power side of things is ok but I am also getting the same 23.5VDC across 'C10', should this not be lower? my electrical skills are very basic so I don't know if 'Q1' and / or 'Q2' could potentially be damaged, what do these components actually do? Literally any help would be most appreciated :eek:
 
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I'll have more time later but you need to check that the supplies to the opamps are correct as a first step. Measuring from ground check voltage on pin 8 and pin 4 of Opamp U1. If you have 23 volts across C10 (that sounds OK) then you should have plus 11.5 on pin 8 of the opamp and minus 11.5 on pin 4 of the opamp as measured from ground. Q1 and Q2 look as though they are used as switches to disconnect the amp from the PSU as the battery runs low. Check the supplies are present and correct on U3 and U4 as well (pins 4 and 8 again and measured from ground as before)

More time later....
 
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OK, you need to do a quick recheck here.

If you look at the circuit then C10 (the one with 23.5 volts across it) is effectively across the opamp supply pins (4 and 8). So that means you should measure 23.5 volts across pins 4 and 8.

The two readings of 11.76 (plus voltage, yes ?) and 5.49 ??? don't tie up. Pin 4 should be minus 11.5 ish.

Just do a quick recheck :) If those readings are actually what you are getting then there is some lack of continuity somewhere.

Also check (as a validity check) that you have plus 11.5 on one end of C10 and minus 11.5 on the other end of C10. Those voltages are measures with respect to ground.
 
Im so sorry, C10 is NOT 23.5v, I am very confused now, I did remove opamps earlier to check readings without them in their sockets and then with and I don't think I did but it's a possibility that two different opamps were installed in the wrong sockets, highly unlikely but it's the only reason I can think of for getting the different results now?
Opamps are currently in the correct locations and I am getting the following results as of now:

23.5v across C15

C10 has ONLY 6.07v when measured across it...
between ground and one end of C10 I get +11.68v,
between ground and the other end of C10 I get +5.61v.

Also I checked U2 and get -11.8v on pin 4 and +11.7v on pin 8.

Thanks for your help by the way.
 
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Its no problem :) (first time I've seen this circuit too)

The good news is that +11 (we'll round them up) and -11 on the opamp pins is correct. Those same voltages should be present on C10. Don't take my word for everything, look at the circuit :) Pin 4 is V- and pin 8 is V+. Yes. And those points go to C10.

Lets assume all is OK supply wise. The -/+ 11 volts (that's how we write it) is correct on the opamps. If you are certain that is OK then look at each opamp on the circuit. Do you see there is a - and a + input on each and an output at the point of the triangle. The voltage on all three of those points on IC1, IC3 and IC4 should all be approx zero volts measured from ground.

So if you are sure that there is -/+11 or so volts on each opamp then that is the next thing to check.
 
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I'll have to leave it for tonight but those are the important things to check.

1. Be sure that there is a correct plus and minus supply on the opamps (pins 8 and 4). As far as opamps are concerned the supply voltages are not critical and the circuit would work with anything from -/+ 3 or 4 volts upward.

2. Check the DC voltages on all the opamp pins. Discounting the opamp in the power supply all the others should have zero volts on all the other pins.

3. Could any physical damage have occurred such as broken print ? I'm thinking particularly of any ground connections or perhaps a ground that is floating. The hum/buzz sound that you are getting could point to something like that.

4. Don't assume anything. Make sure that the input grounds (and all other parts that go to ground) really do have continuity to that main ground shown on the PSU. Check that with your meter on a low ohms range and with the unit switched off.
 
OK so with the amp powered on I get continuity between pin 8 of U2 to pin 8 of any other opamp and I get +11V between every opamp pin 8 and ground.

Pin 4 on the other hand only gives -11V on U2, the other opamps, (U1, U3,U4) only read 5.6V on pin 4. Is there a problem between U2 and C10 or am I not seeing the bigger picture :confused:
 
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The +11 is fine. Seems there is a problem with the negative rail though. From what you are describing it appears Q2 could be open circuit.

What you can try (switch it off first) is to apply a short (a piece of wire) between pin 4 of U2 and pin 4 on any other opamp. Look at the circuit again. Doing that bypasses Q2.

Provided there is no short or faulty opamp anywhere then hopefully that will power up and prove its all otherwise OK.

As you are not familiar with fault finding there is a small risk in doing that (if there were a short elsewhere) and so you should be ready to measure the -11 volts immediately at switch on. If it comes up OK then all is well. If it doesn't then switch off again immediately.

Gotta leave it for tonight... try that though... I suspect that whatever you shorted has zapped that transistor.