Between pins 4 and 7 of U2 it reads 260K Ohm, if I remove the comparator from the socket then re-measure between pin 4 and 7 on the socket I get exactly the same resistance (260k). This is with no power to the amp, is that correct or should I power it up to take the readings?
OK. That's not conclusive at all.
Remove both FET's and lets see if the drive from the comparators is OK or not. That means having -11 volts on pin 1 and +11 on pin 7
OK both fets removed.
Pin 1 = -11v
Pin 7 = -11v also
(Pin 8 = +11v I dont know if this helps)
Pin 1 = -11v
Pin 7 = -11v also
(Pin 8 = +11v I dont know if this helps)
The drive is still wrong.
Now its time to pull that LED out (leaving the FET's out). You say you have some LED's ? A red one ? Do you know how to identify the leads on it ?
Now its time to pull that LED out (leaving the FET's out). You say you have some LED's ? A red one ? Do you know how to identify the leads on it ?
Right, just found my bag of led's, they are clear but light up red from what i can remember, I am not sure on polarity though? Will these be suitable?
Going to have to leave it for a little while... edit just seen your reply.
Yes they should be suitable. Even if not and even if fitted incorrectly no damage will occur to the amp.
If you have an LED then testing it on your meter on the "diode" range should get it to light. When it lights, the red meter lead will be connected the LED anode and that is the end that goes to R6 and pins 3 and 5.
Fit the LED and confirm that pin 7 is now high at 11 volts.
Assuming it is you can now test the comparator action. If you have a resistor of something like 2.2k to 33k, anything, and dab it across R9 then both the comparators should change state. What was high will go low and vice versa.
If that's OK then refit the FET's and your good to go.
Test a few LED's on your meter and pick a bright one if you can.
I'll look in later 🙂
Yes they should be suitable. Even if not and even if fitted incorrectly no damage will occur to the amp.
If you have an LED then testing it on your meter on the "diode" range should get it to light. When it lights, the red meter lead will be connected the LED anode and that is the end that goes to R6 and pins 3 and 5.
Fit the LED and confirm that pin 7 is now high at 11 volts.
Assuming it is you can now test the comparator action. If you have a resistor of something like 2.2k to 33k, anything, and dab it across R9 then both the comparators should change state. What was high will go low and vice versa.
If that's OK then refit the FET's and your good to go.
Test a few LED's on your meter and pick a bright one if you can.
I'll look in later 🙂
OK I fitted the LED, it's not as bright as the old one but does work (The old one was specced as a high efficiency and the BOM says 'see docs before fitting a different LED', I can't find any more info on why though?).
I now get -11v on pin 1 of U2 and pin 7 now gives +8.4v (should this be +11 or is the +8.4v ok)?
I didn't carry on, thought i'd wait to see what you say?
I really appreciate your help by the way 😉
I just found the following information regarding the LED (copied and pasted from nvawguy's blog):
POWER LED: You might think this doesn’t need a mention but there are some special requriements. First, the normal forward current for most LEDs is 20 mA. That’s as much power as the entire rest of the amplifier needs! So a 20 mA LED would cut the battery life in half. The O2 uses a “HE” high efficiency red LED that is sufficiently visible with only about 0.5 mA. Second, it’s powered symmetrically from the rails (18 – 24 V) on purpose even if that seems less efficient. Otherwise one battery will drain slightly faster than the other. That gets you nothing except mismatched batteries. Finally, the LED’s forward voltage is a critical element of the power management circuit. You can’t change to a different color (especially white or blue) without making other changes as that will require more current and the different forward voltage means the power management circuit has to be altered.
I'm trying to find a suitable LED replacement on Farnell's website...
POWER LED: You might think this doesn’t need a mention but there are some special requriements. First, the normal forward current for most LEDs is 20 mA. That’s as much power as the entire rest of the amplifier needs! So a 20 mA LED would cut the battery life in half. The O2 uses a “HE” high efficiency red LED that is sufficiently visible with only about 0.5 mA. Second, it’s powered symmetrically from the rails (18 – 24 V) on purpose even if that seems less efficient. Otherwise one battery will drain slightly faster than the other. That gets you nothing except mismatched batteries. Finally, the LED’s forward voltage is a critical element of the power management circuit. You can’t change to a different color (especially white or blue) without making other changes as that will require more current and the different forward voltage means the power management circuit has to be altered.
I'm trying to find a suitable LED replacement on Farnell's website...
Its looking good.
Pin 7 voltage sounds OK. If you look at the circuit there is a 270k and 1.5M ohm which feeds into the junction of R5 and R9. So we have a voltage divider action plus the loading of the meter as you measure. so the measured sounds OK.
Did you check the comparator changes state OK ? Just connect any value of resistor below around 33k across R9 (or even short out R9) and the voltages on pins 1 and 7 should flip the other way giving near to -11 on pin 7 and plus 11 ish on pin 1
If that is OK then refit the FET's and test the amp.
The absolute ratio of R5 and R9 (they form a voltage divider) sets the trip point at which the low battery cut off works. It trips as soon the voltage on pin 2 (which varies as the battery discharges) reaches the same voltage or lower than that on pin 3 (which the LED holds constant). So any LED will work as long as you select R5 (or R9, one or the other) to match the LED voltage at the battery voltage you want it to trip at. So that is easily alterable once have a bright enough LED fitted if needs be.
Pin 7 voltage sounds OK. If you look at the circuit there is a 270k and 1.5M ohm which feeds into the junction of R5 and R9. So we have a voltage divider action plus the loading of the meter as you measure. so the measured sounds OK.
Did you check the comparator changes state OK ? Just connect any value of resistor below around 33k across R9 (or even short out R9) and the voltages on pins 1 and 7 should flip the other way giving near to -11 on pin 7 and plus 11 ish on pin 1
If that is OK then refit the FET's and test the amp.
The absolute ratio of R5 and R9 (they form a voltage divider) sets the trip point at which the low battery cut off works. It trips as soon the voltage on pin 2 (which varies as the battery discharges) reaches the same voltage or lower than that on pin 3 (which the LED holds constant). So any LED will work as long as you select R5 (or R9, one or the other) to match the LED voltage at the battery voltage you want it to trip at. So that is easily alterable once have a bright enough LED fitted if needs be.
Yes, that's saying the same thing.
A different LED needs circuit changes, which is just either R5 or R9. That's no worry. The LED brightness is fixed by R6 which limits the current. So at the expense of battery life you could reduce R6. How much it affects battery life depends on how much current the LED draws in relation to the rest of the amp. To give an idea, 1ma running from 1000mah batteries would last for 1000 hours before they were flat.
Another break 🙂 back later....
A different LED needs circuit changes, which is just either R5 or R9. That's no worry. The LED brightness is fixed by R6 which limits the current. So at the expense of battery life you could reduce R6. How much it affects battery life depends on how much current the LED draws in relation to the rest of the amp. To give an idea, 1ma running from 1000mah batteries would last for 1000 hours before they were flat.
Another break 🙂 back later....
ok shorting R9 did change the state of U2 pins 4 and 7 so fitted both fets and the amp seems to work perfectly. Anything else I should test?
Final checks would be to confirm that the voltage measured across C10 drops to zero when its powered off proving the FET's were switching on/off OK
The low battery cut off point is determined 100% by the LED characteristics and so using a different LED will alter the cut off point slightly. I would suggest seeing if you are happy with the present LED (its brightness and colour) and if not replace it with something else.
You could then replace R9 with a 100k trimmer pot and set the cut off point exactly, and having done that remove the trimmer and replace it with a new fixed resistor close to the value that the trimmer is set at.
All sounds good though 🙂
The low battery cut off point is determined 100% by the LED characteristics and so using a different LED will alter the cut off point slightly. I would suggest seeing if you are happy with the present LED (its brightness and colour) and if not replace it with something else.
You could then replace R9 with a 100k trimmer pot and set the cut off point exactly, and having done that remove the trimmer and replace it with a new fixed resistor close to the value that the trimmer is set at.
All sounds good though 🙂
The amp is NEVER used with battery power, AC only so could I just leave things as they are, I can live with a dimmer LED as long as it has no detrimental effects to the rest of the system?
C10 does drop to zero when powered off BTW.
If you never use battery then you can leave it be or fit any type or colour LED you want.
Reducing R6 will increase the brightness. Only proviso with "any LED" is that white ones and maybe some blue have a high volt drop and so you might need to fiddle the value of R9 if the amp didn't switch on.
Its all 100% then... we got there in the end 🙂
Reducing R6 will increase the brightness. Only proviso with "any LED" is that white ones and maybe some blue have a high volt drop and so you might need to fiddle the value of R9 if the amp didn't switch on.
Its all 100% then... we got there in the end 🙂
Thank you so much for taking up your time to help a stranger, can I buy you a beer (Paypal you a fiver), pm me your PP address if so or is this not allowed on the forum? I don't want to break any rules, im just so very grateful you helped me. 😀
Your very welcome and its a very nice offer, thank you, but technical challenges are what we enjoy doing here 😀
Its payment enough to know its fixed and that hopefully you learnt a little along the way 🙂 Enjoy your amp... and don't blow it up 😛
Its payment enough to know its fixed and that hopefully you learnt a little along the way 🙂 Enjoy your amp... and don't blow it up 😛
I definitely learn new things about circuits and I'm so very happy the amp is now working (I have my headphones on / listening to the amp while typing this). I will be more careful when handling it in future. Thanks once again, you are a true gentleman.
Objective 2 help
hi, im new to this forum but i have just completed building an objective 2 headphone amp. However, its not working. I did the initial resistance test and it was fine but when it came to measuring the voltages from the battery leads i got a very low reading. The wall adapter is putting out 13.5VAC but when i connected my dmm to the positive and negative leads of the battery terminals it read only 11.5VDC instead of the 23-24vdc that NwAvGuy said it should in his blog. Also, when i checked other voltages they were all a lot lower than they were supposed to be. I tried looking over the solder joints and reheating them as well as making sure all components were correctly installed, but the problem persists. Any help would be appreciated, thanks.
hi, im new to this forum but i have just completed building an objective 2 headphone amp. However, its not working. I did the initial resistance test and it was fine but when it came to measuring the voltages from the battery leads i got a very low reading. The wall adapter is putting out 13.5VAC but when i connected my dmm to the positive and negative leads of the battery terminals it read only 11.5VDC instead of the 23-24vdc that NwAvGuy said it should in his blog. Also, when i checked other voltages they were all a lot lower than they were supposed to be. I tried looking over the solder joints and reheating them as well as making sure all components were correctly installed, but the problem persists. Any help would be appreciated, thanks.