The Objective2 (O2) Headphone Amp DIY Project

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deckguy

Member
Joined 2021
Hi everyone,
I was browsing NWavGuy's blog about the creation of the O2 and I am wondering, could someone please share the design files?
Like a lot of folks here, I too am trying to build such an amp, and hope to get a real quality device in the end.

I have tried via the initial Google drive, as pointed by Nwavguy's blog, however that requires nowadays a password to get them/there.
Thanks,
Appreciate it,
Ron
 
BamboszeK

BamboszeK

Member
Joined 2012
That's not true. You should take care when selecting alternative LED if you want to use amplifier with batteries.
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.

POWER MANAGEMENT: The idea is to shut down the amp when the the 9 volt (8.4 volt nominal) batteries drop to somewhere in the 6 – 7 volt range. U2 is a low power comparator. The circuit prevents potentially damaging headphones with DC. The “A” section compares the total power supply voltage to the LED voltage. The LED in this application has a relatively constant drop as the batteries discharge so it forms a “free” voltage reference without any added power consumption or other components. It’s plenty accurate for this application although if you substitute a different LED it may alter the shutdown voltage. C1 removes noise from the battery voltage and also provides a slight turn on delay. When the voltage at pin 2 is higher than pin 3 the comparator output pulls to the negative rail which turns on power MOSFET Q1 and also causes the output of U2B, operating as an inverter, to go high turning on MOSFET Q2. C16 and C21 provide a controlled turn on reducing the transient “click” at power on. If the battery voltage falls too low, or one battery is disconnected, pin 3 will be higher than pin 2 and Q1 and Q2 rapidly switch off shutting down the amp. R25 provides hysteresis to help prevent the amp from turning right back on again as the now unloaded battery voltage rises. Power consumption in shutdown is under 1 mA.
Click to expand...
http://nwavguy.blogspot.com/search?q=led#circuitdescription
 
availlyrics

availlyrics

Member
Joined 2011
Salt5haker said:
The led listed in the BOM isn't available, so is using any red led fine?
Click to expand...
You can use other type of red LED, provided it has forward drop of 1.72V, brightness is immaterial, I used Red LED from old modem with R6 value changed from 40.2K to 33K. That red LED is super critical for reference voltage , do not substitute with any other colour as then th fwd drop voltage will change. Do it only if you are not newbie and have basic electronics knowledge. Else stick to original BOM. Whatever you do , DONT PLUG YOUR HD650 , for trial runs, stick to elcheapo headphones till all voltage checks are done multiple times
 
K

ktre

Member
Joined 2020
I think I destroyed my "fixed regulators" MC7812ACTG and MC7912ACTG...My Objective o2 desktop kit is only working with the batteries included. The 15V AC is not showing any signs of current at all. I tried in an audio shop another 15V AC power adapter, also nothing.

Long story short: I want to replace the fixed regulators, can I use those from the links:

https://www.conrad.at/de/p/stmicroe...linear-to-220-positiv-fest-1-5-a-1184699.html
https://www.conrad.at/de/p/stmicroe...-negativ-fest-12-v-1-5-a-to-220-3-156077.html

Or anyone knows another source in EU? (I'm also asking at head n hifi but I have a Conrad store nearby)
 
K

ktre

Member
Joined 2020
Mooly said:
I would be surprised if you have destroyed the regulators.

7812 and 7912 three pin TO220 outline regulators are generic parts. You can use any versions so those you link to seem fine :)

Have you got the correct DC voltage going into the regulators?
Click to expand...

The pins of the the regulatros where touching each other so I suppose this could have destroyed them maybe?

All right, didn't know that, I thought because of the "ACTG" it has something to do with the AC :D
 
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