The Objective2 (O2) Headphone Amp DIY Project

[Twinster]

Sorry I meant P1 just beside J2. So yes I'll try to remove one of the ground wire and leave one RCA groung only. Thanks for the input.

J1 is the DC power jack. That is why you have hum if you connected to the ground there,. There are some really big current spikes moving back and forth between the power jack and the filter capacitors.

The ground ring of the RCA connectors goes to the center (middle of jack, towards the front of the board) pin of J2, the input jack that you connect to your music source. And you should only use one wire, not two, or you risk a ground loop. Just run one wire from that pin on J2 to the ground ring on either RCA jack. The other RCA jack is touching the case with its ground ring, and you just grounded the case with your other RCA jack ground ring and your sanding, like MrSlim said.

If you just made a typo and really are connecting to J2 instead of J1 then the two wires, rather than one, may be the cause of the hum. Also try to route that single ground wire away from the area around the DC jack and 4 filter capacitors C2 - C5 to avoid hum pickup.

 

[agdr]

Hello gents,

I would have a question regarding capacitors C6 and C7. The BOM states 1.0uF whilst the circuit diagram states 0.22uF. Which values are the correct ones?

Thanks, Stefan

The BOM spreadsheet is correct. I think RocketScientist posted once that he was having some trouble getting the schematic document updated on Blogger. The spreadsheet is dated Dec 2011 but the schematic is Aug 2011.

 

[swalter]

Thanks, so I will refrain from soldering the 0.22uF ones in that came with the kit and look through my parts stock at home for some 1uF ones instead.

Stefan

 

[agdr]

Thanks, so I will refrain from soldering the 0.22uF ones in that came with the kit and look through my parts stock at home for some 1uF ones instead.

Stefan

That is actually a modification I came up with that RocketScientist adopted. If you already have the 0.22uF you are OK. The negative regulator chip U6 shows a 1uF in the data sheet which would help suppress oscillations under some odd circumstances. But if you put in the 0.22uF and your U6 isn't getting abonormaly hot, which would probably be the case 99% of the time, you are fine. If someone is buying the parts the first time though might as well get the 1uF since the price is about the same.

 

[swalter]

...if you put in the 0.22uF and your U6 isn't getting abonormaly hot, which would probably be the case 99% of the time, you are fine...

I guess I use the 1uF then, better save than sorry Thanks for the detailed explanation!

 

[MrSlim]

Sorry I meant P1 just beside J2. So yes I'll try to remove one of the ground wire and leave one RCA groung only. Thanks for the input.

I've mounted the RCA's this way:

Colored isolating ring on the outside, Screw it down tight with nut #1 (if you use the part# RS specified you get 2), add the ground lug ring, then add the second nut. Throw away the the inside washer..

I also use a double ground, one to each RCA twisted tightly with the signal wire, with the RCA's right above the input connector, wires are about 1.5 inches long (with a 3 pin connector). I get no hum..

 

[Twinster]

So you also used a ground wire to each RCA jack.

My wires are about 2" long and I have also twisted them around the signal wire. Maybe I need to sand down more inodize paint and I'll redo the nut order like your.

Thanks again.

I've mounted the RCA's this way:

Colored isolating ring on the outside, Screw it down tight with nut #1 (if you use the part# RS specified you get 2), add the ground lug ring, then add the second nut. Throw away the the inside washer..

I also use a double ground, one to each RCA twisted tightly with the signal wire, with the RCA's right above the input connector, wires are about 1.5 inches long (with a 3 pin connector). I get no hum..

 

[wendell c]

just finished my O2 and I had a question about charging, does the led light up when charging or when batteries are fully charged? thanks

 

[agdr]

I also use a double ground, one to each RCA twisted tightly with the signal wire, with the RCA's right above the input connector, wires are about 1.5 inches long (with a 3 pin connector). I get no hum..

Good to know! Certainly can't argue with good results.

 

[Davey]

just finished my O2 and I had a question about charging, does the led light up when charging or when batteries are fully charged? thanks

The LED has nothing to do with battery charging. It just indicates whether the unit is on or off.

Batteries will charge if the AC wall-wart is connected, they won't if it's not.

Cheers,

Dave.

 

[Splatem]

I am building a desktop case for my o2, where all sockets and buttons will be mounted off the PCB. I was wondering if twisted pair cat5 cable is the best option for wiring, or if shielded audio cable would do a better job at isolating noise. Twisted pair is effective at isolating hum for balanced operation, but I am unsure how it helps minimize noise in this unbalanced application?

 

[Atilla]

For the input signal to make it through to the output without power, given the common ground, it would pretty much have to either punch through the signal path, or out to the power supply rails via the internal circuitry of the op amp(s).

No change from the volume control should rule out the signal path. The power supply bypass capacitors C8 and C9 should also do an effective job at bypassing any signal current on the supply rails, leaving just whatever voltage results from signal current x capacitor series resistance.

Well that is an interesting one! I don't have any ideas.

Well. I've looked at it, twisted and turned it, inspected all areas for potential mistakes and I can't say I can find anything that might give me a clue about how could I possibly get mysterious output with power off. What a funny, strange thing to debug.

If anyone accidentally hears output, when the amp is off, let me know

 

[Atilla]

Hm, actually. There is 1 thing. I forget what I gain resistors I set exactly (yeah, I know) but they were 2x and probably 6x. The output from my sound card is high enough to drive the 6x into clipping immediately. I haven't really measured it, since it's just fine on 2x with my headphones.

If I lower the volume from the PC a bit, then I don't get this coupled in.

Still ... what the hell is going on exactly?

 

[agdr]

Hm, actually. There is 1 thing. I forget what I gain resistors I set exactly (yeah, I know) but they were 2x and probably 6x. The output from my sound card is high enough to drive the 6x into clipping immediately. I haven't really measured it, since it's just fine on 2x with my headphones.

If I lower the volume from the PC a bit, then I don't get this coupled in.

Still ... what the hell is going on exactly?

Here is something else to try testing. If you have a DMM measure the DC voltage between pins 4 and 8 of U3 when it happens (with the amp still off). If the incoming signal is large enough that the amp would be clipping if it were on, maybe the input voltage is making it through the U1 op amp internals to the power supply rails and actually charging the rails to a volt or two. Looking back into the mosfets with the amp off would be an open circuit, cutting off the LED as a DC load.

If that is what is happening the solution may be adding more power supply load after the mosfets. A 4.7k resistor between the positive end of c8 and the negative end of c9 under the PCB would pull about 5mA at +/-12.0Vdc, or 400uA at +/-1.0Vdc.

Something else to try would be back-to-back 2.2V zener diodes across R14 and R20

http://www.mouser.com/ProductDetail/Central-Semiconductor/1N4616/?qs=sGAEpiMZZMstCHp3EWKGlwYVHJ9qG6LpPmeqxzyI0f8%3d

That would hard-limit the incoming signal to around +/-2.8V peak. Back to back red LEDs may work too.

 

[Atilla]

Back-to-back LEDs would be fun as they'd be a warning indicator. Well, if the source can feed them enough current to light up.

I'll open and measure it when I bring it to my tools. Sounds quite plausible.

 

[Twinster]

Got the O2 working like a charm now. It was a bad ground. I sand down the inodize paint properly this time and mount the RCA jack like you recommend and it fixed my hollow sound.

Thanks for the help!

So you also used a ground wire to each RCA jack.

My wires are about 2" long and I have also twisted them around the signal wire. Maybe I need to sand down more inodize paint and I'll redo the nut order like your.

Thanks again.

 

[sgrossklass]

I am building a desktop case for my o2, where all sockets and buttons will be mounted off the PCB. I was wondering if twisted pair cat5 cable is the best option for wiring, or if shielded audio cable would do a better job at isolating noise. Twisted pair is effective at isolating hum for balanced operation, but I am unsure how it helps minimize noise in this unbalanced application?

Definitely go for shielded here. Make sure you don't add any ground loops when wiring up the volume pot (or anywhere else for that matter), or just extend that one mechanically.

 

[billyk]

I am building a desktop case for my o2, where all sockets and buttons will be mounted off the PCB. I was wondering if twisted pair cat5 cable is the best option for wiring, or if shielded audio cable would do a better job at isolating noise. Twisted pair is effective at isolating hum for balanced operation, but I am unsure how it helps minimize noise in this unbalanced application?

Shielded is good, I get all my wire from this guy items in John's Silver Teflon Wire Shop store on eBay! It is just so nice to work with. I have been buying this wire from him for 10 years!

 

[jfrana]

I do know that a click and thump is expected on turn off and on (as per documentation), and while it is not startlingly loud, it is a solid click and thump. Voltage measurements are close to expected (if a tad low throughout, especially U2-p7 at 8.5v). Hopefully it is nothing to worry about. Voltage at output is under 4mv, with ac, no batteries, and no input. Measuring voltage at on and off, the most extreme value I have seen was -7 volts at turn off which quickly comes below a volt and decreases to < 10mv within a minute.

All good? Would you risk $1000 headphones without further testing?

With headphones plugged in it is an entirely different story. The highest I saw was -170mv and dropping to 0 within a second of turning off. The turn on max was less than -100mv and dropping to 3.4mv within a second.

Yes the -7v was a surge for a moment. On turning off, with probe on P2 and headphones unplugged, the DMM has -5v (or -6.5 or 2.1) immediately on turn off and is below 200mv within a couple seconds. On turn on, P2 comes to 3.4mv within a second and holds steady, never to rise above 4mv until the surge at turn off.

U3, pins 4 and 8 on turn on nearly immediately rise to -11.7 and 11.7. At turn off, both drop below 1v within a few seconds. (Same observation from U4, pins 4 and 8).

Thanks agdr for you attention and shared knowledge. This project has been fun and educational.

After resoldering and many tests, measurements remain the same. Is that on/off transient spike of up to -7v (with no headphones) likely to damage headphones? This might be a question for RocketScientist, I have completed the tests indicated in the "details" page.

"Check the U2 pin 1 and pin 7 voltages" p1= -11.72, p7=8.5
"battery removal test" U4 pin 4 and pin 8 are below 1v at .68
with the amp on the stable output voltage is below 4mv
But still up to -7v surge at turn off from P2 square to each channel of P2.

Any concerns? (Are there tests to determine if Q1, Q2, C1, C16, and/or C21 are damaged? Do my U2 readings indicate that mine likely are?)

 

[agdr]

jfrana - yep that is definitely one for RocketScientist or someone else who knows a lot more about transient voltage effects on headphones than me.