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Old 21st April 2014, 07:42 AM   #3911
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by yetanothersteve View Post
Regarding measuring the DC Offset thump on power off, is this acceptable way to measure it?

1. Plug in headphones, plug in power, no source.
2. Turn on power of O2.
3. Place ground (black) of DMM at ground [square hole] of P2. Place Voltage meter (red) of DMM at the round hole furthest away from the ground hole. (Left channel I believe.)
4. Turn DMM to measure DC mV and then turn on MAX function. I have a Fluke 115.
5. Turn off O2 and note MAX value.
6. Repeat for other round hole second furthest away from the ground hole. (Right channel I believe.)

My headphones have an impedance of 120 Ohm and I measured:
Off thump Left 17.7 mV
Off thump Right 15.3 mV
I've never seen an O2 PCB so I'll have to take your word for the test points you are using. You need to be measuring between ground and the junction of the 1 ohm resistors on the opamp outputs. You don't need headphones plugged in for this.

Any steady state DC present is down to the natural offset of the opamps and it will vary. The design makes no attempt to minimise this offset. To reduce it to zero would need the input bias currents of the four output opamps equalising and that would be done by breaking the direct connection between output and inverting input for each, and fitting a resistor there instead such that the theoretically identical steady state currents flowing out of the two inputs for each opamp were each working into equal impedances.

That would reduce the ever present DC offset but that isn't the same as switch on/off thump.

The thump is measured the same way but unfortunately a meter isn't fast enough to record the values. That needs an oscilloscope. You can prove that to yourself by measuring the voltage across a battery. If the battery reads 1.5 or 9 volts or whatever steady state, now try "flicking" or touching one lead on the battery very quickly. The voltage applied to the meter is still the same but it won't register it correctly. The meter needs a certain time to acquire and display the reading. The thump noises may vary depending on the position of the volume control. That is because of the added "thumps" from the earlier stages.

Nothing you can do to the O2 circuit will stop this thump. To make it silent would need a dedicated mute arrangement on the output.
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Old 21st April 2014, 12:43 PM   #3912
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If the Thump concerns you, unplug your headphones before turning the amp off.

Last edited by TheGimp; 21st April 2014 at 12:48 PM.
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Old 21st April 2014, 11:45 PM   #3913
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Quote:
Originally Posted by TheGimp View Post
If the Thump concerns you, unplug your headphones before turning the amp off.
The thump does not concern me, it is not loud at all, I was more curious about how to measure it.

I do not believe that unplugging or plugging headphones in to a powered on amp is a good idea. Can create distortion as the plug moves past the connection points in the jack. Also, the jacks will eventually wear out if the manufacturers spec sheet is to be believed.

Quote:
Originally Posted by Mooly View Post
The thump noises may vary depending on the position of the volume control. That is because of the added "thumps" from the earlier stages.
The thump sounds the same without regard to volume position.

LIstening to music through the O2 now, no problems. It has been an educational first build.
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Old 22nd April 2014, 12:51 AM   #3914
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Could someone tell me where to find the BOM for the O2? I have a couple of boards that I got from Qusp, I haven't seen or heard from him for a long time now. I am looking for the BOM and circuit layout and I have seen that there is a trouble shooting guide in one of the threads. Thanks in advance to whoever can point me in the correct direction. These boards are for my two kids to build and they are bugging me now to get this done! I wouldn't imagine there are any more group buys of all the components, any info is appreciated.

Steven
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Old 22nd April 2014, 01:21 AM   #3915
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Quote:
Originally Posted by Kindhornman View Post
Could someone tell me where to find the BOM for the O2? I have a couple of boards that I got from Qusp, I haven't seen or heard from him for a long time now. I am looking for the BOM and circuit layout and I have seen that there is a trouble shooting guide in one of the threads. Thanks in advance to whoever can point me in the correct direction. These boards are for my two kids to build and they are bugging me now to get this done! I wouldn't imagine there are any more group buys of all the components, any info is appreciated.

Steven
They are all on the blog here NwAvGuy: O2 Details

BOM https://docs.google.com/file/d/0B52A.../edit?hl=en_US

Schematic https://docs.google.com/file/d/0B52A.../edit?hl=en_US
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Old 22nd April 2014, 01:32 AM   #3916
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mcandmar,
Thanks for the links. I already gave them to one of the kids.
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Old 23rd April 2014, 03:54 PM   #3917
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I noticed that when you push the power button slowly, the amp makes a very loud crack sound before it turns on. I guess this is because the dual pole switch doesn't work in sync. Has anyone else noticed this?
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Old 23rd April 2014, 09:14 PM   #3918
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My switches have been on backorder for two weeks from Mouser, so I have the switch shorted. I install the batteries, then plug in the headphones.
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Old 25th April 2014, 06:40 AM   #3919
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Quote:
Originally Posted by UnknownAX View Post
I noticed that when you push the power button slowly, the amp makes a very loud crack sound before it turns on. I guess this is because the dual pole switch doesn't work in sync. Has anyone else noticed this?
"Toggle" switch are better in this regards. I leave near coastal region & all "push" type switches needs to be cleaned/lubricated quarterly by something like WD40.
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Old 14th May 2014, 12:34 PM   #3920
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The switches are simple bifurcated slide switches. The contacts have a thin coating of what appears to be dielectric silicon grease.

I took my gain switch apart when I had a problem with the low gain position. The left channel dropped considerably below the right channel, and I could find no errors with the resistors. After taking it apart, I could find no problems, so I re-assembled it, tested it, and put it back on the board.

My O2 Headphone amp now workes fine. Go figure.
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