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Old 14th March 2013, 04:15 AM   #3191
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Quote:
Originally Posted by weslito View Post
Bookcase?! I'm confused.
book·case [book-keys]
noun
a set of shelves for books.
Origin:
1720–30; book + case2

Bookcase | Define Bookcase at Dictionary.com

Pls see attch. image(The offer does not include the beautiful model)
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Old 14th March 2013, 06:48 AM   #3192
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Originally Posted by Namenlos View Post
Two things, is there an output jack that fits the pcb can accept TRRS plugs? I drives me nuts sometimes.
Get a TRRS to 3.5mm convertor
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Old 14th March 2013, 07:26 AM   #3193
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Originally Posted by weslito View Post
... If so, the easiest way to achieve even lower gain would be swapping R7 and R3 for a higher value and resizing C11 and C12 appropriately, as you say....
Increasing R7, R3 will increase the Johnson noise according to RS (critical in gain stage). You can solder 2 pairs of resistors in the empty 6 holes of VR1 vol. pot such that they are in parallel with VR1. One can start with 4.7K+4.7K or any other suitable configuration(not to go below total combination of 2K, the lower limit for JRC2068 loading).One can get a rough idea of the value of voltage divider resistors by measuring the resistance of VR1 w.r.t. vol. knob position.
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Old 14th March 2013, 09:22 AM   #3194
weslito is offline weslito  United Kingdom
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Quote:
Originally Posted by availlyrics View Post
book·case [book-keys]
noun
a set of shelves for books.
Origin:
1720–30; book + case2

Bookcase | Define Bookcase at Dictionary.com

Pls see attch. image(The offer does not include the beautiful model)
I don't have a bookcase, but I do have a benchcase
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Old 16th March 2013, 11:06 AM   #3195
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Quote:
Originally Posted by availlyrics View Post
Increasing R7, R3 will increase the Johnson noise according to RS (critical in gain stage). You can solder 2 pairs of resistors in the empty 6 holes of VR1 vol. pot such that they are in parallel with VR1. One can start with 4.7K+4.7K or any other suitable configuration(not to go below total combination of 2K, the lower limit for JRC2068 loading).One can get a rough idea of the value of voltage divider resistors by measuring the resistance of VR1 w.r.t. vol. knob position.
Then I would have to add a resistor between the pot and the NJM2068 right?
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Old 17th March 2013, 09:11 PM   #3196
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Default ODAC with O2 grounding

Hi all, first post... ...this thead is something of an epic, but I did try the search function. If this has already been asked before, I apologize.

I just received an ODAC to use with my O2, and I'm pondering how to enclose it. NWAVGuy seems adamant both in the project notes and in the schematic that the O2 should be connected to the enclosure at one point only (the input jack), presumably to avoid ground loops. However, if I fit the ODAC into the same case and then expose the USB socket via a hole cut in a metal panel, it seems that if the USB socket touches the panel the O2 is now connected to the enclosure at both the USB jack (via the signal ground wire from the ODAC) and via the 'regular' thin wire front-panel connection. Is that right? If so, is this not really so important after all?

I also have a couple of general questions about grounding in an amp like this. I'm not clear if the Triad AC/AC adapter's secondary is grounded at one end, or if the whole arrangement floats. But in any case, I'm assuming that on battery operation with a battery-powered source everything must be floating with respect to to mains earth - unless a mains-powered source is connected. If I run on batteries and then connect a USB source, for example, signal ground is now referenced to the USB ground of the source. Is that correct?

Thanks. I'm more than happy to do the reading / thinking myself if someone wants to point me in the right direction!

Last edited by Earl Grey; 17th March 2013 at 09:18 PM.
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Old 17th March 2013, 09:33 PM   #3197
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Oh, and one more before I forget, although this is really a 'pure ODAC' question... ...the traces from the line-out header to the pads for the 3.5mm socket on the ODAC board have jumpers on the PCB that need to be bridged with solder to get the signal to the 3.5mm socket. Why not just run the traces to the 3.5mm socket and the line-out header with no jumper? If the 3.5mm socket is not fitted, what's the disadvantage to having the unused pads still connected to the output signal?
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Old 19th March 2013, 07:24 AM   #3198
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Quote:
Originally Posted by Namenlos View Post
Then I would have to add a resistor between the pot and the NJM2068 right?
Add resistor in parallel with the pot(magnify the image in last post)
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Old 19th March 2013, 05:32 PM   #3199
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Originally Posted by availlyrics View Post
Add resistor in parallel with the pot(magnify the image in last post)
This can't work, you just increase the load of the preamp. You have to create a voltage divider somehow.
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Old 19th March 2013, 07:43 PM   #3200
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Quote:
Originally Posted by Namenlos View Post
You have to create a voltage divider somehow.
The easiest way is to increase the values of R3 and R7, which form voltage dividers with R20 and R14, as described here:

O2 amp CRC, diode, cap, and heatsink mods

I find that I need around 10K for 50% attenuation with my sources. When you increase those resistor values you have to correspondingly decrease the values of C11 and C12 to keep the corner frequency of the input RF filter the same, as the link describes.

Below is the AC sim plot of the O2 amp RF filter with the original values. The second plot is with R3 and R7 increased to 10k, for a 50/50 voltage divider, and C11 and 12 decreased to 22pF. The capacitors should be 50Vdc MLCC COG types with 2.5mm lead spacing, like Mouser part number 594-k220J15C0GF53L2.

Ideally it would be best to add a 10K resistor in series between the first op amp and the 10k pot, since the buffer stage following has 1x closed loop gain and won't increase the resistor noise. But that requires cutting PCB traces. The assumption here is that you are probably using 1x gain anyway in the first stage since you need attenuation, so even that won't matter. Easiest just to replace those two existing resistors and caps.
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File Type: png existing O2 RF filter plot.png (25.5 KB, 192 views)
File Type: png voltage divider O2 RF filter plot.png (22.0 KB, 177 views)

Last edited by agdr; 19th March 2013 at 07:55 PM.
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