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Old 7th April 2013, 04:29 PM   #31
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Made the following changes:

Changed the I/V resistor to Vishay S102c 61.05 ohms and the parrallel capacitor to 13.7nf Rifa PFE225 polystyrene giving a cutoff frequency at 190.29Khz. Sound was really good but the volume low so i etched a board of Yardbird from Russ White

YardBird OPA627/637 BUF634 preamp/headphone amp

I replaced R2 with a piece of copper lead and replaced R5 value to 15kohms RN55 dale. Now the sound is of proper volume but less trasparent maybe due to the fact that i use generic metal film resistors and a standard lm317 lm337 psu to power the Yardbird (soon to be replaced by a Salas V1.0).

Are the resistor values in yardbird correct for this kind of use? Can i use lower values to lower resistor noise and retain the amount of gain?
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Old 7th April 2013, 10:17 PM   #32
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Quote:
Originally Posted by marce View Post
Isolated power for the crystal/osc either with an LDO or minimum cap, ferrite, cap.
Isn't it desirable to have a ferrite or inductor on the LDO's input before the input cap? The LDO's regulation bandwidth is probably 10s of kHz max, meaning high frequency chaff from the digital side passes through the junction, package, and layout capacitance parallel with LDO's input and output. I haven't seen any measurements of this above 10MHz or so but regulator PSRR is generally fairly degraded by a few MHz. For a 100mA or so LDO that'd be used to power an XO a 10 cent chip inductor and some decent layout yields an LC input lowpass that's good to a few hundred MHz. Hits peak attenuation in the tens of MHz, which is right where you'd want it for isolating typical audio clocks.

Quote:
Originally Posted by skouliki View Post
Would an opamp gain stage after passive i/v be a bad idea? (I have some LT1361 and opa627 laying around )
As Ken and Abraxilito have mentioned it's a good idea. Figure 10 of the datasheet uses a pretty typical second order multiple feedback lowpass that's a reasonable starting point for either voltage or current out. IMO the additional image rejection from third order is worth it but there's no hard requirement to filter before the op amp inputs---both Burr-Brown (TI PCM) and ESS use instrumentation amplifier type topologies where the DAC output goes directly to the op amp summing junctions to achieve DNR 15+dB greater than the AD1865's with basic parts like the NE5532. There is usually a shelf in the instrumentation amp front end's feedback but it's not normally more than 25 or 30dB. One of the more interesting variants of this topology is the one Analog uses with the AD1955, which also has a cap in parallel with the IV resistors.
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Old 8th April 2013, 01:21 AM   #33
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Quote:
Originally Posted by skouliki View Post
About using an RC filter on the opamp inputs (excuse my cluelessness! ): Inserting a capacitor to ground after the I/V resistor forms an RC filter or not?
RC filtering doesn't provide much protection of sensitive opamp input stages from the copious RF coming out of a DAC. Nor, as Ken's pointed out, does it do much for image rejection.

I've recently put up on my blog an active elliptic filter with about 30dB gain which could be a viable solution here, though I've not yet built it myself. The opamps can be very fast ones (I'm planning to try with AD8014, very low power for the GBW) and hopefully the 50dB stop-band rejection will help considerably with IMD in the downstream amp. I'm curious to find out how it'll sound compared to a similar response passive filter.
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Old 8th April 2013, 10:44 AM   #34
JLOP is offline JLOP  France
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Quote:
Originally Posted by skouliki View Post
Thanks everybody for your suggestions.
Creating a double sided version with a ground plane is a very interesting idea (and easy enough for my amateur level).
Hi Skouliki

Did you make a double sided design in the end ? Would you share it with us, i am much interested !

Thanks

Jean-Louis, from Geneva Area but born in Athens (;-)
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Old 8th April 2013, 02:08 PM   #35
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I created a ground plane version on ExpressPCB (havent etched it yet) but i am hesitant to post it here
I will post it tomorrow so be gentle with my silly mistakes
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Old 8th April 2013, 02:08 PM   #36
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Quote:
Isolated power for the crystal/osc either with an LDO or minimum cap, ferrite, cap.
Not both either, though a pi filter with a ferrite will remove any high frequency noise from other and is often used especially where LDO's follow a switch mode usually where multiple voltages have to be generated for a processor, FPGA or the like.
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Old 8th April 2013, 02:44 PM   #37
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Quote:
Originally Posted by skouliki View Post
I created a ground plane version on ExpressPCB (havent etched it yet) but i am hesitant to post it here
I will post it tomorrow so be gentle with my silly mistakes
Dont worry, I am pretty much a newbie there too, although trained as EE I spent most of my time with stuff like SAP Siebel...
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Old 8th April 2013, 10:11 PM   #38
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Ok posting the boards... hoping i havent done something really stupid

In the 24bit board i plan to add the 3 MIC5207 regulators later
Attached Images
File Type: jpg 24bit.jpg (115.7 KB, 180 views)
File Type: jpg 24bit192khz.jpg (114.0 KB, 176 views)
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Old 9th April 2013, 07:32 AM   #39
marce is offline marce  United Kingdom
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First thoughts:
power tracks could do with being a bit thicker, same width as signal traces.
The SMD caps next to the pins on the large IC could do with moving away from the pins a bit, this will ease assembly especially soldering.
Try and keep traces either 90 or 45 degree, a minor thing, keeps things looking tidy.
Vias look close to decoupling caps on chips on the bottom of the board.
Large capacitors cover SMD caps footprint on large chip, high frequency SMD decouplers need to go next to pins, larger ones can move away from pins.
Don't use thermal relief on Vias, it is not needed.
Other than these minor points well done for a first attempt.
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Old 9th April 2013, 07:46 PM   #40
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Thank you marce for your kind words and suggestions. Really appreciate it
I will do modifications based on your suggestions.
The tracks have all the same thickness (0.51mm), maybe resizing the screen capture caused some distortion on the image presenting some tracks thinner.
Thanks again
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