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Old 31st January 2013, 04:03 PM   #11
Crumboo is online now Crumboo  Sweden
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Quote:
Originally Posted by qusp View Post
the board above is a standard opamp based IV, nothing tricky there, look for the es9008 application note, there is a multichannel schematic and good layout recommendations/application note. again it should be able to be applied with decent results.
Thanks! Is this the one you are referring to?

I assume it is just to use 8 of the schematics in fig. 1. I don't know about the differences between ES9008 and ES9018, are the component values the same for ES9018?

And also (sorry I'm a real newbie of electronics): what does the "offset" to the left in that figure mean?
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Old 31st January 2013, 10:31 PM   #12
qusp is offline qusp  Australia
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yes thats the one, offset refers to the DC offset. the ESS is a V+ only dac, no neg voltages, so the output swings around a DC offset of Half the AVCC supply voltage. youve probably seen the term AVCC/2 mentioned?

well the idea is you take a feed from the AVCC regulator directly (I run at 3.6v, but the spec is 3.3v) and feed this to a voltage divider to get AVCC/2 and send that to the opamps as pictured to delete the DC offset. you can see this trace if you look closely at the layout, is fed to the opamps in between the + and - power planes on the top layer. track it back to see the voltage divider. its hard to see if you dont know its there. I say again, pay close attention to the recommendations on page 5

yeah the values are still about right, depends on how much gain you want if any. the sabre tends to run a bit hot as far as voltage. I like it that way but it depends on your amps I guess. there are a few things I would do differently for sure, but its a good start. for starters I would run a separate AVCC for left and right and feed it from a dedicated supply instead of taking it from the digital supply. I would also avoid the AD797 unless you know what you are doing, not the easiest chip to get right at low gains and the PCB layout is critical.

pay very close attention to the ground plane, it should be solid uninterupted to the opamps and AVCC should be a solid uninterupted power plane from its reg to the AVCC pins. use as small (size) decoupling caps as you can solder. basically read page 5 many times and dont look at some of the other cheap stereo PCBs on the forum for inspiration...

good luck, I gotta say for an electronics noob, this isnt really the best project to start with and could turn into a pretty large money and time pit.

Last edited by qusp; 31st January 2013 at 10:36 PM.
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Old 19th March 2013, 07:02 PM   #13
Crumboo is online now Crumboo  Sweden
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Wow...finally some time to work on this again. And more questions...

Also, thank you for your comments earlier!

I had a look at the schematic of the TP IVY III and was thinking about using this (with the parts according to the BOM). Now I wonder: if I don't need single ended outputs, can I just ignore the parts within the red lines in the attached picture (this is just for one channel)? Or are there other modifications that should be made?

Click the image to open in full size.

Best regards
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Old 20th March 2013, 05:07 AM   #14
pos is online now pos  Europe
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Crumboo, if you end up with a good solution and a 8 channels PCB, and decide to do a group buy, please count me in the list
Meanwhile, I will be using my BIII without I/V stage...
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Old 31st May 2014, 06:35 PM   #15
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Hi pos,
I think Crumboo has managed to build an active I/V stage for his BIII. Do you feel the urge to have one, too? Or are you happy with running your BIII in voltage mode?
In which configuration do you use it, how is it connected to your amps? Do you have a passive RC low pass? Or just a resistor?
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Old 2nd June 2014, 01:02 PM   #16
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[QUOTE=qusp;3350720]yes thats the one, offset refers to the DC offset. the ESS is a V+ only dac, no neg voltages, so the output swings around a DC offset of Half the AVCC supply voltage. youve probably seen the term AVCC/2 mentioned?

well the idea is you take a feed from the AVCC regulator directly (I run at 3.6v, but the spec is 3.3v) and feed this to a voltage divider to get AVCC/2 and send that to the opamps as pictured to delete the DC offset. you can see this trace if you look closely at the layout, is fed to the opamps in between the + and - power planes on the top layer. track it back to see the voltage divider. its hard to see if you dont know its there. I say again, pay close attention to the recommendations on page 5

QUOTE]

Qusp/Anybody
I have the standard "evaluation board" IV stage. See schematic below:
http://hifiduino.files.wordpress.com/2013/01/iv.png
Based on Qusp's explanation above, there should be zero DC offset after the IV op amp as it is nulled by the 1/2 AVCC voltage feed to +ve input of the opamp?
On my board I measure 1/2 AVCC Volts on the negative signal opamp input (from the DAC) and the same on the output pin of the opamp. This is the same on all 4 IV opamps.
What could cause this to happen?
Any help or clarification appreciated.

Edit - Apologies for this off topic post.
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