Hello everyone!
New to the forum ... I've learned SO much from you guys and have much appreciation for all who post here.
I don't have an EE background, but hopefully I can share some useful information.
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So I made it a pet project to try an trick out my ada8000 after reading this thread and a few others on this forum...
Here's what I did:
I followed Joe's mod and sent the line input signal directly to the a/d chip by bypassing the whole input section, including the opamp. I replaced the two 22u electrolytics with some Black Gates and the 3n3 with 4n7 Wima poly per the al1101 data sheet's recommended value.
On the output I replaced the stock opamp with a MC33079 per jacob's suggestion.
Here are some sound samples I made which might help illustrate the differences w/ the mod and without.
I only modded one channel, so in this test, I ran a Beck tune in a loop back test through the modded converter and then a separate test through the stock converter.
In order to magnify the differences I repeated this loop back test 32 times per file!
Although this is not something you would normally do in a mix situation, I think it's a good way to test how robust a converter is in it's ability to accurately reflect the source signal.
(FYI - all files in this test were clocked by the Apogee Big Ben external clock)
Here are the sound clips:
http://www.musicians-samples.com/Original Track.wav
http://www.musicians-samples.com/Mod x 32.wav
http://www.musicians-samples.com/Stock x 32.wav
I urge you to listen to them with good monitors or headphones... I'll let you interpret the results.
A couple things to note:
The ada8000 I have does not exactly match Jacob's schematic. A few revisions were made since Jacob drew out the original... the 3n3 capacitor between the + and - of the A/D inputs is now a 1n8, and a few resistor values were changed or just plain missing from the board (from what I could tell).
A couple questions:
1. The input on the modded channel is very hot, and adversely the output on the modded channel is very low. I want the input and output to reflect +18dBu = 0dbfs.
Can I just change the 220R resistors on the input (Joe's mod) to a higher value to knock back the gain? Can I change some of the resisters on the output to a lower value to increase the gain?
2. What exactly is the capacitor between + and - doing? The data sheet says "To remove high frequency noise at the differential inputs, the capacitor between the differential inputs should be located as close as possible to the input pins."
Is this capacitor acting as a lowpass? Can I use a lower value cap to allow in more high frequency information?
Thanks again for everyone's input! Experimenting with this mod was a lot of fun, and I got my first shot at working with SMD parts. (They hurt your eyes!)