The new boards changed by:
1. caps added in parallel to R3 for better CMRR
2. smaller caps used for DC blocking, allowing for a much less cramped layout
3. revised trace layout
I had an EE friend look over the board and give me feedback. Functionally it can be built exactly the same as my original version, but these tweaks should allow for better performance overall. The fader option was fixed in the last revision, so it didn't change for this one.
Talked to the PCB fab guy today and he said they are in production now. That means I should see them early next week.
I think the prices will end up at 40 USD for the 8 channel main board and 3 USD for the switch board (or 20$ for 8 boards). There will be a white market thread as soon as I have them in hand and confirm they are up to snuff.
I have had some questions regarding PSU's and opamp compatibility.
PSU - there are kits available on this forum for various projects. Alot of these will work. What you need is a bipolar supply capable of +/-15v-17v (a larger range is fine as well). Remember, the opamps give their power specs for what they want at their pins not what the supply puts out. So if you are using the blocking diodes (if you don't understand what these are doing please use them, if you do then I'll leave it to you to make your own informed decision) then you are losing a volt there. Meaning you will want to trim your supply to +/-16v instead of 15. Jeff's summing board uses the same blocking diodes, so if you are confident you will not wire the supply lines backwards you can skip these diodes on both boards and have no need to compensate. As for draw (Amp capacity) you need to look at the spec of your desired opamp and multiply it by the number you will have in the box. So a SL Red Dot, for example, draws 60ma MAX. If you have a 24 channel box then you have 28 (channels+summing) opamps potentially drawing 60ma (this is the totally over mic'd drumkit crushing mix down). So that is 28*60=1680ma. So unless I am completely burnt out right now, that is 1.7A potential load. This mixer needs a PSU with more capacity than 1.7A. How much more? That's your call. I used my own 3A PSU and my box, 24 Gar2520's and 4 SL Re Dots, always seemed happy, but that was my mixer, in my studio. I think the guys who have shared their builds in the thread have mentioned what they are using for a supply, so be sure to go through the thread.
Opamp Compatibility - The circuit was designed from the original 2520 application sheet and then tweaked based on the input section of an old API desk. Anything designed to be a drop in replacement for the API 2520 SHOULD work. I have only personally used the Gar2520's I made from Gary's kit. If you want to use another opamp with the same footprint (990, gar1731, Forsell, or any of the many other) I would send the stereotype schematic to the designer and ask if any values should be adjusted. If you do that, please be sure to report back any info here in the thread. I am excited to see some of the mutations people will inevitably be building.
ok, that's enough typing for this post
don
