As an update for the prototype team, tomb is going to send me the PCM chips to me to include with the boards when I mail them out. I will ask for the money for shipping and the addresses of those that are getting protos when I am ready to ship.
I've been asked about op-amp rolling, and here are my thoughts on that.
So far, I've attempted to use three DACs on the board
- THS4222 - extremely high speed, but was not stable at all
- LMH6643 - this is the current choice for me. Personally, I think the board sounds so good with this, I haven't seen the need to try others
- LME49721 - this is a "slow" opamp and seems to be stable when the other start to be unstable and show offset issue.
There is two choices for the rails and this can affect your choice of op-amp
- Regulated - using the BOM parts, you get +-2.5V on the rails.
- Unregulated - don't install the regulators, and jumper 1A to 1B and 2A to 2B. You have to understand what you're dealing with though. USB input power can be 4.75 - 5.25V. The charge pump will invert that to equivalent negative voltage. The issue comes that the charge pump droops as it gets more loaded. The rail will droop up to 0.5V, so the negative rail could end up at about -4.25V. When considering op-amps, for safety, I would consider +-4V as working, but they need to be able to handle +-5.25V.
If you are looking at the regulated option, consider rail-to-rail output amps to ensure that you don't have issues.
As I mentioned in the initial post for prototypers, the layout of the board, may be prone to offset issues. My board is stable with the LMH6643, but I have built one (where I put the output resistors on the bottom side) and it had offset issues on one channel (150mV IIRC). You will need to check your build for offset, and if it is excessive, the easy fix is to replace the LMH6643 with a LME49721.
Minor offset, depending on your setup, you can live with. If there is any stage that is AC coupled, you are good, If you have a DC servo in your signal chain, you are also likely good depending upon its range. If you are full DC coupled with no servo, you'll have to look at the offset and your gain and decide upon your risk tolerance.