Modding, and how to get started....

Ok, here's a few thoughts based on 12 years of design experience..

Nobody divides analog and digital supplies. Nobody divides analog and digital grounds anymore either. Decoupling and poor layout are the primary issues with consumer level designs, so you can at least improve the decoupling in most of these designs.

The bulk cap in the power supply is usually sufficient. Where the designers go cheap is the local decoupling and bulk cap in the rest of the circuit. Good caps aren't cheap, so they'll use the bare minimum of the cheapest caps possible.

The rule of thumb is that you'll have a low ESR ceramic cap to ground on EACH power supply pin and one across the pins. That's 3 caps per opamp just for decoupling. Within 2 inches of each opamp should be a bulk cap reservoir of at least 47uf. Also, if a part has multiples of the same power input, use a cap on each pin regardless.

For SMD parts and super high speed stuff, like I design, it's the largest value cap in the smallest package available so that you minimize the parasitic inductance of the package. For me, this is commonly 1uF in an 0402 size SMD package. For audio, 100nF in a leaded part is probably fine.

Schottky rectifiers are nice however aren't necessarily needed. Don't ever add the snubbing caps that a lot of people add across the diodes. This only serves to create RF trash. The problem here is usually poor layout, where folks don't provide good grounding to the rectification circuit and you end up with ground bounce. Check your ripple at the bulk caps while under load. Keep adding bulk cap until the ripple is as low as possible.

Also, paralleling caps with films is perfectly fine to do, but yields little benefit in the audio world. Some people swear by it, but I've never found it to improve anything, in audio or in RF. Just use a higher quality electrolytic and you'll be fine, or switch to tantalum.

Decoupling the CS8420 is a good idea. Figure on doubling the decoupling since these datasheet schematics tend to only account for the circuit at hand, not an integrated design.

Replacing the 4580/4558, etc is probably more of a taste thing. On paper they look decent enough, but since they are cheap, they get used in cheap designs, which probably means they don't have sufficient decoupling, bulk cap, and clean power rails. I bet if you fix these things, those opamps will sound OK.

I agree totally with not bothering with super speed opamps. It's a marketing thing. Once you get above 10V/us slew rate, you aren't going to gain anything but needing to fix everything else to reject RF!

Overall, have fun, but don't expect anything special. Mixed signal design is hard to do correctly and it's almost a certainty that Behringer didn't spend the time and money to do a good multi-layer PCB and proper decoupling. Some products just aren't good bases for modding.


Check out Dr. Howard Johnson's book www.amazon.com/High-Speed-Digital-Design-Handbook/dp/0133957241 for some really good info on high speed designs.  Once you learn high speed design issues, everything else is easy.