Preampssssssssss

[Vinny]

Haha wow, never expected a thread revival. I think I'm gonna go with either the Octopre or the Octane (Can't find the Mackie anywhere), and am leaning more towards the Octane.

 

[Omega_Void]

Wow, I would have thought ad/da conversion and all that would be a lot more confusing.

All the confusion's hidden inside the DAC chip. I bet the tricky part is making sure it doesn't oscillate, when you think about the frequencies involved.

That Wavefront chip's unusual in that it has the clock source built in, which should help by reducing the number of RF-carrying traces on the PCB.

What do you want to know about the output on it? Looks pretty standard, the opamps get the signal up and give the 2 phases needed for the balanced output.

Yeah, but look again dude -- the DAC chip already has hot and cold outs!

The first opamp stage is a differential summing with gain, converting the balanced out from the DAC to single-ended, and the second opamp stage converts it back to balanced.

Would have been simpler just to use two gain-stages. Why go balanced to unbalanced and back to balanced? Uses more parts for no advantage I can see. (But maybe I speak to quickly?)

They've done the same on the inputs as well. Why not just have the balanced out from the discrete pre driving the balanced in of the ADC directly? AL1101 datasheet shows 160k typical input impedance, so it's not a heavy load.

I wonder if using say, an OPA4132 would make the whole unit sound better. In my experience with opamps, the OPA's allowed more high end and low end through.

I've tried OPA2134 a couple of times, I'm not a big fan. Sounds more "hi-fi" than TL07_ (it's certainly an upgrade) but has an easy-listening kind of vibe. There doesn't seem such a big choice in audio opamps if you're looking for FET inputs.

Bipolars might be worth trying, but the highish source impedances wouldn't be good for noise, and you'd get DC offset on the cold out unless you added a resistor between +in and ground.

Swapping out the BJTs in the front end might be worth doing.

Although knowing behringer they probably use surface mount chips instead of using a socket so it may be tough to do.

And surface-mount thick film resistors, I shouldn't be surprised. About as noisy as carbon comps, with added non-linearity too. There's only so far you can mod this before it starts getting more sensible to start from scratch.