What's everyone using for output protection these days?

[jaymz168]

Hi all, I got into Eurorack recently after years of resisting the GAS. Most of these Euro mixers have single ended outputs and you end up having to eat up space with modules to balance the outputs. So I'm planning to make a little tile thing that can sit inside the case and go between the module and the output jacks. Since I want to make this as small as possible I'm going to use SMT wherever I can and I'm trying to optimize the output protection diodes. I'm using THAT1646 output devices and they recommend 1N4004. Alternatively I've been looking at RS1B/D, BAT60 or even BAT160 dual common cathode.

Here's a screenshot to show the kind of dimensions I'm working with:



What's the part du jour for this at the moment? Any other suggestions? Thanks!

 

[ruffrecords]

Why not simply impedance balance the outputs?

Cheers

Ian

 

[jaymz168]

Why not simply impedance balance the outputs?

Cheers

Ian

• The additional signal level from both legs driven is appreciated
• I personally just want to do something with this chip. I'm also designing a 1U Eurorack mixer in parallel that will have something similar on board. This more simple project is something of a pilot for that. If you have suggestions for a more contemporary part with better performance I'd love to hear them. I'm not an EE, just a guy tinkering around.
• It's my first PCB that I plan to have made and assembled and I'd like to have an active design to test some things out. At that point I might as drive both legs and according to the datasheet the THAT 1646 is fine driving into single-ended loads so that covers that scenario.

As for the protection scheme

• The reasoning for output protection is simply because of deployment in various environments where device and/or operators may not be operating correctly. Mistakes happen, gear gets broken.
• The two AES papers on the subject focus on the mic preamp side of things but demonstrate currents of ~4-5A from 47uF coupling caps on the preamp side. That's the design spec I'm going for. I've been sorting for peak surge current around 10A so I think that should do it.
• THAT specs 1N4004. I'd like to go smaller. Also my reading suggests that leakage current is related to increased distortion so I want to minimize that
• In the typical parallel diodes to the rails setup the current returns to the rails. I have 12V rails and the caps are 25V parts. I'm guessing this means that I have to bump them up to 60V parts to handle conducting this? Also my bulk filter caps are only 10uF, do I need to bump them up to 47uF to handle the charge going through?

 

[Newmarket]

Smaller - as in physical size ? If so then SMT is your friend - without checking my facts here look at BAS116 for low capacitance.

 

[jaymz168]

Smaller - as in physical size ? If so then SMT is your friend - without checking my facts here look at BAS116 for low capacitance.

I've been strongly considering RS1B with its low leakage current but its capacitance seems to be 10pF compared to the 2pF. What's the significance of the capacitance in this application?

 

[merlin]

Also my reading suggests that leakage current is related to increased distortion so I want to minimize that

That applies when there is a significant resistance in series with the signal, before the diodes. This doesn't apply to output protection diodes since the source impedance of the IC is extremely low.

 

[jaymz168]

That applies when there is a significant resistance in series with the signal, before the diodes. This doesn't apply to output protection diodes since the source impedance of the IC is extremely low.

That's good to know. If that isn't a concern, then is there any other parameter I should optimize for other than size and surge current? Newmarket mentioned capacitance, what is its significance in this application? I'd guess it forms a single pole RC filter in conjunction with the output resistance/impedance? For two RS1B in parallel and 50R output impedance I'm looking at an F3 of like 160MHz so that doesn't seem like a concern in this application.

 

[merlin]

Newmarket mentioned capacitance, what is its significance in this application?

Again, same deal as with leakage. Capacitance would matter an an input, but at an output it is negligible / swamped by cable capacitance.