AC coupling caps

??? I'm supposed to provide 20 Hz at the low end. WTF are you talking about???

This design is not so cost constrained, but it is very size constrained. Higher value caps can be larger. Also, with the issues of supply, locking into a higher value cap in the smallest possible footprint, means you have no alternates if supply gets tight.

No, this is not a trivial design decision for some cases. In designs like Larkin's, where he uses a board some three times the necessary size and loves to marvel at the beauty of the regular arraignments of the long traces, you can use any part you want, and even provide multiple footprints for alternates. I don't have that luxury. I barely have room for vias. They are actually a PITA, taking up room on all layers. Too bad I can't just eliminate them.

Imagine, then, the problem of IC design in the days before multilayer metallization; examine the old TTL books, the circuit diagrams don't ever show wires crossing, except for wires crossing a resistor (buried layer) or multiple connections to a transistor base (also a buried conductive layer).

you can get blind and buried vias and you remove the annular ring on unconnected layers

Blind and buried vias are expensive, $$$. Even removing the annular ring on unconnected layers still leaves a large keep out zone around the via hole. The point is, this impacts layers that have no routing for that trace, just the via. Adding vias can approach being counter productive, using nearly as much space than they free up by using other layers. It's much better to route on a single layer, where possible.

If I have a fixed format, like VME or PXI or a 1U rack, there may be lots of room for parts, so a board can be neatly arranged. Sometimes, in a limited area, we want to pack as many channels as possible, and do that using both sides for parts. Often thermals dominate density.

But all schematics and board layouts should be beautiful, because that's satisfying and because beautiful things work better.

Audio is pretty simple, given objective, measurable goals and not measurement by golden ears. Most cable drivers are low Z, receivers are hi-Z, and there is no concern for the characteristic impedance of the cables. The 600 ohm thing is an artifact of lossy open wires on telephone poles from a couple centuries ago.

Hey, let's sell a special DVM just for audiphiles. (Audiophils?) It would be very expensive and gold of course.

This board probably is half the density of mine. I see gobs and gobs of room for more components if you needed to add them. With my board, it's hard to add anything, without removing something else.

Yes, and 1 Mohm in an op amp circuit can add high frequency roll off from parasitics, as well as DC offset from the input bias current. I had to lower the FB resistor from 200k to 100k to get that under control. So now the input is 36k. But that's not the point. The input impedance is 50 ohms when used single ended and 600 ohms differential. That requires much larger input caps than 1 uF.

Assuming you know or can measure the inter-stage impedances, why not just Spice it?

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So here is a 6V reference too, for the op-amps. Make that the common ground and the need for DC isolation of ins and outs disappears.

...... Phil

Sorry, I'm not sure what you are getting at??? I have a spice simulation. What are you trying to say about it???

Common ground for what? The input? Sorry, that won't work. The rest of the system is using the ground as ground. Even though the inputs are differential, there is a single ended signal mode. That has to be ground referenced. Also, the CODEC can't be referenced to the 6V virtual ground since the ground is shared between analog and digital. That's the second DC block required. The output side has two DC blocks, codec to op amp and op amp to output.

It's only the DC block on the 50 ohm terminator I'm concerned about. I was thinking about this on the flight today and am thinking of removing the caps in front of the terminators, and just using the ones after the terminator (3.3 uF). I will beef up the power handling on the 50 ohm terminator. The 600 ohm termination should be ok. It takes a lot more voltage to kill it. (0.5W * 600)^0.5 ~= 17 Vrms (0.5W * 50)^0.5 ~= 5 Vrms

So I'll bump the 50 ohm termination to 1W which should allow short term voltage inputs of up to 10 Vrms. Eliminating the pair of 1206 caps will more than make up the increased size of the resistor.

The existing design has 1 uF into a 51 kohm input impedance. This design will use a 39 kohm input impedance at the op amp, so a 1 uF will work ok, as will 470 nF.

Mea culpa. I didn't read your original post properly soz about that.

it adds a few more production steps but since your board is probably small it might not add that much per board

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..... Phil

Except that it doesn't. One of my concerns is that applying something like 5V or 12V to the input will exceed the rating of the 50 ohm input impedance resistor. The 600 ohm input impedance resistor requires a much larger input to over drive it. The design is very, very tight, so I was focusing on keeping the resistors small. But the caps are much larger. It's probably best to increase the wattage of the 50 ohm resistors to suit. I need to check to see at what input the switches are over current though.

It's nice when you have total control over a design. This design is a 4 square inch daughter card, with an FPGA, RS-422 I/O, and two channels of audio input and output with selectable impedances. I suppose I could construct a daughter card for the daughter card. Otherwise, there is no place possible to add a -12V supply.

Actually, -12V is supplied to the daughter card, but it's only a few mA, originally intended for biasing serial port drivers. These analog I/Os have up to ~50 mA each and there are two differential outputs. If you can design a circuit for this, that will fit in 0.25 inches square (single sided components), I'd be happy to use it. Oh, wait, it would probably need to use the +12V line and that might be close to maxing out.

it's just a mismatch. And the ratio can be large.

If that were true the load impedance would always be as high as practical. That is clearly not the one aim or the practice.

low Z driving high Z is by definition a mismatch. And is mostly no problem with audio.

indeed.

ok

the reduction of coupling caps to trim off the bottom end to enable speakers & amps to perform better. It's common practice.

Do you mean setting the value of the coupling cap to provide a high pass filter? That's rather a DUH. There are two reasons to reduce the value of the coupling cap. One is to make it cheaper, the other is to tune the filter to the frequencies of interest. But setting the corner frequency of the filter has to take into account all other aspects of the use, which is where this thread started. Perhaps you might want to read the first few posts again.