op amp gain; vary Rg or Rf, and why?

[emrr]

next dumb question. The C in series with RG, almost always seen on the ground side. Is that placement important? If placed on top of the R, or even between two R's, do we have a biased condition that causes problems? Leakage enough to still cause DC gain?

I could keep this rework more simple if I could add variable R between C and ground, with fixed R on other side of C, going to op amp.

[JR.]

Series components can be in any order and will electrically be similar, but if one end of the network gets connected to ground there may be an advantage to connecting the larger component of the two to ground for any self shielding available.

Old school electrolytic caps had the outer metal case connected to ground, new school caps not so much.

JR

[mediatechnology]

Doug - What John said.

Old school electrolytic caps had the outer metal case connected to ground, new school caps not so much.

You might still want to determine which terminal. + or -, is the outer wrap even though the can floats. (I think you'll find it to usually be the - terminal.) I found large differences in "terminal to can" capacitance in large value Cs such as those used with typical Cgain values. The one with the larger C to case is probably the side you want grounded or driven by the lower impedance. I found it to be less of an antenna that way.

I had a drawing with those measurements but can't find it.

[emrr]

It's tantalum beads in this case, rather than can type electrolytic. Only problem I could imagine was potential build up of a bias at the cap, if it was above ground, and whether or not that might have a bit of micro-leakage that might still thump the gain switch. As I drew it in the alternate plan, I show the R and the C as separated, but I'd have to cut a trace to do that. The amp and Rg network is on a daughter card, and only connects to common when plugged in. Inserting additional R at that connector junction would be the simplest modification.

[JR.]

Electrically there is no difference in what order two components are in. There are only subtle differences related to mechanical or stray effects caused by different physical location.

JR

[emrr]

This sucker is already old style 'spread out all over the place' layout, so improvements are pretty easy.

[JR.]

Don't assume they didn't know what they were doing, while modern components may have slightly different quirks, like the floating electrolytic cans, etc.

JR

[emrr]

I usually assume they knew exactly what they were doing, but the original gain method now gives me pause, as does the fixed 590 ohms presented to the mic.

Have a look.

[JR.]

I suspect the external components are not the weakest link in that signal chain.. While it's always possible to make something worse...

My dad was a recording engineer for RCA back in the 50's I wonder if he worked with stuff like that?

Have fun, and don't lose too much sleep over the minutia

JR

[emrr]

UTC A-24 and API 2520. I meant to mention this is the only RCA broadcast preamp I've encountered which has less than a bridging input Z, and I've pretty much had or seen them all. These things start in 1969, and run until at least 1974, meant for custom TV consoles specifically. They kept selling the standard discrete fixed gain BA-72's to the general broadcast market.

I'd think your dad would have used the octal tube stuff. RCA didn't introduce SS to broadcast until '59 or '60, and kept selling the 9 pin tube stuff on through the late '60's.