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

[mediatechnology]

An assumption definitaly not to make is to assume that the carbon comp resistors really are the correct value in 2010.

I just threw out a bunch of 5% carbon comps that were off in the 20-30% range. Others from the same period, depending on value manufacturer etc., were spot on.

[emrr]

Oh yeah, certainly. The critical values are all 1% types, and measure dead on. What did they used to use for 1% back then?

[emrr]

Also with old carbon; the amount of change after soldering. You can hand pick a value, but you can't count on it after heating.

[JR.]

Old precision Rs were often wire wound, but I wouldn't presume to guess what is in yours.

I recall when I was spending a lot more time on the bench, I was very impressed by ROHMs carbon films, since they were very good (5% nominal values typically measured <2%) and I have had some red hot that returned close to original value when they cooled down... I fought to keep using them at Peavey and survived several cost saving attempts but finally lost to a cheaper brand. Fractions of a cent, make a difference that can't be ignored for the volumes we were using.

JR

[klangumwandler]

I usually assume they new 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.

Hi Doug: I assume this is a BA-103 or other variant—on the schematic I have, the secondary of the A-24 is shunted with 44k2, which should present about 1k7 load to the mic. I take it you don't want to take out the source select switch? Otherwise a reverse log pot there would make sense.

Best,

James

[emrr]

590 ohms for the 200 ohm tap; correct? Or is my math wrong? Every unit I've seen is hooked up for 200 ohms. Anyway, I covered it all earlier in the thread.

[klangumwandler]

Right—I assumed it was using the 600 ohm tap. On the schematic it is listed as 600:15K, with a gain of +13. I haven't seen any of the mic preamp modules--I have some BMM-100 line modules with the UTC A-20 input and Painton fader.

[emrr]

The 200 taps all appear stock, with no evidence of 600 connection. Strange. I think I'd rather lose or change the secondary load resistor and see what happens, especially if introducing a 20 dB U pad on the input.

The trend continues; all I have to do is hand draw a schematic, and the real one appears. My own private magic trick.

[emrr]

Looking more closely, I think you're right, and someone moved all the wiring on all of mine to the 200 ohm taps, very very cleanly. The 500 terminals look untouched at a glance, but not so under closer scrutiny. So, less weird than I thought.