Poor input circuit?

[somejohndoe]

I have an pro-amp that has an unbalanced line input that consists of a 10ohm drain resistor and a 220uF blocking electro just before the op amp. These values seem odd to me (cap too high, resistor too low) and I get a corner freq of about 72Hz which seems awfully high. I'm thinking of changing the resistor to 10k and the cap to a 2.2uF tantalum. Is that desirable?

[Frank Berry]

Schematic?

[somejohndoe]

Right. First off, apologies for posting and running but life, unlike amplifiers, is rarely linear . Secondly, my old eyes have once again failed me, well, that and in-circuit testing. The input resistor R108 is, in fact, 100K and not 10 ohm. (missed the fourth orange band). Measurement in-circuit says 11 ohm but if I pull the ceramic bypass 5C8 then it measures 100K. So... another apology on the false alarm, however my gut still tells me that the 220uF electro 5C22 is too high. A 2.2uF puts the corner at just over 0.7Hz, which is fine.

Screen Shot 2011-10-27 at 12.48.49 PM.png

5C8 10nF ceramic
R108 100K
5C22 220uF
R89 220ohm
R88 100K
5C25 10nF ceramic

[Michael Bean]

If you're measuring DC resistance, C-5C8 should read as an open circuit, and C-5C22 should read initially as a low value resistor that rises in value until it is fully charged to the meter's probe voltage and then read as an open circuit also, so after C-5C22 fully charges you should be seeing only the resistance of R108. So if DC resistance reading isn't somewhere around 100K, you have some bad caps in there. Also, I don't recommend using tantalum caps anywhere in audio circuits, not even for power supply decoupling due to there extremely high distortion characteristics, use a good quality film cap instead.

Mike

[somejohndoe]

Thanks. I suggested tants because that is what I have on hand and didn't want to spend more money on this. I have some Sprague 3.3uF axial films that I can make fit but the board is fairly tightly packed so I doubt radial films will work. I'll just add that when I measured, 5C22 was already removed so only 5C8 was present. Now that its gone (it will be replaced for safety sake), the resistor measures fine.

[AndrewT]

R108 discharges current leakage through the DC blocking cap.
6C8 adds in a bit of VHF attenuation.