Location: Bucks
Posts: 71
I'm Alex.
Location: Seaford UK
Posts: 1,861
I'm Dennis.
Doing some calcs but the hay fever is so bad I cannot concentrate or see properly.
Last edited by Pharos; 26-06-2018 at 13:41.
I wouldn't worry about the mistakes Andrew - we all make them, as I did in one of my earlier posts. Anyway, the points you made were correct, even though the example you used was a bit extreme.
One can readily buy in-line attenuators fitted with RCA phono connectors. They use a simple L-pad and do the job very well. They are not expensive either.
Barry
Location: Seaford UK
Posts: 1,861
I'm Dennis.
OK, amazing how out of practice I have become, and I also had to fix my 43 year old calculator.
I would tend to ignore the frequency dependant stuff because I don' think that filters have been present on many of the I/Ps to amps I have dealt with, but it may of course be a reality in any one case.
I agree, placing one series R at the power amp end does dispense with cable capacitance effects.
If we take two values of resistor w.r.t. Johnsons or thermal noise, say 1K and 1M, we gat the following;
Vn = root (4KBTR) volts, where K = Boltzmanns constant, 1.38 x (10 -23) J/K, B = bandwidth, T = temp Kelvin, and R = resistance.
(10-23) is my way of representing indices, 10 to the power of -23.
Assuming an ambient temp of 20C which is 293K.
For 1K Vn = root [(4 x 1.38 x (10-23) x 2 x (10-4) x 293 x (10-3)]
= root [8 x 1.38 x (10-16) x 293]
= root [3.2 x (10-13)]
= 5.6 x (10-7)V = 0.5 microvolts
This is -124dB w.r.t. 1V, and -118dB w.r.t. 0.5V.
For 1M Vn = root [4 x 1.38 x (10-23) x 2 x (10+4) x 293 x (10+6)]
= root [8 x 1.38 x (10-13) x 293]
= root [3.235 x (10-10)]
= 0.18 microV
This is -94.9dB w.r.t. 1V and -89dB w.r.t. 0.5V.
These figures are not at all bad even for the worst case of a 1Mohm resistor, and the rather unlikely low value perhaps of the 1K resistor.
Location: Seaford UK
Posts: 1,861
I'm Dennis.
You've probably dealt with many more than I have, and I accept your word on that.
The Nelson Jones 10 + 10 Class A went up to 1MHz, and didn't suffer from oscillation problems.