Re: All right, Patrick

"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> wrote in message news:4437E75B.9E408083@xxxxxxxxxxxxxxxxxxxxx
alright, but with 22.55k and 0.13 uf, the product is 2,931 uS, not
3,180 uS. so -3db is 54.2 Hz.

I neglected to consider the effect of the 3.2K resistor, which adds
to the total series resistance seen by C1. This lowers the corner
frequency.

if C2 wasn't there, the attenuation at R1 output should be negligible
at 10Hz, but by 10kHz should be exactly -20dB; you should have
poles at 50hz and 500Hz. so R1 = 9R2, so if R2 = 3.2k, R1 = 28.8k
can you show by first principle reasoning that I am wrong?

But C2 is there, and the network never hits a flat response of -20dB.
At midband (1000Hz), where RIAA is specified to be -19.911dB, the
reactance of C2 is a non-negligible 4.8K Ohms.

I ain't very good with equations with reactive j quanties, vector
analysis etc. I just measure and trim; its quicker than calculations.

It's certainly quicker if you're no good with equations.

Everywhere i look around the web there are different relevant values
of R&C which are supposed to give the same error-less outcome, but
they are just not all right; and especially RIAA feedback networks.
They all blather on with schematics and calculations, but few
measurements.

If you account for all strays and second-order effects, calculations
will be exact. It's easier to use a reasonable approximation to get
within, say, 0.5dB and trim the network on the bench to get it
spot-on.

Well that is about true. But trimming should be not necessary when
a computer is trying to see us right; we employ computers to be
accurate within 0.1%.

We would have no RIAA curve in the first place without calculations.
I don't deny that you can get good results without math, but in this
case I think your lack of math background causes you to be unduly
suspicious of computers and calculations

The noise generated by the network after the first gain stage is
quite negligible compared to the noise already in the amp from
V1 grid input noise and flicker noise and shot noise.

The 20dB loss introduced by the network adds directly to the noise
figure measured at the input to the second stage. If the first stage
had only 20dB of gain, at midband the signal level at the input to
the second stage would be the same as at the first stage, and so
the second stage noise would be as significant as the first's. There
doesn't seem to be any advantage to degeneration in the cathode
(or source) of the first stage except as required to set the bias
current and to insure sufficient input overload margin. Every dB of
gain in the first stage lessens the effective noise contribution of
the second stage.

-Henry


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