> Who wants a mixer with just one input used?
That's just argumentative.
Very few real mixes have all mikes at the same gain. (A choir may be an exception.) If one is more than a few dB hotter than the rest, it dominates the output hiss. Anyway it only assumes that the mix-network designer has done reasonably well. If mix-node impedance is similar to mike impedance, we can simplify and say that mike-amp gain must be greater than the number of inputs. An 8-in only needs 26dB mikeamp gain. Since the card in question goes down to 35dB which is 1:56, 16- and 24-in mixers should be dominated by mike-amp not mix-amp hiss.
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> the measurement conditions: a - 50 signal was input to a microphone preamp, and the gain thru the console adjusted to produce +4 dBm at the output. All other faders down. The signal was removed and replaced by a 200 ohm resistor. Then the noise on the console output was measured over a 20 to 20 kHz bandwidth. It was guaranteed to be 80 dB below the + 4 dBm output.
So: input at "-50" (dbM or dBV or dBu?), total product adjusted for 54dB gain.
If "-50" is dBu or pseudo-dBm-in-600, that is 2.45mV in, a reasonable realistic level.
Now shunt 200 ohms, find output noise at -80 below +4dBm. -76dBm.
Referred to input, (-76)+(-54)= -130dBm.
Which is 0.245 micro-Volts.
A weeny 2N5087 at 0.3mA will do 0.280uV in 20KHz. The 0.28uV is datasheet typical; some will be worse, and some will do a bit better (without breaking the Law). For the time and budget, it is not hard to weed-out the hissy half and get a better average. A fatter device will do better. The very-spiffy LM194 seems to be under 0.2uV at 0.3mA.
0.245 apparently-measured, versus 0.28 or 0.2 for readily available devices, is tight but hardly worthy of scorn and doubt.
I do have quibs (not full quibbles). Is there a transformer? Is the "-50" input referenced to 600r, 200r, 150r, or what? What is the -50's source impedance? (If 50 ohms, then effective gain drops a small amount with the 200r resistor; if a 600r pad then gain rises on 200r source.)
I _am_ reminded of some other old designs with odd action. The first BiFet low-price consoles were a 1:1 tranny into an opamp INVERTER with 470 in and 0-500K gain-pot. Maxed, it had gain of 1:1,000; but if you DIS-connected the input it was unity-gain and utterly silent. Which is not proper measurement technique but actually a useful thing in live-sound where inputs did get pulled loose. (Easier on the audience, harder on me finding why the cowbell was gone.)
That's just argumentative.
Very few real mixes have all mikes at the same gain. (A choir may be an exception.) If one is more than a few dB hotter than the rest, it dominates the output hiss. Anyway it only assumes that the mix-network designer has done reasonably well. If mix-node impedance is similar to mike impedance, we can simplify and say that mike-amp gain must be greater than the number of inputs. An 8-in only needs 26dB mikeamp gain. Since the card in question goes down to 35dB which is 1:56, 16- and 24-in mixers should be dominated by mike-amp not mix-amp hiss.
_________________________
> the measurement conditions: a - 50 signal was input to a microphone preamp, and the gain thru the console adjusted to produce +4 dBm at the output. All other faders down. The signal was removed and replaced by a 200 ohm resistor. Then the noise on the console output was measured over a 20 to 20 kHz bandwidth. It was guaranteed to be 80 dB below the + 4 dBm output.
So: input at "-50" (dbM or dBV or dBu?), total product adjusted for 54dB gain.
If "-50" is dBu or pseudo-dBm-in-600, that is 2.45mV in, a reasonable realistic level.
Now shunt 200 ohms, find output noise at -80 below +4dBm. -76dBm.
Referred to input, (-76)+(-54)= -130dBm.
Which is 0.245 micro-Volts.
A weeny 2N5087 at 0.3mA will do 0.280uV in 20KHz. The 0.28uV is datasheet typical; some will be worse, and some will do a bit better (without breaking the Law). For the time and budget, it is not hard to weed-out the hissy half and get a better average. A fatter device will do better. The very-spiffy LM194 seems to be under 0.2uV at 0.3mA.
0.245 apparently-measured, versus 0.28 or 0.2 for readily available devices, is tight but hardly worthy of scorn and doubt.
I do have quibs (not full quibbles). Is there a transformer? Is the "-50" input referenced to 600r, 200r, 150r, or what? What is the -50's source impedance? (If 50 ohms, then effective gain drops a small amount with the 200r resistor; if a 600r pad then gain rises on 200r source.)
I _am_ reminded of some other old designs with odd action. The first BiFet low-price consoles were a 1:1 tranny into an opamp INVERTER with 470 in and 0-500K gain-pot. Maxed, it had gain of 1:1,000; but if you DIS-connected the input it was unity-gain and utterly silent. Which is not proper measurement technique but actually a useful thing in live-sound where inputs did get pulled loose. (Easier on the audience, harder on me finding why the cowbell was gone.)