" bravely wrote to "All" (19 Mar 05 10:51:01)
--- on the heady topic of " Reducing hiss by changing op-amps ?"

It is relatively easy to measure noise. The simplest tool to use is
the very amplifier the noise is in. First measure the amplifier's
gain, then view the noise on a calibrated scope graticule. The RMS
value will be roughly equal to the Peak-To-Peak value divided by 5.
Depending how well you judge the peaks, the input noise will be equal
to the estimated RMS value divided by the amplifier's gain previously
found. Easy as pie.

A*s*i*m*o*v


pc From:
pc Xref: aeinews sci.electronics.design:4378 sci.electronics.repair:43450


pc Pooh Bear wrote:
Would be interested to know what Q101 and 102 are though.

pc They are 'KTC2240BL' (the component list is on the bottom left of the
pc schematic).

pc As for measuring the noise properly, I'm afraid my methods are
pc probably next to useless ! All I have to gauge it by are my ears, and
pc my extremely un-scientific 'soundcard method'. i.e. I'm plugging the
pc output from the mixer directly into the line-in on my PC's soundcard
pc and measuring the signal level from within an audio editor (Cool Edit
pc Pro). As none of this is really calibrated, the numbers are probably
pc meaningless, but the differences between the numbers should be ok.

pc e.g. -
pc with the cable to the soundcard shorted, I get -76dB showing.
pc with the cable connected to the mixer output (but nothing assigned to
pc the output and all faders to zero), I get -71dB.
pc with a microphone connected to channel 1 (heavily wrapped up in cloth
pc in a silent room - as I have no resistors lying around !), ch1 fader
pc and master fader set to 0dB, pad switch OFF, input trim ZERO, I get
pc -68dB
pc As above, but with the input trim set to max gain, I get -37dB

pc Like I said, I don't know if any of this is helpful, but it's all I
pc can offer right now ! :-)

pc Cheers,

pc Kev.


.... A stereo system is the altar to the god of music.