brad.sanders at circellar.com brad.sanders at circellar.com
Sat Mar 22 20:05:43 CET 1997


A>2) The problem is with making low level measurements of say =
A>"almost-to-CD quality". A 14 bit (cheesy $59 CD) system has a 78dB
A>SNR, = or almost 1 part per million. Thats 1uV (microvolt) of noise on
A>a 1 volt = signal (RMS).

Yes - but with a noise amp we no longer HAVE a 78db S/N in the "meter." 
If we stick a 40db amp in front of that cheapo soundcard, what we have 
is 78db (for example) + 40db, or 118db S/N. Of course, this would 
require about 100V "0db" level! But for making noise measurements
we don't NEED to be able to input a 0db level.

So, with a good 40db gain amp feeding the soundcard, we still only have 
78db or so dynamic range - but (figuratively speaking) we have the 
BOTTOM 78db... where the noise is. Short the input to the amp, take a 
reading; connect the DUT and take another reading, and the added noise 
is coming from your DUT.

A> So now we are back to the "if I can't hear
A>it, it doesn't = exist" arguments of rec.audio.high-end....

OOOOH no. All I have to say on THIS topic (about five years worth) is in
the archives. I've weaned myself from this world quite well, thank you. 
I don't even wanna see the letters "rahe"...

A>3) There was a HP Distortion meter, basically a notching vector =
A>voltmeter, that was in use for about 14 years. It was  absolutely =
A>sufficient for measuring any audio equipment pre CD.

Actually, many of us still use them things. A good notch filter, noise 
amp, and a low distortion sine oscillator can get one to .003% or so.

A>I am redesigning the DKI Synergy synth (Wendy Carlos' favorite). They
A>= use 24 bit math, a (at that time) $250 16bit DAC, and 5534 output
A>amps. = But guess what drives the 1/4" outputs? A LM348 quad 741! Why
A>in the = world?

This is alot more common then most folks realize. AD even has appnotes 
on employing the slew limiting response of cheap opamps in the post 
filter of DACs to help reduce parts count. By carefully biasing the amp 
for proper slew limiting, you can add two poles to a "regular" three 
pole lowpass opamp filter, thereby saving one extra opamp stage while 
still reducing ultrasonic noise output.

In fact, this is used in about every consumer CD player I've ever worked
with... just call it a "feature."

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