eacj@batcomputer.tn.cornell.edu (Julian Vrieslander) (03/21/90)
I recently started a design project requiring high quality audio op amps. It's
been a couple of years since I updated my files on these products, and I was
surprised by the performance of the newer chips. Here's a few notes on some
that looked especially impressive. They may be worth consideration by folks who
like to run op-amp tasting parties in their CD players, etc. Note that the
specs provided below are taken directly from mfr's data sheets.
To keeps the winds of controversy blowing, I calculated values for
Vth = SR / (2 * pi * GBW),
where SR = slew rate, in Volts/microseconds,
and GBW = gain-bandwidth product, in MHz.
This quantity has dimension Volts, and been used by some authors as a measure of
an input stage's tolerance for transient overload. They claim that, when other
specs are comparable, amps with higher values of Vth have a cleaner sound.
Analog Devices AD744 (single), AD746 (dual)
FET input op amps with better specs than the OP-42 used in some audiophile CD
decks.
THD: 0.0003% (Vout = 7 Vrms, 1 kHz, gain = 10, load = ?)
Noise: 18 nV/root Hz at 1 kHz
Slew: 75 V/us
Vth: 0.92 V
Settling: 0.5 us to 0.01%
Analog Devices AD846 (single)
Current feedback bipolar amplifier with a low impedence, "highly linear" input
stage. The AD manual has schematics using it in a MM/MC phono stage and in a
line driver, both with DC servos.
THD: 0.0005% (2 Vrms, 100 kHz, gain = -1, load = 500 ohm)
Noise: 2 nV/root Hz at 1 kHz
Slew: 450 V/us
Vth: 1.6 V
Settling: 0.11 us to 0.01%
Precision Monlithics OP249 (dual)
JFET inputs. Features very linear large signal response, with symmmetrical
(equal) positive and negative slew rates. High open loop gain (120 dB). The
spec sheet is unusually complete, even including a node and net list for SPICE
modelling the OP249.
THD: 0.002% (Vout = 7 Vrms, 1 kHz, gain = 1, load = 600 ohm)
Noise: 17 nV/root Hz at 1 kHz
Slew: 22 V/us
Vth: 0.75 V
Settling: 0.9 us to 0.01%
Burr Brown OPA627 (single)
Fast, low noise FET input op amp. OPA637 is faster but stable only for A > 5.
THD: 0.0005% (Vout = 7 Vrms, 1 kHz, gain = 10, load = 600 ohm)
Noise: 5.2 nV/root Hz at 1 kHz
Slew: 55 V/us
Vth: 0.55 V
Settling: 0.6 us to 0.01%
Linear Technology LT1028A (single)
Bipolar op amp. Probably the lowest noise monolithic amp for use with low
source impedances (MC cartridges, tape heads). Very high CMRR and PSRR (both
exceed 120 dB).
THD: 0.0002% (Vout = 7 Vrms, 1 kHz, gain = 10, load = 10k)
Noise: 0.85 nV/root Hz at 1 kHz
Slew: 15 V/us
Vth: 0.03 V
For comparison, I'll include specs for a couple of chips that are often seen in
commercial hi-fi components:
National LM833 (dual)
THD: 0.002% (Vout = 3 Vrms, 1 kHz, gain = 1, load = 2k ohm)
Noise: 4.5 nV/root Hz at 1 kHz
Slew: 7 V/us
Vth: 0.07 V
Signetics NE5534 (single), NE5532 (dual)
Noise: 5 nV/root Hz at 1 kHz
Slew: 9 V/us
Vth: 0.14 V
Finally, here are the numbers for one of my favorites, a 10 year old design:
Jensen JE-990 (single)
This is an audio op amp module made with discrete components. Used in pro audio
gear and in some very expensive audiophile components. Note the combination of
very low noise and high current output capability.
THD: 0.005% (Vout = 12 Vrms, 1 kHz, gain = 10, load = 75 ohm)
Noise: 1 nV/root Hz at 1 kHz
Slew: 16 V/us
Vth: 0.05 V
Price: $80
I've heard good things about the AD748, another new monolithic amp, but I have
not yet seen its spec sheet. If anyone has info about it, or other new amps
with good audio band performance, I would appreciate a follow-up.
--
Julian Vrieslander
Neurobiology & Behavior, W250 Mudd Hall, Cornell University, Ithaca NY 14853
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