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PRR · 9th January 2005, 07:33 AM

You got good advice.

The Jensen-design 990 is still hard to beat in many critical audio uses. There are several pin-alike "990"s; some don't reach the 990's noise performance or VAST output current (while perhaps being better in other ways). And it isn't a chip but a chunk.

The 5534/5532 is often 98% as good for 2% of the price. But there seem to be several "vintages", some considered better than others. The balance of process specs needed for best audio are different from what most non-audio chips need: most chips want minimum currents, in audio we often want as much current as we can stand, at least in input and class-AB output stages. A foundry that does not dig audio may be "optimizing the process the wrong way" for out needs. I have not done critical comparison, but NJM makes a 5534/5532 copycat and audio is NJM's main business.

LM833 and LM837 -seem- to be reinterpretations of the 5534, generally similar though not the same (and not rated for the high power voltage). Note that 5534 is not unity gain stable (but in a summer, you would only run unity gain with all channels muted; even then you could add a dummy resistor with minimal loss of performance). I had not seen tINY's LM837 MHz oscillation but such problems are common in hot amplifiers.

I have not seen an AD797. The specs look promising. Astonishingly low noise voltage for a chip. If you do DC-couple, it has much better DC specs than most of the audio standbys: 25uV instead of 1mV. THD is low and can be gimmicked lower.

I'm also too old to know the OP275, but I know people who love it.

For eight 10K mix resistors, a fat vacuum tube can be pretty low noise, and has "glow". It is harder to get effective feedback to kill mix interaction, but you may not need theoretical zero-Z mixing in a monitor mixer. A 6DJ8 gain stage with 6DJ8 or 12AY7 (or HV NPN) cathode follower fed-back to unity gain (from one 10K input) would give a summing impedance around 300 ohms, mix interaction of a few tenths of a dB, and drive a reasonably sensitive power amp easily. If the gain stage has Gm over about 5,000 microMhos and you weed-out noisy tubes, overall noise is similar to the summing resistors alone.

Of course as a monitor mixer, mundane details like stability, grounding, and connections are more important than how cool your chip is.

9th January 2005, 07:33 AM	#5
PRR Gear interested Join Date: Jan 2005 Location: eastern USA Posts: 2	You got good advice. The Jensen-design 990 is still hard to beat in many critical audio uses. There are several pin-alike "990"s; some don't reach the 990's noise performance or VAST output current (while perhaps being better in other ways). And it isn't a chip but a chunk. The 5534/5532 is often 98% as good for 2% of the price. But there seem to be several "vintages", some considered better than others. The balance of process specs needed for best audio are different from what most non-audio chips need: most chips want minimum currents, in audio we often want as much current as we can stand, at least in input and class-AB output stages. A foundry that does not dig audio may be "optimizing the process the wrong way" for out needs. I have not done critical comparison, but NJM makes a 5534/5532 copycat and audio is NJM's main business. LM833 and LM837 -seem- to be reinterpretations of the 5534, generally similar though not the same (and not rated for the high power voltage). Note that 5534 is not unity gain stable (but in a summer, you would only run unity gain with all channels muted; even then you could add a dummy resistor with minimal loss of performance). I had not seen tINY's LM837 MHz oscillation but such problems are common in hot amplifiers. I have not seen an AD797. The specs look promising. Astonishingly low noise voltage for a chip. If you do DC-couple, it has much better DC specs than most of the audio standbys: 25uV instead of 1mV. THD is low and can be gimmicked lower. I'm also too old to know the OP275, but I know people who love it. For eight 10K mix resistors, a fat vacuum tube can be pretty low noise, and has "glow". It is harder to get effective feedback to kill mix interaction, but you may not need theoretical zero-Z mixing in a monitor mixer. A 6DJ8 gain stage with 6DJ8 or 12AY7 (or HV NPN) cathode follower fed-back to unity gain (from one 10K input) would give a summing impedance around 300 ohms, mix interaction of a few tenths of a dB, and drive a reasonably sensitive power amp easily. If the gain stage has Gm over about 5,000 microMhos and you weed-out noisy tubes, overall noise is similar to the summing resistors alone. Of course as a monitor mixer, mundane details like stability, grounding, and connections are more important than how cool your chip is.