AD797 vs. 990C shootout

[CoteRotie]

I initially built my J99 with the AD797 op-amps. The AD797 is an extraordinary op-amp, the input-referred noise is amazingly low, it has reasonable GBW, good offset voltage and current, and is a fantastic achievement overall. A great choice for an audio amp.

But still, I planned to swap out the 797 for the 990C when I got the chance, and to compare the sound between the two.

I fully expected to find liitle or NO difference between the two circuits, since I thought that both amplifiers were excellent, and lots of the character would come from the transformers, not the amplifiers.

I recorded some acoustic guitar before and after swapping the 797 and the 990C.

I was really surprised to find that the 990C was very noticeably more defined in the high end and seemed to give more presence than the 797. Both ways the pre is excellent but I really preferred the 990C version. This was not a blind test so take it for what it's worth, but that's my 2 cents.


Regards,

John

[cwalcott]

how hard was it to make the change between the 797 and the 990? seems like there's a lot of stuff that you have to remove from the board.

- chris

[CoteRotie]

how hard was it to make the change between the 797 and the 990? seems like there's a lot of stuff that you have to remove from the board.

- chris

It depends on the equipment you have available. I'm in the electronics industry so I have access to professional de-soldering stations and it was pretty easy. If you don't have good equipment I'd just snip out the components (a socket, a cap and a resistor for each op-amp) and remove the remaining leads with some solder wick and pliers. Then you add the 2 compensation caps and pin sockets if you didn't put them in already and plug the 990Cs in place. You have to re-do the offset current calibration, but all in all it's not a big deal. You can snip the plastic between the pins on the sockets to isolate the pins then remove them with needle nose pliers if you can't get them out whole.

Regards,

John