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Thread: LME49720NA for ART Digital MPA?

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    LME49720NA for ART Digital MPA?

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    I ordered some LME49720NA opamps so I could use one in a little preamp that I got from mrbowes. I had tried the OPA2134 in it and liked it, but I also like the LME49720NA, because it has a "faster" sound to it.

    Any reason not to try them out in the ART Digital MPA? There are a *bunch* of OPA2134 in the unit, and I'm sure they're not all directly related to signal amplification, but I guess I could try them one by one and see what happens.

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    Yes, there are a few reasons! First off, unless I have badly erred, the National chip is BJT input, whereas OPA2134 is FET input. The MPA uses only one 2134 per channel, and it's used for the instrument input. That means it needs to be high input impedance, which means you need a FET input opamp. The LME's input impedance is listed as 30K, vs. 10^13 for 2134.

    Reason #2, I said that OPA2134 is used for the instrument input, and that's true but not completely true--the mic signal is also routed through the 2134, but only after a discrete stage and a stage of good ol' 5532. Why is this done? Because a mic doesn't need the high input impedance of a FET, and BJTs are generally quieter than FETs, so it's done to get the MPA's nice and low input noise spec. Going straight into OPA2134 generally gets you something on the order of -120dBA, vs. the -134dBA spec (or so, going off memory here) of the MPA.

    OK, moral of that story is that before the 2134, there are two other stages beforehand, although I think the 5532 is unity gain. After the 2134 comes the tube, then a couple more 5532s. There is gain at the first discrete stage, the 2134 (responsible for most of the input stage gain), the tube (supposed to be +20dB, but the +20dB switch is actually a pad going into the tube, and there are way too many components around the tube for me to calculate the tube gain), and the final 5532 which adds a hair of gain--10dB I think.

    Whew, that's a lot. It's not too likely that swapping anything for 2134 will make much difference, because the signal level is high enough that you won't gain much in noise performance, the slew rate is the same, and of course there's that input impedance problem.

    What's more interesting would be to replace the 5532s with the LME chip, especially at the output, where the slew rate might be able to help a bit. Or even at the input, for improved noise performance there (although again, I don't think that will matter too much). But the 5532s are soldered in, so they aren't as easy to swap . . .

    #1 on my list of MPA improvements is to rewire the output level knob so it comes right after the tube, rather than after two opamps after the tube. That should allow hitting the tube a lot harder without clipping the following opamps. But it's a complicated circuit, and somewhat involved surgery, so I need to study that a bit more first.

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    interesting -- it was actually a NE5532 that was in the Gaines preamp (that I got from mrbowes) originally, and I replaced it first with a OPA2134, and then with the LME49720NA -- too bad those are soldered in (and presumably SMT? I'll have a look - if not, I can socket'em - I've gotten handy at that.).

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    Quote Originally Posted by antichef View Post
    interesting -- it was actually a NE5532 that was in the Gaines preamp (that I got from mrbowes) originally, and I replaced it first with a OPA2134, and then with the LME49720NA -- too bad those are soldered in (and presumably SMT? I'll have a look - if not, I can socket'em - I've gotten handy at that.).
    I don't think there is any SMT on the MPA, except for the digital section of the DMPA. You will have to pull the main board though (except for the input stage 5532, which is on the easily accessed gain pot board), and its screws are glued down

    I never socket anything myself . . .

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    Quote Originally Posted by mshilarious View Post
    I never socket anything myself . . .
    That's because you know what you're doing

    Rats - bricked it I destroyed a bunch of 5532s on their way out of the board, and perhaps the high heat of my iron internally damaged even the ones that made it out with all 8 pins. Anyway, the LME49720NA ones didn't work *at all* (I socketed and replaced all 8 5532s) - I just got hiss in one channel and bizarre motorboating in the other when the tube was engaged (and silence when it wasn't). I tried putting back the 5532s without luck - a little mic signal and a lot of noise in one channel, and nothing in the other . I'll get some more. Visually anyway, I did a nice job with the sockets, so hopefully all's well except the opamps.

    Good thing those group buy pres are working -- just at the moment, I'm feeling like I should return to my non-career as a musician

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    I guess it's closing the barn door after the cows are gone, but never try to desolder 5532s. I mean, they are what, $0.50 each? Cut them out, then remove the solder from the holes. Much safer than trying to save a bunch of jellybean chips.

    You probably destroyed a trace somewhere; you can try to find it, although if these are four layer boards (I don't think the mini-gain boards are; I've never pulled the main board), you could have broken a power or ground plane connection which you can't see.

    Start with (power off) measuring continuity between all pins 4 and -V and pins 8 and +V, with the OPA2134 sockets as your reference. If you find no problems there, move on to the signal pins and try to trace where they go. If you find a problem, run a jumper from another point that is still good.

    The motorboating could be caused by oscillation, even if you cut an input trace somewhere so there is no signal. It's not a huge surprise with a faster chip. You should add bypass capacitors to all chips you are replacing.

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    I'll give it a shot - I suppose I could also test that way with my new sockets?

    I also contacted ART about getting a replacement board or even sending it to them.

    Multi layer boards -- the bane of newbs!!

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    answer #1 from ART - no replacement boards! I'll find out about service next.

    I'm starting to get my courage back up regarding trying to fix it, and I got a bunch of crisp new 5532s, so I can restore the stock circuit -- I have one of those "pens" that is full of conductive material - are those worth considering?

    I recognize that if it's a multi-layer board, and the trace I messed up is on the inside, that I have some challenges.

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    What's more interesting would be to replace the 5532s with the LME chip, especially at the output, where the slew rate might be able to help a bit. Or even at the input, for improved noise performance there (although again, I don't think that will matter too much). But the 5532s are soldered in, so they aren't as easy to swap . .
    yes thats best, but there are more stable chips. The motorboating is oscilations, if you socket chips you really need to use machined sockets the flat pin ones are bad for oscilations and moaterboating and usually need decoupling caps across power an ground

    Machined-contact sockets provide lower contact resistance and greater
    retention.


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    I did use machined sockets (not because I knew what I was doing, but because that's what they were selling at the electronics store)

    anyway -- wish me luck

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