ADA Preamps > Original MP-1
OD1 Issue
Moff40:
Hi All;
This issue has been persistent for a while now, and I did talk briefly with MJMP about it a few months ago. I noticed it after I reversed the 3.666 mod on my first MP-1 (I have 3 now), so I probably messed something up, but maybe not, too. Here goes:
On any voicing (SS Clean, tube clean, or dist), if I run OD1 somewhere around 5.0, and give the guitar a hard strum, the OD1 LED goes on solid, and the whole thing gets distorted, in fact, even SS clean will act like I've stuck a fuzz box in front of the amp. If I stop playing, it will "calm down" after a second or two, as though extra voltage is being dissipated (bad cap?). When it started months ago, MJMP and I went through a few things, including measuring voltages at various ICs, and swapping chips between this unit and one of my working ones, to no avail. Then I gave up, thinking I could live with it - just use it in my studio, and not dial OD1 higher than the "tipping point", but now it's become problematic.
The tube board in this unit is in rough shape, with lifted and repaired traces everywhere, but it works OK (I don't trust it in its current state for road/live use though), and since this issue occurs even in SS mode, when the tube board isn't involved, I'm thinking the tube board isn't likely to be the source of he problem. I've ordered some VTL5c4/2's from Small Bear, just to have a couple on hand (they're getting harder to find, so thought I'd stock up), but I doubt they're the culprit. From what I understand, vactrols either work or they don't; they're not usually flaky like this. Maybe I'm wrong on that.
Any ideas? I'll also email MJMP later today.
Moff40:
Update:
After thinking about it, I figured that whatever was happening, was happening before OD1. I took a look at the schematic, and there isn’t much before it in the circuit.
Remembering I had installed a socket at U4, and having a couple of vactrols lying around that were pulls from another MP-1, I decided to re-flow the solder on U4, and replace U5. That didn’t help. Then I remembered that when I socketed U4, I had broken a leg on the stock NE5532 while removing it, and had replaced it with a BB OPA2134. I had since purchased a few NE5532s from China (probably pulls from recycled gear), so I installed one in place of the OPA2134, et voila! problem solved. Just to see if maybe the OPA2134 was flaky, I tried another, and the problem came back. An OPA2604 didn’t fare any better. Put the NE5532 back in, and all is fine. I can now run OD1 wide open without the craziness. Yes it clips and distorts, but in a way you’d expect; it doesn’t clip and then stay clipped for any amount of time.
So it looks like I’ve solved it myself. However, if anybody can offer a reason why the BB OPAs don’t like to be in there, I’d be happy to hear their input.
rnolan:
Good news that you fixed it. The OPA2134 and NE5532 have different pinouts and internal circuit (https://www.utmel.com/components/opa2134-vs-ne5532-how-to-differentiate-the-opa2134-and-ne5532?id=1685).
The OPA2134 is a Fet Input op-amp, while the NE5532 is a Bipolar Input op-amp. Seems the MP1 circuit needs the Bipolar Input :dunno:
Moff40:
Interesting. If I’m not mistaken, the OPA2134 (or maybe the OPA2604, I don’t remember) is what MJMP sent me as part of the noise mod. I’m sure he wouldn’t have sent one if it wasn’t compatible. I’d love to find out more, but at any rate, it seems to be humming along now. I think this particular unit’s remaining life will be spent taking it easy in my studio, along with the ElevenRack.
rnolan:
I suspect it depends on the circuit the op amp is in. They basically do do the same job albeit a little differently but if the circuit is such that it's using the other pins on the NE5532 then maybe it's not a direct swap, where as in another circuit (or part of the circuit) it may make no difference and either can be used?
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