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Author Topic: Which opamps are in the signal path?  (Read 171 times)

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Overtone85

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Which opamps are in the signal path?
« on: Time Format »

HI All

I don't remember if this is my first post. Been so long.
Anyway, I got my first MP-1 from ebay last October. it was a v1.38 unmodded, and I installed the MDRT and "big" noise mod from MJMP + latest EPROM and replaced broken output jacks.
Everything went well and I've used it for a few months now.

I noticed that when the gain is very high on OD1 or OD2 there is still quite a bit of noise and a little bit of squeal/whine? especially when OD2 is above 5-6.
As far as I remember the big noise mod replaces one of the opamps (the input buffer, correct?)

But if I wanted to experiment with different types on other opamps on the board, which ones are involved in the signal path? (U-number?)

I will replace the 100k plate resistors on the tube board with 1/2w metal film to start with.
Do you suggest to replace all the electrolytics on the signal path on the main board as well? The noise mod only took care of the power supply and the tube board as far as I remember in terms of electros.

I also noticed that if I want to prevent the OD2 clipping LED from turning on I have to keep OD1 gain low and increase the OD2 gain, but that's how I get most of the annoying noise.

Cheers
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van Sinn

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Re: Which opamps are in the signal path?
« Reply #1 on: Time Format »

Been a looong time since my MP-1, so bear with me..

I always suggest replacing every electrolytic capacitor anywhere in older electronics.
If you were to unsolder and check them with a capacitance/inductance meter, you'd find a lot being well below specs.
As such, your device will no longer adhere to how it was originally designed.
Unfortunately, most will replace only the power supply caps.

Good idea replacing higher Ohm resistors, especially in the tube sections, as you're already doing.
WRT opamps, I have no idea as to which to experiment with. The standard mod descriptions has some suggestions, so I'd start there.
However, WRT noise, do bear in mind the noise is being summed through a circuit, and that the first stage creating noticeable noise will be the one setting the overall [noise] stage.

(can't get to grips with bear/bare.. this morning - if there are too many bears in my text, bear with me..  :crazy: )
« Last Edit: Time Format by van Sinn »
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..but it's buggy!
- it's software..

MarshallJMP

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Re: Which opamps are in the signal path?
« Reply #2 on: Time Format »

To actually test the mp-1 for noise you test it without anything connected to the input. The noise you hear will be the noise generated by the mp-1.
If you plug in a guitar and turn the vol pot up ,the noise you hear is added noise picked up by the PU's.
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