MP-1 classic MOD

[pgmlml]

Hi guys!
briefe introduction to the mod:
When I got my Mp-1 Classic, I made some direct recordings with the stage outputs, for playback with IRs.
That's when I found there was a dip around 7khz, in the frequency response of the preamp, in the tube distortion voice, only.
This is something that I've never seen been, and I've been working as sound designer for more than 10 years now. I compared the frequency response to some direct recordings of an original Mp-1 and other tube preamp outputs (from an Orange JR#4 and Peavey 6505 and PRS MT15) that dont show this!

Started digging into the schematics and found an "anti-sizzle" circuit in the EQ section. The manual refers to EQ bands changing with voicing to reduce harshness and sizzle, and realised this is a fixed EQ band that is only active on the tube distortion voicing (removing around 7khz)

The mod is very simple, change R253 from 10k to 5k (I opted to solder a second 10k in parallel to achive 5k). This reduces the gain ratio on the OP-AMP therefore, reducing the amount of frequency that is taken away.
The Tube distortion voice now sounds very natural and open :)

Be very carefull soldering around those ICs!!!

I dont know how to post pictures here,  so you can see my post on Faceb**k ADA MP-1 group
https://www.Faceb**k.com/groups/324620841942701/user/1050629104/

cheers

[rnolan]

Hey Pedro, cool (though I don't have a classic).  Easiest way to post a pic (well the way I do it) is to add it as an attachment to the post (+ Attachments and other options)

[pgmlml]

thanks! I was looking for the attachments :) but I couldn't find it!

[Harley Hexxe]

Hey Pedro.

      Just a couple of questions to be sure I understand this.

      Instead of removing the 10K resistor and replacing it, you just added another 10K resistor in parallel?

      The 2nd question is: Does this apply only to the stage outputs, or is this in general with the Distortion Tube voicing across all outputs?

      Since there have never been any mods done before this to the Classic, it makes me wonder about something else that has bugged me about the Classic vs. the Original MP-1. The Original MP-1 has more Bass and Mid frequencies than the Classic does, and I can't help but wonder if it might be possible to mod those frequencies in a way similar to this? So far, the only thing that I have found to warm up the low end of the Classic a little bit was to change the tubes to long plate Mullard reissues, which helps just a little bit.

      Thanks for posting that.

Harley 8)

[Dante]

Hmmmmm....very interesting....

[MarshallJMP]

This will affect all the outputs.

[Harley Hexxe]

This will affect all the outputs.

   Are you going to try it on yours?

[pgmlml]

This will affect all the outputs.

The anti-sizzle it's an EQ band, that it's only ON (cutting frequencies) in the tube distortion voicing. Yes, it will affect all outputs. It has nothing to do with the speaker emulator circuit.

I believe this anti-sizzle was there to compensate for the speaker/power amp somehow, because the preamp doesn't boost that frequency by any stretch of imagination. Nowdays, it way easier to see what we're hearing... or not to see ;)

[MarshallJMP]

Not for the moment Harley, I rarely use my classic. Maybe it's time that I use it more  ;)

But I did measure it also and PGM is right , there is a dip.