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