Nobels PRE-1 [schematic]

[FloPoeKo]

I´m not quite sure if a 49€-pedal qualifies as cheap, but to me it does.
I acquired a Nobels PRE-1 for 30€ used and I figured I´d do a few things to it. It´s built rather sturdy.
As a starter: how do I change the frequencies of the treble and bass bands.
Can anyone explain what the circuit is called and how to calculate the values for different frequencies? The manual states 25Hz and 10kHz.
What type of EQ is this anyway? Shelving?

[FloPoeKo]

here comes a gutshot (main board, component side). It is connected to a daughter-board which holds potentiometers and LED.
A few guesses here that should be tested (any thoughts and comments are greatly appreciated).

Opamp
The first thing I noticed: Contrary to what the schematic sais, this production model (silver casing) utilizes a 8DIL-Opamp instead of a 8SIL-Opamp (U1: JRC4558D). That opens it up to easy upgrades in that spot with the usual suspect for neutral clean pedals, for example a Burr-Brown OPA2134.

Electrolythics
Also, all the electrolythics could be upgraded to non-polar Nichicons or Panasonics for ultimate clarity. I wonder if conversely tantalum could be used to give it a bit more grit and flair (of course only in those parts of the circuit which are not affecting the bypass-feed). While at it, if the voltage rating is taken into consideration one could go to/beyond 25V to allow operation at higher supply voltages for more headroom.

switching logic and maximum operating voltage
The only obstacle is the now obsolete HCF4007UBE (used here for switching the FETS), which is best operated at up to +15V, although it can take +18V. Since we roughly loose around 0.7V with the polarity-protection diode D101 that might be manageable. I thought it would be more ideal using a 18V Zener Diode like the Fairchild 2N5248-B instead: for protection against spikes and with a loss of 1.2V due to its forward voltage one should be fine using a 18V PSU with that chip (if the electrolythics have the appropriate power-rating). The Burr-Brown OPA2134 could handle ±18V, so that should be fine too...

[lolbou]

The only obstacle is the now obsolete HCF4007UBE

CD4007UBE will be fine...

The circuit looks like an active baxandall (see here) which actually cuts or boosts the highs and lows...

[Dirk_Hendrik]

I wonder why you first want to change the electrolits for types that "claim" a more hifi approach and immediately after that suggest using tantalum which is about the biggest non-hifi change you can suggest. Are you modding with an idea of what this pedal should sound like when done or are you modding for the sake of modding. In the latter case every change that sounds different and different is always better in pedal-modding-country. In the latter case as well I'd suggest leaving that pedal alone since tinkering without focus or destination is rather futile... at least imho.

[alexradium]

I wonder why you first want to change the electrolits for types that "claim" a more hifi approach and immediately after that suggest using tantalum which is about the biggest non-hifi change you can suggest. Are you modding with an idea of what this pedal should sound like when done or are you modding for the sake of modding. In the latter case every change that sounds different and different is always better in pedal-modding-country. In the latter case as well I'd suggest leaving that pedal alone since tinkering without focus or destination is rather futile... at least imho.

+1000

[FloPoeKo]

I wonder why you first want to change the electrolits for types that "claim" a more hifi approach and immediately after that suggest using tantalum which is about the biggest non-hifi change you can suggest. Are you modding with an idea of what this pedal should sound like when done or are you modding for the sake of modding. In the latter case every change that sounds different and different is always better in pedal-modding-country. In the latter case as well I'd suggest leaving that pedal alone since tinkering without focus or destination is rather futile... at least imho.

C´mon Dirk, I get your point, but that comment was really below you (You're my hero after all!)...

I wonder why you first want to change the electrolits for types that "claim" a more hifi approach and immediately after that suggest using tantalum which is about the biggest non-hifi change you can suggest. Are you modding with an idea of what this pedal should sound like when done or are you modding for the sake of modding. In the latter case every change that sounds different and different is always better in pedal-modding-country. In the latter case as well I'd suggest leaving that pedal alone since tinkering without focus or destination is rather futile... at least imho.

+1000

And shame on everyone applauding it reflexively... No offense, my fellow tinkerers, but this becomes a little shtick lately
I usually hate entering discussions about the what the the exact focus is (after all tinkering is the most fun part, right?!) or "who would want something like that" when my initial question was about ideas what could be done and go different directions...
But, as we´re at it
My thought was in no way contradictory: Making a boost/EQ/preamp-pedal sound "superneutral" is one destination (serving as a buffer with a neutral boost/EQ-option or as a preamp for acoustic guitars).
On the other Hand, making it (especially in this case) decidedly coloring has a very valid point to it: I´ve played through a lot of boosters lately, some of which which have a tendency to color the sound in a way that makes them not exactly the prettiest sound in a situation where you want it "just a bit louder", but a little colorful boxyness can go a long way, foremost in situations where they act as a boost for overdriven sounds... IMHO
Since I favor using sockets for the tinkering-stages (and please, I know your/agree on your comment on your homepage on hitting the boutique-pedal on the table until the components come loose and that that´s not an option for a pedal that´s finished/road-ready, that´s not what I mean) this can be done easily and without wasting/soldering components in-out... So please, hook a brother up with some ideas...

[alexradium]

i don't talk about shame on other people that i don't know personally,i just speak from experience in the field after many experiments in a proper electronic lab,and that is real fact not fiction or religion to believe in.
What i find pretentious is the will to change something that people don't even know how it works,to get that elusive sound that only golden ears of the chosen by God is able to hear....
Good luck with that.

[FloPoeKo]

i don't talk about shame on other people that i don't know personally,i just speak from experience in the field after many experiments in a proper electronic lab,and that is real fact not fiction or religion to believe in.
What i find pretentious is the will to change something that people don't even know how it works,to get that elusive sound that only golden ears of the chosen by God is able to hear....
Good luck with that.

I haven´t said anything about elusive sounds that only golden ears will hear... Please, can we discuss the pedal?

[phatt]

It's not a complex circuit ,, just bread board the basic audio path (Q1 and u1A/u1B) as that is the heart of the tone,,the rest is switching and buffer which is not going to change the sound.
Try changing the values of the caps (C21,,C24) until you are happy as that's enough to give you an idea of what the end result will sound like.

Be aware that most pedals lack headroom and are already close to the limit so any boosting will colour the sound somewhat as it will hit the rail faster and add more edge.
Now if the bandwidth of the whole system is HiFi wide (a common issue) that extra fizz will start to sound very unnatural.
If using Acoustic with Piezo active preamp then you can easy turn even the most expensive acoustic into a tin toy tone.

If you want to boost (Electric or Acoustic) you must remember that the wider the bandwidth the faster you will run into trouble maintaining well rounded tone. If you want good control with acoustic you need a Parametric EQ and use deep notch CUT somewhere in the mid band and restrict the hi end to keep it mellow. Para EQ works best in Cut mode,, boosting often trashes everything.

Back to your quest,, go download some simulation software and take the time to learn how that works. It will be hard to start but ten times faster once you learn ,,,I'm speaking from experience.

I could simulate your circuit in about half an hour and change the values many times over to give a super fast understanding of what values will have the most effect.
Saves lots of solder and I'm not friggin around inside often delicate tiny little boxes.
Phil.