Overture

Hello there !!

I'm a proud owner of a Cort Artisan A4 FMMH bass.
(And of course  the proud father of my daughter Elettra.....)

You can see both of them here in the pic , the day the bass arrived... 
 ...........few days before the start of the COVID mess....

This instrument will be a baseline for some modifications, reverse engineering
& restyling that I will post irregularly here as  tracking of my working progresses.

I will basically perform a FULL XRAY analysis of the whole thing, trying to
create something more unique...that Elettra will then inherit  at the end of my crazy life...
(More or less around the Year 2120.. next covid.....)
:-)






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Overview

Well ... here is the bass as it was shipped to me:

I personally think it's stunning beautiful !!!!! Isn't it ?

Where to start ?  Something simple ....

Yes !!

The Factory specifications !!

CORT A4 FMMH Open Pore RED finish

Body: Mahogany
Top: Flamed maple
5-Piece neck-thru-body: Maple/Wenge
Fretboard: Panga Panga
Dot fretboard inlays
Scale: 864 mm (34")
Nut width: 38 mm
24 Frets
Pickup: 2 Bartolini MK-1 humbuckers
Preampickup: Bartolini MK-1 with 3-band EQ
1 Volume knob and 1 balance knob
Platinum hardware
Hipshot Ultralight machine heads
Hipshot Trans Tone Dual Access bridge
Active / Passive(??) mini switch  
Ex-factory stringing: XL D'addario Nickelwound .045.  -  065  -   .085  -  .105
Finish: Thin open-pore lacquer
Colour: Black cherry

...........

The specifications are fully  correct ....................apart the fact this bass.................................................. IS NOT ACTIVE \ PASSIVE:

The toggle switch will only bypass the Equalizer ..
but will leave active the 1st preamp stage of pickups amplification
and related mixing .............as  We will see in the next posts !!!

So.......No battery.....No party .... just to be clear.....
:-)





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Get rid of the awful plastic black truss rod cover

One of the first thing I noticed as unacceptable detail for a such well designed instrument was ........... this :

I mean ...... what the hell is  this fat plastic condom fitted with 1 screw on the top my bass headstock ????       Unacceptable.... nononono... I need to get rid of it.

So let's put over the counter some Italian re-styling (YepI'mItalian..) skills and design something more serious......  

Something in warm wood... maybe mid dark solid beech tree..

Kind of This:

Of course In my basement there was (by chance) an old small rusty cnc machine waiting for some new.....glory! 
:-)

After some carving...............

...........here the raw result:
 Please note the tab that will slide into the truss-rod groove \ hole keepling the cover super stable even keepling the original 1 screw compatibility design.
.............................................. (In a way You can later mount back the black  fat condom...)

Look promising.... A bit of oil finish .... and We can proceed with the installation:

And the final result is....

Wow .... this is really a worthy upgrade !!!!  Tally and skinny.
What You think?   :-)

In case someone is interested ...I can build  and ship  a small nr of pieces customized with the model type like A4 B4 A5 but I need to know some parameters like the groove width, screw hole position etc etc,  so  leave a comment or contact me (kkprince) on talkbass forum If You wish.

(The plate cover is wood carved...not printed ..each one take 4\5 hours of slow machining , 2 sides)






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MK-1 Preamp Anatomy (INTRODUCTION)

The heart of many Cort and several other brands of basses is a Bartolini MK-1 preamp.

I've read tons of comments on this little thing.... basically soo many that I've decided to make a full and NEUTRAL reverse engineering of the whole device.  

This will be time consuming , boring and potentially destructive for my bass..... but the other face of the medal is that....once done I will have FULL CONTROL of my tone and infinite possibilities of modifications & optimizations.

Yes because little thing not buried into a black epoxy block is ideal for being optimised...

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--
---
----
-----

Where to start?  

Pretty simple ...

Let's remove  the cavity cover... for a first overview  of the electronics.

This super mess is the  ..... electronics:
:-)

You can notice the cavity is  shielded with some microns of conductive paint.
(I will review the whole shielding later , not on the top of my list for now ;-)
Lets remove the preamp and perform some initial review.
There is a small adhesive foam to remove to expose the component side of the preamp.

HI-DEF pic of the connector \ rear side side:

HI-DEF pic of the component side:

The signal pocessing is performed "left to right" as we will see in next posts.
The 3 active components are the omnipresent TL062 dual Operational Amps.

What to say?  was TL062 engineered in mid 1970...used in tons of battery powered music devices .... for decades and decades .... till now ! It has a trade off to consider:

• Very low power used
• Relative acceptable-noticeable Noise Level
• Low cost
• Easy replacement

Yes ... easy replacement:
I will definitely replace them at the end of the reverse engineering with something more actual, like a triade of super expensive LT1352 ....


Generally speaking:

• The leftmost Opamp 1 is used as pickups 1st stage amplifier & mixer.

• The center Opamp 2 is  a decoupler and a  Baxandall "Bass & Treble" Equalizer.

• The rightmost (half used) Opamp 3  is a Baxandall Mid Range Equalizer.

• The diode  soldered "on the fly"  in parallel with the power supply is to me useless as in my bass there is a schottky idiode in the battery cavity able to protect the circuit.

• The smd circuitry is using  a medium density scale so with some care is fully possible to apply some modifications , upgrades and of course some repair.

In the next post we'll see an overall description of the whole circuitry reversed engiinered.


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a little contribution using my paypal account:

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*******************************************
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:-)

MK-1 Preamp Anatomy (OVERALL DESCRIPTION)

After a very long  & boring  decoding session I finally managed to reverse engineer the whole circuitry .............................................................!!!!
;-)

 We can then see in the pic below the overall  organization of the MK-1 ensemble.
A kind of Google Maps Satellite vision of it...

Even if this implementation is maybe specific for a Cort A4 , I will bet my Hamster balls You will find more or less  the SAME setup on several other basses with very few variations.


If You are interested I have the fullsize 4000x4000 pixel image I  can share.

OVERALL DESCRIPTION

The preamp itself is based on three dual OpAmp TL062 . A total of 5 Operational Amplifiers are effectively used. In fact only half of the rightmost TL062 (IC3) is employed.  

This suggests me that sooner or later I will get rid of the half used Ic3,  obtaining a better organization of the things..... Less distortion , less noise,  less power consumption... :-)

We'll see...

As a sort of patch I found a diode added in parallel to the power supply. In my case this diode is meaningless as I have another  (schottky) diode in the battery compartment  in series to the +9v line. I phisically and digitally removed the diode (a common 1N4001).

In the whole circuit a virtual reference mid tension ground is used for driving correctly the  Operational Amplifiers in rail to rail mode.  A 4 components (2x 22uF + 2x22k)  resistive-capacitive divider is employed for creating the half tension Vref:

Be aware that externally all the potentiometers, input jack and pickups are grounded to the REAL GROUND , not to the vref virtual one!.


In the next post We'll see the organization of the preamp INPUT STAGE

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MK-1 Preamp Anatomy (INPUT STAGE )

INPUT STAGE

The 2 signals coming from the 2 pickup are processed entirely by the leftmost IC1, used as non inverting HIGH impedance dual Operational Amplifier. 

Impedance seen by each pickup is 470K Ohms.  

Each pickup is respectively independently amplified by a factor of 2 (6db).

GAIN = 1 + (3900Ohm : 3900Ohm)  =  2

The 2 amplified signals are then MIXED externally by mean of the BALANCER, a  dual Audio tape <-> reverse Audio tape potentiometer organized in a way that any rotate mix position will imply same load and  no changes in the overall volume.
I think this is very well designed.

The mixed signal out of the potentiometer is then passed to the TOGGLE SWITCH:

• In the PASSIVE POSITION  the mixed signal is routed directly to the VOLUME potentiometer and the entire Equalizer is then BYPASSED. BE AWARE there is no REAL passive configuration here ...........as the IC1 will remain active for mixing and amplifying  the 2 pickups.

• In the ACTIVE POSITION  the mixed signal is routed to the Equalizer input branch, processed , received back from the Equalizer output branch and finally sent to the VOLUME potentiometer.

In the active position half of the IC2 is used as non inverting gain \ impedance adapter and decoupler just before entering in the equalization section. 

The factory gain here is 1 but  this  can be incremented using a proper resistor to be placed in the circled post We can see in the picture:

• FACTORY GAIN = 1 + 3900 : Rx( when no present ->∞ )   =  1

• CUSTOM GAIN = 1 + 3900 : Rx  = .....

There is no reason to change the gain unless finding differences in the Volume perceived in ACTIVE\PASSIVE mode. 

In the next post we'll start to describe the Equalizer section!

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a little contribution using my paypal account:

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MK-1 Preamp Anatomy (EQUALIZER STAGE)

EQUALIZER SECTION

When the toggle switch is on the ACTIVE position, the mixed (Bridge+Neck) signal, once  amplified by the input stage (IC1)  is routed to the equalizer section of the preamp.

The signal processing is based on a classic 2 ACTIVE cascade BAXANDALL filters .

This is done apaarently for minimize the interaction of hosting a three band filters on a single active unit. 

To sum up here is the serial sequence of the operations:

• 1st  Half of IC2 is decoupling the signal coming from the BALANCER potentiometer

then feeding the

• 2nd Half of IC2 , providing the ACTIVE BASS+TREBLE Equalization filter

feeding the

• 1st  Half of IC3 , providing the ACTIVE MID range Equalization filter

The 2nd Half of IC3 is doing....NOTHING.

:-)

At the end of the equalization process, the signal is sent back to the TOGGLE SWITCH and finally routed to the 500k linear Volume potentiometer.

Schematic of the BASS - TREBLE Equalization stage rev-engineered:

Schematic of the MID Equalization stage rev-engineered:

In the next post We will model all the Equalizer curves to see and prove how the pream behaves also in respect of  the specifications declared semi-officially by the Factory.
If We will have a match this will mean the reverse engineering is mostly correct and We have a solid base to start with for applying future enhancements !

If You like my work please consider
a little contribution using my paypal account:

Donate!
*******************************************
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*******************************************


THANKS !!
KKPRINCE
:-)

MK-1 Preamp Anatomy (EQUALIZER SPECIFICATIONS)

Now that We have completely dissected and reverse engineered  the whole  little thing.......
I need to be sure about the Factory specs of the equalizer and plot some graphs that We will use as STARTING POINT for any future optimization.

There is and old graphic  I found over the Internet (unknown surce) displaying the overall equalizer "factory" curves of the MK-1:

We need to find OUT the naked truth.... so let's model the 2 Equalizer sections using a Circuit Simulator (Multisim in  this occasion) :

Performances  of the BASS - TREBLE  1st Equalization stage model:

The bass & treble filters branches are using & sharing the 2nd half of IC2 Operational Amplifier.

• BASS BRANCH SECTION

• TREBLE BRANCH SECTION

Performances  of the single separated MID  2nd Equalization stage (IC3):

The MID RANGE  filter , centered on 355 Hz  is using the 1st half of IC3 Operational Amplifier.

.....  now let's put ALL THE THINGS TOGETHER :-)

OK.....
This will be Our starting point. In fact...If  We make a comparison with the specs declared by the Factory.... We have a quite good MATCH and this shows the foundation  of  the reverse engineering  !

:-)


In the next posts We will review  the practical usability and performances of the MK-1 defining paths for implementing some definitive enhancements of it.


Stay tuned!





If You like my work please consider
a little contribution using my paypal account:

Donate!
*******************************************
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*******************************************

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KKPRINCE
:-)

MK-1 Preamp Anatomy (EQUALIZER PERFORMANCES)

The complete reverse engineering of the equalizer section of the MK-1 is the following:

Once more:

• The Bass & Treble controls are sharing the same active Baxandall stage with possible undesird interactions
• The Mid control is using a separate stage not interacting with Bass & Treble

Review

What's the best and fast method for demonstrate the MK-1 is a bit ... weird ?

To make a very long story short using a very direct and intuitive example.....let's imagine to grab Our bass and roll for a 1\4 turn clockwise both BASS & TREBLE controls....

This is a typical setting for amplify a bit both the edges of the bass audio spectrum...

Le't forget about the MID control for the time being..

What You may expect to see the graphs is:

• A descending slope on the  BASS-leftside

• A small valley in the MID-dle area 

• An ascending slope on the TREBLE-rightside

.
..
...
....
.....
......

The reality is instead the FOLLOWING:

More that an equalization curve  .....this looks like a flat PIZZA more or less levitating over 4Db........reference line........   :-(

THIS undesired behaviour is due to the excessive interaction between bass & treble controls.... as We can easily understand looking at the consistent common area  underlined in light blue:

Just for reference  We can give a look at the specs  of other well known bass Preamps:

We can see both of them trying to minimize the COMMON INTERACTION AREA and this is one of the 1st goal of the modifications We Will perform in the next posts.
We can also see the EXCESSIVE power of the Treble control that is way too much invasive. Basically this is acting as a kind of volume control instead of a proper treble filter... 



In the next posts We will see how to modify  the MK-1 in a way to achieve a more nice and effective equalization and a better tone definition.   

We will start with the 1C-MOD , the most easy mod possible in the universe of the mods.








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a little contribution using my paypal account:

Donate!
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KKPRINCE
:-)

MK-1 Preamp (1C-MOD)

This is an interesting  MOD that can  be performed by just replacing ONLY 1 SMD capacitor located on the Treble filter branch  of the MK-1 Equalizer section. (1C => replace  1 capacitor)

For more details of the MK-1 Preamp please have a look at the previous posts in this blog.

Here in the pic the details of the proposed optimization:

WARNING :

SMD USED ARE 0805  size, VERY SMALL .. then GOOD SOLDERING SKILLS ARE REQUIRED  to perform the job!!!    With some extra attention ALSO A 1206 smd can be used.

The effect of this fix is to MOVE the bell curve of the Treble filter AWAY  from the Bass  curve avoiding to share too much interaction of the respective controls and .... at  the same time calm DOWN just a little the excessive PREDOMINANCE of the Treble itself.

The new curve is then shaped like this:

Let's now see if a bit of clockwise roll of both  Bass & Treble controls will generate the flat PIZZA described in the previous post ( https://corta4reloaded.blogspot.com/2020/04/mk-1-preamp-anatomy-equalizer.html ) or not:

No more flat PIZZA .... We start to to see some personality rising !!!!

We can see the overall new performances here, please note the cross area is at 400Hz:

For a quick comparison here the pic of the original MK-1 preamp 
response curve with the crossing area centered at 250Hz:

If You like my work please consider
a little contribution using my paypal account:

Donate!
*******************************************
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KKPRINCE
:-)