OVATION OP PRO: DEFINITIVE OUTPUT GAIN FIX

Hello all, with this post I definitely close the discussion on
how to modify the OP PRO FOR GETTING a standard  output level.


We will sum  2 experiences:


 a)
 Increase the piezo available input signal properly repartitioning
  the 2 input resistors to a correct convenient value (68k & 1.2M)
 b)
 Increase the gain  of the differential output stage
 changing the gain from 1.5 to 2 factor. (Or 3 to 4 factor for XLR connection).


Mod will be valid for both XLR or "Jack Only" equipped guitars!


Reading the blog will be easier to modify the gain to
an higher value if desired, keeping attention on having
equal gain for  the two XLR branches.

Personally I  want to preserve the design and
have "room" for not saturate the preamp and not increase too much the
distortion and signal to noise ratio.
 
The mod itself will be easy, but need to be done
on very small smd components ... so a proper "hand" and good
 sight\magnifying glasses  are required.


Of course You will do it at Your risk ...
But I definitely think  this will give FULL LIFE to this Preamp Unit !!!!!!!!!!!!!!!!!!


A) Modification (Change Piezo partition network)

Replace R16 FROM 680k(684) to 1.2Mega Ohm(125)

Replace R15 FROM 560k(564)  to 68K Ohm (683)

In this way from 54% of the piezo signal We pass to 94% signal AVAILABLE.


This mod is passive , gain itself is not increased! 
The op amp of this stage (1\4 of a tl064 quad opamp) just acts
as impedance adapter with gain 1 and will remain as is.



B) Modification (Increase differential stage gain)


Replace R50 FROM 20K(203) to 10K(103)
This will increase gain of  jack output & "in phase" XLR output from 1.5 to 2.

Pay attention on desoldering R50 as is partially covered by glue like insulator.


Keep available the original R50 20k resistor.


Replace R25  FROM 15k(153) to 20k. (Using the original 20K resistor removed from R50)


This will increase gain of inverted phase of The XLR output (when present on the guitar)
 from  1.5 to 2 .

  33% more volume will be available with this gain mod..

This is basicall ALL Folks

Just for let You know the volume potentiometer
on the Op PRO is 100k linear, non AUDIO TAPE.

Here is the final pic with  mods applied.

The overall volume gain in volt will be around 2.5 (8db) if compared with the original circuit.

NOTE:
FOR a More robust voltage gain factor (3):
R25 30k (Or 33K standard value)
R24 10K
R49 20K
R50 10K




Enjoy
:-)

KK

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