My Blofeld modification for higher Line out level

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8 Monate 3 Wochen her - 8 Monate 3 Wochen her #1 von rolfdegen
Hello Blofeld friends ..
I was wondering if there was a technical possibility to increase the output level at the line output and possibly to replace the operational amplifier.
To do this, I first examined the OpAmp circuit for the line-out output and drew a sketch.





Here the OpAmp circuit for Line Out in the Blofeld (left channel). It is a 2nd order low-pass circuit. Unfortunately I could not determine or measure the capacitor values ​​in the circuit.
When calculating the new gain, the offset voltage at the output of the operational amplifier must be taken into account.
An operating voltage of Vcc 11.78V results in an offset voltage of approx. + 3.5 V at the opamp output. Now if you increase the gain by changing the two resistors R1 and R2,
then the offset voltage at the OpAmp output also changes.

According to my calculation, the gain should be 2.4 so that an offset voltage of + 5.2V is created at the output. This corresponds to about half the operating voltage!
R1 would then have to be replaced by a 1.5KOhm resistor. Or you can solder a 3.3KOhm resistor in parallel to R1.

In my calculation, I took into account the input voltage divider consisting of R3 and Rx 10K for the offset voltage. This is + 2.1V at the junction of R3 and R4.

Calculation of the old circuit with R1 3.3KOhm
V = 1 + (2.2K / 3.3K) = 1.66
V offset at the output = 2.1V * 1.66 = 3.5V

Calculation for a new circuit with R1 1.5KOhm
V = 1 + (2.2K / 1.5K) = 2.46
V offset at the output = 2.1V * 2.46 = 5.2V

Before I convert my Blofeld, I made a few measurements on the OpAmp output of the NE5532.
For the measurement I programmed an init sound with three sine oscillators and a level of 127.

The first picture shows the offset DC voltage at the quiescent output of the OpAmp NE5532. It's exactly 3.54 volts


The second picture shows the OpAmp output when a note is played


The 3rd picture shows the overdriven OpAmp output when playing two notes


But now it can also be that the DAC output in the Blofeld is already overridden with two notes. I can only check this by making the disconnection between DAC output
and OpAmp input and measure with the oscilloscope at the DAC output.

The fourth picture shows the DAC output when playing two notes (Voffset + 2.47V)


DAC out there is a slight overdrive when playing two notes. The OpAmp output does not seem to have reached its control limit yet.

Continued on next page..

 
Letzte Änderung: 8 Monate 3 Wochen her von rolfdegen.

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8 Monate 3 Wochen her - 8 Monate 3 Wochen her #2 von rolfdegen
I will now reconnect the DAC output to the OpAmp input and increase the gain of the OpAmp by reducing R1 to 1.5 KOhm.  By changing R1 to 1.5KOhm, the gain of the OpAmp has changed and with it the offset voltage at the OpAmp output. As you can see in the picture, the offset voltage at the OpAmp output has increased from + 3.54V to + 4.94V.  The output amplitude when playing a note has increased from 2.80Vpp to 3.90Vpp. So there is still some margin upwards until the offset voltage is half ofthe operating voltage (+ 11.78V) achieved. Will try it with a 1.2KOhm.    
With 1.2KOhm the offset voltage at the OpAmp output reaches a value of +5.98 volts. I think that's ok. 
  

Continued on next page..
 
Letzte Änderung: 8 Monate 3 Wochen her von rolfdegen.

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8 Monate 3 Wochen her - 8 Monate 3 Wochen her #3 von rolfdegen
The output amplitude when playing a note has increased from 2.80Vpp to 4.72Vpp compared to the old circuit

 

The maximum output level of the NE5532 is now in the range of + 2V to + 10V. With a Rail to Rail OpAmp you would have just under 4 volts more
.
 

I have now compared both output channels. The left channel (yellow) is modified and the right channel is still in its original state.You can easily see here that the left channel (yellow) has a significantly higher output amplitude and is not distorted.In comparison, the right channel (blue) goes into OpAmp saturation and becomes distorted with negative aplitude values.For the measurement, I pressed two notes and turned up the system volume control until the amplitudes start to distort. 



For the modification, the two 3.3KOhm resistors (red marking) must be unsoldered and replaced with two 1.2KOhm resistors. Or you solder a 2.2KOhm parallel on top of the respective resistor. 


 

I would still suggest replacing the OpAmp NE5532 with a Rail to Rail OPA 1612. This has better sound properties, very low distortion and a high level of controllability (output voltage swing). 

OPA 1612 Characteristics
• Superior sound quality
• Very low distortion: 0.000015%
• Low noise: 1.1 nV / vHz
• Real FET input: IB = 5pA
• High speed:- Slew rate: 27 V / µs- Bandwidth: 40 MHz
• Large supply range: ± 2.5 V to ± 18 V 

German blog: www.sequencer.de/synthesizer/threads/wal.../page-2#post-2300849 

Greetings from germany. Rolf
Letzte Änderung: 8 Monate 3 Wochen her von rolfdegen.

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8 Monate 3 Wochen her #4 von rolfdegen
I got this circuit diagram from another forum. It's a little different but shows the capacitor values.


I made filter measurements on the Line output with Osc 1- 3 rectangle and value 127. Play one note.
Yellow original circuit and Blue is mod circuit.

Cutoff 127, Resonance 0


Cutoff 80, Resonance 70


I simulated the filters and found that changing the gain through R1 and R2 had no effect on the filter frequency.

Greetings Rolf

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8 Monate 3 Wochen her - 8 Monate 3 Wochen her #5 von rolfdegen
Hallo Blofeld friends 
I have now developed a volume increase for the  Blofeld .

This is fed by the internal + 12V supply voltage in the  Blofeld . The OpAmps are special audio OpAmps with low distortion and little noise.
The OpAmps are supplied with a bipolar voltage of + -8V. The Amp gain is 3.75 which allows a maximum output level of 12.75Vpp.




Greetings from germany. Rolf
Letzte Änderung: 8 Monate 3 Wochen her von rolfdegen.

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8 Monate 3 Wochen her #6 von rolfdegen
Bode Filter plot (yellow dot line 40KHz -3dB)

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