Presonus MP20 help

[bxt403]

Greetings,

I've had a Presonus MP20 (early version that already has a Jensen trafos installed) for about a year now. 
I contacted Presonus support to see if they might help with mod , and they told me to  :-* their a$$ (actually, I was told that if I had any problems with my unit, I have to ship it to them for repair and user mods are a big no no).   

I installed AD797's since the consensus on the web seems to be that they're the best suited for a Jensen Twin Servo (not to mention the schematic for an IC twin servo pre on the Jensen website also shows AD797s in it).  I figure since the MP20 is a "twin servo mic preamp" the Jensen schemo is identical (or at least very similar) to the MP20 cct.  I managed to adjust the rails to +/-18V on the power supply on my own.

The only problem I'm having now is that there is a tiny square pot (looks like a lego block) on each pre channel that seems to adjust the LED meter's response or perhaps locking of the servo (since the signal seems to degrade I'm thinking it's a combination of the two).  I'm not sure how it should be metering (ie: are the red/clip led's indicative of clipping at the input or the output?). Adjusting the gain knob also changes the level of the LED meter so the logic follows that when I feed a test tone into the unit I should calibrate the LEDs to the amplitude of the signal I'm putting in or does the gain setting need to be put into account?  Should I calibrate and set the LED meter's to an external converter input? 

All help is appreciated.
Thanks.

 

[tk@halmi]

I installed AD797's since the consensus on the web seems to be that they're the best suited for a Jensen Twin Servo (not to mention the schematic for an IC twin servo pre on the Jensen website also shows AD797s in it).  I figure since the MP20 is a "twin servo mic preamp" the Jensen schemo is identical (or at least very similar) to the MP20 cct.  I managed to adjust the rails to +/-18V on the power supply on my own.

The schematics are similar, but not identical. The MP20 uses a FET between the transformer and the dual servo circuit. It enables the use of the cheapest Jensen input transformer, and it adds that pointless Idss adjustment circuit.

The only problem I'm having now is that there is a tiny square pot (looks like a lego block) on each pre channel that seems to adjust the LED meter's response or perhaps locking of the servo (since the signal seems to degrade I'm thinking it's a combination of the two).  I'm not sure how it should be metering (ie: are the red/clip led's indicative of clipping at the input or the output?). Adjusting the gain knob also changes the level of the LED meter so the logic follows that when I feed a test tone into the unit I should calibrate the LEDs to the amplitude of the signal I'm putting in or does the gain setting need to be put into account?   Should I calibrate and set the LED meter's to an external converter input? 

I would leave them alone. Surely, you can hear when the preamp clips.

 

[bxt403]

I installed AD797's since the consensus on the web seems to be that they're the best suited for a Jensen Twin Servo (not to mention the schematic for an IC twin servo pre on the Jensen website also shows AD797s in it).  I figure since the MP20 is a "twin servo mic preamp" the Jensen schemo is identical (or at least very similar) to the MP20 cct.  I managed to adjust the rails to +/-18V on the power supply on my own.

The schematics are similar, but not identical. The MP20 uses a FET between the transformer and the dual servo circuit. It enables the use of the cheapest Jensen input transformer, and it adds that pointless Idss adjustment circuit.

The only problem I'm having now is that there is a tiny square pot (looks like a lego block) on each pre channel that seems to adjust the LED meter's response or perhaps locking of the servo (since the signal seems to degrade I'm thinking it's a combination of the two).  I'm not sure how it should be metering (ie: are the red/clip led's indicative of clipping at the input or the output?). Adjusting the gain knob also changes the level of the LED meter so the logic follows that when I feed a test tone into the unit I should calibrate the LEDs to the amplitude of the signal I'm putting in or does the gain setting need to be put into account?   Should I calibrate and set the LED meter's to an external converter input? 

I would leave them alone. Surely, you can hear when the preamp clips.

That's the problem: I turned the knobs thinking they were for the power- until I looked at power section and noticed three other trimmers! 

I'm of the mindset that believes if the LED meters work, I want them to do just that and have a 100 percent operational unit.

Oh well, I suppose I setup the good ole' 400hz tone on my generator and get my calculator out...I was hoping I didn't have to do any math (my physics and calculus classes have me all factored out for the day :-\ ).

 

[bxt403]

I managed to figure out the issue with the VU meters:  it seems that the trimmers nearest the op amps needed to be adjusted (where adjusting the output of the last 797 in the signal chain to 0.0mV corrected the DC offset) . 

In any case: the VU meters started behaving correctly (and the preamps were clipping when the red LEDs on the VU meter were lit). 

Presonus weren't very helpful (and said I had to mail the unit in for repair) but cross-referencing with the Jensen IC twin servo, and DOA twin servo schematics and John Hardy's made me realize I needed to correct the DC offset. 

The design of the preamp is nice enough, since it looks like the VU meter's calibration relies on the DC offset adjustment..

 

[ppa]

this transformer does not like an high current noise so for this reason there is an fet buffer after it.
Infact its low ratio needs a low voltage noise of the amp stage, so, in this amp stage is needed to interface this transformer, or a very low noise FET opamp  or a fet buffer that is followed by a low noise bipolar input opamp, as the ones that is used on this preamp.