sbeach
Active member
Hi,
I have been excitedly planning a new diy project ... a mic preamp based around the Studer 169 mixer mic input amplifier. I have heard many people praise the sound of these mixers - especially regarding the mic preamps. At my workplace (SC Educational Radio & Television) we have an old Studer 169 mixer that has been "out of service" for over a decade and a half. It needs a thorough re-capping, etc. so I doubt it will ever be revived - no funds for costly repairs. I have heard many classical music recordings made through this mixer & they do sound terrific! So, I decided to try a DIY approach for getting a similar sound for my own recording projects.
I downloaded the .pdf service manual for the 169/269 mixer series from Studer, found the mic input pages, & copied the circuit pictures. One odd thing about the way the schematic was drawn is that it has the positive supply at the bottom of the page, and the negative DC supply at the top of the page. I found this very confusing as I have always drawn & worked from schematics that are the reverse of this.
Anyway, I have re-drawn the circuit to reflect my preference & have made a few "simplifications" in the feedback loop. I have replaced the original "switched-plus-variable trim" gain scheme with a single 35 ohm resistor to ground & a 10k linear pot in the feedback loop. This new feedback loop approach should give a continuously variable gain at a range from +6db to +48db. This assumes that an input transformer (or possibly an INA217 opamp stage) will provide an additional +12db to +15db of voltage gain for a total of +60db or more.
Originally, the next stage in the mixer, after this amplifier, is the Bass-cut EQ. This amplifier circuit was not intended to output directly to the real world as an unbalanced output. But, that is how I'm hoping to use it. I seldom find the need for balanced outputs as I never need to drive long lines. I also do not wish to use an output transformer if it's not necessary. In checking the spec. sheet on the BC239C transistor, it's maximum collector current is 100 Ma. This seemed OK for driving loads down to around 2K & I'm hoping not to need an additional current booster buffer stage. Does this seem reasonable to you more experienced folks?
Here is the original Studer schematic ... http://groupdiy.twin-x.com/displayimage.php?pid=1629&fullsize=1
Here is my redrawn version ... http://groupdiy.twin-x.com/displayimage.php?pid=1628&fullsize=1
Do you see any potential problems or issues with either the redrawing or the overall implementation plan? Any helpful comments or insights would be greatly appreciated. Thanks for looking.
Skip
I have been excitedly planning a new diy project ... a mic preamp based around the Studer 169 mixer mic input amplifier. I have heard many people praise the sound of these mixers - especially regarding the mic preamps. At my workplace (SC Educational Radio & Television) we have an old Studer 169 mixer that has been "out of service" for over a decade and a half. It needs a thorough re-capping, etc. so I doubt it will ever be revived - no funds for costly repairs. I have heard many classical music recordings made through this mixer & they do sound terrific! So, I decided to try a DIY approach for getting a similar sound for my own recording projects.
I downloaded the .pdf service manual for the 169/269 mixer series from Studer, found the mic input pages, & copied the circuit pictures. One odd thing about the way the schematic was drawn is that it has the positive supply at the bottom of the page, and the negative DC supply at the top of the page. I found this very confusing as I have always drawn & worked from schematics that are the reverse of this.
Anyway, I have re-drawn the circuit to reflect my preference & have made a few "simplifications" in the feedback loop. I have replaced the original "switched-plus-variable trim" gain scheme with a single 35 ohm resistor to ground & a 10k linear pot in the feedback loop. This new feedback loop approach should give a continuously variable gain at a range from +6db to +48db. This assumes that an input transformer (or possibly an INA217 opamp stage) will provide an additional +12db to +15db of voltage gain for a total of +60db or more.
Originally, the next stage in the mixer, after this amplifier, is the Bass-cut EQ. This amplifier circuit was not intended to output directly to the real world as an unbalanced output. But, that is how I'm hoping to use it. I seldom find the need for balanced outputs as I never need to drive long lines. I also do not wish to use an output transformer if it's not necessary. In checking the spec. sheet on the BC239C transistor, it's maximum collector current is 100 Ma. This seemed OK for driving loads down to around 2K & I'm hoping not to need an additional current booster buffer stage. Does this seem reasonable to you more experienced folks?
Here is the original Studer schematic ... http://groupdiy.twin-x.com/displayimage.php?pid=1629&fullsize=1
Here is my redrawn version ... http://groupdiy.twin-x.com/displayimage.php?pid=1628&fullsize=1
Do you see any potential problems or issues with either the redrawing or the overall implementation plan? Any helpful comments or insights would be greatly appreciated. Thanks for looking.
Skip