Studer mic preamp - modified diy

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sbeach

Active member
Joined
Jun 27, 2006
Messages
31
Location
Sunny & Humid SC
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
 
As redrawn, when the gain is set to minimum, the output is driving a 70-ohm load (the 35 ohm resistor in series with the gain pot, and the 35 ohm resistor just to the left of there). I don't see that in the Studer schemo.

Peace,
Paul
 
Hi Paul,

Thank you for responding & pointing out what should have been more obvious to me. Yes, the original Studer schematic doesn't have exactly the same resistor values as I redrew, but, in looking at the original schematic, the S1b switch seems to have one position in which a single 82 ohm resistor would be placed between the feedback loop & ground. Still a bad idea if it indeed does just that!

Elsewhere in the Studer service manual, it indicates that this amplifier's gain is set by the ratio of the 10K linear pot to the resistance set by the switch. I confess that the S1b switch architecture is puzzling - I can't figure out how it mechanically should work.

As a more workable solution I have redrawn the schemo again to use a more standard gain-setting approach. A static 10K resistor in the feedback loop & an anti-log 10K pot tied to ground for gain adjustment. Thus at the pot's minimum resistance (maximum gain) the amp's output sees 10K to ground.

original Studer schematic: http://groupdiy.twin-x.com/displayimage.php?pid=1629&fullsize=1

2nd redrawn schematic: http://groupdiy.twin-x.com/displayimage.php?pid=1634&fullsize=1

Otherwise does the scheme look OK?

Skip
 
Hey, it´s a nice desk that you came along with. A few remarks; I recognise the circuit wich is pretty much the one from the Revox A77
period and its widely used,but you can be certain that the sound from this desk is very much from the transformer´s both in and out. So be careful.
If you look at the gainsw. its doing 2 things. R19 is setting dc gain and the pot is more trimming. Both the feedback and the emitter to ground is changed getting diff.gain settings. Cheers Bo
 
Phew, only three transistors but what a bucketload of caps! :shock:

At first the circuit made me think of the pushpull CFP from the D.Self site but when looking it up it's still a bit different:
http://www.dself.dsl.pipex.com/ampins/discrete/cfp.htm
(fig 13)

What kind of input-TX (ratio) was used in the original ?

Regards,

Peter
 
Hi Again,

Peter - the mic transformer is at 4:1. I understand that as being +14db voltage gain. I'm hoping for about +45db to +48db gain from the circuit. And yes Peter, it is quite a lot of caps. I too was aghast at this, but these preamps do sound really good. I can only guess that using high quality caps such as polystyrene and mica for the smaller values, plus bypassing the electrolytics with polypropylene, will work at least as well as the original.

As for the preamp's fine sound being primarily a result of the particular transformers Studer used, well I can only hope this is more conjecture than fact. I had read (on this forum) that the original mic input transformer was prone to microphonics & should be abandoned in favor of a suitable substitute.

See ... http://www.groupdiy.com/index.php?topic=5135&postdays=0&postorder=asc&highlight=studer+169&start=15

As for the original gain setting scheme, here's an explanatory graphic from the service manual ... http://groupdiy.twin-x.com/displayimage.php?pid=1635&fullsize=1
So, I think the second redrawn gain-setting method will work as well or better than the switch/trimpot combo in the original circuit.

Thank you all for your kind & insightful responses. If you see anything else amiss or questionable about this project, please don't hesitate to mention it.

Skip
 
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