Neumann OSVD summing amp -adding input tranformer

[lauritz]

Hi All

I'm building a 24 bus summing amp around a neumann OSVD line amplifier. I have attached a picture of the module. It is impossible to find technical specs for the cards and even the nice people at Neumann could'nt help with schematics ect. I have found out that it runs on +15/-15 V, which is different for most neumann cards running on 24 V

I have been tracing the card for pin out configuration and it seems that the In+ of the OA10 opamp is tied directly to ground -which makes the input unbalanced?

If so, i want to add a couple of line transformers  after the passive resistor(5k1 ohm) network going into the board. I have a couple of cool Jørgen Schou(from a vintage danish broadcasting console) line input transformers rated at 5k:5k or 20k:5k.
Can i just add these in between to make the input balanced? Anything i need to consider regarding input/output impedances? 

thanks in advance
Lauritz

 

[[silent:arts]]

http://groupdiy.com/index.php?topic=44805.msg769171#msg769171

 

[lauritz]

http://groupdiy.com/index.php?topic=44805.msg769171#msg769171

Thanks a lot! That gets me going Regarding my other question...is it correct that i can just throw in those transformers? Would it be a good idea to add input caps on the primary side to protect the transformer -or would that be overkill since the resistor network in front of the board is not connected to any dc current.

thanks again.

best
Lauritz

 

[[silent:arts]]

"because of the needed low input impedance the input is unbalanced"
"if you need a symmetrical input use both amps on the card"
(the description is a little bit confusing with series and parallel here, I would say it means use one amp for hot and one amp for cold resulting in a fully differential summing amp)

If there would have been a way just to add a transformer there would be an option for this transformer, or, if space would be an issues (it isn't with the OSVD), there would be a model number to use external.

With this information from the data sheet, and knowing it is zero ohm summing, you can't put a 5K:5K transformer between the summing network and the (indeed unbalanced) input of the card.

 

[lauritz]

The translations helps!...guess i will have to do some reading on the principles of current summing to better understand what goes on excactly. I appreciate your inputs. Maybe i will come back with a couple of questions once i understand better

best
Lauritz

 

[lauritz]

Hi

Just want to post a couple of pictures of the finished unit. I got it working thanks to your pointers and some reading. Ended up adding a buffer stage (ina217 chip) after the input resistor network to convert the differential signal to match the unbalanced input of the neumann OSVD amp. This buffer also gives the possibility of adding gain before the amp if needed or to drive the amp with a hotter signal. This can be adjusted via a precision stereo stepped potentiometer.

I think the unit works well -it sounds great and very transparent.

Thanks for the help!
best
Lauritz, DK

 

[lauritz]

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