Amek CIB Trafo right before virtual earth summer, why?

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Rogy

Well-known member
Joined
Jun 3, 2004
Messages
247
Location
Belgium
Hi all,

I have this Amek Channel in a Box.
It has a trafo-coupled line input that can be reconfigured as a virtual earth mixer, by simply bypassing the 5.1K resistors between the XLR and transfo in (see attach).
Two questions:
1) What is the advantage of having a trafo in front of the virtual earth summer, other than galvanic separation? The trafo secondaries are in the feedback loop of the 2134's, what's the benefit?
2) If I were to use the virtual earth summer as a 4-input line mixer, does each of the 4 inputs need to have 20k4 resistors to end up with a total parallel of 5.1K for optimum source impedance ?

Thanks in advance!

Rogy
 

Attachments

  • CIB LINE IN AS VIRTUAL EARTH.JPG
    CIB LINE IN AS VIRTUAL EARTH.JPG
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I think (not sure though...) that the trannys in the feedback loop, and that the opamp can account for the non-linearities in the tranny and compensate... There are a couple of pages at the lundahl website that talk about doing this on output transformers...

Either that or it could be a zero ohm summing topology similar to that used in Neumann v475 or Lawo v975... Search for either 'neumann pimping' (not a joke!!) or lawo and there are links in here... Kubarth also has info on his page (forgotten the link ???)

Maybe a combo of all the above, maybe nothing near (still learning here!!)...

Mo
 
ps, it sure looks like a similar topology to the Calrec Polar UA8000 output board that Gyraf posted somewhere on here... Which uses the trafo on oa fb loop to linearise it... There's a good Analog devices/Walt Jung app note on this page: http://waltjung.org/ADI_Books.html you want to download the opamp applications, there's a spiel about it in there...

Sorry if this is completely unhelpful!!

Mo

*EDIT* Got the book name wrong.... Bad day...
 
Rogy said:
Hi all,

I have this Amek Channel in a Box.
It has a trafo-coupled line input that can be reconfigured as a virtual earth mixer, by simply bypassing the 5.1K resistors between the XLR and transfo in (see attach).
Two questions:
1) What is the advantage of having a trafo in front of the virtual earth summer, other than galvanic separation?
The xformer is there just for galvanic isolation and increasing CMRR. Galvanic isolation is very often requested by broadcast companies.
The trafo secondaries are in the feedback loop of the 2134's, what's the benefit?
This topology is called "zero-flux" because actually the FB counteracts the induction generated by the input current. The main advantages are:
a) THD is very low because there is no saturation of the magnetic core
b) the xformer operates with almost zero-voltage so it has a lower nominal impedance than a standard line xfmr, thus it has less leakage inductance and stray capacitance, so it can operate up to very high frequency (200kHz is quite common)
2) If I were to use the virtual earth summer as a 4-input line mixer, does each of the 4 inputs need to have 20k4 resistors to end up with a total parallel of 5.1K for optimum source impedance ?
No. If you want unity gain, each input resistor must be equal to the FB resistor. In practical terms, these resistors need be 5k1.

BTW, if you could post the full schematics of the CIB, I would be delighted (as many others)...
 
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