preamp output transformer

[lauritz]

Hi

I'm currently constructing a rackmodule around a vintage neumann v470 line amp. This module accepts both line/instrument signals via a input impedance switch.
I want to add a two preamps to the box as well, so i am planning on using the "5 dollar" circuit based on the TI ina217 chip -for simplicity and just to try it out.  (http://www.ti.com/lit/ds/symlink/ina217.pdf)

So the question....the output of this mic preamp circuit is unbalanced. The neumann module input has transformers at the input and takes in a balanced signal according to the manual(can't get hold of the actual schematic, but the manual/specs is attached) Do i need an additional transformer(unbalanced to balanced) before feeding the signal to the line amp or can i just connect it directly in some way?

best
Lauritz

PS. Some people use the v470 as a preamp itself, but then you lose the flexibility of the 3 seperate output signals, which is very cool for parallel processing and signal splitting in generel.

 

[Matador]

You could just go impedance balanced on the output of the INA217.

 

[lauritz]

Great...that's what i'll do then.  I guess there is plenty of transformer sound in there already, and i will lower the cost too.  Thanks for the help 

 

[abbey road d enfer]

....the output of this mic preamp circuit is unbalanced. The neumann module input has transformers at the input and takes in a balanced signal according to the manual(can't get hold of the actual schematic, but the manual/specs is attached) Do i need an additional transformer(unbalanced to balanced) before feeding the signal to the line amp or can i just connect it directly in some way?

You don't need to balance the signal from the INA217 because you run only a couple of inches; a transformer accepts indifferently balanced AND unbalanced signals. Going impedance-balanced would only increase the source impedance seen by the transformer's primary, which would increase noise and distortion. You may have to DC protect the primary with a coupling capacitor, though.

 

[lauritz]

Just to understand correctly...you suggest skipping the (impedance balancing) resistor between ground and the "negative side" of the primary input transformer, and instead maybe put in a coupling cap between output of the mic amp and the "positive side" of the primary?

And another quick question...in the schematic for the INA217 mic pre there is a OPA opamp circuit that is suppose to set the dc offset at 0 V. Would that not make the coupling cap redundant?

Thanks for the insights...

 

[abbey road d enfer]

Just to understand correctly...you suggest skipping the (impedance balancing) resistor between ground and the "negative side" of the primary input transformer, and instead maybe put in a coupling cap between output of the mic amp and the "positive side" of the primary?

  Yes, exactly. I couldn't find the schemo, but according to the datasheet, there are two options for the input.
The 600 ohm input is direct to the transformer's primary. You don't want to use this possibility, because of the risk of frying the primary in case anything goes wrong with the INA217 (and many things can happen with a direct-coupled mic pre subject to phantom power). In the other mode (>= 5k) the input is determined by two resistors, that would more or less protect the primary in case of problems. But since the impedance is perfectly controlled in that case, I think capacitors are an inexpensive and efficient addition. You would prefer a bipolar cap of about 10-47uF, or two polarised caps head-to-tail of 22-100uF, rated 16V minimum.

And another quick question...in the schematic for the INA217 mic pre there is a OPA opamp circuit that is suppose to set the dc offset at 0 V.

That's the servo DC-null. That is an option.

Would that not make the coupling cap redundant?

Yes, although it can fail.  The capacitors would still be there for ultimate protection. Somewhat similar to fuses. One can devise the most sophisticated protection devices, in the end it's the fuses that prevent putting the house on fire.
Ther are two main justifications for DC-null servos:
a- when they allow replacing bulky and/or expensive capacitors, which is not the case here
b- when an audiophool decides coupling capacitors degrade the performance

 

[lauritz]

Thanks for your thorough considerations abbey road d enfer. Very helpful indeed...just about to order some component, and i will throw in some caps as you advice...and maybe ditch the DC-null altogether.

Ther are two main justifications for DC-null servos:
a- when they allow replacing bulky and/or expensive capacitors, which is not the case here
b- when an audiophool decides coupling capacitors degrade the performance

I take from the subtleties of  your comment, that you are no be fan of that arrangement in this particular situation 

thanks again

 

[abbey road d enfer]

Thanks for your thorough considerations abbey road d enfer. Very helpful indeed...just about to order some component, and i will throw in some caps as you advice...and maybe ditch the DC-null altogether.

Ther are two main justifications for DC-null servos:
a- when they allow replacing bulky and/or expensive capacitors, which is not the case here
b- when an audiophool decides coupling capacitors degrade the performance

I take from the subtleties of  your comment, that you are no be fan of that arrangement in this particular situation 

thanks again

DC-null servo is a creative idea; it is another tool in the designer's toolbox. Just like any tool, it should be used purposefully. I'm not sure in the circumstances it really brings a genuine improvement. In many cases I felt that servo DC-nulled line outputs are at best an interesting designer exercise, at worst fodder for the mktg dept. There is a case for output stages used to drive transformers though.
Now if you have a PCB with provision for implementing the DC-null servo, why not use it? The cost is minimal. But don't expect miraculous improvement over a good old cap.

 

[lauritz]

Cool. I've ordered components for both arrangements, and plan on doing the circuit on a breadboard, so no big deal fitting in everything and changing arrangement. Fairly new to electronics, so it will educational either way around 

thanks and regards
Lauritz