Pad network to feed mic transformer with line levels

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abbey road d enfer said:
This is what you should end up with.
Yes, I tried to left the "center" wire floating (the grey one) but the trafo. didn't work.

In this configuration it just outputs higher freqs, really attenuated.

To let it work I must connect the Grey wire to ground or to the signal source, in both cases it works smooth.

It's weird, did you already experienced this behavior?
 
Herva said:
Yes, I tried to left the "center" wire floating (the grey one) but the trafo. didn't work.

In this configuration it just outputs higher freqs, really attenuated.

To let it work I must connect the Grey wire to ground or to the signal source, in both cases it works smooth.
I don't understand; what do you mean "to the signal source"? Hot or cold?

It's weird, did you already experienced this behavior?
No, this is not normal. Have you checked the DC resistance between:
  • white and grey
  • grey and black
  • white and black
 
I checked all connections, here belows are the only ones that wasn't open circuit:

- Black/White around 55 ohm
- Red/Grey around 1,5 kohm

So at this point I think that primary is not center tapped and it's between Black and Withe + the secondary is between Red and Grey that's why is I left the Grey floating I had no output.

I'm sorry I made this mistake since in the original schematic Grey and Green where both grounded so the color code was logical in this way to me: Black/Grey/Whith for primary and Red/Green for secondary.

Anyway the Green wire shows open circuit with all the other wires and the shielding, I don't know its purpose, what you think?
 
Herva said:
I checked all connections, here belows are the only ones that wasn't open circuit:

- Black/White around 55 ohm
- Red/Grey around 1,5 kohm

So at this point I think that primary is not center tapped and it's between Black and Withe + the secondary is between Red and Grey that's why is I left the Grey floating I had no output.
OK, so mystery solved.


Anyway the Green wire shows open circuit with all the other wires and the shielding, I don't know its purpose, what you think?
It's probably an electrostatic shield, that ideally should be grounded to earth (chassis), not audio ground.
 
abbey road d enfer said:
OK, so mystery solved.

It's probably an electrostatic shield, that ideally should be grounded to earth (chassis), not audio ground.

It make totally sense!

Anyway about the padding resistor + secondary loading what would you suggest?
 
Herva said:
Anyway about the padding resistor + secondary loading what would you suggest?
You must experiment; first disconnect the resistor and Zobel from the secondary. Feed the primary via a resistor, then increase the resistor until you see significant LF loss. I would start with 200 ohms or so, and a level of about -20 dBu.
There should be a resistor value that creates a 3 dB attenuation respective to midrange; the choice of LF frequency is yours, if you want to design for 20Hz, measure at 20 Hz, if you want to design for 30 or even 40 Hz, do as you please. Let's say with 2400 ohms in series, you have -3dB at 20 Hz, and you want to achieve -1dB, the resistance must be halved, so you must make sure that the source impedance, either direct or via an attenuator, must be less than 200 ohms. In this particular case, some mics would have too high impedance for proper LF response.
This is the basic high-pass effect of an R-L circuit (see attached).
For an attenuator of more than 20 dB, it's the shunt resistor that governs the output impedance. With the example values, that means that the shunt resistor should be smaller than 200 ohms.
That is without loading the secondary. There is no good reason to load the secondary significantly. Using a resistor to damp the xfmr's resonance is brute force. A Zoble is a much more elegant and productive solution.
In a previous post, I explained briefly how to work the Zobel, but the results you presented show that something is wrong in your set-up.
If the secondary is loaded by a resistor, this resistor is reflected at the primary, in parallels, so the effective impedance seen by teh primary is the result of the source Z in parallels with the reflected Z. This may be the reason why the secondary was so heavily loaded originally. It's a dirty trick to make a less-than-decent xfmr look like a half-decent one. Of course, in practice, there is the added attenuation, that often results in less-than-optimum S/N ratio.
 
abbey road d enfer said:
Not can, does. Optimization of a Zobel network implies providing the correct source Z to the device. A mistake commonly made by younger designers is to let their generator at its lowest impedance and optimize the Zobel there; I know, I've been there.
Indeed, with a non-constant-Z attenuator, compromises are to be made.

The 'correct' source is often  a variable these days. Most modern condenser mics present a source impedance of around 50 ohms whilst most dynamics will present 150 to 200 ohms.

Cheers

Ian
 
ruffrecords said:
The 'correct' source is often  a variable these days. Most modern condenser mics present a source impedance of around 50 ohms whilst most dynamics will present 150 to 200 ohms.
Indeed; that's the reason why the Zobel tuning is necessarily a compromise.
 
abbey road d enfer said:
You must experiment; first disconnect the resistor and Zobel from the secondary. Feed the primary via a resistor, then increase the resistor until you see significant LF loss. I would start with 200 ohms or so, and a level of about -20 dBu.
There should be a resistor value that creates a 3 dB attenuation respective to midrange; the choice of LF frequency is yours, if you want to design for 20Hz, measure at 20 Hz, if you want to design for 30 or even 40 Hz, do as you please. Let's say with 2400 ohms in series, you have -3dB at 20 Hz, and you want to achieve -1dB, the resistance must be halved, so you must make sure that the source impedance, either direct or via an attenuator, must be less than 200 ohms. In this particular case, some mics would have too high impedance for proper LF response.
This is the basic high-pass effect of an R-L circuit (see attached).
For an attenuator of more than 20 dB, it's the shunt resistor that governs the output impedance. With the example values, that means that the shunt resistor should be smaller than 200 ohms.
That is without loading the secondary. There is no good reason to load the secondary significantly. Using a resistor to damp the xfmr's resonance is brute force. A Zoble is a much more elegant and productive solution.
In a previous post, I explained briefly how to work the Zobel, but the results you presented show that something is wrong in your set-up.
If the secondary is loaded by a resistor, this resistor is reflected at the primary, in parallels, so the effective impedance seen by teh primary is the result of the source Z in parallels with the reflected Z. This may be the reason why the secondary was so heavily loaded originally. It's a dirty trick to make a less-than-decent xfmr look like a half-decent one. Of course, in practice, there is the added attenuation, that often results in less-than-optimum S/N ratio.
Hi and much thanks for your detailed answer, I've taken these days to experiment with your advices.

Since I need to interface it only with line signal to avoid big voltage drops I decided to keep at the primary side a series resistor of 2,2 kohm (that seems a good compromise to me) + a pot of 1k as shunt resistor to tune the source impedance manually.

I decided to try a design for a -3db loss at 20 Hz, anyway this lead to compromises..

Using your approach If I want a good frequency response (flat square wave), the secondary output voltage  results more than halved compared to the signal input (measured at the input of the padding network).

With a "brute force zobel" (50kohm series 100 nF and no further load) I've been able to reached around -1db a 20 Hz whit the voltage at the output of the secondary almost at the same level of the input (1:1).

I'm having a very little ringing with this loading but I can deal with it.

On the original circuit I think they made a similar trick since the secondary was loaded with 47 kohm +  in shunt 68 kohm in series with 10uF.

Maybe they where forced to do it since it's just a budget transformer that without this loading can't handle basses frequencies properly.

Anyway for S/N ratios the brute loading seems the way to go for me, what you think?

As always thanks for your time and advices!


 
abbey road d enfer said:
Indeed; that's the reason why the Zobel tuning is necessarily a compromise.

I've experimented some more and now I get what you said, the response is quite linear and I'm totally happy with it.

I designed to have a source imp. of  around 47 ohm and loading secondary with 47 kohm I didn't needed any zobel compensation.

Much thanks Abbey, your advices were totally rights, now the transformer sound less stressed and more linear!

By the way I suggest to give a read to:

http://www.foxaudioresearch.ca/Audio%20Transformers%20Chapter.pdf

It gives a nice overview on the argument.

Cheers!
 
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