One more Bridge T attenuator question

[dirty1_1garry]

Hi!

I want to make stepped mic pre input pad using bridge T attenuator. Seems to be a good idea. But what kind?

This type of attenuators  are used in many classic amplifiers like Sta-level, UA176... 1176 uses this type of attenuator at the input before the input transformer. But all of these devices use a bridge T attenuator. But bridge T attenuator is an unbalanced attenuator, if it's right to say. So attenuation became only on hot wire, right? Is it so unfundamentally?

Where it could be more suitable to use bridge T and where Balanced Bridge T attenuators?

 

[abbey road d enfer]

Bridged-T attenuator is a very bad idea for microphone preamplifiers.
Briged-T attenuators have identical input and output Z, which makes a lot of noise.
The preamp wants to see 200 ohms but the mic wants to see 2kohms.
Use the well-known balanced-L attenuator (also called U-attenuator).

 

[dirty1_1garry]

Bridged-T attenuator is a very bad idea for microphone preamplifiers.
Briged-T attenuators have identical input and output Z, which makes a lot of noise.
The preamp wants to see 200 ohms but the mic wants to see 2kohms.

May be that's the reason why I never seen bridge T in mic pre's.
So Bridge T bad for mic pre but good for line level devises that work on stable impedance. Right?

 

[dirty1_1garry]

Use the well-known balanced-L attenuator (also called U-attenuator).

May be something like on attached file.

 

[abbey road d enfer]

Correct!

 

[dirty1_1garry]

Both of this pads give same -20dB
But which is better?

 

[emrr]

which one gives a load to the mic closer to bridging impedance?

look up the Jensen Transformers front end, something in that range probably gets used more than anything else. 

the real answer is the one that loads both the preamp and the mic the best. 

 

[abbey road d enfer]

Both of this pads give same -20dB
But which is better?

The first one shows te transformer a better (lower) impedance, but the input impedance is only 1k, which is lower than the recommended load.
The second one gives the correct input impedance (about 2k) and shows the transformer an also correct impedance (about 2ààr).