bridged t attenuator

[seva]

Hi guys,

Can somebody help my understanding this design?

I have some problem with building a stepped attenuator switch. The design is based on this bridged T schematic from this thread:

https://groupdiy.com/index.php?topic=31472.0

schematic:

I found many web informations about attenuators also in this forum but when i tried out this circuit
it doesn`t work correct. Maybe I did some mistakes?

 

[seva]

I use a 3 pole 4 position switch.

here is the first type I have build up. It´s representing the schematic I think so but it doesnt work.

 

[seva]

here is the second type wich I have modded. That works correctly in terms of db loss (attenuation 2db steps) I don´t know what happen with the impedence right now.


I am driving a ballanced output signal from an opamp output, 50Ohm to this 600Ohm ballanced attenuator. I will design the pad to 50Ohm after understanding whats going on. but I thing the 50 to 600 ratio is no problem and no reason for this.

Thanks a lot for help

 

[emrr]

It attenuates the correct steps with a 600 ohm source and a 600 ohm load. 
It attenuates almost the correct steps with a 50 ohm source and a 600 ohm load. 
It attenuates much less than the correct steps with a 50 ohm source and a 10K ohm load. 

You do not want to make it a 50 ohm design. 

 

[trashcanman]

I use a 3 pole 4 position switch.

here is the first type I have build up. It´s representing the schematic I think so but it doesnt work.

Isn't it supposed to be a 2 pole 12 position switch?

 

[abbey road d enfer]

Hi guys,

Can somebody help my understanding this design?

I have some problem with building a stepped attenuator switch. The design is based on this bridged T schematic from this thread:

https://groupdiy.com/index.php?topic=31472.0

schematic:

I found many web informations about attenuators also in this forum but when i tried out this circuit
it doesn`t work correct. Maybe I did some mistakes?

We could probably help you better if you said what "doesn`t work correct".

 

[abbey road d enfer]

I am driving a ballanced output signal from an opamp output, 50Ohm to this 600Ohm ballanced attenuator. I will design the pad to 50Ohm after understanding whats going on. but I thing the 50 to 600 ratio is no problem and no reason for this.

How are you measuring it? The attenuation is correct only if the output stays balanced (or floating).

 

[seva]

Ok, as it seems, that the placement of the first switch is right ! But it does not work properly in my circuit.

What I meant with "the first switch worked incorrectly" was: after I have switched to the attenuation steps only a minimal attenuation took place. In addition, the signal peaks start to flicker and has behaved abnormally.

I did the measurement in the studio. I have connected the circuit to a DA / AD converter and sent a sinus at 1kHz out - + 4dbu. The AD input I measured with penguin SPPM and VU meters to see the dB steps. I could clearly see a flickering of the level peaks and the RMS value, so I have the switch modified until it has the correct values ​​displayed My tools are limited here what measurement technology is concerned and I have unfortunately not the necessary expertise around everything theoretically to understand. Therefore, I always try out so long until it works.

OK, then I will get me now a multimeter and measure the outputs to be sure that it is a symmetrical signal going to the attenuator. But should it be otherwise I would probably have a level drop at the input of the AD converter at + 4dbu input, I think. hmmmm?


Here is the output schematic. I wanna place my attenuator after the output. Some guys told me its best to design it for 50Ohm when the ouput impedance is 50Ohm too. I planing to build another attenuator which is feeded by 150Ohm Output. What do you think is the best Impedance for designing an ballanced output attenuator for 50Ohm or 150Ohm. Is there any ratio law to understand. Some people told me that a 1to10 ratio is the minimum for matching input and output of every kind of circuit. so i decided to use my attenuatior in 600Ohm. It goes from 50 to 600 to 10k or whatever the next input has.

 

[abbey road d enfer]

Ok, as it seems, that the placement of the first switch is right ! But it does not work properly in my circuit.

What I meant with "the first switch worked incorrectly" was: after I have switched to the attenuation steps only a minimal attenuation took place.

That's normal, you have connected it in the wrong direction. As it is, it has no attenuation because the 100uH inductors by-pass it.

  In addition, the signal peaks start to flicker and has behaved abnormally.

It looks like you are overloading the opamp and it starts to act erratically. This will disappear when you connect the attenuator correctly.

I did the measurement in the studio. I have connected the circuit to a DA / AD converter and sent a sinus at 1kHz out - + 4dbu. The AD input I measured with penguin SPPM and VU meters to see the dB steps. I could clearly see a flickering of the level peaks and the RMS value, so I have the switch modified until it has the correct values ​​displayed My tools are limited here what measurement technology is concerned and I have unfortunately not the necessary expertise around everything theoretically to understand. Therefore, I always try out so long until it works.

Your method is adequate.

Here is the output schematic. I wanna place my attenuator after the output. Some guys told me its best to design it for 50Ohm when the ouput impedance is 50Ohm too.

This is not a good recommendation. Although the actual output Z is lower than 50 ohms, it wants to see a minimum of about 300 ohms.  I would recommend 600 for minimizing distortion.

I planing to build another attenuator which is feeded by 150Ohm Output.

Is it really 150 ohms? Can it drive 150 ohms without significant loss? Describe it. 

What do you think is the best Impedance for designing an ballanced output attenuator for 50Ohm or 150Ohm.

It depends if you want matching or bridging. Matching means 600 ohms drives 600 ohms, bridging is Zload >10 times output Z.

Some people told me that a 1to10 ratio is the minimum for matching input and output of every kind of circuit.

No. Matching is equal impedance, resulting in 6dB loss; it comes from landline telephone; it's obsolete in audio.
People give advice, they do not formulate it correctly, and do not realize that it can have disastrous consequences. In science, precision and using the correct words is of paramount importance.

[/quote]so i decided to use my attenuatior in 600Ohm. It goes from 50 to 600 to 10k or whatever the next input has.
[/quote] This is called bridging.

 

[seva]

thanks man

simple changing plus and minus ? thats it?

or do you mean changing the hole direction of the attenuator circuit? out is in and in is out?

OK when I understand you right,  an attenuator circuit should be bridged that means min 10 times of input impedance. matching is not common nowadays

Quote
I planing to build another attenuator which is feeded by 150Ohm Output.
Is it really 150 ohms? Can it drive 150 ohms without significant loss? Describe it. 

One of my mastering gear (manley backbone) has an 150 Ohm output. so i planing a attenuator which can handly very hot signals up to 22dBU.

Would you prefer a 1,5K Ohm for this attenuator?

 

[emrr]

It attenuates the correct steps with a 600 ohm source and a 600 ohm load. 
It attenuates almost the correct steps with a 50 ohm source and a 600 ohm load. 
It attenuates much less than the correct steps with a 50 ohm source and a 10K ohm load. 

You do not want to make it a 50 ohm design.

 

[abbey road d enfer]

simple changing plus and minus ? thats it?

No.

or do you mean changing the hole direction of the attenuator circuit? out is in and in is out?

Yes.

One of my mastering gear (manley backbone) has an 150 Ohm output. so i planing a attenuator which can handly very hot signals up to 22dBU.

Would you prefer a 1,5K Ohm for this attenuator?

I think that would be correct, although the Manley litterature is not very well documented in this respect.
BTW, the max output level is quoted at +29dBu.

 

[seva]

understand

what about resistor dimension (Watt)

+27dBu equal to 21,83V  it´s over 20V to attenuate to get +4dBu

 

[seva]

29dBU

 

[abbey road d enfer]

29dBU

22v  into 1.5k=> 0.3W. Since the power is shared by at least two resistors, you can use 1/4W.

 

[seva]

OK, sounds great, I will try it out 8)