Design for 600R Bridged-T Pad Attenuator 24 Step Rotary Switch!

[emrr]

Just trying to visualize and understand, so If a 2.5k Bridged T was used only between a low impedance source (100-200r) dac or other piece and a 20k load? Would that satisfy the 10x bridging thing?
It's also unclear on what the relationship is to the actual attenuation steps as it relates to mismatched impedances. From what I understand, the steps of attenuation won't actually be what is calculated for 2.5k for 1dB since the source and load wouldn't be matched. Is there reading on this? If the first question of the bridging makes it useful in that regard, are the attenuation steps being out of whack the worst thing that will happen? Again, is there reading on this and calculations to find compromises? I feel it's a futile exercise, just curious.

The bridged 'T' is used all sorts of places without matching impedances - see: Hairball/CAPI preamps and compressors. They work fine, and in those cases they aren't stepped so you don't notice the change in range. (they're also not 'really' proper 'T's, that's another subject that's been covered elsewhere here - but it hardly matters in most cases). Stepped 'T's just change the step size with mismatch, that's it. The question is really whether a 'T' is too complex for the need. If you aren't inserting it in a matching Z position, is there an easier way? If buying a Hairball/CAPI type is the easiest path for cost/time/labor, and you can afford the minor introductory loss, why not? In your case, repeatability, why not just do a 5K stepped pot?

 

[scott2000]

The mathematical answer is very complicated, but fortunately we have simulators.
The result, in brief, is that the first steps are the ones that are the most shifted. E.G. a 2.5k attenuator driven by a 100r source and driving a 10 input sees its first position (which is actually -6) shifted to -0.086dB, the -6dB position shifted to -3.4, its -12 position shifted to -8.6 and the -18 position shifted to -14.5
So the biggest discrepancy is in the first steps, where the 6 db range becomes compresed to -3.4

Great. Thank you! Anything you can point me too or terminology I need to use to search for what it is you're describing?

In your case, repeatability, why not just do a 5K stepped pot?

Not against it. Just hearing that the bridged T doesn't alter frequency depending on attenuation position or something like that but, I've never paid that much attention to that.
Guess I'm so vested in making one it's clouding my better judgement. When talking stepped pots, we mean like the series Goldpoint ones like the attachment? or something else? An L Pad? Trying to familiarize myself with the differences to be honest.
Like what would Heikki's stepped lorlin attenuator be considered?

 

[emrr]

Just hearing that the bridged T doesn't alter frequency depending on attenuation position or something like that but, I've never paid that much attention to that.

there's capacitive leakage effects depending on the physical layout and materials used, shows up at more extreme attenuations.

https://groupdiy.com/threads/t-attenuator-response-interactions-at-higher-attenuations.70980/

 

[abbey road d enfer]

Both your diagrams are L-attenuators.
Indeed, improperly designed L-attenuators can result in HF loss in certain positions, but it's just a matter of proper choice of impedances. OTOH constant-impedance attenuators don't have this problem, at the cost of increasing the load on the output stage.

 

[scott2000]

there's capacitive leakage effects depending on the physical layout and materials used, shows up at more extreme attenuations.

..Just came across this article too...

"The bridged-T has a constant input and output impedance throughout the attenuation range and has a zero dB insertion loss when the input and output impedances are equal. In the higher losses above 30 to 40 dB, the value for R1 (see Fig. 2B) becomes so high that it is more susceptible to the effect of any stray capacitance across it."

https://www.rfcafe.com/references/e...tenuators-pads-may-1966-electronics-world.htm

 

[abbey road d enfer]

..Just came across this article too...

"The bridged-T has a constant input and output impedance throughout the attenuation range and has a zero dB insertion loss when the input and output impedances are equal.

The formulation is very debatable. Although the attenuator by itself does not attenuate at its maximum position, there is 6dB attenuation due to the voltage drop created by the source impedance feeding the identical receiver impedance.
Thus, the position markings are usually in reference to -6dB. In other words, -1 in an impedance-matching environment is actually 7dB attenuation.

 

[scott2000]

What exactly is he talking about here as it relates to the bridged T? Some additional pad? or just the circuitry involved

"with the use of an impedance-matching network at one end, they can be used between unequal impedances with the necessary insertion loss due to the impedance match"

 

[abbey road d enfer]

What exactly is he talking about here as it relates to the bridged T? Some additional pad? or just the circuitry involved

"with the use of an impedance-matching network at one end, they can be used between unequal impedances with the necessary insertion loss due to the impedance match"

I believe he meant if the source impedance is too low, it is possible to increase it with a series resistor and if the load impedance is too high it is possible to reduce it with a shunt resistor.
Since the original article dates from 1959, it is deeply entrenched in an impedance-matching world.
Very poorly formulated, if you ask me...

 

[scott2000]

lol...Thanks...
So this extra resistor ,if that were the case ,would mess with the attenuation as a whole? Is that what he means by insertion loss?
Would this then make the steps of attenuation where they should be on the switch as it relates to the Bridged T itself and it's chosen impedance. Sorry... Just trying to visualize it. Hope it makes sense what I'm asking.

 

[emrr]

Well, yes, -6 as '0' because that's what matching does, so you call it '0' and get on with your life, at '0' or less if needed, stepped positions acting as advertised on the tin.

 

[emrr]

1x2 splitter with pad and atten

 

[abbey road d enfer]

1x2 splitter with pad and atten

And with duplicated male and female connectors, needed in times where many broadasters adhered to the german paradigm of female outputs and male inputs (same as mains cords).

 

[emrr]

I’ve seen so many similar DIY problem solvers in broadcast junk rooms. Usually “why did they need that?”

 

[emrr]

Why does this particular switch have 2 sets of in,c, and out terminals?afaict they are one in the same?

Always measure and deduce. Some are, some aren't.

 

[scott2000]

Always measure and deduce. Some are, some aren't

Thanks... getting .15r between each set of in, c, and out.... guessing they're the same?
Confusing stuff for me atm... I appreciate it....

 

[abbey road d enfer]

In the "off" position, is there supposed to be audio coming through?

According to this schemo, "OFF" must be taken as "no attenuation".

 

[Brian Roth]

<g>

Bri

 

[Walrus]

When the centre pole of the switch is in the OFF position isn't the destination effectively shorted out?

 

[abbey road d enfer]

Got it. So there is never really complete attenuation to silence?

Sorry, I misread the schemo. OFF is no signal.

 

[scott2000]

I've done something wrong then. Hopefully a mis wire. I get signal at first position. Thinking something to do with the cold leg going straight through... need to look...
Are my step positions labeled correctly?
tbh, seems that completely off would mean an abrupt jump in level when going to next position...Now "off" it's just lower than next position...