Bridged-T attenuator

[NewYorkDave]

There seems to be a lot of call for a DIY switchable bridged-T around here (as input or output attenuators for a number of classic compressor circuits, for example), so I drew this up today. It gives 0-20dB in 2db steps, plus "off", and uses a common two-pole, twelve-position shorting rotary switch. (Mouser sells some cheap Alpha rotaries that work well).

Bridged-T attenuator (12kB) GIF

 

[NewYorkDave]

Put some Milena Velba in there, too, and I'll be a happy man! :green:

 

[peter purpose]

Dave... you're a diamond... :thumb:

 

[Rob Flinn]

Nice one Dave !

 

[Flatpicker]

:thumb:

 

[daArry]

More goodies...great stuff NYD! :thumb:

I think i want to implement these in my dual 1176 for the i/o levels for easier level matching in stereo mode, can i drop it in as a 1:1 replacement for the pots ye?

Cheers!

 

[gary o]

Cooool! thanx

Gary O.

 

[rlaury]

Is anybody making a bridiged T rotary like for the 1176?
Interstate wants $60.00 ea for a ModPot version they build.

 

[Scenaria]

nope

that price is about right..

$60

 

[MikoKensington]

Thanks again Dave!

 

[martthie_08]

Dave, thanks for posting this!

This circuit is for unbalanced use with the upper connection on the diagram being the signal lead - correct?

I was looking for something to interface with my v72's, but it would have to be balanced - any suggestions?

I had a quick look at http://www.uneeda-audio.com/pads/ and the way I see it, a balanced pad would need a 4-pole switch?

 

[PRR]

> a balanced pad would need a 4-pole switch?

3-pole.

However, there is no connection to ground. If you keep the wiring tight and inside a metal box, you CAN use an "unbalanced" pad in balanced 600Ω lines. We old-timers did it all the old-time. It is "floating", the only un-balance is stray capacitance, and with a tight layout it is not a problem in 600Ω lines.

 

[gang of elk]

FWIW, i've been pouring over many old tube varimu schematics and i see that all the ones i can find, ie: collins, gates, GE, federal, have balanced I/O connections but use bridged T's instead of H attenuators @ I/O.

i'm currently racking a collins 356E-1 and thought i'd need H attenuators for I/O until i saw what other similar units use. i'm going w a pr of T's a la dave. (thanks dave!)

 

[Guest]

Since we're on this topic I have a small question relating to the proper implementation of the various attenuator types. What I'm trying to do is build an attenuator to go in between the outputs of a pair of White 4400 eq's and the input of a Bryston 4B(4400 o/p impedence is 300, 4B input is 20k), the point being to provide an accurate and repeatable level control in the high end of our main monitors. That way, if a client comes in and feels the room is too bright the mains can be tweaked slightly to suit them. I was thinking the bridged T would be the right one for the job, but would any of you fine gentlemen care to point me in the proper direction if it isn't? I'm still a little behind on my understanding of a lot of basic theory(what can I say, I learned to run before I could walk), so thanks for the help.

Zach

 

[adrianh]

Does anyone have a URL for the Interstate Modpots?

 

[guavatone]

[quote author="PRR"]> a balanced pad would need a 4-pole switch?

3-pole.

However, there is no connection to ground. If you keep the wiring tight and inside a metal box, you CAN use an "unbalanced" pad in balanced 600? lines. We old-timers did it all the old-time. It is "floating", the only un-balance is stray capacitance, and with a tight layout it is not a problem in 600? lines.[/quote]


How would you connect this given NY Dave's drawing? Hot and cold on each side and sleeves connected to each other?

 

[Brian Roth]

[quote author="adrianh"]Does anyone have a URL for the Interstate Modpots?[/quote]

I'm guessing the correct name is STATE Electronics, and here's their potentiometer website:

http://www.potentiometers.com/

Easy to remember <g>.

Bri

 

[ap123]

Edit:

 

[glbaudio]

There seems to be a lot of call for a DIY switchable bridged-T around here (as input or output attenuators for a number of classic compressor circuits, for example), so I drew this up today. It gives 0-20dB in 2db steps, plus "off", and uses a common two-pole, twelve-position shorting rotary switch. (Mouser sells some cheap Alpha rotaries that work well).

Bridged-T attenuator (12kB) GIF

the value of 66,5 ohm is correct? or should be 6,65?
thanks in advance, anyway, this save my ass! ;-)

 

[Kingston]

the value of 66,5 ohm is correct? or should be 6,65?

66,5ohm is correct. It's because the scale jumps directly from -20db to off/inf. Otherwise it would be divided to lower ohm -30dB, -40dB etc. values.