Bridged-T.. Not sure I completely understand it!

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

NewYorkDave

Well-known member
Joined
Jun 4, 2004
Messages
4,378
Location
New York (Hudson Valley)
R3 and R4 in FM System's diagram are equal to the characteristic impedance. For example, these would be 600-ohm resistors in a 600-ohm attenuator.

The bridged-T is designed for equal input and output impedances. This is not the attenuator to use if you want unequal impedances.
 
Hi Jaakko!

You will need to set the input and output resistors to the same value in the bridged-t....unbalanced they would both be 5k and balanced would be 2k5 each for +-. The source and destination will see this impedance only I think.

If you want an attenuator with an unequal impedance I think you will always have some loss...... IIRC normal t-attenuators can be designed for unequal impedances but I believe the noise level of the atty varies with level.

Has anyone ever heard or pi or delta attenuators, these would work too? Also U-attenuators are primarily designed to be useful for matching high to low impedances...but the loss is secondary to the matching. I think these may not be useful for variable attenuation because of changing impedances but I'm unsure about that. I think the bridged-t is best because it reflects a constant impedance in both directions at all times.

Do you want a high-ish input impedance for the DA to drive and a low ouput for your monitors???

Any experts care to comment of the issues surrounding feeding the monitor amp with a 5k impedance as opposed to something like 100ohms directly from the DA?

Also with the bridged-t and a fourpole switch, even when the shunt and bridge resistors are switched out (zero attenuation position), the source will see the series resistance......surely there must be a voltage drop here?

I hope some of that could help
Tom
 
i started to get it all with the help of The Lab forum and

http://www.microwaves101.com/encyclopedia/attenuators.cfm

this one has a nice calculator,
http://beradio.com/notebook/passive_attenuators/
 
Hi Jaakko!

It is my understanding (after discussion over @ Brads forum with DC) that the 5k (or 2k5) bridged-t will be the most suitable for what you need. It always reflects a constant Z to both source and destination and should easily be driven.

I have been wondering if it would make any noticeable difference in performance to get the output Z as low as possible. For example, our Lynx2 DA is 100ohms.

Maybe a normal T (non bridged) will help you out if you really want to match the input impedance to your DAW and output to your monitors. From reading on the Shallco site, they state the normal T attys are slightly better than the bridged version but more expensive.....this I don't understand? Maybe it requires more decks as all resistors must switch?

Anyone else?

Oh I just played with the calculator, a normal T atty with 5k input and 100ohm output Z, 2dB attenuation, requires negative resistance so that ain't gonna happen! The minimum loss will have to be around 23dB before 'real' resistances can be used with these values...so I don't know.

Cheers Tom
 
> bridged-T attenuator... goal would be 5k input impedance and output as low as possible.

Then ignore all the classic constant impedance pads.

You want a U pad. That gives highest ratio of input to output impedance possible. It also needs the least number of wipers/poles.

U-pad.gif


The 5K variable resistor could be a pot or a buncha resistors on a switch. Audio-taper pot is suitable.

With no output load, the input impedance varies from 5K at "silent" to 10K at "full-up". Minimum loss is 6dB voltage.

With 10K output load, the input impedance varies from 5K at "silent" to 8.33K at "full-up". Minimum loss is 8dB voltage.

With source impedance assumed to be much-less-than 5K, the output impedance varies from "zero" at "silent" to 2.5K at "full-up".

This pad gives "silence" by trying to short-out the two 2.5K resistors with the pot/switched-resistor. It is not really possible to reduce level to zero because there is always some stray resistance when you turn the pot or switch to zero resistance. Dirty wipers/contacts will give deafening blasts. When switching, make-before-break is essential in this rig.

A standard potentiometer, or equivalent switch, is probably a better plan.

If you actually must present over 5K to the source and 100 ohms to the load, without a transformer, the minimum loss must be at least 40dB voltage. You can not maintain voltage over such a radical change of impedance without an amplifier.

Why do you think you need a 100 ohm output impedance? No standard audio power amplifier needs anywhere near that low. For short cable runs, 5K impedance seen by the power amp input is usually fine.
 
Back
Top