12-step balanced bridged-T attennuator 600 Ohm

I finally found some rotary switches I could afford for building some 12-step (2dB/step) attennuators. I did a circuit drawing and I think it is correct, but it would be really great if you guys could look over it, before I get to work with the soldering iron.

calculations: http://homepage.mac.com/marten.thielges/prodigy/12step balanced pad page2.pdf

circuit: http://homepage.mac.com/marten.thielges/prodigy/12step balanced pad.pdf

[edit: updated link and made correction in circuit diagram]

Will 1/4W resistors be ok for all values? I am not quite sure on how to do the math on that...

If anyone is interested, the switches are from monacor, I got them in Germany for 3,64? from http://www.schlotzhauer-versand.de/, look for RS-3312. One "problem" with those is, that they don't stop, meaning that you will be able to switch from -22dB to -0dB. If I had the cash, I would have gone for ELMA or Grayhill...

Any help really appreciated!

Hi Martthie,
you're links are down.
Sleeper

sleeper, sorry for that, I had deleted them by accident. They are back on the web, the links should work again! Thanks for pointing this out, I wouldn't have noticed and would still be wondering, why nobody answeres this thread

i dunno about it but i am also interested as I need to build something similar!.

> Will 1/4W resistors be ok for all values?

Is this a small mike-amp putting out +10dBm (0.010 Watts)? Or a POWER amp putting out +40dBM (10 Watts)?

A "0.010 Watt rated" output might make 0.1 watts in gross overload, but still the total attenuator dissipation is much less than 0.25 Watts so 1/4W resistors are clearly ample.

You might run a +40dBm 10Watt amp at less than 0.1 Watts peaks, much less average. Or you might run it at that level for years, until your idiot assistant plugs a tape output into a mike input and the system forces 20 Watts out of the 10W amp, and the BLAST makes him too stupid to pull the plug.

Exact power computations take more work than the resistors are worth. I think worst-case is with maximum attenuation. At that point the two left 300 ohm resistors (plus a small resistor) are in series across the source. If they are 1/4W each, it can eat 1/2W total. They match a nominal 600 ohm source. So you can be feeding 0.5 Watts or 500 milliWatts. A modern amp rated +24dBm at hard-clipping will output +27dBm or 0.5 watts of gross square-wave distortion (rememeber your idiot assistant).

I think to harden this against modern-studio abuse, you might want to make the four 300Ω resistors 1/2 Watt. Or since 150 is a standard value, make each one out of two 150Ω 1/4W in series to make 300Ω 1/2W.

If you are choking the output of one of the twin-6V6 broadcast limiters/ phone-line drivers, you should probably use 1/2W for all the odd values, and 1W or even 2W for the 300Ω parts.

Thanks for your words, PRR, I will give it a go next week. The pads are on the output side of TAB V76 and V72 preamps, btw.

I will be back here once I get it done up and running.