Another Input/Output Pad thread (-6dB attenuation values?)

[Randyman...]

So, I made eight mic level pads I'm happy with. They are -14dB and use a 470 on each leg and a 220 across them. I ran across these values as posted from some Neumann developer, and they seem to work well when needed on the input side.

Now, is it possible to build a pad with a satisfactory in/out impedance for something like a -6dB output pad? Even -9dB would be OK, as I really want to get the preamps driving the output iron pretty good, and my converters cap out @ +19dBu. I've tried the -14dB pads on the outputs, and I have to sacrifice lots of level (preamps clip hard around -6dBfs or lower), but it does work.

I've tried to get some descent values on the H-Pad/T-Pad calculator, but the values get a bit crazy with smaller attenuations. Any suggestions for an appropriate 3-resistor line-output pad in the -6 to -9dB range that uses standard resistor values, or should I suck it up and get a pair of LittleLabs 8-channel variable-pad units (not cheap)?

I'm guessing all of the seventh circle modules (N72/A12/J99/C84) could handle 600 Ohm output impedance as long as I pull the internal 600 Ohm jumpers, so is that a recommended starting point? I'm driving a pair of RME ADI-8 converters which don't mention their preferred input impedance...

Thanks for any help

[Randyman...]

Shameless Bump! Tim - Any suggestions for the resistor values for a -6dB to -9dB output pad for your fine modules?

[Randyman...]

FWIW - I got some values from Tim via email, and from MC on the RME forum. Both of these guys are so helpful!!!

Tim recommended a 150-300-150 resistor pad, and MC recommended a 470-1K-470 pad (basically a modification to my current 470-220-470 -14dB mic pads). The 150-300-150 will apparently "load" the transfomer more, and the 470-1K-470 will not damp the transformer as much.

MC indicated the 470-1K-470 will provide a 5.7dB pad, and I'd assume the 150-300-150 that Tim recommended would be in the same ballpark.

I'm going to build about 5-6 of each and see which ones I like better. I will, of course, be pulling the modules' 600 Ohm jumpers to avoid double-terminating the transformers with 2 loads in parrallel.

This might help someone in the future as I could not find ANY simple info on what resistor values would work best for a smaller attenuation line-output pad involving transformers...


Enjoy

[tpryan]

Assuming a load resistance much larger than the bridging resistor (R2), the attenuation in dB is 20*log(R2/(R1+R2+R3)), so 150-300-150 gives -6.02dB.

Again assuming a large load resistance (it's probably 10K), total load on the output is R1+R2+R3, so 150-300-150 gives 600 ohms, same as the loading resistor on the board.

[Randyman...]

Hey Tim,

All the store had was 150's and 330's - I'd assume this will net a hair less attenuation overall, and raise the overall load to 630 Ohms, correct? I built 4 of those 150-330-150's, and 4 of the 470-1K-470's. I'll try them out over the weekend...

PS - I received my 2 new A12's and got them assembled and tested yesterday - so now I have an entire chassis of 8x A12's!!! 2 more N72's will be in my future to fill up the other chassis

[tpryan]

All the store had was 150's and 330's - I'd assume this will net a hair less attenuation overall, and raise the overall load to 630 Ohms, correct?

Exactly.

[niccolo]

Hey Tim,
while we all should do our math I guess that a sticky in the general forum with all the infos (values and placement) on how to build line output pads of the most common values would be very welcome!
values could be -6, -12 and -24 for example.

As Randyman pointed out, the net is quite scarce on infos about resistor values for attenuators and, since we'd be using the pads with your preamps, who knows what values to use better than you?

sincerely

niccolo