Hint: note the ratio between the parallel and series resistors. It's 3:1 for -10dB and 10:1 for -20dB. I'd be shooting for something like 30:1. Also notice how the parallel value doubles going from the -10 to the -20. Just off the top of my head, assuming the values you give are correct, I would try a 27M on the series and an 870ohm on the parallel for -30dB.

What a mess this lot is. None of the figures given actually take into account what the load impedance on the driver should be. The tech's figures for a 10dB pad would give ~12dB attenuation with an input impedance of just over 1kOhm while the figures for the 20dB pad would give ~21dB with an input impedance of just over 6kOhm. Both these calculations are unloaded figures.

Bouldersounds figures would actually give you ~90dB with an input impedance of 27MOhm. I think he meant 27kOhm which would be much much closer! :

Let's use maths instead of intuition for a more reliable result...

Lets target an input impedance of around 10k so make Rseries = 10k and Rshunt = 330Ohm. With a reasonably high input impedance on the output (say 10k or more), then attenuation is 29.9dB and input impedance is 10.33 kOhm with no load.

For 25dB, same 10k and 560Ohm will give 25.5dB at 10.56kOhm unloaded. Maybe change the 560 to 620 for lower impedance loads.

For completeness for 20dB I would use 9.1kOhm and 1kOhm for a 10.1kOhm input impedance and consider changing 1kOhm to 820Ohm.

For 10dB, use 6k2 and 3k6 to get 8.7dB at 9.8kOhm input but with a 10kOhm load, this would become 10.5dB with an input impedance of 8.8kOhm.

Note that with higher shunt values, load impedances are more significant hence actual attenuation and input impedance values will vary more. Larger attenuations are more immune to this effect due to the lower shunt values. This may account for the lower values provided by the tech. but this also results in a very low input impedance - in this case possibly too low for some outputs. For instance it would NEVER work on a passive guitar output. Then again neither would my values as the input impedance is still far too low!

Aha, I didn't answer the original question!

For a quick unloaded calculation, take the shunt resistor value, divide it by the sum of both resistors, take the logarithm and then multiply by 20.

I.e. for 30dB, using my values of 10k and 330Ohm, you get 330/10330 = 0.03194.... Log of this is -1.4955... Times 20 = -29.9117... which is approximately -30dB.

You can of course reverse these calculations to work out values needed for any particular attenuation.

Thanks MrEase!!

I know nothing of the mathematics of electronics so that's why I ask here. I only know how to use a soldering iron and build a sound studio :-)

Thanks for your very precise answer. I'm though a bit confused as how to use it in my case. What I have is a condenser mic going into a field mixer that I connect with a video camera with a stereo-minijack input. The dB cut box is supposed to take the signal from the field mixer output, lower the level, and send it further to the camera. This is because the mixer has too high an output to be able to calibrate the input in the camera gain.

On the specifications of the mixer it says so:

Azden FMX-20 Field Mixer
Main Output
0.5dBu with 600 ohm load 0dBu ref @ 0.775Vrms
0.5dBu with 1000 ohm load 0dBu ref @ 0.775Vrms

More info here

I don't understand why they specify two ohm loads and how should I use this to calculate my dB cut box?

Thanks for your help and time! :-)

That series of Azden field mixers has a -50dBu stereo output on a 3.5mm jack designed for feeding the stereo mic input of a video camera. You don't need attenuators with that - just use a minijack stereo cable.

From what passes for a user manual for the FMX-20:

This output is stereo (dual-channel). It is recommended to use a stereo-to-stereo minicable (not supplied) from the UNBALANCED OUTPUT (16) to the mic or line input of your video camera or audio recorder. The UNBALANCE OUTPUT (16) is designed for video cameras or audio recorders, which have mini-jack microphone or line inputs. Because the FMX-20 has low-impedance XLR inputs and a mini-plug output, users of mini DV cameras that have mini-plug microphone inputs can now use high-quality microphones with XLR outputs.

MrEase, post: 375285 wrote: Bouldersounds figures would actually give you ~90dB with an input impedance of 27MOhm. I think he meant 27kOhm which would be much much closer! :

Right k, not M.

bouldersound, post: 375299 wrote: Right k, not M.

Hey, no problem from me but others may not know the difference so I guessed it should be pointed out! This type of slip is so easily done - I did it myself in the clock jitter thread and Boswell kindly pointed out that error.