Stepped gain stage for stereo XLR in/out

[davidjnichols]

Hey guys.  I"m interested in building a stepped gain stage for mastering.  It would be extremely simple.  Two inputs (left and right XLR) each with its own switch for controlling amount of gain to two outputs.  I would like to use something like this for the switch: http://www.grayhill.com/assets/1/7/Rotary_53,57,59.pdf  #53, the 24 option rotary switch.  I'd like to have each turn of the knob be +.5 dB.  But the thing is, even though I know it's simple, I have no idea how to do it!  I'm good at building off schematics for guitar pedals, but I still don't understand exactly how everything works.  Could any of you point me in the right direction for getting more info on building something like this?  Any help is appreciated. Thanks!

[PRR]

Do you know how a regular pot works?

Do you know how much 0.5dB is in voltage-ratio?

[davidjnichols]

Do you know how a regular pot works?

Do you know how much 0.5dB is in voltage-ratio?

I do know how a pot works, but no, I don't know how much 0.5dB is in voltage-ratio.

[Bill Mountain]

This should help.

http://www.muzique.com/schem/gain.htm

[PRR]

> This should help.

No insight.

Here's where I would go.

One dB steps are good enough.

-1dB is very-near 90% or 0.9.

If we had a 1 ohm pot, and turned it to -1dB (and pot loading may be neglected), we'd find the wiper at 0.9 ohms.

1 ohm is an awkward load for practical reasons. Scale-up to 10K ohms. Now -1dB on a 10K pot is 9K up from the bottom. And above the -1dB tap is 10K-9K or 1K.

Already we have two steps, 0dB and -1dB.

The -2dB point is 0.9 times 0.9 or about 0.8. Our third tap is 0.8 up, or 8K. So we have 1K 1K 8K for 0dB -1dB -2dB.

Cheat: having figured the 1dB steps, you can simply split the differences and call them 0.5dB. It's wrong, but the error is far beyond hearing.

Of course 0.9*0.9 is not quite 0.8. Anyway -1dB is not quite 90%. More like 89%.



You can figure a 0dB...-3dB switch-pad on a matchbook; but much more and you want a full pad of paper and a sharp pencil. Calculator is handy because there is a lot of repeated arithmetic where each step depends on all the figuring before it, and rounding errors accumulate.

> stepped gain stage for mastering

OTOH, for _this_ purpose you do NOT need exact 1.000dB (or 0.5000dB) steppings. If you figure -1dB=0.900, half-dBs are half-ways, and take that out to "-12dB", you get to -11dB. Re-calibrated, +/-0.5dB total error, and insignificant differential errors. If you know your times-nine tables, you can figure it on a large matchbook.

[ashcat_lt]

Are you going to unbalance the input first, or get out the old meter and spreadsheet to find the matching pairs of resistors?