Input and output pad for my MB-1

[drjet]

Hi there,

3 month ago I build my Pultec MB-1 clone, which I use and like a lot. 
Have a look right here:

http://groupdiy.com/index.php?topic=59296.0

But using the instrument input with my bassguitar gives me a very hot output even at lowest gain setting, which clips my audiointerface line input (Focusrite Sarlett 18i6). Because I really like this slightly saturated sound I decided to implement an output pad. I think three steps (0, -6, -12 dB) are enough, so I opted for a  3P4T rotary switch.  I calculated the U-Pads as described on this page:

http://www.uneeda-audio.com/pads/

This is the way I calculated for about 6 dB output pad:
I wanted to have a series resistor of about 10k,

K-factor( for-6dB)=10´`(6dB/20)=1,9953
series-resistor(R1):  10k2 (2x5k1)
shunt-resistor (R2)= R1/(K-1)=10200/(1,9953-1)= 10047Ohm; I choose R2=10k Resistor

K-factor=(R1/R2)+1  =>  K=(10200/10000)+1=2,02
pad= 20*log(K)  => pad=20*log(2,02)= 20* 0,3054=6,1 dB

for  aprox. 12 dB output pad:

K-factor( for-12dB)=10´`(12dB/20)=3,9811
series-resistor(R1):  10k2 (2x5k1)
shunt-resistor (R2)= R1/(K-1)=10200/(3,9811-1)= 3421Ohm; I choose R2=3k3 Resistor

K-factor=(R1/R2)+1  =>  K=(10200/3300)+1=4,0909
pad= 20*log(K)  => pad= 20* 0,6118=12,2 dB

Did I do it the right way?

I also calculated a three-step input-pad just the same way.
Please have a look at the attached schematic.

Best wishes from Germany
Jojo

 

[RuudNL]

Keep in mind that the load is in parallel with the 10K resistor in the output pad. (Position 2).
This might give more attenuation than expected.
Also the input pad varies the (microphone) input impedance a lot. This might change the sound of certain microphones.