+1 dB/ 0 dB/ -1 dB Trimmer for the Sweet Whispers Stepped Attenuator

Someone asked how to get a fine volume adjust in the Foreplay's stepped attenuator. I suggest adding a SPDT center off switch for each channel to add an independent +/- 1 dB channel adjustment. The center switch position is the "off" or no change position. One possible switch is the Digikey CKN1026-ND (C&K 7103syzqe) for $4.26 each.

In the model below, the 90.9K is the existing resistor between the stepped attenuator and the selector switch. The 10K is the value of the stepped attenuator (5K + 5K = 10 K).

If you want to try this just for kicks but don't own a Sweet Whispers Attenuator yet, put a 90.9K resistor between the existing 100K pot and the selector switch. Then install a 11.0K resistor across the two end lugs on the 100K pot. Now the volume pot looks like a 10K resistor to the 90.9K dropping resistor. You don't get the benefit of driving the tube with a lower impedance, but you can have a 1 dB trim switch to see if you want to make it permanent. Both the 90.9K and 11.0K should be good quality parts. Don't use a pair of $5 part here if you are keeping the original 100K volume pot. Before you do that, do the smart thing and buy the Sweet Whispers kit first.

I have no idea where to mount the "SPDT center off" switch. The only place that looks feasible at the moment is centered between the tube socket and the original volume knob. By adding this switch you have the same resolution as a 34 step 1 dB per step attenuator. Also, I am not in a rush to install this in my Foreplay at this time. However, it was an interesting exercise.

Several useful combinations of resistors are listed below. I calculated the exact value needed and then used the closest value Michael Percy carried for the calculations. The attenuation is given at full volume. The nominal (center) attenuation needs to be at least -2 dB or the input resistance of the volume control will vary too much for comfort.

Top Resistance Bottom Resistance Top Trim Resistor Bottom Trim Resistor Up min Attenuation Center min Attenuation Down min Attenuation Input R, Up Input R, Center Input R, Down

12100 100000 0 88700 0.00 -0.99 -1.99 100000 112100 59106

26100 100000 23200 169000 -1.01 -2.01 -3.02 112282 126100 88925

41200 100000 69800 237000 -2.00 -3.00 -4.01 125908 141200 111526

59000 100000 143000 301000 -3.03 -4.03 -5.04 141767 159000 134062

100000 100000 357000 412000 -5.01 -6.02 -7.02 178118 200000 180469

22100 10000 118000 56200 - 9.13 -10.13 -11.13 28614 32100 30589

90900 10000 665000 73200 -19.08 -20.08 -21.09 89969 100900 99698

169000 10000 1370000 77800 -24.11 -25.06 -26.05 160442 179000 177861

309000 10000 2490000 80600 -29.09 -30.08 -31.06 284887 319000 317896

First version before 8-1-2000, Last change 9-15-2000

Top Resistance	Bottom Resistance	Top Trim Resistor	Bottom Trim Resistor	Up min Attenuation	Center min Attenuation	Down min Attenuation	Input R, Up	Input R, Center	Input R, Down
`12100`	`100000`	`0`	`88700`	`0.00`	`-0.99`	`-1.99`	`100000`	`112100`	`59106`
`26100`	`100000`	`23200`	`169000`	`-1.01`	`-2.01`	`-3.02`	`112282`	`126100`	`88925`
`41200`	`100000`	`69800`	`237000`	`-2.00`	`-3.00`	`-4.01`	`125908`	`141200`	`111526`
`59000`	`100000`	`143000`	`301000`	`-3.03`	`-4.03`	`-5.04`	`141767`	`159000`	`134062`
`100000`	`100000`	`357000`	`412000`	`-5.01`	`-6.02`	`-7.02`	`178118`	`200000`	`180469`

`22100`	`10000`	`118000`	`56200`	`- 9.13`	`-10.13`	`-11.13`	`28614`	`32100`	`30589`
`90900`	`10000`	`665000`	`73200`	`-19.08`	`-20.08`	`-21.09`	`89969`	`100900`	`99698`
`169000`	`10000`	`1370000`	`77800`	`-24.11`	`-25.06`	`-26.05`	`160442`	`179000`	`177861`
`309000`	`10000`	`2490000`	`80600`	`-29.09`	`-30.08`	`-31.06`	`284887`	`319000`	`317896`