+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.

The step attenuator in question has a 100K input. The volume control consists of a 10kohm stepped attenuator and 90.9K resistor feeding the 10K attenuator. 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 parts here if you are keeping the original 100K volume pot. Before you do that, do the smart thing and buy the Bottlehead 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.  I have more info in my attenuator page. Volt's 12 Position Shunt Regulator Notes

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.