Capacitance ?

I am currently using a vintage Adcom 555 pre-amp in a simple stereo set-up. On the rear of the pre there is a switch with three settings : Capacitance
55 pf
175 pf
235 pf
What are these settings for? Yesterday I set the switch to 235 pf from 55 pf where it had been set for almost a year unnoticed. And, it may be my it sometimes is with the 'audio' senses , but it really seemed as if the sound opened up a bit when I moved that switch to the 235 pf position. Somebody please help me understand.
That switch probably has to do with phono cartridge loading. As such, it should not affect the line section or any other input in the least. That is, if it is properly designed. Sean
The manual is available at, and Sean guessed right. If you heard a difference (while not using a turntable), it may have been due to something else you inadvertantly changed (unplugging and re-plugging in a cable, etc.). Try listening with the switch in different positions again.
Pug what source were you using? If it was the phono input then yes it could / probably would sound different. As Sean suggests, this is a phono stage loading option & would not affect any line level inputs. Look again to see if there is another switch for cartridge loading resistance. You might see values of 47k-Ohm (standard value) & then some much lower values - 100 Ohm, 50 Ohm etc. If you use a moving magnet cartridge then the likely recommended termination is 47k. For moving coils, try the lower values & determine which sound that you most prefer. Experiment with different combo's of capacitance & resistance too.
I agree with Sean. Never had an Adcom but would bet the switch is to match capacitance with your phono cartridge. If you like vinyl look in the cartridge manual for suggested setting. Will likely have to include tone arm wire and cable capacitance in the mix.

Do not change the switch when your pre and amp are on.

Have you checked the Adcom web site for info? That is the only place to go. Manufacturers often have all the manuals and stuff online if you have lost yours.

I remain,
I owned one and Clueless is correct right down the line.