PL pot wiring

[laatsch55]

When Jer did his 700 LED lights he noticed the the leads coming off the wiper went to the RCA jacks(input). From the schematic it shows the wiper lead going to the driver board. I did not check that before I sent it to Jerry. I will from now on. Just got done checking the 700 of Suttons, it's backwards too.

Joe----- what happens when its backwards, noisier??

 

[Gepetto]

Perhaps Lee it would be noisier. At reasonable listening levels it may increase the input impedance to the first differential stage and could cause it to be noisier than when the wiper is wired to it. At minimum volume setting, this wiring config would effectively short the preamp signal output to ground. At maximum volume, you would probably not notice any difference at all as that would again be straight through.

 

[laatsch55]

At full volume no contribution to impedance then? If you were doing away with the pot the pre-amp would see the normal 50 K input impedfance??

 

[Gepetto]

Hi Lee
If you do away with the pot, you will effectively see the 10K in series with 39K for a total of 49K (reduced only slightly by the near negligible contribution of the long tailed transistor pair input impedance).

This is the normal input impedance of the PL400 amp with the PL14 original board installed.

The White Oak board also uses this same input resistor arrangement.
Joe

 

[jbeckva]

Heheh.. Lee and I are having a discussion but I'll put 'er out here to ya Joe. What I'm thinking is bypassing the pot altogether. But if I do that, then I "think" I need to wire a 100K fixed resistor in it's place across the input.

The reason is because if the pot is taken out, it also eliminates the 100K the pot itself would present to the input in parallel.

I'm thinking that this here would be the equivalent circuit of both i.e. the pot as it is but turned all the way up.. OR... the pot taken out and R53 now becomes a "fixed" resistance, with the same resistance, so as to not change the input charateristics or overall impedance..

[attachment=0:ixkty1d6]Input stage.jpg[/attachment:ixkty1d6]

BUT.. and here's my question to ya, Joe... Do I really need the fixed 100K to keep the same characteristics of the input, or do you already have a load resistor on the WO driver board where I don't need it?

 

[Gepetto]

Hi Jerry
Short answer, you do not need it. Using your schematic snippet above as reference, R1 on the WO board is 10K, R3 on the WO board is 39K, C1 is 120pF. Thus it will present a DC impedance to your preamp of 49K ohms. If you bypass the volume controls, it will be the same input configuration as the PL400 amp, exactly.

Hope this helps
Joe