Can someone tell me what this adjustable pot is for ?

[WBFAir]

I was having a problem with a brand new sub making kind of a buzzing sound from the speaker during normal operation from day one.

I have a identical sub and swapped all the internal parts and narrowed it down to what I believe is the main amp.

After finding that out I noticed a white adjustment pot on the amp and saw that on the good sub it was in a different position.

I adjusted the pot on the bad amp to same position as the good one an wallah...it stopped the buzzing.

I've set it now as best as possible to the point where the good sub was but now of course I just have to know what that pot is for.

I've attached a pic of the schematic for the control board for help and I know its a bit fuzzy but it the best I can do

As best as I can tell it is the part labeled "P3" with a "2K" below it and location on the pic in on the very bottom, just a bit to the right of the middle. It's leads markings are labeled, 3 on the top left, 2 on the middle right and 1 on the lower left and it has an arrow point to the left from the middle lead marked 2.

As I said I adjusted it as best as possible but I'm assuming it must have some proper way this should done via some meter, tool or process so maybe if anyone has a clue as to the part or how to adjust it, I would greatly appreciate any passing of knowledge.

Thank you

 

[MJL21193]

I have the funniest feeling of deja vu....

 

[WBFAir]

Yeah MJL21193 its me again, still trying to exercise the ghost in the machine of my subwoofer life.

BTW I got the Seas speakers today and have just got one in so far and I don't like the sound at all.

I think I have gotten a good sense now of what the difference is between a subwoofer and a woofer.

The Seas just seem a lot muddier as I think with the higher Fq possibility there producing more sound which I guess is what there supposed to do. The old drivers seemed clearer with more pop.

I'm gonna try it for a while and see if I can use the crossover adjuster to tweak it but I have a feeling I'm gonna go back to original drivers.

Anyway any help on the my main post would be helpful.

Thanks

 

[DSP_Geek]

This is just a guess, but could it be a bias control? Check the potential difference between the test point (labeled TPsomething. Can't miss it just on the other side of the resistor between the two output devices) and the speaker output to see whether that changes when the control is moved. If so, set it to the same value as you'd measure in the good sub.


Cheers,
Francois.

 

[bjorno]

...but could it be a bias control?

I also think T5 is a constant current source that has a dual purpose for the Power-amp with a gain of 11 times.

First to define emitter current to the long tailed pair and partly defines mirrored collector current gain, secondly trims output DC offset at the same time by balancing the idle collector currents for T10and T8, thus if equal, balances TP1 to zero output offset voltage.

The voltage over the pot (max R) is constant = 0.6 V when amplifier is on and base bias exists on T5, but zero if no input signal is present at the input of the amplifier or the pot is set at min R.

Max Ie=Ic is 0.5 mA (Upot= 0(V)) when the pot is at min and about 0.3 mA when max).

b