Bias Adjustment on the White Oak Audio Design PL14_20 Board

[mlucitt]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

One pot is almost dead in the middle. The other is closer to the low end of the pot. but I have adjustment room and that is all I need. I could probably go down to a 320 Ohm resistor on R62L to put the pot in the middle but hey, that is a project for another day.

 

[laatsch55]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Yep, if it's still inrange it's not time to do it anyway.

 

[Gepetto]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

I think you want to go the other way Mark if you desire to center the pot more than it currently is. It is counterintuitive.

The CCW end of the pot is with it maxed out (shunts the smallest amount of base current away). The CW end of the pot is the zero ohm end.

 

[mlucitt]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Right, I forgot that. Maybe the R62L should be 470 Ohm? I'll try it and see. The bias resistor is new, the drivers and outputs are new, the board is new, I don't know what could have caused the pot to not be in the center unless the pot is bad. I'll test R20L resistance now that the amplifier power is off.

 

[Skratch]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Mark, The reason the pot is adjustable is to let you get your bias where you want it. In tube amps alot of them are fixed bias. It is a close enough scenario. Bias is not that critical in most situations because there are too many factors that come into play. If you ever had a Phase that the bias was even close to where it was supposed to be you were lucky. That is why Phase sets there at between 300 and 400 mv.

 

[mlucitt]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Thanks, Mark.
Is there such a thing as an ideal bias? The service manual cautions about raising the bias above 350 to reduce distortion. I guess that would have the driver transistors conducting all the time. That increases current draw and the amp runs warmer. I would think for professional sound reinforcement where there are plenty of watts available but reliability is a concern, the bias could be down around 250-300.

Thoughts.

 

[Gepetto]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Hi Mark
I found something interesting about this forum. If two people post (nearly) at the same time to the same thread, one of the 2 messages gets thrown away. You won in this case...

As I was saying, I apologize for an error in an earlier post. I did not pull the schematics when you talked about changing R62, I mistakenly thought you were referring to R19. Reducing the value of R62 OR increasing the value of R19 will help center the bias pot on the one that is CCW of center.

As for your question about the pot being defective. I doubt it very much. There are many contributors to the off center setting of the bias listed in this order.

The beta of Q6
The forward base-emitter junction voltage of Q11 and Q12
The forward base-emitter junction voltage of Q10 and Q7
The forward P/N junction voltage of D6-D8
The tolerance of R38
The tolerance of R62, R19 and R20

 

[laatsch55]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Mark, I have supplied a lot of oilfield shops in Gillette with Phase amps and other stuff so they had tunes while working. In that situation I set most of them up to run at 270-275, for the fact that most of the shops were pretty warm for electronics, and the other being they were usually npretty noisy so no reason to have the distortion at audiophile levels. Saved many an amp.

 

[mlucitt]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Joe,
Thanks, you had me going there for awhile. Lee and I were talking about changing the value of R19 and you convinced us R62 was a better choice to fool around with. Hey, it's what we do.

Lee,
Good to know and you confirmed what I thought.

Mark

 

[laatsch55]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

In a perfect world, no tolerance nor pots needed.

 

[Gepetto]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Sorry for the bum steer Mark. Next time I will pick up the schematic first. And yes, changing R62 is the better way to go than messing with R19.

 

[speakerman1]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Professional audio design is a studio solutions company that provides pro audio equipment and service assistance from design and construction.

LOL OK

Larry

 

[laatsch55]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Rather pretentious wasn't it??

 

[Gepetto]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Lee
I think DWS123 may have jut misposted his reply to your intro question on this thread rather than the one earlier today...

 

[laatsch55]

Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

There was one earlier today? This was posted late last night. i have not seen all new posts though.