Bias Adjustment on the White Oak Audio Design PL14_20 Board

[laatsch55]

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

Not at that resistance. If the board hesitates for long it'll be just smoke left anyway.

 

[mlucitt]

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

Rod used discreet devices rather than the DC sensing IC. He said his tests showed a max delay on relay opening of 3.5 msec. Pretty fast.

 

[laatsch55]

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

Fast enough to save speakers I would guess.

 

[Gepetto]

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

Too fast. I don't think that is the real trip point. Half period of a 20 Hz wave is 25msec so it has to be slower than that.

A woofer can handle hundreds of milliseconds before damage sets in. The woofer is the only thing you are protecting since the mid and tweeter are protected by the capacitor in the crossover network.

 

[Skratch]

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

Physics are not debatable. I triamp and have a frequency cap in line with all my components

 

[ksrigg]

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

Are we saying it is a good idea to change out the 2N1304/05 with a newer transistor during a rebuild? Or is there a viable alternative to this pair?
How fast is a fuse in line to protect speakers?

Fast enough to prevent damage? Or is the cap a better idea? How do you calculate the value of the cap?

 

[laatsch55]

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

The 1304 and 5 styay Sutton. He was talking about the DC protect he built from a Rod Elliot board. A fusse is way to slow to protect speakers unless it's so far below the max you're not closed anyway.

 

[mlucitt]

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

Sorry for the confusion. I used a quote out of context. The 3.5 msec was the delay from momentarily connecting DC from the rail to the connector at the speaker output of Rod's DC protect board. He was basically timing the electronics, which is almost instantaneous, and the relay coil collapse due to a transistor turning off. His first test showed the relay contact opening in 1.2 msec with no back emf protection and the collapse of the relay coil generated a 500V spike. Adding a diode increased the contact opening time to 7.2 msec. Rod finally settled on a diode - resistor combination that opened the contacts in 3.5 msec with a maximum back emf of -30 V.

He also timed the "Loss of AC" function of his board. When the AC power is removed, the relay drive circuit is de-energized within about 50 msec.

 

[Gepetto]

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

Hi guys, some dialog on Q6.

The arrow on the schematic indicates the CCW rotation direction. When the pot is fully CCW it is at the 5K end of the pot so it puts 6K ohms across the Q6 base - emitter junction and this corresponds to the MINIMUM bias setting achievable.

Conversely when the pot is all the way CW, it is at the zero ohm end putting 1K across the Q6 base emitter junction.

Failure to achieve a low enough bias is generally associated with 2N3403 that is too low in beta and will not head toward saturation with low base current. I swap these tired 2N3403s out with a new 2N5088 which I know has good beta characteristics. If you do not want to swap out the 2N3403 device, then you can experiment with decreasing the value of R62 from 390 ohms to a smaller value (I would not recommend going below 150 ohms). This will provide Q6 with more base drive (through D6-D8 and then through R62).

I find that there are usually issues at the low end of the bias rather than the high end of the bias setting.

Hope this information is helpful to you.

 

[nakdoc]

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

On older transistors, close inspection of the 3 leads will reveal a slight corrosion. This forms a leakage path. Clean the corrosion by 'cutting" between the leads at the transistor body.

 

[Gepetto]

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

This is good advice, especially on devices that have silver plated leads. Silver tends to grow conductive dendrites which are reasonably low ohmic (generally 10s of ohms) shorts. Can drive you crazy trying to find the cause. Humidity and ionic contamination on PC boards and components tends to aggravate this growth.

 

[mlucitt]

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

How about spilled beer? Does the low alcohol content cut the dendrites? Maybe the resulting dried goo is a sort of protectant? I could dry large quntities of beer and then sell it as a corrosion preventative compound, come to think of it - maybe that's what it is!

We used it in the Navy, especially on aircraft exposed to salt spray.

 

[laatsch55]

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

It's used in the oilfield as a general anesthetic.

 

[Gepetto]

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

One of those nights No serious talk here, that is good.

 

[laatsch55]

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

Must be, been sick with the flu for 4 days now. Only spent a few hours in the field yesterday and today. May not spend too many out there tomorrow.

 

[Gepetto]

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

Take it easy Lee and get back running on all eight cylinders again. Sounds like you were pushing it too hard in the last couple of weeks. That stuff catches up with you eventually.

 

[laatsch55]

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

Yes Joe it does. Sooner than in the past. Damnit!!

 

[jbeckva]

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

Must be, been sick with the flu for 4 days now. Only spent a few hours in the field yesterday and today. May not spend too many out there tomorrow.

You too? Being sick sucks ass. Been that way since Monday.

 

[laatsch55]

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

Ah, misery loves company. That was a song, Michael Stanley Band , off the album---"Cabin Fever"

 

[Skratch]

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

You are not supposed to put beer in electronics, makes them sizzle. TRUST ME Happened to me one night when I was DJ at a wedding. You are supposed to drink it