Setting the BIAS on the White Oak PL14_20 PCBA

[Gepetto]

Setting the Bias on the PL14_20 Board
Initial setting of the amplifier bias is a very simple procedure with the White Oak Audio PL14_20 board assembly. The bias is not all that critical and will drift around somewhat as the amp warms up. Do not be concerned that the setting is rock solid. It is a feedback control system using Q6 as a thermal sensor in the board circuit. As the amp warms up it is natural to have some movement in the bias voltage setting by design. The final bias setting you are working to achieve is between 0.300V and 0.400V. Most Phase Linear users find a setting near 0.350 – 0.380V close to ideal.

1. Ensure the amp is powered off.
2. All measurements will be taken with NO inputs connected to any source and NO outputs connected to any speakers.
3. Open the top cover to expose the top edge of the PL14_20 PCB. Be careful not to drop any hardware down into the amplifier enclosure.
4. Turn both bias pots located on the top edge of the board assembly fully counter-clockwise (minimum bias setting). These pots are labeled Left Bias Adjust and Right Bias Adjust.
5. Attach your DVM positive test clip to the left channel bias setting test point. It is located in the upper left corner of the PL14_20 PCB and is marked Bias - L.
6. Attach your DVM negative test clip to the LEFT channel WHITE speaker output binding post.

1. Power up the amp and allow it to warm up and stabilize for approximately 5 minutes
2. Adjust the Left Bias Adjust pot to achieve a bias setting between 0.300 and 0.400V. Most Phase Linear users find a setting near 0.350 – 0.380V close to ideal.
3. Carefully move and attach your DVM positive test clip to the right channel bias setting test point. It is located in the upper center of the PL14_20 PCB and is marked Bias - R. This test point is located just to the right of the upper mounting nut holding the board into the chassis.
4. Attach your DVM negative test clip to the RIGHT channel RED speaker output binding post.

1. Adjust the Right Bias Adjust pot to achieve a bias setting between 0.300 and 0.400V. Most Phase Linear users find a setting near 0.350 – 0.380V close to ideal.
2. Power down the amp.
3. Remove test leads from the DVM.
4. Reinstall the amplifier top cover. Be careful not to drop any hardware down into the amplifier enclosure.
5. Bias setting complete.

 

[laatsch55]

Joe, can the bias circuit be latched out to be unresponsive?? Ok, some background, I have exerienced low bias of 50 to 78 mv after an event such as shorted input or driver. Is that the protect circuit shutting off bias voltage?/ Was driving me buggy, so the last time this happened, I unsoldered Q6 and soldered a bare 2N5088 directly to the board. The bias voltage then became active and adjustable. I desoldered the 2N5088 and resoldered the aforementioned q6 back in circuit. Same result, bias was again active and adjustable, with no changesd of anykind betyween those 2 events. The protect circuit should unlatch after powerdown correct??

 

[Gepetto]

Joe, can the bias circuit be latched out to be unresponsive?? Ok, some background, I have exerienced low bias of 50 to 78 mv after an event such as shorted input or driver. Is that the protect circuit shutting off bias voltage?/ Was driving me buggy, so the last time this happened, I unsoldered Q6 and soldered a bare 2N5088 directly to the board. The bias voltage then became active and adjustable. I desoldered the 2N5088 and resoldered the aforementioned q6 back in circuit. Same result, bias was again active and adjustable, with no changesd of anykind betyween those 2 events. The protect circuit should unlatch after powerdown correct??

Not saying no Lee but need to know more. By protect circuit, do you mean Don's board or the onboard 2N1304/1305 protection?

 

[laatsch55]

The 1304/05 set.

 

[Gepetto]

Hi Lee
The 1304/5 set is non-latching, that is it protects temporarily only when needed. Once the overload clears, it releases its clamping action on the output drive. This pair of transistors has little to do with the bias so what you observed is strange. The action of the 1304/5 set is to short out the base-emitter junction of Q7 and Q10 starving the drive to the output predriver and main drivers.

 

[laatsch55]

Then WTF?? It wasn't the first time it's happened. But it took desoldering and soldering a bare 2N54088 in to do it. What I don't know is if it would have done the same thing just by resoldering in the bias tranny that was already there,.. And the solder joints were good as the bias was behaving before the shorting events...

 

[laatsch55]

Can you spice a shorted output??

 

[Gepetto]

Lee
What do you mean by a shorted input?

"I have exerienced low bias of 50 to 78 mv after an event such as shorted input or driver."

 

[Gepetto]

Lee
I assume you first fixed the shorted driver and the bias was still reading low. Is this a good assumption or was the shorted driver still under debug and not located yet.

If you have a shorted driver, then yes I agree that you will have low bias because the 2N1304 and 5 will be active and starving the predriver.

 

[laatsch55]

Lee
What do you mean by a shorted input?

"I have exerienced low bias of 50 to 78 mv after an event such as shorted input or drive

I meant shorted output Joe.

 

[laatsch55]

Lee
I assume you first fixed the shorted driver and the bias was still reading low. Is this a good assumption or was the shorted driver still under debug and not located yet.

If you have a shorted driver, then yes I agree that you will have low bias because the 2N1304 and 5 will be active and starving the predriver.

I had located the shorted driver and replaced it, and still had a low bias...

 

[Gepetto]

Lee
In the cases you cite, can you recall whether the output went to the positive or negative rail? I do not think it matters but will allow me to think of possibilities of what you might have been seeing.

 

[laatsch55]

In one case positive the other negative, should narrow that down eh??

 

[Gepetto]

Well, alrighty then

 

[laatsch55]

Downright neighborly wasn't it??