Bias Adjustment on the White Oak Audio Design PL14_20 Board

Gepetto

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#1
Here are some pictures of the left and right channel DMM connection points for the White Oak Audio Design PL14_20 RevC Control board. Be careful not to short out meter probes when taking these readings, turn off power when attaching the meter clips then apply power to avoid damaging the amp.
 

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#2
Re: Bias Adjustment on the White Oak Board

Here are some pictures of the left and right channel DMM connection points for the White Oak Audio Design PL14_20 RevA Control board. Be careful not to short out meter probes when taking these readings, turn off power when attaching the meter clips then apply power to avoid damaging the amp.
 

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laatsch55

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

Ah .... so that's what TP1 is!!!!! Joe, you must hace a sick mind cause your bench is wayyyyyyyy to neat!!
 

mlucitt

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#4
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Joe,
The Rev C board looks good. When are you ready to ship those?

Mark
 

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#5
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Sorry Lee. Funny my wife does not share your opinion on my neatness, or lack thereof :)

Mark, I am ready to ship now. I have some web page work to do to update my photos. Way behind...
 

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#6
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

If the price is the same, I will order five from the website. If the price is different, let me know.
 

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#7
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Hi Mark
The website pricing is for the RevC. I no longer have RevA, out of stock and replaced by RevC.

RevA has now been relegated to 'collectors item' status. But I still have S/N 1 :)

RevC has all the things in it that you guys have come to love and then some.

I will soon post the release notes for RevC
 

mlucitt

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

I powered up my PL 700B this evening to do some adjustments to the DC protect board that I obtained from Rod Elliott. As a precaution, I was using the DBT and the VARIAC. As I adjusted the DC bias, I noticed the DBT would get brighter if I increased the bias voltage and it would get dimmer when I decreased the bias adjustment. This may be completely normal, but I was still entertained by it.

As a side note, the right bias is 280 mV as I type this and the left bias is 532 mV with the adjustment pot all the way down. does this mean the 2N3403 bias transistor is getting tired? Should I go for the 2N5088 instead? If I do that I will probably replace both right and left, but I don't think it is necessary to do both if the other channel is OK.

Thoughts?

Mark
 

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

Symmetry Mark, Symmetry. The bias is a current adjustment, hell yeah it's gonna drive the bulb crazy. I have found myself wondering how much bias it would take to blow the bulb!!!!Have you measured accross the BE/BC junctions so we might get some baseline on what differences make the too much or not enough?? I keep forgetting to do it myself. Something inside the little buggers aint right, wonder if its measurable??
 

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

How was the DC offset? Any other issues that might cause one side to run warmer than the other? On the one that won't go below 532, have you done a drive by with the heatshrink gun?
 

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#11
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Mark, When I do a board upgrade, I always put in new bias trannies. I put in 5088's with #24 wire. Joes new board has the Phoenix connectors for the bias also. All the boards that I have done the bias pots are very close to the middle setting for 380mv. Its like PM on the amp, you changed all other small transistors
 

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#12
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Also, how about a review on the Rod board??
 

mlucitt

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#13
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Lee, I knew you were gonna ask me that. I hand matched the TIS97 input pair. The DC offset on the left channel is 6 mV and on the right channel the DC offset is 9 mV.

The amp has been on with a .7 V 2kHz input for 24 hrs and the bias is still high at 518 mV (left) while the right is 285 mV, pretty steady.

The Rod board is good but the turn-on delay is very fast, probably half a second. He lists some RC combinations to adjust the delay and I am going to play with those. The only drawback I see is that his board cannot handle a supply voltage above 60 VDC. He designed it for normal amps, not our power-hungry PL 400 and 700s. So I had to use a small PS to give the board and relay 40 VDC. It works well, he said to test it use a small resistor on the rail and touch it to the speaker outputs, the relay drops the output jacks to ground immediately.
 

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

That's a damn good DC offset. You know I bought 5000 97's don't you??
 

mlucitt

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#15
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Lee, I may take some of those 97s off your hands if it will help with the finances. BTW, you taught me how to grade and match them, that process works well.

Skratch, this PL 700B was a bit rough when I got it, it still smells a bit like beer (not a bad thing). Still has the original XPL909s and I wanted to see if I could bring it back to life because it had several problems. This was the amp that we tried the Gepetto Mod on and it worked! I just received 36 MJ21106G and 12 MJ15024 transistors from Mouser and now may be the time to switch them out and try some new bias trannies.

I still would like to learn why one side will not go lower than 518 mV or is the bias tranny the only determining factor?
 

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#16
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

That and R20 and R19. the gain of Q6 is more of a factor than anything.
You could measure accross the jct's of all biased devices and see if the total resistance is more than the other side??Na , too many variables there too. But bwhen ya think about it, as a device grows older it may get harder to turn on, kinda like me.
 

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#17
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Mark, i got em for 7 cents each, bouight 5000 TIS-93's too. All original Fairchild. Quite a stash. That's where I got the break, the 10,000 mark.
 

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#18
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Small resistor?? As in 1/8 watt to avoid disaster??
 

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#19
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

It is usually that Q6 gain is too low that causes the bias adjustment to not to be achievable within the normal setting range.

I think environment takes its toll on these devices over the years, since the packaging is not hermetically sealed in a TO-92 package. No proof of that, just no other real causal since the device is conservatively rated within this design.
 

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#20
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo

Rod said to use a 100K 1/4 watt resistor to connect the + rail to the speaker + output to check the DC protect circuit. Does that seem dangerous?
 
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