F5 v3.0 PSU and Bias Help Needed - diyAudio
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Old 3rd May 2014, 12:07 AM   #1
feds27 is offline feds27  United States
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Default F5 v3.0 PSU and Bias Help Needed

Hi Folks,

I just finished building my F5 but have yet to get it working properly.

1) I'm getting +/-26V for V+ and V- off of the Universal PSU v3.0. I see that Papa has specified +/-23V. I'd like to get the supply as close to +/-23V as possible to match the First Watt specs. What should I check as the problem?

2) I also tried to adjust the bias on both channels but I'm not able to get past .57V on the right channel and .47V on the left channel. Offset was steady at 0V. I'm thinking I may need to check my MOSFET Vgs and maybe change R7/R8 (formerly R11/R12 on v2.0?) to higher values? I see some people have gone up to 3.3k to fix similar bias issues on the F5 v2.0 PCBs.

I'll have to check the voltage at R7/R8 (formerly R11/R12 on v2.0?) and see if I get .6V. I'll also have to check R5/R6 (formerly R3/R4 on v2.0?) and see if I get 42V.

I used Papa's F5 schematic on page 2 of his F5 Turbo article to build these F5 v3.0 boards. FYI, I set P3 to dead center by checking resistance before installing and I'll adjust it once I get everything else sorted.

Thanks for your help.
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Old 3rd May 2014, 07:20 AM   #2
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chance R5/R6 to 3.3K or 4.7K then you will reach the bias point. don't mind the railvoltage. nothing bad ever comes from a little higher rails (unless you don't have enough cooling).
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Old 3rd May 2014, 09:17 AM   #3
AndrewT is online now AndrewT  Scotland
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Feds,
measure the voltage drop across each of the jFET source resistors.
From that you can estimate the jFET current.
Measure the total resistance across the mosFET Gate to Source.
Multiply the jFET current times the GS resistance to estimate the expected maximum Vgs.
Measure the mosFET Vgs to see if that is what you are getting.
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Old 3rd May 2014, 01:17 PM   #4
6L6 is offline 6L6  United States
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Quote:
Originally Posted by feds27 View Post
1) I'm getting +/-26V for V+ and V- off of the Universal PSU v3.0. I see that Papa has specified +/-23V. I'd like to get the supply as close to +/-23V as possible to match the First Watt specs. What should I check as the problem?
If you are measuring the PSU with no load, there is no problem, it will reduce with load.

Anyway, unless you have 25V caps in the PSU, it genuinely doesn't matter.

Quote:
2) I also tried to adjust the bias on both channels but I'm not able to get past .57V on the right channel and .47V on the left channel. Offset was steady at 0V.
That's awfully close striaght away (and kudos for determining how much bias you can get with zero offset). Anyway, you should only need to increase the parallel resistors across the bias pots by 2 or 3K to give enough resistance to set the bias properly. Heck - I have an F5 with V grade jfets and it took 10K in parallel with the pot to get things operating happily. Sounds and measures great, BTW.

Quote:
I set P3 to dead center by checking resistance before installing and I'll adjust it once I get everything else sorted.
A good place to start. You can mess with the pot once everything else is happy by counting turns in one direction or another. Take notes every time you adjust P3. And also, if you ever get lost, just bottom the pot in either direction and turn it back 12.5 turns - that will be close enough to the center to be no matter.

May we see some photos of your build?
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Old 5th May 2014, 01:25 AM   #5
feds27 is offline feds27  United States
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Hi Folks, thank you for the advice. I won't be able to try any of these suggestions out until Tuesday.

More info I forgot to provide on the build:
Used lsk170 and lsj74 B grade matched at 9 right and 9.8 left
Used ztx550 and ztx560 didn't bother matching since I read that boarding would compensate.
Used irfp240 and itfp940 didn't bother matching. Thought boasting would compensate.

I'm not sure if the polarity of the LEDs changed from v2.0 to v3.0 but I installed my blue LEDs long lead as positive into round pad and it didn't work for me. Flipped it to match the diode symbol and it worked.

Pictures to follow on Tuesday. Sorry about that, I knew better than to post without =D
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Old 5th May 2014, 02:31 AM   #6
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If you wish to figure it out methodically, do what Andrew has suggested and measure the voltage across R3/R4 first and compute the ma passing thru each Jfet.

Your matched Jfets are 9ma and 9.8ma??
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Old 5th May 2014, 01:24 PM   #7
feds27 is offline feds27  United States
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Sorry, iphone auto correct changed 'biasing' to 'boarding' and 'boasting'

Yep, 9ma and 9.8ma. I wasn't able to get all 4 at the same value so I matched a pair for each side.
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Old 5th May 2014, 05:38 PM   #8
feds27 is offline feds27  United States
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FYI, I used the method described in the following links to match my JFETs. 6L6 and Tinitus describe the method and provide links.

Posts #11244 #11245 in the F5 Amp thread.

F5 power amplifier

Transistor matching

I'll try what AudioSan, Andrew and 6L6 have suggested and will get back with more info.

Thanks guys.
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Old 5th May 2014, 07:14 PM   #9
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I just think that 9.0ma and 9.8ma are not that close a match. Spencer's Jfets are within 0.2ma of each other at 10vdc. I would be interested in learning how many ma's are flowing thru the Jfets as suggested earlier. I suppose pot P3 could also be used to fine tune this.

nash
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Old 5th May 2014, 07:23 PM   #10
Zen Mod is offline Zen Mod  Serbia
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use what you have , in 20-25% ballpark

our dear Baobabwana made his first F5 lookalike with J309 and 2SJ108 years ago ...... you'll get much better match ....... due to balancing nature of circuit
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