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Old 31st January 2009, 05:55 PM   #2611
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dcb,

i already gathered that from your 50w post, mine was a addition to the question of the "smacking" Tea-Bag.
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Old 31st January 2009, 06:09 PM   #2612
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Default Bridged F5's

Jacco - cool. I think the last thing one wants to mess with is Papa's choice of the bias point on his selected MOSFETs.....my guess is Pass does a LOT of testing and listening to determine this, and I don't presume to think I can do any better.

I do think a "double-bridged" F5 would be a pretty cool project and would make one hell of an amplifier. In fact, given the simplicity of the circuit, ease of building these things and the high input impedance of the JFET's Pass has chosen, I suspect "scale-up" was an implicit design objective.
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Old 31st January 2009, 11:39 PM   #2613
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Quote:
Originally posted by Zen Mod



if nothing else - go to Papa's site ( Pass Diy ) and download few pdfs ...

I mean that in "discrete OP for DIY " ( or something like that ) file you can find how to test jfets and mosfets .

use your imagination , and learn something ......

Thanks Zen Mod.

I'll build the jfet testing circuit and see what comes of it. I just thought someone might know right away if putting Q2 in backwards would fry it or not. Anyway.... I've ordered some more Q1 and Q2.

If Q1/2 check out ok on the test circuit I suppose the next step is to look at the current limiting parts. Test TH1/2. I'm a little concerned that I've actually used NTE128/129 instead of the ZTX450/550 (which is what mouser threw up for ZTX450/550 part). So that might also be the problem.


Thanks for the help.. will hope to do some testing tomorrow and will report back here with the results.

Fran
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Old 31st January 2009, 11:44 PM   #2614
tms0425 is offline tms0425  United States
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You can probably just take those current limiting parts out, replace the thermistors with 4.7k or so and see what happens. At least to eliminate those as sources of problem. After that, there aren't many parts left to go wrong Tom
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Old 31st January 2009, 11:48 PM   #2615
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Thanks Tom,

I know what you mean about the parts - thats why I was so p***ed off when I saw what I'd done!

Anyway, more investigations tomorrow. For tonight, its me and some Aretha, Otis and Carla Thomas. Chill out time!


Fran
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Old 1st February 2009, 01:07 AM   #2616
tms0425 is offline tms0425  United States
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I was tapping and drilling on my F5 chassis tonight after a long day and decided it was time to just kick back and relax... lest I make a(nother) mistake One more heatsink to go and hopefully fire it up with no smoke. The power supply with Christian's board and the AnTek toroid works just fine so far at +/-25 unloaded.

Just remember stuff happens to all of us. Hang in there. It should be easy to fix and it will be worth it!
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Old 1st February 2009, 02:10 AM   #2617
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Quote:
Originally posted by woodturner-fran
I just thought someone might know right away if putting Q2 in backwards would fry it or not.
If you check that link, you might think that it shouldn't: http://www.diyaudio.com/forums/showt...52#post1202052
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Old 1st February 2009, 09:34 AM   #2618
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Thanks Peter,

some testing to follow today. To be honest I'm kinda hoping that it is the problem! Otherwise theres another issue....

Fran
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Old 1st February 2009, 10:57 AM   #2619
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hi..what is the input voltage to get 25watt class a and 50watts class ab? thanks
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Old 1st February 2009, 02:19 PM   #2620
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OK, so I done some more testing. First up was to check out Q1/Q2. I made up the circuit that someone posted a little back up here (Idss) - and I got 10mA for SJ74 and 9mA for SK170. So it would seem that they were alive still.


Next I decided to look at the current limiting circuit. I removed all the components from the circuit as detailed earlier in the thread. Now I found I could get the bias set ok across R11 and 12. Its set as 0.6V on both but my offset at the speaker terminals is high. The mosfets are heating up nicely, but they aren't that hot. I've left it on for a little while and will go back and check it again. The offset I'm seeing is "floating" ie moving around a good bit while the bias values seem pretty stable. I get 4.4-4.6V G-S on Q3 and 4 and I meaure 16V D to S on Q1 and 2.


Does this sound OK?

Fran
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