Bride of Zen fuse blowing

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Hi there,

I'm building a BOZ preamp. The parts are as described
in Mr Pass' original BOZ article, except for two changes:
I'm using 2200 uF (100V) cap instead of the 1000 uF
ones and the transformer is a 300VA dual secondary (30V)
toroidal. I tested using 1/8 A slow-blow fuses, which seem
to blow immediately (?) after I apply power.

The PC board shows no sign of damage. I haven't yet
found problems with the parts. Anything in particular I should
look for?

I'm now wondering if this could be caused solely by
inrush current. Any suggestions on what I should try?
As a related question, does the larger capacitance on the
PS require a higher current fuse?

Hello Dennis. When I powered up my BOZ using a 200VA transformer, the fuse also blew. But when I used the recommended transformer VA, the fuses are OK. IMHO, yes, it is due to the inrush current, if the circuit has been properly constructed. Try a larger slow-blow fuse, maybe 1/2 A? Good luck.
Lesson learned.

A bad good experience or was it a good bad experience ? :boggled:
Anyway, after testing my Aleph30 with a 400VA 16-0-16 transformer for a while, I eventually got my REAl transformer last night, a 625VA 20-0-20.
So 1st I did some comparison tests between the the 2 trafo's like unloaded AC and DC readings. During this time everything work smooth. Then I took out the test trafo and fit the new 1. Check everything again and switch on. (Here is where the bad experience starts) :darkside: :bomb: :flame: :yell:
All I can remember, is a crackling sound and a flash, which seems like came from FET #2. Now usually when these signs appear, you also get a "something went wrong" smell, or a "sww" marks, like burn marks or melting marks, or all of them. Instead of observing for the latter, my 1st reaction (obviously still new at this), was to panic. :worried: (FET's destroyed, caps destroyed, big money stories) :crying: :yell: O.K. so I remove pcb's and start testing. Seems like there are shorts on all windings of trafo. even caps seems faulty and FET's DO NOT look healthy anymore. To make sure, test trafo without anything connected. Fit new 3A fuse - blow, 4A - blow, 5A - blow. Then small bell rings softly. :idea: INRUSH CURRENT!!!. Fit 5A resetable fuse. Switch on, transformer gives grunch sound, vibrates a little bit, go quite, pirrs. Test output - :sing: :cheerful: :D O.K. 1/2 way back in bussiness. Connect bridge and caps. Nice. Test FET's again, 1x1, follow circiut - see why I got 3.1k where I thought should have been infinity - :ashamed: ;) All O.K. :sigh: Now it was 01:30 -again. Switch off, go to sleep, sweet dreams.

Lesson learned - when stuff go's wrong - DO NOT PANIC!!! , yet. I did fit slow blow 2A fuse from start, did not last the 3rd time for some reason. Also use thermistor as on A30 circiut.

1. What must I do to avoid this problem , I don't like 5A resetable fuse for permanent option, think 5A is a bit too high.

2. Found this idea this morning, is it good?
Look here, interesting stuff:

3.If good, what should R1 value be?


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Hi JDev,

Have you tried using a thermistor on the primary side of the transformer as used on the original Alephs.
If that is used then you should be fine with a 4A slow blow fuse. I believe the type used by NP was a 10R/5A ntc thermistor (Thermometrics CL60). I have heard others have used Rhopoint SG200/170 thermistors, Mark Finnis used them on his Aleph 5.

Hope this helps,


Sorry to jump on the thread with an off topic question. Do you find it difficult to locate a 5K audio potentiometer for the volume control on the BOZ? I can not find one anywhere. I have searched for alps and others but have come up short. Peter Daniel use a 100K alps at the input, but I don't really understand how it still works. If you used a 5K pot. for the volume I would greatly appreciate information on where you got it. Thanks
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