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error0 12th January 2009 06:22 PM

yet another bosoz
I'm atempting to build a BOSOZ but have run into problems.

First I built the possitive side of the PSU. all is fine and output is about 60V

Next I attempted to build the negative side of the PSU. I made some mistakes and some magic smoke escaped :dead:

I did some trubble shooting and found that I had run Q102 (IRF9610) without connecntion to source-pin. Some of the zeners in the stack had been placed the wrong way round.

Now the transformer, zeners and the transistor runs verry hot and I only get -20ish Volt.

I need some help to find the fault.

hifimaker 12th January 2009 10:16 PM

Is the 20vdc you measured from the + side or minus side?

What is the size of your transformer. Let us know the make and VA rating.


error0 12th January 2009 11:22 PM

Thank you David for taking the time.

-20ish at minus side of PSU. I've only run minus side of PSU a few seconds

Transformer is a Sedlbauer AG 30VA 2x30V.

Will Q102 (IRF9610) fail without a sourcepin connection?

hifimaker 13th January 2009 12:44 AM

I assume you are using 2 transformers, one for each rail side + -. Another assumption is that you are measuring the PSU while not connected to the preamp board.

Those transformers should work. Nelson originally specified 2 30+30 transformer due to availability at the time. You can also run a single 100va 60+60 with 1 bridge rectifier.

Test everything on the negative side. start by disconnecting the secondary side of the transformer and measure it's unloaded AC value, if you concerned about the transformer. Probably not the problem, but always good to start at the beginning. Use an ohm meter to test the diodes. Reconnect the rectifiers to the transformers and verify the unloaded DC output. Verify the zeners are oriented correctly and of the right value.

Nelson provided some known values on the PSU schematic. If everything checks out, then let us know the voltages at the points in the schematic where these values are present.


error0 13th January 2009 10:29 AM

I found the problem.
I forgot to connect the zenerstack to resistor R102 and R104. D'oh :clown:
All zeners on negative side are dead. :headbash:
Today I lerned not to try to be smart while I'm tierd.

error0 13th January 2009 10:33 AM

Next question

I'm unable to source the 750ohm resistors. Can I substitute for two 1,5khohm i parallel? If so what imact will that have on the circuit?

eLarson 13th January 2009 12:23 PM

Should be just fine to parallel two 1k5 resistors to effectively get a 750R resistor.

Buhl 13th January 2009 02:36 PM

Two 1K5 in parallel are just fine - use it myself.
And while you are at it - upgrade the resistors - 5 watts each are not overkill i my opinion - everything on that pre runs hot! (almost).

Also, I used an aluminium L-profile and mounted the four FETS to this - then a large heat sink the sice of the preamp board on top of the L-profile - and the FETs are still to hot to touch - use all the sinking you can (elegantly) get on the FETs - just my two cents :)

You could also do the X mod while the iron is hot - it's just 4 resistors - the pre should be in 20dB gain though....

Kind regards

hifimaker 13th January 2009 02:38 PM

I'm glad you got it sorted out. Take Hans advice and make sure you have to-220 heat sinks on the 4 preamp mosfets. Some builders just mount the mosfets to the case with thermal insulators. I used heat sinks for my BOSOZ and it's been a reliable build for almost 4 years.

After you have the preamp up and running, enjoy it for a while. It's actually a very nice pre, better than what a normal person could justify or afford to build. After becoming more familiar with the circuit it can be upgraded with a mosfet constant currant source instead of the 750 ohm resistors at the bottom of the circuit. Later on you can add the X resistors Hans also describes. Some people like the X mod and some don't.

Have fun for a while enjoying it as is to get familiar with the sound.

Edit: Heatsinks options that work for the BOSOZ


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