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Old 25th November 2001, 09:41 PM   #1
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Join Date: Apr 2001
Location: Brisbane Australia
I've just finished the BOZ preamp and have a few questions regarding its operation.

First of all when I fired it up for the first time I couldn't get enough bias voltage to turn on the mosfets. I traced this back to the fact that I was getting nearly 40V drop across the power supply resistors (47.5KOhm part of the RC filter) leaving me with only 20V rails. This puzzeld me as the rest of the circuit seemed to be working OK. I bypassed these resistors with wire and the circuit works great except.....

The second problem is that when there is no input to the preamp a loud hum is emitted through the speakers. I am guessing this is either a grounding problem or removing the filter resistors has introduced a lot of noise into the circuit.

Any ideas about either of these problems?

Cheers

Dan
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Old 25th November 2001, 10:13 PM   #2
Geoff is offline Geoff  United Kingdom
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Dan

You say the power supply resistors were 47.5 kohm (47k5). If so, it is no wonder that you are short of voltage as they should be 47.5 ohm (47R5). Shorting out the resistors will increase the supply rail ripple, causing hum, since you are removing the R element of an RC Pi filter.

Geoff
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Old 25th November 2001, 10:22 PM   #3
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Thanks for the tip geoff. Did I read the schematic wrong or is their an error? Why is it that the hum is only present when no signal is supplied?

Thanks

Dan
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Old 25th November 2001, 10:25 PM   #4
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Well don't I feel the fool. I just checked the schematic and your right. I can't believe I made such an oversight.

Thanks again I'll make the necessary alterations tonight.

Dan
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Old 25th November 2001, 10:52 PM   #5
Geoff is offline Geoff  United Kingdom
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Dan

Glad I could help (even though I have never considered building a Pass design). A kohm resistor in the supply rail just didn't make sense. I wondered at first if there was a typo in your post.

It could well be that you have a grounding problem as well, so if the hum is still there when you get the supply rail voltage correct you will have to investigate further.

Geoff
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