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Old 24th February 2002, 11:03 AM   #11
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Dear Paulb

Hum problem....:

the original szekeres does not use a decoupling cap for the MOSFETs bias circuit.. that could very well reduce the hum......

also 200mA DC with 2mA AC though a 0.1 Ohm ground lead causes enough hum to be audible so carefull gronding is nesscery..

you could also put the MOSFET inside you opamp feedback loop... an opa134 could be stable at a gain of 3 .. or use a compensation cap around the feedback loop to give it a 100KHz roll all .. should be more then enough... This is what I want to try if I had just a litle more time for things. :-(


greetings,
thijs
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Old 24th February 2002, 03:07 PM   #12
paulb is offline paulb  Canada
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Thanks, tschrama, I've been pretty darn careful with my grounds but I may have messed something up. The hum is too low to see on my scope, but it's audible at zero volume setting. I haven't had a chance to look more closely at it yet, too busy listening to the power amp section.
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Old 24th February 2002, 10:55 PM   #13
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When I checkt my amp and found a DC offset I thought I messed up.. then I realized the DC current and the tinny wires I used... anyways I use a 22000uF/16V cap ... at these low voltages these are not expensive (10$ ??) that takes care of the hum ... no regulated supply needed :-)
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Old 24th February 2002, 11:36 PM   #14
paulb is offline paulb  Canada
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The hum is definitely not from the power supply - I'm using a capacitance multiplier that reduces the ripple from about 100 mV to essentially 0. Works like a charm.
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Old 25th February 2002, 09:05 AM   #15
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hmmmmmmm... nice!!

I leave my chassis floating, but that shouldn't matter... the only ground connetion is at the audio inout connectors..
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Old 25th February 2002, 06:49 PM   #16
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Really nice job, Tortello!
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Old 26th February 2002, 08:20 AM   #17
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Thanks Nelson, you're very kind,
... even if all this flour is from Pass' bag!!
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Old 26th February 2002, 09:56 AM   #18
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Default Nice work!

Very nice work I'd say. I would greatly appriciate if you would send me a better quality pdf file.

Thanks in advance...

//A guy from Sweden
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Old 26th February 2002, 03:52 PM   #19
paulb is offline paulb  Canada
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Just a quick update on my hum problem.
I found that it was the ripple coming in to the cap multiplier that caused the problem. While the multiplier did a very effective job of removing the ripple, the input to it was close enough to the headphone amp board (2 inches or so) that it induced ripple into the amp.
The power supply is in a separate enclosure, so I added a simple RC (4R7, 1000 uF) there. Voila! The ripple was cut down by about a factor of 10, and it softened the edges a bit. The hum is now inaudible.
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Old 26th February 2002, 04:30 PM   #20
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It's good to read you solved your hum problem. Can you explain a bit more.. I would very much like to understand what was going on..
Is this inductive indused hum, maybe due to the high impedance of the MOSFET and bias resistor?

Happy listening :-)

thijs
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