how low is 'low DC' on SOZ outputs? - diyAudio
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Old 3rd March 2002, 07:31 PM   #1
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Question how low is 'low DC' on SOZ outputs?

a simple newbie question: i just finished my SOZ, getting 0.3-0.4 VDC at the output speaker terminals.
is that 'low' enough? or do i need to match my MOSFETs better?

thanks in advance for any info.

-p
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Old 3rd March 2002, 08:54 PM   #2
arnach is offline arnach  United States
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Default Hello

.4 is almost half a volt! That's way too much, and should produce an audible buzz. I don't know about matching MOSFETS but in the prototype SOZs i've built when I had DC on the terminals I added a pi filter and more capitance until it went away. My final version had .01V.

-- AaRon
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Old 3rd March 2002, 09:11 PM   #3
PedroPO is offline PedroPO  Portugal
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arnach

How do you measure your DC at 0.01V?

Do you use a digital multimeter? with mine I cant get a fix reading.

Pedro
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Old 3rd March 2002, 09:41 PM   #4
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Default DC Buzz

You need to match your fets closer or add some provision to adjust the DC offset at the input which is not too hard to do. Also look at the DC offset of your preamp and make sure that is not contributing to the offset of your amp. I have heard that many high sensitivity speakers like to see less that 50 millivolts of DC offset for the best sound. DC offset will not contribute buzz to the sound. Excessive power supply ripple will.

H.H.
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Old 3rd March 2002, 10:47 PM   #5
arnach is offline arnach  United States
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Default Hmm

I built my SOZ prototypes when i was 13 or 14 and had little knowledge of electronics. What I did was set the multimeter on DC mode and take the reading across the terminals with and without a load/source connected.

I found a direct coalition betwen this reading and the amount of "buzz" noise present in the speakers.

Then I beefed up the power supply and the reading went down and the buzz all but went away.

Nothing particullary technical.. I never bothered to match the MOSFETS.

-- Aaron
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Old 4th March 2002, 10:51 AM   #6
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Hallo,

first of all we need to clarify a few misunderstandings.

when you have a buzz or any other noise you have riple or oscilation or somehting and you can see this either with a multimeter when it is a buzz and measure ac voltage. Or with an osciloscope if you have sound sof what so ever due to oscilations.

when we have dc offset there are NO sounds. You can see this on the woofer when it moves in one direction. This is DC offset and can damage your woofer when it is excessive. A few millivolts are only good. maximum maybe 50-100 mV. I would say less then 70mV.

The best is to match the mosfets or trim the dc offset changing the resistor that bias the FETs till you have 0mV at the outputs.
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Old 8th March 2002, 11:23 AM   #7
ralf is offline ralf  Germany
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Default Re: how low is 'low DC' on SOZ outputs?

Hi,

i used a capacity multipler circuit (to save money for a big amount of Capacitors) and matched the Mosfets.
I tested it against big capacitors: The Multipler-Circuit is even better than 600000uf per channel!
The Soz is absolutely quiet. But this has to do with capacity and nothing with Dc offset.

My Dc Offset is at 10mV

Regards

Ralf
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Old 8th March 2002, 12:10 PM   #8
arnach is offline arnach  United States
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Default What

What works: works!

Could you post a link to that capacitance multiplier circuit?

Thanks,

Aaron
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Old 8th March 2002, 01:39 PM   #9
paulb is offline paulb  Canada
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Default Re: What

Quote:
Originally posted by arnach
Could you post a link to that capacitance multiplier circuit?
Project 15 at www.sound.au.com
Also see a related article at www.gmweb.btinternet.co.uk
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