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Old 29th August 2006, 11:35 PM   #1
johndiy is offline johndiy  Greece
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Default noise floor problem

I have built this zen circuit as a headphone amp, all works
well except for its noise floor i use sensitive headphones and
these make the problem worse

in the power supply i have used a total of 10000uf capacitance
to tame the ripple but noise floor is still too high

whats the problem?

can you help

john
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Old 30th August 2006, 08:00 AM   #2
MikeB is offline MikeB  Germany
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You might try a resistor to the gate. Eventually the mosfet oscillates.
Shouldn't the mosfet have a small resistor at the source ?

Mike
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Old 30th August 2006, 08:12 AM   #3
AndrewT is online now AndrewT  Scotland
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Hi,
just guessing, but, have you snubbered the PSU rectifier?

How about RCRC type smoothing after the recifier?

Headphones are very critical of noise and demand a very good PSRR of the amp and/or very low noise from the PSU.
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Old 30th August 2006, 09:26 AM   #4
johndiy is offline johndiy  Greece
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Quote:
Originally posted by AndrewT
Hi,
just guessing, but, have you snubbered the PSU rectifier?

How about RCRC type smoothing after the recifier?

Headphones are very critical of noise and demand a very good PSRR of the amp and/or very low noise from the PSU.

"snubbered the PSU rectifier" yes

rcrc no i dont want voltage drop at the filter

i have use 10000uf should i go higher?

john
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Old 30th August 2006, 09:29 AM   #5
johndiy is offline johndiy  Greece
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Quote:
Originally posted by MikeB
You might try a resistor to the gate. Eventually the mosfet oscillates.
Shouldn't the mosfet have a small resistor at the source ?

Mike

it is not oscillation, small resistor at the source? why?

john
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Old 30th August 2006, 09:45 AM   #6
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Default .

The source resistor is in order to put a feedback for bias current control and stabilization.
Your amp has the problems of all too much simples circuits.
A low noise circuit is suggested for headphone amp.
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Old 30th August 2006, 09:51 AM   #7
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What is your actual problem? Is it actually noise (which is broadband) or is it hum? If it's hum then it may simply be a grounding issue and nothing is wrong with your circuit or PSU. I know headphones can be sensitive but 10000uF should be more than enough by a long way.
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Old 30th August 2006, 10:22 AM   #8
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The voltage used is not known but still...

Use an LM317 circuit with cap at adj pin or use a capacitance multiplier with a darlington.

Gajanan Phadte
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Old 30th August 2006, 10:49 AM   #9
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The philosophy of this circuits is the employ of the minimal count of the components that they are in series with the audio signal.
A voltage stabilizer integrated circuit it is not adapted to this philosophy.
This amp have a very low PSRR and the supply noise
(above all without any circuit of ripple rejection) it must to be tolerate....
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Old 30th August 2006, 11:06 AM   #10
AndrewT is online now AndrewT  Scotland
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Hi,
poor PSRR, simple C PSU (snubbered), and headphones are a conflicting set of requirements.

Something has to give.
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