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PostPosted: 23 Feb 2018, 20:00 
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I have a tube amp and I saw the structure is pretty similar to the NP-100v12 - 12AU7 / IRF510 design. Sounds is good, but when I use low impedance high sensitivity headphone or earphone(like 64ohms,106db/MW ),I can hear the electronic noise from amp.The e-noise is increasing by volume,and reach the maximum at 70-75%volume(2pm-3pm position).Is there any way to reduce the noise like change the tube to 6922,ecc88,ecc84 with Conversion seat or there is other way?When I change this kind of tube do I need to readjust the operating point by using the two blue potentiometer.


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PostPosted: 27 Feb 2018, 20:42 
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Hi and welcome to the forum.

Quote:
when I use low impedance high sensitivity headphone or earphone(like 64ohms,106db/MW ),I can hear the electronic noise from amp


The output stage of the headphone amplifier is an IRF510 and can drive an 8 ohm speaker. Guessing here, I think the noise is independent of the load and you are just hearing more noise because of the high sensitivity of the headphones.

The power supply is likely your source of noise. Are you using a regulated supply? Have you tried other power supplies? Another option can be to test with batteries.

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PostPosted: 02 Mar 2018, 13:26 
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It is a Switching Mode Regulated Power Supply, not a good power supply.


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PostPosted: 11 Mar 2018, 21:43 
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Try out the head amp with a regulated power supply. That will cut back on the noise significantly.

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PostPosted: 24 Apr 2018, 16:21 
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Gio wrote:
Try out the head amp with a regulated power supply. That will cut back on the noise significantly.

Thank you. I have another question when the volume potentiometer goes high(like over 65%).I can hear big distortion, and after 70% the bass is like sound come out from a broken speaker. I try use foobar2000 preamp set -10db, and the problem is repaired from volume 0 -100%. Does all the hybrid tube amp have the same problem or it's only mine? I find the similar Circuit diagram on other website. Does the problem from gate resistance?


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PostPosted: 24 Apr 2018, 17:07 
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When your volume is over 65% your bass is distorted, because input volume/voltage is very high, this means the valve is driven into cut off.
Where the input voltage becomes very negative that it cannot swing below zero volts.

You can bias up the grid to a volt or few then change change the cathode resitor untill its center biased. This shold fix the plorbem.
Once you bias up the grid/input, a extra input capacitor is required to block the dc voltage at input.

Second choice is to run on higher voltages, this will make the valve grid to cathode more negative and allow more swing however is much less efficent compared to biasing grid up.

Fixing this would be biasing the gate of valve to a volt or two and redesigning the input schematic.
The schematic isn't perfect, because the annode load of the valve is a constant current source, typically constant current source loads have high distortion compared to pure resistor.
When the annode resistance changes, voltage gain of valve changes.


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PostPosted: 25 Apr 2018, 04:05 
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Typically small valves have arround 1 to 2 volts more at the cathode, this means a maximum voltage swing of 1to 2 volts can occur at input(grid) before the valve cuts of (cannot swing below 0v because there isn't a negative vcc)


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PostPosted: 07 May 2018, 08:37 
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KochiyaYamato wrote:
Typically small valves have arround 1 to 2 volts more at the cathode, this means a maximum voltage swing of 1to 2 volts can occur at input(grid) before the valve cuts of (cannot swing below 0v because there isn't a negative vcc)

Great, thank you very much.


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