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Old 22nd November 2018, 05:44 PM   #101
Nikon1975 is offline Nikon1975  Austria
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Finally I assembled two channels and I am trimming the first one. I have the configuration with All FET NGF, single power supply. Here are my questions:
1) What voltage should I use ? Can I go straight to 24 V ?
2) How close to zero R11 should be +- 5mV is enough or should I be more obsessive ?

I did not solder the output devices yet. Some porn later on.

Thanks,

Davide
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Old 22nd November 2018, 05:53 PM   #102
Nikon1975 is offline Nikon1975  Austria
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IMG_4986.jpg

D
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Old 22nd November 2018, 06:19 PM   #103
EUVL is offline EUVL  Europe
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I used +/-24V with current limit during test.
But it will work well enough still at say +/-15V to allow you to check things as a first start.
Eventually you need to run it at operating voltage to trim DC offsets, etc.

For trimming R11, I would aim at < +/-10mV minimum, 5mV preferred.
After that you need to trim the power devices using the two biasing resistors.


Patrick
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Old 23rd November 2018, 03:51 PM   #104
Nikon1975 is offline Nikon1975  Austria
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Any particular recommendation for a power supply? I would like to use 24V , but I think I could replace the on board resistors with regulated stage.

D
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Old 23rd November 2018, 03:56 PM   #105
EUVL is offline EUVL  Europe
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Plain cap multipliers would be just fine.


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Old 6th December 2018, 09:14 AM   #106
Nikon1975 is offline Nikon1975  Austria
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So,

First channel is running. I still have to adjust the output devices. The offset set around 5mV without correction with a 190mA bias. The Jfet are 10 mA. One thing I noticed is that it took quite some time to settle. It started at +130mV, then went to -40mV and, in 40 minutes, went down to 5mV. Did other builders observed this ? The heatsink temperature settled at 34C. It was a bit cold in the room.

Regards,

Davide
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Old 6th December 2018, 11:19 AM   #107
EUVL is offline EUVL  Europe
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I don't have so much offset at the start.
But then I have only about 140mA bias.

So probably you need better coupling to the heatsink (I use Kerafol).
But a SHPP will protect your headphone against the 130mV.
Anything below 50mV is no issue IMHO.
I assume you use over-the-ear phones.


Patrick
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Old 6th December 2018, 11:51 PM   #108
Nikon1975 is offline Nikon1975  Austria
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There is something wrong: as soon as I disconnect the ground from the input, I get 20v on R11 and on the output.

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Old 7th December 2018, 12:45 AM   #109
EUVL is offline EUVL  Europe
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Is that still the case if you connect the input to Gnd with a 10k or 100k resistor ?


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