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Old 8th January 2012, 11:57 AM   #1
veteran is offline veteran  Poland
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Default Universal FET buffer / preamplifier

For a long time I was looking for a good supplement for my input selector project (described in this thread) an here it is

Click the image to open in full size.

In the beginning I wanted to make modified Nelson Pass B1 buffer (which sounds quite good in my system) but then I decided to upgrade it a bit to make it more universal. I designed PCB which can be:
  • standalone FET buffer
  • op-amp preamplifier (for example LM4562)
  • op-amp preamplifier with FET output stage
  • op-amp preamplifier with FET output stage in the feedback loop
There is also dc-servo circuit so the output capacitors may be omitted.

Here is the schematic of one channel.
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Old 8th January 2012, 12:04 PM   #2
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Measurements made with my e-mu 0202 shows that the THD+N is less than 0.001% for entire band.
This is how it looks for 1kHz, +-17V supply voltage, 10k load and 1.55V RMS (4.42V Pk-Pk) Vout.

E-mu 0202 loop:
Click the image to open in full size.

Standalone FET buffer:
Click the image to open in full size.

Op-amp preamplifier with FET output stage:
Click the image to open in full size.
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Old 8th January 2012, 12:06 PM   #3
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20kHz square wave from my GAG-810 generator:
Click the image to open in full size.

Click the image to open in full size.
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Old 8th January 2012, 07:33 PM   #4
gootee is offline gootee  United States
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I can't open your jpg image.
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Old 8th January 2012, 07:38 PM   #5
ddietz is offline ddietz  United States
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Gootee, its a PDF and opened fin for me.
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Old 8th January 2012, 07:46 PM   #6
veteran is offline veteran  Poland
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Schematic is in PDF, rest of the files are in PNG format.
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Old 8th January 2012, 11:11 PM   #7
gootee is offline gootee  United States
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Sorry, it was the JPG photo at the top that wouldn't open. But it's fine, now. Thanks.

I would be interested in seeing how you routed the grounding, and the signals and power, on the PCB. But if you don't want to post that I will understand.

Last edited by gootee; 8th January 2012 at 11:17 PM.
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Old 9th January 2012, 07:36 AM   #8
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Quote:
Originally Posted by gootee View Post
Sorry, it was the JPG photo at the top that wouldn't open. But it's fine, now. Thanks.

I would be interested in seeing how you routed the grounding, and the signals and power, on the PCB. But if you don't want to post that I will understand.
There are two separate grounds for signal and power connected in one point. I'll post more details later.
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Old 9th January 2012, 11:18 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by veteran View Post
There are two separate grounds for signal and power connected in one point. I'll post more details later.
Can you label the schematic to show the different grounds/returns?
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Old 9th January 2012, 12:51 PM   #10
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Quote:
Originally Posted by veteran View Post
Measurements made with my e-mu 0202 shows that the THD+N is less than 0.001% for entire band.
This is how it looks for 1kHz, +-17V supply voltage, 10k load and 1.55V RMS (4.42V Pk-Pk) Vout.

E-mu 0202 loop:
Click the image to open in full size.

Standalone FET buffer:
Click the image to open in full size.

Op-amp preamplifier with FET output stage:
Click the image to open in full size.
I'm curious to know what measurement software you are using with your e-mu 0202? Also, how satisfied are you with that measurement solution?
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Last edited by Ken Newton; 9th January 2012 at 12:53 PM.
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