Re: 80dB now but still needs improvement at 1KHz

On Wed, 6 Nov 2024 23:20:07 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

On 11/6/24 17:42, john larkin wrote:

On Wed, 6 Nov 2024 17:40:08 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
 

On 11/6/24 17:26, john larkin wrote:

On Wed, 6 Nov 2024 10:04:42 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
>

On 11/6/24 03:39, Edward Rawde wrote:
[Snip!]

Now approacing 90dB if you believe the simulation with with this experimental circuit.
You'll want to find something else to do while it simulates.
How to inprove it further?
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Version 4
[Deleted....]

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You are chasing ghosts. Simulation doesn't prove anything at this
level. Build it and measure it.
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Jeroen Belleman

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Simulation won't asolutely prove what circuits would produce low THD,
but it will pretty definitely show which circuits won't.
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Of course, a sim like this needs good opamp models.
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If one builds it and measures it, what would you use to measure the
distortion?
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I'm old school: A passive notch filter. It's likely to be fiddly.
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Jeroen Belleman

 
LC? Just make sure that the inductors and capacitors are all linear.
 
Even twin tee can generate distortion.
 

>
Twin tee is what I was thinking of, indeed. Of course, it's essential
to use carefully matched components of good quality. It would be
fiddly, yes.

One could use a good resistive divider to halve the signal amplitude.
If the filter is linear, the THD shouldn't change.

Or add a bit of DC bias.

Ceramic caps can be awful.

>
In some distant past, I've used a box we called a 'Null Detector'
which did just that. You'd null out the fundamental, and what was
left was the distortion. I don't recall if it was good enough for
-80dB though.
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Jeroen Belleman
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"Selective Voltmeters" were cool. I think I still have an old HP.