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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: 80dB now but still needs improvement at 1KHz
Date: Wed, 6 Nov 2024 23:20:07 +0100
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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?
>>>>>
>>>>> Version 4
>>>>> [Deleted....]
>>>>
>>>> You are chasing ghosts. Simulation doesn't prove anything at this
>>>> level. Build it and measure it.
>>>>
>>>> Jeroen Belleman
>>>
>>> Simulation won't asolutely prove what circuits would produce low THD,
>>> but it will pretty definitely show which circuits won't.
>>>
>>> Of course, a sim like this needs good opamp models.
>>>
>>> If one builds it and measures it, what would you use to measure the
>>> distortion?
>>>
>>
>> I'm old school: A passive notch filter. It's likely to be fiddly.
>>
>> 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.

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.

Jeroen Belleman