[sdiy] Simple discrete Unity-Gain Follower ?

[Czech Martin]

I always thought that distortion is defined as follows:

Put an ideal sine wave into the system (amplitude maximum
allowed or designed value). Wait until system has reached
steady state (i.e. all transients from "switching on" the
sine have disappeared). Now record system output.
Look at the spectrum and determine fundamental
and harmonics. The ratio of both (in terms of RMS)
gives the distortion figure, usually given in %.
Better would be to display the DFT plot as well.
Real systems have 1/f and Johnson and shot noise,
so THD is computed. Total harmonic distortion + noise.

So, basically I thought that distortion is only meaningfull
in terms of steady state.

I wonder why:
1.) distortion spectra are usually not given in specs
2.) intermodulation is completely left away

For a transient response, like impulse or step response
I always thought that we could consider slew rate, or
overshot, ringing and the like. But I have to be carefull:
After some experiments I have made higher frequency ringing
is not audible. E.g.: if the system is so bad that the feedback
shows ringing at 10kHz, some kind of "whistling" can be heard.
If considerable ringing exists at 30kHz, I can not hear any artefact.
I think that is the reason why digital brick wall lp filters, which
have considerable ringing can not be heard as artefact, the ringing
is (usually) too high.

Am I wrong here?


m.c.

-----Original Message-----
From: Tim Ressel [mailto:madhun2001 at yahoo.com]
Sent: Freitag, 2. Mai 2003 17:53
To: Don Tillman; grichter at asapnet.net; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Simple discrete Unity-Gain Follower ?


Yo,

My $0.02(US) worth,

--- Don Tillman <don at till.com> wrote:
>    > Date: Thu, 01 May 2003 11:39:20 -0500
>    > From: Grant Richter <grichter at asapnet.net>
>    > 
>    > Most of the "audiophile" distaste for feedback
> loops refers to
>    > the failure of the feedback loop, not when it's
> working properly.

Feedback loops tend to make designers lazy. They will,
in theory, clean up any distortion in your amp. And
when you measure with steady-state signals, it does.

But music is not steady-state. Feedback always has a
delay because you are adjusting what is going to
happen next based on what happened last. This delay is
audible in real-time to the human ear.

Good to excellent designers make their amp designs as
clean as possible without feedback, then add in as
little feedback as possible. Note that DC feedback for
zero-point determination is usually nessessary.

I try to give as many words for $0.02(US) as possible.
I concider it 'value added'.   ;-)

--Tim

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