[sdiy] Better testing techiques for audio circuits

[Neil Johnson]

Hi,

Justin Owen wrote:
> One follow up question to this...
>
> How useful are simulations like SPICE in showing noise and distortion
> level in a circuit?
>
> ...or more specifically - if I was to simulate a 'bare bones' version
> of a simple, discrete circuit and a 'precision' version of the same
> circuit - how useful would simulations be in showing any differences
> between the two with regards to improvements (or drops) in noise and
> distortion?

It really depends on what you're looking for in terms of improvements. 
And what quality your spice models are.  And whether you remember to 
include all the circuit parasitic elements that a real circuit has.

SPICE, used properly, can give indications of what might happen in reality.

It really, really depends on what you're looking for though.  If you 
could give some indication of what performance you're trying to achieve 
then that would help identify the tools and methods that would help you.

For example, in your opening email you mentioned looking at waveforms on 
a scope.  It is generally reckoned that you can spot by eye about 3-5% 
distortion.  To go better you need proper analysis equipment.

Then you mention your mixer having <0.0007% THD.  Its unlikely that 
you'd be able to simulate to that extreme in SPICE.  To measure it in 
the real world needs expensive kit (Audio Precision 2700 series). 
Simply because at that sort of level EVERYTHING has an effect on 
performance, from choice of components to PCB layout to power supplies 
to connecting wires.

In the middle you have kit like the venerable HP 339A, which can go down 
to around 0.003% THD+N.  Or the HP 8903 if you prefer automatic 
operation and GPIB.

Neil
-- 
Modules and more :: www.cesyg.com
Homepage :: www.njohnson.co.uk ::