Pics for Phil

Hi Phil

I mentioned to you some equipment originally used at the
British Aerospace acoustics lab at Filton UK.

Here are some pics.

Not a bad setup Iain, unfortunately it will only provoke
Phil into hating you even more as you have better gear than
He does.

I did not mention that these items were priced at UKP 5 each!
I still need a very low distortion oscillator.
Iain

My oscillator project is on hold at the moment: too many other things
to do right now

Understood. We all have the same problem, too muich to do,
and too little time to do it in.

But, when you get round to it Mark, I shall be keen to have
one.

The SPL meter is battery operated and also has a
mains pack., The mic stem is also the ON/OFF switch.
When the mic is vertical the unit is ON, and when it
is folded down horizontal to the meter face, the unit is
OFF.

Patrick Turner gave me the idea of using a third-octave
filter at the output of the test generator. It's a a good tip,
and cleans up the 2H and 3H nicely, reducing the generators
own distortion from 0.04% to 0.02% (6dB)

** The audio bench oscillator I use every day has a basic THD of 0.001% -
rises to 0.0025 % at the extremes of the audio band. The design was
published as a project in Electronics Australia magazine:

Low Distortion Oscillator #1 - Feb 1989

Low Distortion Oscillator #2 - Mar 1989

That's a pretty serious ocillator Phil. I wish I had something like that.
Most of the industrial generators seem to be at about 0.1%, and those
from the far east much worse, typicallty 0.4%.

I would be interested to have a very low distortion oscillator.

I use Wien bridge tuneable notch filter for THD measurements - it can
resolve down to 0.0006% at 1 kHz with an input signal of 2 volts or more.

But you are a professional - and perhaps need that kind of resolution.
Valve amp building for me is a hobby. If I can measure down to say 0.01%
(that's at the bottom of the 0.1% scale) for a cathode follower then I
am happy.

Both these units have exceptional frequency stability due to use of wire
wound control pots, metal film resistors and polystyrene caps in the high
frequency ranges - essential for the notch frequency the match the
incoming frequency at rejections of 100dB. It typically takes less than a
minute to null the reading and get a THD figure.

Except for the audio generator itself, the lowest THD audio devices I come
across are CD players - figures of < 0.002 % THD being typical at 1 kHz.

Phil you test the audio circuit from the output of the D/A I guess, but do you
also test the sound coming off of the CD itself? A test CD? I wonder what
difference you could expect... there is probably a minimum spec that can't be
beat due to the design of the system, IE the number of bits.

The lowest THD figure for a power amplifier I have tested was a Crown D150
Series 2 - 0.0013 % at any level up to full power into 8 ohms at 1 kHz.

Do you actually need to measure that low in your daily work?
I would take such a task to the broadcast lab which I frequent.

Thanks for an interesting post.

Regards to all

........ Phil

Wouldn't 6 dB down be a quarter of the original distortion?.

The band analyser is a useful mains and battery powered
unit. You simply tune to the frequency of the harmonic you
wish to measure.

These three pieces of equipment are all in pristine condition, and
had calibrtation certificates from Sept last year.