On 01/05/2015 06:17 PM, Phil Allison wrote:
dave wrote:


Look on the bay for a Heath IG-58 (several variations). They are
very low distortion for the price. (Typically 0.04 to 0.02 % THD,
depending on level and frequency). You can get a guaranteed one for
$125 or less. Or wait for a bargain and grab one for $50.


** You meant a Heath IG-18 right?

Yeah - that looks pretty good, except for the lack of a decent
frequency dial.

My generator has a 3inch aluminium disk that turns with the frequency
pot, marked across 290 degrees, turned by a 6:1 reduction drive using
ball bearings.

I also have a ancient, valve Wien Bridge generator that uses a large,
dual tuning gang coupled to a 30:1 reduction worm drive - as you
might imagine, frequency adjustment is very silky and resolution is
genuinely infinite.

Lotsa bench generators use dual tuning gangs for frequency, cos they
give ultra smooth, noise free adjustment that lasts for - unlike most
dual pots.

But watch out if you try to float the internal circuitry and just
leave the metal case safety grounded, it does not work. It is
essential that that case and shielding around the tuning gang all be
at the same potential or supply frequency leakage from the power
transformer secondary injects straight into the oscillator and causes
major amplitude modulation.

.... Phil

Yes. I misread my memory. Thanks. I have an early prototype HP200AB with
a big knob, which is great for sweeping arrays for resonances but too
fuzzy for absolute distortion testing. I am using my Sound Technology
generator because it's 10 times quieter than the Heath. I have a crappy
little Tek function generator which is great for scifi sound effects. My
iPod Touch v3 has a sine wave generator.