19-01-2015, 11:05 AM
Nice box, David - I'll have to up my game again and start doing finger joints instead of dovetails 
Mahogany too! I'm only using lowly oak!
Regarding the circuit - the use of a MES lamp to stabilise the gain of the loop is a good (though standard) trick - the use of this form of AGC goes back a long (long!) way.
Although not stated in the article David cites, the circuit is a form of a Wein Bridge originally described by William R. Hewlett in his 1939 masters thesis - and, yes, its that "Hewlett" (of him and Packard fame). HP started their life making good lab audio oscillators based on this circuit (the HP 200A - also see http://www.hp.com/hpinfo/abouthp/histnfa...index.html).
I would refer everyone to Jim William's exemplar series of Linear Technology application notes, specifically AN43 (1990), Page 29, where the exact circuit David is using is described in fig. 39 - see http://cds.linear.com/docs/en/applicatio.../an43f.pdf - Jim then develops the circuit into an oscillator that by fig. 48 has 0.0003% THD (3 ppm).
I built this circuit (the fig. 48 version, which is widely regarded as an absolute classic of analogue design) many years ago for distortion testing of high-end amps I was then designing and I had some loop stability problems with it. I emailed Jim on the off chance that someone in his exalted position would take pity on simple me, and to his eternal credit he was very helpful and we had an incredibly useful email exchange.
He did admit that the circuit, whilst having extremely low THD, does suffer from loop stability problems, so don't expect too much !
Anything by Jim Williams is worth a read
Here's a photo of that ultra-low THD variant under test on my bench as it was 10 years or so ago...
Mahogany too! I'm only using lowly oak!Regarding the circuit - the use of a MES lamp to stabilise the gain of the loop is a good (though standard) trick - the use of this form of AGC goes back a long (long!) way.
Although not stated in the article David cites, the circuit is a form of a Wein Bridge originally described by William R. Hewlett in his 1939 masters thesis - and, yes, its that "Hewlett" (of him and Packard fame). HP started their life making good lab audio oscillators based on this circuit (the HP 200A - also see http://www.hp.com/hpinfo/abouthp/histnfa...index.html).
I would refer everyone to Jim William's exemplar series of Linear Technology application notes, specifically AN43 (1990), Page 29, where the exact circuit David is using is described in fig. 39 - see http://cds.linear.com/docs/en/applicatio.../an43f.pdf - Jim then develops the circuit into an oscillator that by fig. 48 has 0.0003% THD (3 ppm).
I built this circuit (the fig. 48 version, which is widely regarded as an absolute classic of analogue design) many years ago for distortion testing of high-end amps I was then designing and I had some loop stability problems with it. I emailed Jim on the off chance that someone in his exalted position would take pity on simple me, and to his eternal credit he was very helpful and we had an incredibly useful email exchange.
He did admit that the circuit, whilst having extremely low THD, does suffer from loop stability problems, so don't expect too much !
Anything by Jim Williams is worth a read
Here's a photo of that ultra-low THD variant under test on my bench as it was 10 years or so ago...
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