ultra-low distortion audio oscillator

[anatech]

Hi Brad,

DDS from the PCM1704 could be paired with analog-to-digital conversion from a PCM1804 to provide a flexible analysis tool, instead of just generating pure tones... though it's basically a novelty soundcard now

Hardly!
Now you are beginning to chase an Audio Precision type system. The generator and A/D sections are two completely different projects that could be included in the same case. Note I didn't mention "card", as in "installed in a computer".

To get the low noise and THD you want, plus input and output signal conditioning, you would need a larger box free from switching power supply noise. It would help to ventilate some of the components that would run warm. That's a side effect of chasing the low noise figures. It's no mistake that AP used an external box for all the analog signal processing. It's all done for a very good reason.

My X-fi Extreme music is a sound card. This is an instrument.

-Chris

[EC8010]

Square waves are best generated in the analogue domain. Since (for audio) we're concerned mostly with the rise time or the flatness across the top, there's a very good way of generating suitable waves. Use a mercury-wetted relay switched at mains frequency to direct a floating DC supply to the output terminals. You can now use a 10 turn wirewound pot (with dial) to adjust the DC voltage of the (regulated supply), giving you a very accurately calibrated square wave with a rise time in ps (picoseconds). Definitely good enough for audio. The fact that it can only repeat at mains frequency isn't a problem because we look at the edge for HF and the top for LF.

[jackinnj]

Scott Wurcer chimed in on this thread:

Can we improve this low distortion sine oscillator ?

there's a link to an EDN article to be found.

I built the "Eager Oscillator" (paen to Dale Eagar who wrote the Linear Tech Application Note) -- aka "Uber Oscillator" and describe it here:

http://www.tech-diy.com/TestEquipmen...oscillator.htm




For ease of application, I wouldn't recommend the uberoscillator -- instead look for Bob Cordell's version of the SG-505 on his website.

[anatech]

Hi EC8010,
I have to agree with both your points on square waves. I must admit that I'm lazy and either use my generator output, or an HP pulse generator I bought for that purpose. Your relays may in fact do a better job of creating a square wave.

The buzz let's you know it's on!

Hi Jack,
Completely forgot about the SG-505 clone, I'll have t read up on the "Eager Oscillator".

Thank you for the links!

-Chris

[mome]

Originally Posted by geekysuavo

i would have to agree with that. DDS from the PCM1704 could be paired with analog-to-digital conversion from a PCM1804 to provide a flexible analysis tool, instead of just generating pure tones... though it's basically a novelty soundcard now, lol! i'll look into it, most definitely.

~ brad.

Hi Brad,
I want you to look at this circuit.
Perhaps it is useful to you?.



Low-distortion Audio-range Oscillator using NE5532