Low-distortion Audio-range Oscillator

Samuel,

thanks for the detailed comments.

We already studied (and admirered) the various compensation tricks around the 5534's and found it unlikely to make any improvement there. But will try the class-A mod on U1401 and see if it does something.

We swapped the 3 OpAmps in the SV section to 797's in a quick try, did not work out (got much worse). Of course having the comp pins floating...

Also we tried fine-balancing the gate divider and found that an exact 50% remains to be the best operating point for the FET. Further, in a brute-force fashion, we tried to increase (by 5x) the control voltage smoothing caps by switching additional precharged ones after settling which also did not change anything except a tendency to destabilize the control loop.

We basically are after reducing the higher order components where our guess is that they come mostly from injected control signal spikes/ripple. Therefore, your hint about C1502 and C1603 (and nearby resistors) is very handy (those two in fact were a mystery to us), maybe we can make out something there.

Thanks again,
Klaus
 
We swapped the 3 OpAmps in the SV section to 797's in a quick try, did not work out (got much worse).

The AD797 won't be stable in this configuration. You'd need to add a 220r resistor right in series with its negative input. An OPA1611 would probably be the easier choice.

But first of all find out whether the dominant distortion is in the oscillator itself, or the output stage (i.e. measure at U1401 output through a small series resistor).

Samuel
 
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Hi Demian,

I'm interested in looking at everything. If it's not too much trouble can you post it?

I went back and looked at the older solutions. They are complex with lots of parts but discretes are cheap. I am attaching the oscillator circuit from an Optimation AC125 ac calibrator. The lower right hand circuit is the AVC. It can meet a .002% AC voltage accuracy which is not necessary for this application (dispense with the ovenized zener).

The Fluke 510A is a really interesting solution as well. it captures the peak voltage without a quad signal for triggering. The top half of the schematic is the agc circuit.

Oops, the PDF's are too large to post. PM me for them. The 510a is available on the web but the schematic in it has been sliced into pieces and really hard to follow.
 
But you get a lot of hardware for relatively little money and save building time and effort for the new circuit.

Why are you trying to talk me into doing something I don't want to do? All of that old hardware is not very good, and I would not do it that way anyway. If you want to do that, fine, go for it. I've done similar things in the past with hifi and it's far more work and then you have to deal with a lot of limitations. No thanks. I'd rather start from scratch.
 
Of course--I'd not comment at this detail if I didn't had the background.



Definitely. Building low distortion oscillators is a steep learning curve!

The schematic is only half the rent--bad layout can completely ruin performance.



The former pre-trims the Q of the oscillator such that J1 is working with just as much voltage across it as necessary. The latter adjusts the symmetry of the rectifier to minimize ripple.



An ES9018 will not have -130 dB dynamic range at full scale. This figure is measured at -60 dBFS and degrades above ~35 dBFS. Also it would surely not give 1 ppm distortion at 100 kHz!



A reasonable and realistic design goal for a very good DIY oscillator is perhaps:

* Frequency range 10 Hz to 100 kHz, in 4 decades
* Amplitude range -60 dBu to +20 dBu
* Harmonics below -125 dB for 20 Hz-20 kHz
* Harmonics below -110 dB for 10 Hz-40 kHz
* Harmonics below -100 dB for 10 Hz-100 kHz
* THD+N (20 kHz BW) below 110 dB for 20 Hz-20 kHz
* THD+N (80 kHz BW) below 100 dB for 10 Hz-40 kHz
* THD+N (500 kHz BW) below 90 dB for 10 Hz-100 kHz
* Settling below 1 s for 100 Hz-100 kHz
* Settling below 5 s for 10 Hz-100 kHz
* Amplitude flatness below 0.1 dB for 10 Hz-20 kHz (ref. 1 kHz)
* Amplitude flatness below 0.5 dB for 10 Hz-100 kHz (ref. 1 kHz)

Samuel

I believe you can get those specs from the Cordell oscillator.
The article comes with a nice PC layout too.
Building that oscillator and tweaking it is a good starting point.
 
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Why are you trying to talk me into doing something I don't want to do? All of that old hardware is not very good, and I would not do it that way anyway. If you want to do that, fine, go for it. I've done similar things in the past with hifi and it's far more work and then you have to deal with a lot of limitations. No thanks. I'd rather start from scratch.

If you are making a continuous tuned oscillator the frequency determining components are a real PITA. There are a lot and they need to be of a certain grade and close tolerance (don't forget tempcos and matching the caps to the resistors to improve the system tempco. . .). Also an attenuator that is accurate and frequency constant is not simple.

Replacing the oscillator circuit lends it self to upgrades easily. Separating the AGC from the oscillator will help keep any junk in the AGC from getting into the oscillator (with sample and hold circuits this is an issue).

For a few fixed frequencies building from scratch makes some sense, but Victors oscillator's are so good and cheap just putting them in a box with switching makes a lot of sense.
 
@dirkwright -- please keep us in the loop and provide construction details when you achieve a low-cost, 10Hz to 100kHz tunable oscillator that has 20V p-p output with 0.00006% THD at 1kHz.

However, a really great alternative is the easily built Cordell SV oscillator which offers even lower distortion and higher output. I also buit that -- with a few mods -- into an old Heath IG-18.

I'm not being snarky, here; just pointing out that you have been offered many suggestions of things that work very well indeed....
 
I believe you can get those specs from the Cordell oscillator.
The article comes with a nice PC layout too.
Building that oscillator and tweaking it is a good starting point.

Getting the rotary switches is a pain in the @ss for that design. Modifying it for PC mount relays would not be fun either. It's a shame that designs like this are not available for purchase as PCBs. It's also a shame that the highly skilled designers here refuse to take a leadership role and make their designs available for purchase as a PCB kit. Oscillators are for the most part really old technology. These kinds of things should be readily available, but they are not, as bare PCBs that anyone can purchase and make into something useful. I don't think it's fun to have to learn all of these stupid details about oscillators just to have a good one of my own.
 
@dirkwright -- please keep us in the loop and provide construction details when you achieve a low-cost, 10Hz to 100kHz tunable oscillator that has 20V p-p output with 0.00006% THD at 1kHz.

However, a really great alternative is the easily built Cordell SV oscillator which offers even lower distortion and higher output. I also buit that -- with a few mods -- into an old Heath IG-18.

I'm not being snarky, here; just pointing out that you have been offered many suggestions of things that work very well indeed....

Bob's oscillator is NOT easy to build unless you know where to get the rotary switches. I do NOT want to buy an old Heath oscillator just to gut it for Bob's oscillator. Bob's circuit is not easy to modify for pc mounted relays.

You are one of the highly skilled designers here that could easily offer a ready to go PCB and BOM that anyone could buy, but you don't. You refuse to take a leadership role here and force people like me to try to learn and understand all of the details of oscillators when I don't want to learn that.
 
I believe you can get those specs from the Cordell oscillator.

Not sure--my specs are beyond the Tektronix SG505, and the Cordell oscillator is probably not as good as the SG505. In any case the system specifications shown in his article are considerably worse. We don't know though what comes from the oscillator and what from the analyzer--if someone is willing to borrow me his implementation I'll put it against the AP SYS-2722.

Samuel
 
If you are making a continuous tuned oscillator the frequency determining components are a real PITA. There are a lot and they need to be of a certain grade and close tolerance (don't forget tempcos and matching the caps to the resistors to improve the system tempco. . .). Also an attenuator that is accurate and frequency constant is not simple.

Replacing the oscillator circuit lends it self to upgrades easily. Separating the AGC from the oscillator will help keep any junk in the AGC from getting into the oscillator (with sample and hold circuits this is an issue).

For a few fixed frequencies building from scratch makes some sense, but Victors oscillator's are so good and cheap just putting them in a box with switching makes a lot of sense.

I don't think you understand. I don't want to design my own oscillator. I don't want to do research into all of the different kinds of AGC circuits, and the pros and cons of each one. I just want a nice oscillator for my own use that is better than the Kenwood that I have now. Victor's oscillators are a pain in the @ss to deal with. I can't put them in a box unless I get a custom faceplate made. The board doesn't fit into a standard kind of box, I just about ruined one of them trying to remove some components, and he requires a stupid 35 volts to operate it. I wasted my money buying his oscillators in spite of the fact that they are very good performers.
 
Oscillator discussion

Oh boy,

FYI, people are not going to spend time to design something if there is no market = profit in it. And even if they do design/build it, for themselves, what is in it for them to give it away for free?

Complaining about having multiple switches/relays, is just part of the facts or necessary evills in implementing one of these designs!!

Dick M. & Bob C. are people that I admire, they actually offer much insight into an implementation and give away things for free, hats off to them!!

Dirk, I find it quite comical that you are the one spear-heading the continuation of this thread and you were not the originator.
You have to be reasonable, in that this forum is limited in what it can offer you. I think that there has been enough information offered for you to fab something for your own use. If you want to make it available for others to use, this is your option. Once you get done with your oscillator design, layout, parts, build, test, package, we shall see if you are willing to give it all away for free/charity or if you can find a market for it and make a profit from all your hard work. Welcome to the real electronics world!!
I am not trying to be critical and don't take my advise the wrong way, but is is my humble opinion of where this thread has lead too.

Cheers
Rick
 
Hello,
I have just failed to upload my KiCad zipped project file of schematic and PCB because it is too large at 593 kBytes. Even my imperfect design took a fair few hours to do.
An amatuer just doesn't have the design and development time to match the professional units that cost many thousands.
There are a number of designs on this thread to build or give inspiration.
 

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Hu PChi,

I can read the Adobe schematic files, for the rest I need a version of the software, it seems.Little hard for me to read the schematic, unless I print them..
You are so correct, to do a proper electronic design.you need lots of time and some times lots of $'s.
I find building the libraries takes a lot of time. People take it for granted, until it is you who has to do the work. I never trust the libraries that come with all the CAD pkg's.
Anyways, good luck with your oscillator project and make sure to post your results for us!!
Yes, I have learnt much from this thread. Good stuff everyone, wish I had more to add, sorry about that.

Regards
Rick
 
Oh boy,

FYI, people are not going to spend time to design something if there is no market = profit in it. And even if they do design/build it, for themselves, what is in it for them to give it away for free?

Complaining about having multiple switches/relays, is just part of the facts or necessary evills in implementing one of these designs!!

Dick M. & Bob C. are people that I admire, they actually offer much insight into an implementation and give away things for free, hats off to them!!

Dirk, I find it quite comical that you are the one spear-heading the continuation of this thread and you were not the originator.
You have to be reasonable, in that this forum is limited in what it can offer you. I think that there has been enough information offered for you to fab something for your own use. If you want to make it available for others to use, this is your option. Once you get done with your oscillator design, layout, parts, build, test, package, we shall see if you are willing to give it all away for free/charity or if you can find a market for it and make a profit from all your hard work. Welcome to the real electronics world!!
I am not trying to be critical and don't take my advise the wrong way, but is is my humble opinion of where this thread has lead too.

Cheers
Rick

There have been plenty of group buys on good designs on this forum, why not for an oscillator? That's all I'm saying, in addition to the fact that I'm not qualified to design an outstanding oscillator.

I'm not asking anyone to give away something for free. I expect to pay for the circuit boards, and a modest profit is fine.

If Pete Millett can design a sound card interface, have a thread here on it, and sell it at very modest cost as a service to the community, then my hat goes off to him. My complaint is that there are highly qualified people on this thread that could easily do the same thing with an oscillator, but apparently don't want to for some reason. They don't seem to care that much about this community. Everyone here seems to be in this for themselves in one way or another. There's no thought about the group, but that's typically American anyway so I guess I shouldn't be surprised.

So no, you are wrong. People do design stuff for the benefit of a community. It's a shame that this isn't happening here.
 
Well, this is the "Do-It-Yourself-Audio" forum.....

Honestly Dirk, I chose to do the rebuilds I did in order to keep things as simple and straightforward as possible. I never intended to provide a more elegant or more modern design, but rather to simply help folks get to an outstanding result with the fewest possible mechanical roadblocks.

Sorry that didn't help you, but I suspect a few others here have benefitted from all the discussion, which is really what this board is about. Many thanks to all who have contributed; I especially want to thank Sam Groner for his excellent comments on the SG-505 -- really helped me.
 
Group buying is great to a point, once it is over & it is not available anymore, for instance the "Wire Amp" now what? I am designing my own, as a result.Will I offer it for sale, release the design for the good of man kind, we shall see!!
Probably a time issue, why there is is nothing available for you or everyone is busy doing something else or are happy with what they got. Just the way it goes.

Regards
Rick
 
Not sure--my specs are beyond the Tektronix SG505, and the Cordell oscillator is probably not as good as the SG505. In any case the system specifications shown in his article are considerably worse. We don't know though what comes from the oscillator and what from the analyzer--if someone is willing to borrow me his implementation I'll put it against the AP SYS-2722.

Samuel

It's a looonnngggg way to where you are from the Americas. The shipping would be a killer.