Low-distortion Audio-range Oscillator

[davada]

Originally Posted by RNMarsh

One thing I have tried today was perplexing --- Changing the oscillator opamp (A1U1) to the lower noise and lower thd part was an improvement -- but the following opamp used as a buffer [A1U3) is a puzzel. No other alternative opamp I tried gave lower thd than the original ! Retuning after each opamp change never reached the null in thd that the original does. So, I havent changed that opamp.
But wonder why that one works so well... is the input Z of the other opamps a weird load/phase on the osc? If so, it has implications beyond this oscillator source.

Thx-RNmarsh

The 339a oscillator is fickle thing. Changes you would expect to have any effect do. Like what happened when you changed the level pot. There seem to be a sensitivity with impedance at the output buffer's input. The buffer op amp's loading on the oscillator would have to approach the load the pot put on it, which is 10k, to see any effect.
It's one of those unexplained mysteries.

I got a better disto using the LME49600.

Try tweaking the amplitude trim as well as the disto null trim.

[RNMarsh]

Originally Posted by davada

The 339a oscillator is fickle thing. Changes you would expect to have any effect do. Like what happened when you changed the level pot. There seem to be a sensitivity with impedance at the output buffer's input. The buffer op amp's loading on the oscillator would have to approach the load the pot put on it, which is 10k, to see any effect.
It's one of those unexplained mysteries.

I got a better disto using the LME49600.

Try tweaking the amplitude trim as well as the disto null trim.

The level pot change is something I would expect an effect change as it is known that they do so,,, usually traced to the wiper contacts and then the material used. Like I had said, I did the all the trimming to each buffer opamp. The original opamp buffer gives the best results. The ones I tried are no slouches, either ---> LME49710, LT1468, LT1115 all created more distortion. I'll plug in a LME49600. But there is something to learn here --- typical bench testing conditions and isolated tests may not be showing interfacing issues in a system which could have higher distortion than the sum of the individually tested parts. This is a good example of why I have been advocating on JC's forum some months ago -before this buffer change -to test the entire system as a whole for distortion. Thx-RNMarsh

[davada]

Originally Posted by RNMarsh

The level pot change is something I would expect an effect change as it is known that they do so,,, usually traced to the wiper contacts and then the material used. Like I had said, I did the all the trimming to each buffer opamp. The original opamp buffer gives the best results. The ones I tried are no slouches, either ---> LME49710, LT1468, LT1115 all created more distortion. I'll plug in a LME49600. But there is something to learn here --- typical bench testing conditions and isolated tests may not be showing interfacing issues in a system which could have higher distortion than the sum of the individually tested parts. This is a good example of why I have been advocating on JC's forum some months ago -before this buffer change -to test the entire system as a whole for distortion. Thx-RNMarsh

Just out of curiosity did you drop these op amps in with all the original op amp's compensation components. If so you can't do this particularly with the LT1468.

[RNMarsh]

of course not.

[Samuel Groner]

Do you mean 'The harmonics it causes don't necessarily have a phase relation to the fundamental.' Is this correct?

No! The harmonics do have a phase relation to the fundamental. How could it be that they don't have one? If you look at the residual of the notch filter output with a scope, there's a well defined temporal alignement of the distortion to the fundamental, which is a result of their phase relation.

Samuel

[richiem]

@Richard Marsh -- it may be possible that the level cal rheostat is playing a part in this, and I didn't see any mention of changing or eliminating that part. I'm not even sure why HP used that, given that there's a vernier pot in there too. Maybe try getting rid of that trimmer, and re-test?

[davada]

Originally Posted by Samuel Groner

No! The harmonics do have a phase relation to the fundamental. How could it be that they don't have one? If you look at the residual of the notch filter output with a scope, there's a well defined temporal alignement of the distortion to the fundamental, which is a result of their phase relation.

Samuel

Just wanted to be clear about what you were saying.

There is a very sharp phase reversal at notch center.
Surely if the frequency relative to notch center is skewed to one side and then the other of the notch
center we should see a sharp phase reversal of the fundamental and harmonics.
If the notch center is has a flicker to the skew of the notch center there is one possible distortion source.
It is ugly when the phase is changing rapidly one then the other.

[davada]

Originally Posted by richiem

@Richard Marsh -- it may be possible that the level cal rheostat is playing a part in this, and I didn't see any mention of changing or eliminating that part. I'm not even sure why HP used that, given that there's a vernier pot in there too. Maybe try getting rid of that trimmer, and re-test?

Hi Dick,

If you are referring to the amplitude level trim, this trim is necessary. It set the Jfet gate
voltage for the operating level. It doesn't change the output level. That is set by the DC reference voltage at the input of the AGC. The trim pot effects the loop gain.

I did change this pot.

Rick If I remember correctly the mods I did here were based on the IC Picks Dick used in his build of the 339a oscillator. He got good results.

[RNMarsh]

Originally Posted by richiem

@Richard Marsh -- it may be possible that the level cal rheostat is playing a part in this, and I didn't see any mention of changing or eliminating that part. I'm not even sure why HP used that, given that there's a vernier pot in there too. Maybe try getting rid of that trimmer, and re-test?

Which p/n or where is the level cal?

:-) I am changing the Amplitute Adj today.... found a trimmer that is n.o.s. in my drawer to use but maybe it isnt the best. but, I'll try it first and see. The trim null, like all the others is very sensitive at the null point - so would rather have a multi-turn as a replacement. I have already changed the fixed R50, 51 with multi-turn trimmers and I use them. -Thx RNMarsh

[RNMarsh]

1. I took a relook at the LME49600... that buffer is and must be used within an opamp feedback loop. I measured it a year or more ago and the thd and it was too high for audio. It has great dc characteristics but ac is poor unless it is within a feedback loop to get low overall thd.

2. Has the EDN 11.10.94 issue for an oscillator design which has THD below 1 ppm been tried by someone we know? authored by Jeff Smith at Analog devices.

3. I only report changes that lower thd. However, with this old gear, some parts get replaced to keep it reliable. One is the 1000mfd ripple filter caps at the power supplies. [they had no affect on thd].

4. I use the LT1468 everywhere.

-RNMarsh