Thanks for putting it together... back into perspective.... appeared as just a bunch of ideas, at first. And, I start loosing focus. Hopefully, this will lead to a plan of action sometime along the way... or I loose interest. What further can be done with the 339A and what is, if anything, going to be done along these lines you outlined as discussed here?
Is there another existing product besides the 339A which has even greater potential for making a super source/analyzer. One which has built-in some of the techniques discussed here.
A QA400 will be here tomorrow for me to play with
Changing the worn output level pot on the 339a reduced the 10KHz H3 a lot... leaving only H2. BUT at lower frequencies (1KHz) the H3 came back up to near H2 levels. Will try different pot material, later.
How is the CES? I decided not to go this year... first in 20 years or so. Thx-RNMarsh
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Hi Rick,
I working on my oscillator right now and I'm still waiting on parts for my Twin T(s).
I'm giving the 339A a rest for the time being but I will return to this later.
Cheers,
For all the discussion on the HP339 over the last month I am confused as to what mods have been done and which ones resulted in beneficial improvements. Before everyone abandons this part of the topic, can someone please outline these mods up to this point?
I have been looking for a deal on an HP339. When I finally land one I am sure I would like to make these improvements. Even though the theory behind some of the mods is over my head, I feel that I am capable of implementing any of the improvement modifications if they are clearly defined.
I do want to thank everyone for this discussion. I did pick up some knowledge and enjoyed following it.
Thanks!
I have been looking for a deal on an HP339. When I finally land one I am sure I would like to make these improvements. Even though the theory behind some of the mods is over my head, I feel that I am capable of implementing any of the improvement modifications if they are clearly defined.
I do want to thank everyone for this discussion. I did pick up some knowledge and enjoyed following it.
Thanks!
Hi Rick,
It's not necessarily going to be a long pause. I reached by measurement ability and so can't go further until I expand on that. Twin T etc. Secondly I have reevaluate some of the changes I did to the balanced modulator board. It seem I've lost the tracing ability when adjusting the frequency vernier. I might have tightened thing up a bit too much.
I also need to isolate each stage and evaluate them separately. There are some things happening that don't add up. Unfortunately all of the amplifiers operate with full common mode swing, non inverting, and so distortions rise/fall with signal levels.
There is nothing stopping you from continuing.
Hi Dennis,
What mods you do really depend on how you want to use your 339a. None of the modes I did really benefited the meter reading at all. Rick did find some things that helped a bit.
If you want to improve what you see on the monitor output with FFT then some of the mods will do this. If you want to tap the notch filter then those modes will serve you.
What I did with the balance modulator was to lower the noise and harmonics at it's outputs which helps with the FFT but it won't make the 339a analyzer work any better.
Improving the notch depth was redundant because it neither improved the 339a analyzer and is not needed for the FFT/notch method. We only need about a -60dB notch for that.
I would suggest that the mods be carried out one at a time and evaluated before moving on. there is nothing that can't be undone again if it doesn't work out.
Cheers,
David.
Do you mean 'The harmonics it causes don't necessarily have a phase relation to the fundamental.' Is this correct?.
Is this true in general that harmonics don't necessarily have a phase relation to the fundamental or is this particular to filters?
strange buffer things -
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
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.
System testing show interfacing issues -
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
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
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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.
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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
@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?
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.
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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.
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
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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
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
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