If someone just needed a wide range source generator, you can find -- cheap - on eBay a 339A being sold for 'parts only' and use the gen section only. This is cheaper than the K-H generators and with very little effort you will have a gen with well under .001% THD.
Here are a couple examples I put thru a QA400.
Thx-RNMarsh
Here are a couple examples I put thru a QA400.
Thx-RNMarsh
Still waiting for someone to do a quick "how-to" guide for the IC, cap and trim mods for the unit
ReallY? I thought we did that a couple of times... updates and summary.... as we went along. Plus davada's mod - gain increase.
Do you plan to hook-up to the 339A monitor output to an FFT?
Thx-RNMarsh
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I picked up 3 - 339A along the way. various levels of work on each of them --- so how far do you want to take your mods. On one, I replaced every IC with a socket and upgraded every IC; Same with all the trim pots and electrolytics caps (bipolars). etc. It was possible to tune for extreamly low distortion but needed to be retuned on evey turn of the knob to keep it that low... we said another switch wafer/trim R's would be needed to keep the tune at its best point.
The stock noise level is generally too high to get really low numbers without a gain increase to operate the rms-dc converter at its best. davada told how. If you want to take it that far.
If you wanted to do the minimum, just changing the oscillator opamp (AD797 etc) and replace its control pot with a new multi-turn and add another trim for the fixed resistor that is feedback on the control jFET (d-g) with a multi-turn (for 2H only). But, a must do is replace the output level pot. That pot introduces a lot of harmonic distortion.
Its easier to adjust the HD while looking at the monitor output with an FFT (QA400 or other).
THx-RNMarsh
The stock noise level is generally too high to get really low numbers without a gain increase to operate the rms-dc converter at its best. davada told how. If you want to take it that far.
If you wanted to do the minimum, just changing the oscillator opamp (AD797 etc) and replace its control pot with a new multi-turn and add another trim for the fixed resistor that is feedback on the control jFET (d-g) with a multi-turn (for 2H only). But, a must do is replace the output level pot. That pot introduces a lot of harmonic distortion.
Its easier to adjust the HD while looking at the monitor output with an FFT (QA400 or other).
THx-RNMarsh
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Thanks! I am going for the "level 1" mod per your post, which should be enough for my needs - geetar amps
Davada found this pot to be a source of distortion... I did also. The oscillator's voltage level output pot I found at Mouser [$7 ] It is a Bourns p/n 51SADU25A15L .
Other brands may be equal for less money. But this is the one I used and you can see the physical size needed from the Bourns p/n.
-RNM
Other brands may be equal for less money. But this is the one I used and you can see the physical size needed from the Bourns p/n.
-RNM
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Much of the improvements to the 339A come from modding or replacing old parts in the oscillator. Adding a trim to the Jfet drive allows for tweaking the feedback operating point of the fet. The theoretical 50% FB is not optimal for all frequencies. Improvement can be had by replacing the osc op amp. But not so much from replacing the buffer amp.
The Bridged T oscillator operates at a very high level and this places a limit on it's performance. It also lacks the filtering found in a State Variable. At 20 Vpp the SVO is no match for a Bridged T. But at 2.5Vpp the SVO leaves a bridged T in the dust but requires amplification to get the level up and some of the performance is lost by doing so.
The 339A analyzer is somewhat fixed in performance by design. What it takes to get just a bit more out of it somehow doesn't seem worth it. But Rick found some simple passive component mods which helped considerably. I spent a considerable amount of money on op amp replacement and ended up with the analyzer performing at spec from a monitor point of view. Some distortion improvements can be had but I don't think it enough to justify the cost and this is why I stopped with the mods. The parts can perform better in new design.
The Bridged T oscillator operates at a very high level and this places a limit on it's performance. It also lacks the filtering found in a State Variable. At 20 Vpp the SVO is no match for a Bridged T. But at 2.5Vpp the SVO leaves a bridged T in the dust but requires amplification to get the level up and some of the performance is lost by doing so.
The 339A analyzer is somewhat fixed in performance by design. What it takes to get just a bit more out of it somehow doesn't seem worth it. But Rick found some simple passive component mods which helped considerably. I spent a considerable amount of money on op amp replacement and ended up with the analyzer performing at spec from a monitor point of view. Some distortion improvements can be had but I don't think it enough to justify the cost and this is why I stopped with the mods. The parts can perform better in new design.
Is this correct, You changed the osc opamp to an AD797? I thought you used a LT1468.
--- re 1468 opamp: Just be sure to remove or bend up - pin 8 - so it isn't in circuit.
Here is example of what you can get from your 339A generator/source in THD -- This 339A has the AD797 in place -->
- I have tuned the 2H to be minimum. Only minimum mods done. Except 3H may be from that output level pot - haven't replaced it on this 339A yet - like I did the others. [on order].
Thx-RNMarsh
Here is example of what you can get from your 339A generator/source in THD -- This 339A has the AD797 in place -->
- I have tuned the 2H to be minimum. Only minimum mods done. Except 3H may be from that output level pot - haven't replaced it on this 339A yet - like I did the others. [on order].
Thx-RNMarsh
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This is what you can expect from a stock 339A with just the osc opamp changed (AD797, in this case) and the output C at the front panel coupling cap changed to a bipolar. And, only the jFET control RC network (gate to drain) changed to bipolar and a multi-turn trimmer... to tune for lowest 2H: The THD is -105dB (re .7v/1Khz) or approx. .0005%. But the 339A shows .001% or -100dB... because of the 339A's over-all inherent noise level limits the resolution.
In other 339 models, I removed/reduced the background noise level by replacing all the opamps with modern, low distortion, low noise devices and I could then go lower in the oscillator thd+n and the analyzer output/display. Plus changing gain at strategic place as per davada's instructions etc. But, all of it together is a Lot of work.
With minimal work and an FFT on monitor output you can see below -100dB (if you need to see that low) and have a very useful instrument for minimal cost that is also accurate.
Thx-RNMarsh
In other 339 models, I removed/reduced the background noise level by replacing all the opamps with modern, low distortion, low noise devices and I could then go lower in the oscillator thd+n and the analyzer output/display. Plus changing gain at strategic place as per davada's instructions etc. But, all of it together is a Lot of work.
With minimal work and an FFT on monitor output you can see below -100dB (if you need to see that low) and have a very useful instrument for minimal cost that is also accurate.
Thx-RNMarsh
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THD with 1468 -
and with another 339A and using the 1468 opamp for the osc and using 49710 for output drivers plus a smattering of 1468 elsewhere.... I could get a more complete null of the 2H. I am beginning to think that 3H must be in a passive part somewhere -- maybe the osc freq control dual ganged pot ... its always there no matter what opamp or 339A unit I use and same level. Hmmmm. Boy, if I could get rid of that 3H we would be in heaven at last.
[the output level adjust pot has now been replaced]
THX-RNMarsh
and with another 339A and using the 1468 opamp for the osc and using 49710 for output drivers plus a smattering of 1468 elsewhere.... I could get a more complete null of the 2H. I am beginning to think that 3H must be in a passive part somewhere -- maybe the osc freq control dual ganged pot ... its always there no matter what opamp or 339A unit I use and same level. Hmmmm. Boy, if I could get rid of that 3H we would be in heaven at last.
[the output level adjust pot has now been replaced]
THX-RNMarsh
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