With no signal
I realized something tonight about the operation of the 339a. When there is no signal there is nothing for the notch filter's balance modulators to lock onto. Consequently the notch filter goes to high gain. There is leakage from the oscillator and this is picked up and amplified. Once this passes through the distortion amplifier in high sensitivity settings -80dB range the signal is so strong it puts the distortion amplifier into hard clipping. So hard it can produce a square wave form at the output of the distortion amplifier.
Ill see if I can trace the leakage from the oscillator and confirm the effect by stopping the oscillator. I think this leakage maybe driving the meter reading higher.
Replacing the distortion meter op amp cured the problem with the meter pinning when the LP filters were switched out and the ground problem with the scope cleared up as well.
It was parasitic oscillation causing this. However the 339a does not like my PC ground.
It still drives the meter up.
In the 339a manual in the manual changes section at the bottom there are some changes to the filters to clear up a parasitic oscillation at 5MHz. It might be worth having a look at
this section. Depending on the vintage these changes may have been made.
Cheers,
I realized something tonight about the operation of the 339a. When there is no signal there is nothing for the notch filter's balance modulators to lock onto. Consequently the notch filter goes to high gain. There is leakage from the oscillator and this is picked up and amplified. Once this passes through the distortion amplifier in high sensitivity settings -80dB range the signal is so strong it puts the distortion amplifier into hard clipping. So hard it can produce a square wave form at the output of the distortion amplifier.
Ill see if I can trace the leakage from the oscillator and confirm the effect by stopping the oscillator. I think this leakage maybe driving the meter reading higher.
Replacing the distortion meter op amp cured the problem with the meter pinning when the LP filters were switched out and the ground problem with the scope cleared up as well.
It was parasitic oscillation causing this. However the 339a does not like my PC ground.
It still drives the meter up.
In the 339a manual in the manual changes section at the bottom there are some changes to the filters to clear up a parasitic oscillation at 5MHz. It might be worth having a look at
this section. Depending on the vintage these changes may have been made.
Cheers,
A2U10. Auto set level op amp.
I had changed this to an LT1122 two years ago and had forgotten.
Bad choice at the time, it was oscillating.
One thing to keep in mind with the LT1468 is it has a dual compensation slope. At unity and up to about gains of 35 it has a GBP of 25MHz.
There there is a transition range to a GBP of 90MHz at gains above 40 or so.
Once compensated properly with a feedback cap it's quite well behaved. It it beneficial to include an isolation resistor right at the output pin of the op amp.
This resistor isolates the output from stray capacitance preventing parasitic oscillations.
See Design Features of the LT1468 and the data sheet for feedback compensation. There is a formula.
Cheers,
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Greening of the HP339A - How to:
U10 was on my list to replace. That pretty much cleans up and clears up the old 339A for another lease on life... if it doesnt wear out... cheap-o switches and all. My 400Hz filter IN/OUT switch just quit making contact. Before I replace it, i just jumpered the filter in all the time. May never use this below 400Hz anyway.... but I do have a spare junker 339A for parts....
Parts and IC updates and a few gain shuffling here and there and we have a new THD meter. With a pretty low distortion choice of oscillator frequencies. Thx for working with me on this project. I'll finish it off in the next few days -RNMarsh
U10 was on my list to replace. That pretty much cleans up and clears up the old 339A for another lease on life... if it doesnt wear out... cheap-o switches and all. My 400Hz filter IN/OUT switch just quit making contact. Before I replace it, i just jumpered the filter in all the time. May never use this below 400Hz anyway.... but I do have a spare junker 339A for parts....
Parts and IC updates and a few gain shuffling here and there and we have a new THD meter. With a pretty low distortion choice of oscillator frequencies. Thx for working with me on this project. I'll finish it off in the next few days -RNMarsh
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Hi Rick,
I think there is a lot more that can be done here to improve on things.
Rather than posting on each new find I will just work away at it and then do a final post and summery. Perhaps you can give a summery of what you found so far and post it. What caps were changed and to what value. We need to do a performance check every now and then to be sure we're not just chipping bandwidth away or causing a ripple. That would be cheating.
Cheers,
Perhaps this is Stage One of the upgrades/mods. I have to do more opamp replacements and see how it goes from there and the results.
One can get 10-12 dB improvement by replacing passives with better electrolytic caps (NP) and trimmer pots.
Meanwhile, we sort of monopolized this forum for a stint --- now I'm (temporarily) giving it back to other's oscillators, etc. Thx-RNMarsh
One can get 10-12 dB improvement by replacing passives with better electrolytic caps (NP) and trimmer pots.
Meanwhile, we sort of monopolized this forum for a stint --- now I'm (temporarily) giving it back to other's oscillators, etc. Thx-RNMarsh
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An Ooops Update:
After I repaired the intermittant contacts of the 400Hz filter switch (in series with the sine wave), the THD went back up!
Apparently, the increased series R (switch contact) dropped the signal level during thd reading and it looked better than it is. But, it ain't bad, either.
Now the HD components without the most important IC's being replaced yet, are: .0007% H2 (-105dB) and .0001% H3 (-115dB). THD+N as read on the front panel meter: .0008% or just under the .001% mark. And, it's going to get better! Can't wait. That's, again, with all filters IN. Thx-RNMarsh
After I repaired the intermittant contacts of the 400Hz filter switch (in series with the sine wave), the THD went back up!
Apparently, the increased series R (switch contact) dropped the signal level during thd reading and it looked better than it is. But, it ain't bad, either. Now the HD components without the most important IC's being replaced yet, are: .0007% H2 (-105dB) and .0001% H3 (-115dB). THD+N as read on the front panel meter: .0008% or just under the .001% mark. And, it's going to get better! Can't wait. That's, again, with all filters IN. Thx-RNMarsh
The meter as it is isnt useful in resolving thd for DUT under -100dB.... the bottom 10% of the scale.... but even so, the FFT will get used.... just needed a good low thd flexible signal source. And, then there are the other sources.. like Victors or the one SW just build if DUT is really, really, really low. In either case, I am very curious about the prototype system davada is using.
Thx-RNMarsh
Thx-RNMarsh
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Hi Rick,
I loaded more LT1468 and high frequency noise and parasitic is out of control.
Worse than ever. Each stage just amplifies it more and it keeps getting worse as we go along.
HP compensated each stage narrowing the bandwidth on the 2625's and this is how they made it work. I will have to deal with each stage to tame it and in the end I might just end up with what we have in the first place.
I think I will just focus on the input, buffer and notch filter since this is all fairly well behaved and forget about the rest.
I think Dick has it right with his Twin T running on batteries, a precision pot for an attenuator and FFT.
Lets face it the 339a is a boat anchor. It works well as a level meter and that's about it.
I'm getting good results with my original idea of just using it for a front end.
And please call me David.
Cheers,
Maybe those opamps are not the best to use.... we dont need BW, just low noise and lowest THD. Like maybe the AD711/12/13 would be better in some places or others; more stable and less prone to oscillation.
Its clear from the pcb and wiring layout, that it cant be used for widebandwidth circuits.... long and convoluted wiring and all. <100KHz layout. -RNMarsh
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Hi Rick,
I have the input amplifier tamed for wide band use. If we are to do a THD20 at 20KHz then we need a bandwidth of at leas 400KHz. One thing I noticed is there is not much for input LP filtering. I going to work on this. The LT1468 is particularly low in distortion at audio frequencies. It just needs a little TLC and there are very stable once we get it right.
Even with lower GBP op amps there will still be RF problem to deal with. Any op amp with a GBP 10MHz or greater will go into the 100's of MHz at gains below unity so there is no escaping this. The problem is with the long signal chain.
There is no inter stage LP filtering and so the RF gets passed from stage to stage. I'm going to work on this as well.
I found my unit is not wired the same as any of the manuals I have and this might explain why my unit behaves a bit differently from others.
I by passed the buffer amp at the input which is routed totally differently from the schematic. The noise dropped about half a order magnitude on the meter and with the input shorted the meter goes darn near zero. Of course without signal the auto set level shuts down. Bypassing the the buffer amp does raise the 2nd H a bit because of the added loading on the input amp. I'm going to try a different op amp here maybe a LME49710. Low noise high drive current and low distortion. After all it's just a buffer.
Cheers,
With my SVOscillator I found the LT1468 out perform the LME, LM series op amps. Haven't tried the OPA2604. The OPA1641 and LT1468 are far better when it come to covering a large range of impedance on the noise curve. What we want is a very low current noise figure.
if it wasn't for the high impedance I would use the OPA1611.
And yes the 339a routing is terrible.
Cheers,
The 339A is beginning to see the effects of the THD of the opamps used in itself and also the parts and even contacts of the switches. All issues which an audio amp builder would also come to address at these levels of harmonics. The performance will be limited by the parts we use. I am installing sockets everywhere to try different opamps in different places.... will be much easier until the parts are settled as to which are best ones and stable. Thx-RNMarsh