No. But I might be able to be convinced to do one once my own creation is complete if it pleases Bob. Mine is very DIY. Not cheap to build.
C47 was factory selected and is a different value depending on the vintage of 339a.
You can play with the C47 value and reduce the 10kHz and 20KHz distortion but it's at the expense of higher 1kHz distortion. There is a trade off.
@Richard -- RE PCB for Bob's SVO
If I can find them, I think I still have the layouts for the one on my BIG-18 page. I had them made by ExpressPCB, using their custom software. I'll try and look for them in the next couple of days -- I haven't run the Windows side of my PC in quite a while; I mainly use the Macintosh side. Nag me if you don't hear from me by next week.
If I can find them, I think I still have the layouts for the one on my BIG-18 page. I had them made by ExpressPCB, using their custom software. I'll try and look for them in the next couple of days -- I haven't run the Windows side of my PC in quite a while; I mainly use the Macintosh side. Nag me if you don't hear from me by next week.
A couple years ago I started to replicate the PWB's from Bob's THD Analyzer article, using a modern layout program, and creating a set of Gerbers acceptable to some quick-turn PWB fabricators.
(I wanted to maintain the same outline dimensions, mounting hole locations, and approximate trace routing as shown in the "Audio" articles. Also intended to add copper pours and adjust trace widths where they seemed appropriate, and modified a few footprints to accommodate easily obtained components sted some discontinued models.)
I got about 95% finished with the oscillator PWB when life intervened. If there is genuine interest I could probably be encouraged to finish off at least that first (oscillator) PWB . (You can see my work in the thread "Discrete Opamp Open Design" at http://www.diyaudio.com/forums/anal...screte-opamp-open-design-291.html#post3485352 . Search within the thread for posts by me with attachments).
Dale
Actually got it for free. as it was being disposed of where I work. Everything functions, it probably just needs some TLC, calibration, and maybe some tweaking
.Cheers,
Bob
Hi Richard,
No one that I know of is making boards. We actually sold over 200 sets back in the early 1980's, although I don't know how many were actually built. The switching for the frequency ranges was a real bear for construction, and switches like that would be barely obtainable nowadays, not to mention the cost. I have been tempted on more than one occasion to do an updated design of it, using a different approach to tuning, probably using some on-board relays for at least part of the switching. But it would be a big job.
Cheers,
Bob
Maybe someone here would help you/work with you to get an updated design from you. Then the Expresspcb or whatever can also get posted/published. That will be the best for all DIY for the next 30 years.
Getting the PCB layouts from several people here is a great help to the public of DIY. I know it is also shown in various web sites. But, this is a popular place for DIY'ers and so getting it here helps spread the info so it is easily found.
Good designs need republishing every now and then for each upcoming generation to have current access to the design.
Thx-RNMarsh
Getting the PCB layouts from several people here is a great help to the public of DIY. I know it is also shown in various web sites. But, this is a popular place for DIY'ers and so getting it here helps spread the info so it is easily found.
Good designs need republishing every now and then for each upcoming generation to have current access to the design.
Thx-RNMarsh
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C47
Around midnight, I added a variable cap across C47 and looked at the THD/harmonics. The THD increased. I removed the fixed C47 and found that the THD went down a lot at 20KHz.
With the changes in opamps for the osc and control circuit, it was being over compensated/corrected with the original value.
More details later. need sleep... .
Thx-RNMarsh
Around midnight, I added a variable cap across C47 and looked at the THD/harmonics. The THD increased. I removed the fixed C47 and found that the THD went down a lot at 20KHz.
With the changes in opamps for the osc and control circuit, it was being over compensated/corrected with the original value.
More details later. need sleep... .
Thx-RNMarsh
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Not sure if it is optiumum C47 value but I removed C47 and THD dropped 6dB at 20KHz. I use LT1468 and TL074 for osc/control opamps. THD at 20KHz (2 X 10K setting) is .00045%. [no readjustment of R50-51, R30]. THD at 2KHz (2 X 1K setting) is .0001%
Not bad for a few parts changes/upgrades and three opamp changes (including output buffer/driver). For a cheap oscillator sold as a parts unit/not working with low distortion, variable freq and output, it was fast and relatively painless to get <-100dB as a generator/source.
THx-RNMarsh
Not bad for a few parts changes/upgrades and three opamp changes (including output buffer/driver). For a cheap oscillator sold as a parts unit/not working with low distortion, variable freq and output, it was fast and relatively painless to get <-100dB as a generator/source.
THx-RNMarsh
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I have a lower noise jfet quad op amp in mine. It didn't make much difference over the stock quad 741. Most of the distortion is from the ripple component at the output of proportional / integrator summing amplifier.
Interesting that Rick found lower disto from removing C47. I got the opposite effect using a 620pF cap which lowered high frequency disto. It seems to vary with the vintage of the unit.
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Hi Rick,
Something I haven't done is to monitor the output of the AGC summing amplifier or input to the jfet while making these changes. Perhaps you can investigate this. If you use a jfet op amp to buffer the probe or use an active probe you might get some insight from this.
It is useful to understand what the role is for C45 and why it is there. I was just supposing something like C45 in 'the appropriate place' would have such an affect on distortion. It appears HP figured that out also. The exact value might be found via another trimmer -- for C45. However, I am not interested in 100KHz THD and want to make the compromises for the lowest THD appear below 10-20KHz. So, I am leaving C45 out on this vintage model and just reporting that it can be another place to get some more improvment.
I would have to agree with you that its exact value is a variable (;-))
This all falls into the area I have been thinking about -- injecting cancelling freqs and/or notch out harmonics to get the over-all THD to dissappear into the noise. Of course, that isnt going to happen with the 339 but its a useful test of concept package. It has been done before - injecting 2H to cancell -180 degree 2H. I am really leaning towards rejection/notch more and more as it doesnt require everything in the circuit (R, C, pots, opamps) to be ever more perfect to get THD down.
Thx-RNMarsh
I would have to agree with you that its exact value is a variable (;-))
This all falls into the area I have been thinking about -- injecting cancelling freqs and/or notch out harmonics to get the over-all THD to dissappear into the noise. Of course, that isnt going to happen with the 339 but its a useful test of concept package. It has been done before - injecting 2H to cancell -180 degree 2H. I am really leaning towards rejection/notch more and more as it doesnt require everything in the circuit (R, C, pots, opamps) to be ever more perfect to get THD down.
Thx-RNMarsh
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Hi Rick,
If you want to try some cancellation techniques then try canceling the ripple at the output of the AGC summing amplifier. Then you've taken care of what's causing a large part of the distortion in the first place. It would also probably be easier to do here. A linear multiplier would be the next step in conquering the remaining distortion.
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If reducing the ripple would reduce the distortion a larger shunt cap to ground will show reduced distortion directly.
It would seem that a waveform that is similar to the distortion from the JFET would be ideal. The cap (C45) would not reduce the ripple but applied properly reduces the distortion in some cases. The ripple would be a second harmonic from the rectification if the components are tuned correctly. The whole thing would require a lot of fiddling to get right.
It would seem that a waveform that is similar to the distortion from the JFET would be ideal. The cap (C45) would not reduce the ripple but applied properly reduces the distortion in some cases. The ripple would be a second harmonic from the rectification if the components are tuned correctly. The whole thing would require a lot of fiddling to get right.