I recently acquired a beautiful Panasonic VP-7721A distortion analyser to replace my ageing Sound Technology 1700B. It has had minimal repair work done, as far as I can tell. A poorly implemented PCB fuse re-repair and a new NiCd battery pack and it is as good as new. Literally. Not only are all power supply Voltages within 1% of published specs, but the oscillator meets it's published distortion specs as well. It is a complete delight to use. Very fast and, FWIW, it looks cool too.
Anyway, I was wondering if anyone has modified one of these beasts to elicit superior performance?
I'm happy to leave it alone, but, well, you know....
Anyway, I was wondering if anyone has modified one of these beasts to elicit superior performance?
I'm happy to leave it alone, but, well, you know....
Last edited:
I have one too, and have not found any one or anything that will improve this beast yet. One thing does your have balanced output on the BNC out, I mean do you measure the output signal in +-phase on the BNC ( Shield = phase -) ?
do yourself a favor and........ leave it alone while you're ahead of the game.
Panasonic already performed the extensive research and testing procedures in their labs.
But..... if you think you're qualified and smarter than that huge corporation, be my guest.
I certainly don't consider myself smarter than the Matsushita engineers. The VP-7721A is an exceptionally nicely engineered and built product. However, OP amp technology has moved on significantly. The obvious ones (to me) are a couple of 5534 OP amps in the oscillator section. I suspect that careful replacement may elicit a reduction in oscillator distortion.
I can understand the curiosity and your statement of "technology advancement".
However, I wouldn't be too hasty to judge, particularly from a suspicion viewpoint that an upgrade would show benefits.
IMO you'd be opening up a can of worms and a rabbits hole at the same time.
In all my years of servicing, and occasional curiosity, I've learned that some stuff needs to be left alone.
I applaud DIYers' desire to "hot rod" these old instruments into new, better lives. Just proceed very patiently and methodically.
Be sure the instrument is meeting its original specs before undertaking possible improvements; avoid the temptation to think you'll solve any undiagnosed performance deficiency as you proceed. Otherwise, there's the real risk of obscuring an existing defect in a maze of changes--- finding the original culprit can become even more difficult.
Be sure to characterize before and after each mod; a change that looks favorable at 1kHz may be problematic at 100kHz. Keep good notes (a frequent personal failing).
Try to make your mods substitution-friendly and reversible (e.g. sockets, etc.) so that you can restore to a previous state if appropriate. To the degree feasible, introduce experimental changes one at a time and retain the mod only if it delivers equal or better harmonic or noise-density performance.
New generation opamps will tend to have higher bandwidth; be especially alert to high frequency oscillations, as they may not be obvious in front panel readings.
Cheers!
Be sure the instrument is meeting its original specs before undertaking possible improvements; avoid the temptation to think you'll solve any undiagnosed performance deficiency as you proceed. Otherwise, there's the real risk of obscuring an existing defect in a maze of changes--- finding the original culprit can become even more difficult.
Be sure to characterize before and after each mod; a change that looks favorable at 1kHz may be problematic at 100kHz. Keep good notes (a frequent personal failing).
Try to make your mods substitution-friendly and reversible (e.g. sockets, etc.) so that you can restore to a previous state if appropriate. To the degree feasible, introduce experimental changes one at a time and retain the mod only if it delivers equal or better harmonic or noise-density performance.
New generation opamps will tend to have higher bandwidth; be especially alert to high frequency oscillations, as they may not be obvious in front panel readings.
Cheers!
Warning - In some cases changes by adding an IC socket can introduce oscillation problems.
This is opening up a smelly can of worms, IMO a bad idea.
I've used that piece of equipment, calibrated by Panasonic in their labs to their critical standards, and never felt a need to mess with an already "perfect" design.
Hmm...
Still I would not change/update a thing in this nice device.
On the schematic it shows a Diff.OpAmp setup with a nice Relay/resist network and a Signal/Flow and GND output !? I think opening the thing will make it clear.
Still I would not change/update a thing in this nice device.
Attachments
What performance are you getting from the Panasonic? THD and THD+N at 1KHz and 20 KHz will give a good sense. If those numbers are much below 100 dB then you will be very pressed to make improvements. While the opamps are better getting that improvement through to the output will be challenging. Everything from noise level (determined by resistor values) to analog multiplier/Jfet AGC circuits will be the limitation. I am speaking from experience with several SOTA analyzers. Improving the Shibasoku was Herculean and the improvement, 3-6 dB, hardly worth the effort.
Thanks for your reply. On loop-back, I measure the oscillator distortion at around 0.0015%. Very close to published specs. Which is incredible for such an old instrument. There is a pair of 5534 OP amps driving the output transistors in each side of the balanced output stage of the oscillator. I have not dismantled the oscillator section, as it is quite difficult to access, so I don't know if the OP amps are socketted. There are a number of interesting OP amps available today with distortion figures that as more than 20dB lower. Replacing the 5534 OP amps may make a small, but significant improvement. Or not.
Just what are you reaching for?
0.0015% is sufficient for NASA Space exploration.
And surely sufficent on this planet.
You know, you'd think so. But no, 0.0015% is not sufficient. If I can gain another 10dB without much difficulty, I reckon it's worth the effort.
Congratulations!
You're now nominated for the yearly contest here on DIY known as "obsessive compulsive fanatic of the year".
Sorry, I win. Been tweaking my analyzers to -115 dB THD+N. Really pointless in the real world.
If you can, check upstream from the output amp at the oscillator. Improving the amp if the oscillator is the limit won't be too constructive. And that depends on the performance of the analyzer section. To go down this road you need a better reference. The most cost effective way is to get one of Victor's oscillators. They are better than any available analyzer.
Be careful, madness lies down this road. You have been warned.
Thanks for the tips. Like I said, getting to the oscillator section is a bit of a PITA, so I was hoping to see if anyone else had attempted changes, before I went into it. As the instrument works perfectly now, I may just leave it alone.
Victor's oscillators? Tell me more please.
Yeah, I know about madness. I've been servicing high end audio for the past 40-odd years. I've had my fair share of crazies turn up at my door. My own equipment barely beats 0.1% THD (yes, it's all solid state). That's what zero global NFB does for you. Still, I like the sound. Recently, I've found some small mods have lowered the audible distortion somewhat (under blind testing), but I have not quantified the level.
BTW: My next purchase will (hopefully) be a Shibasoku AD725C, so I will need a really good oscillator.
Victor's oscillators? Tell me more please.
Yeah, I know about madness. I've been servicing high end audio for the past 40-odd years. I've had my fair share of crazies turn up at my door. My own equipment barely beats 0.1% THD (yes, it's all solid state). That's what zero global NFB does for you. Still, I like the sound. Recently, I've found some small mods have lowered the audible distortion somewhat (under blind testing), but I have not quantified the level.
BTW: My next purchase will (hopefully) be a Shibasoku AD725C, so I will need a really good oscillator.
0.1% THD eh?
McIntosh stuff used to hover around 0.25%, yet drew the "elite crowd" because of the status of that brand.
And my lowly Technics is at 0.008% THD, yet gets "put down" by those Mac Worshippers.