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Old 29th August 2013, 01:49 PM   #21
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Originally Posted by RNMarsh View Post
Could we get any updates from Bob before making new versions of his original design. What could be improved, if anything? reed relays instead of switches? Different IC's for oscillator?

Glad it got redone so other people can make it.

Thx-RNMarsh
Folks,

I am truly humbled by your interest in my THD analyzer, after about 30 years. I still use it myself partly because I'm just comfortable with it. I did it simply because I could not afford a good commercial one back at the time. Indeed, even now, I cannot bring myself to spend the big bucks on an AP - and I envy those who have one.

I own an HP339 analyzer (I got it for free) and actually prefer to use mine.

I did some minor upgrades about a year or two ago and I'll try to recollect those. I think one of them was to replace the output average detector with an RMS detector. Another was to put trim pots on all of the JFET control elements to optimize distortion reduction in them (I found that the usual 50% gate feedback is not always exactly optimal).

This biggest challenge with the original analyzer was the rats nest of multipole switches, which would be quite expensive these days. A multiplicity of relays and their associated passive tuning elements down on the PCB would make more sense and greatly reduce assembly labor.

I'd probably tweak the design of the SV oscillator a bit in accordance with some of the things people did on the wonderful audio oscillator thread, and some of those tweaks would likely be applicable to the SV notch in the analyzer section. I could be tempted to lower some of the impedances in the SV tuning circuits to get the noise down a little more.

Of course, adding digital meters and maybe a frequency counter and frequency trim and maybe a PIC processor would also be tempting if they could be done carefully without impacting the noise performance. An external power supply in a separate box would also not be out of the question.

Balanced inputs and outputs would make a lot of sense, too.

Many of these things would not add a lot of cost to the analyzer.

It would also make sense to put it all on a single board and use dual op amps to save some space. Ultimately, a surface-mount-implemented design would make a lot of sense, but would not be as DIY-friendly.

Not everyone needs the increased performance afforded by the tracking residual filters, so they could be optional and replaced by the usual fixed filtering choices on commercial analyzers. The 80kHz filters on commercial analyzers are a joke for THD-20 measurements.

The analyzer could also be equipped with an optionally-engaged class D measurement filter, like the AES-17 filter, but maybe not as complex. I'd probably go for a 7th order Bessel to keep a linear phase characteristic rather than the brick-wall design of the AES-17. All one needs to do is keep the spurious ultrasonic signals from a class D amplifier small enough so that they do not degrade the performance of the analyzer circuits.

Of course, all of this needs to be put in the context of the PC-based analyzer arrangements based on sound cards that are now available. What they can do is truly impressive, and building the necessary interface box like Pete Millet's is not difficult. If need be, I think there is room for improvement in that design, but for most it is great.

Cheers,
Bob
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Old 29th August 2013, 02:41 PM   #22
davada is offline davada  Canada
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Hi Bob,

The PIC controller I'm using with my SVO seems to have the least impact on the oscillator.
They run a a such a high frequency any junk is way out of band. It's more challenging getting USB sound cards to work cleanly with them. That's where they become interfering. Ground isolation becomes necessary. Just for tuning and switching range they are not any more interfering than any other method.
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Last edited by davada; 29th August 2013 at 02:44 PM.
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Old 29th August 2013, 03:09 PM   #23
EUVL is offline EUVL  Europe
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So that means we can eagerly wait for your updated version before too long, Bob ?




Cheers,
Patrick
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Old 4th September 2013, 02:50 PM   #24
RNMarsh is offline RNMarsh  United States
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Congratulations, Bob, on your Retirement !!!

-Richard Marsh
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Old 4th September 2013, 03:20 PM   #25
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I still use mine and find it a superbly designed instrument, wanting for almost nothing. Though I use PC-based instruments, there's something comforting about doing a reality check with the Cordell. If the PC stuff isn't set up just right, the results can be off by a mile. With the Cordell I can look at the residual signal and know what's going on. And happy retirement Bob!
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Old 5th September 2013, 10:07 PM   #26
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Originally Posted by Conrad Hoffman View Post
I still use mine and find it a superbly designed instrument, wanting for almost nothing. Though I use PC-based instruments, there's something comforting about doing a reality check with the Cordell. If the PC stuff isn't set up just right, the results can be off by a mile. With the Cordell I can look at the residual signal and know what's going on. And happy retirement Bob!
Thanks for your very kind words, Conrad.

Tomorrow is my last day. I really look forward to getting out of the rat race and spending more time with Angela (and audio), but I will also miss many great friends from work.

But I have many great friends here :-).

Cheers,
Bob
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Old 2nd October 2013, 05:28 PM   #27
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I have a mouthwatering desire to create a version of this with an LCD display and keypad for selecting the options all controlled by a microcontroller(interfaced with optofets for resistor selecting). It would be a surface mount version with larger components (resisters and capacitors would be no smaller than 0805 packages) so it would step up the difficulty but not be impossible. This would be a large and serious project for DIY'er but I believe I could handle it since I did graduate from an electronic engineering technology program and have job experience in surface mount soldering, PCB design with Altium (couldn't be able to afford it for personal use though), and a good interest in audio and filter fundementals. For me the largest difficulty would be in the programming required for the microcontroller, LCD, and keypad interfacing. As well as preventing high frequency clock noise from the digital portions of the project entering the analog sections.

How on earth would I be able to balance this with an 8-5 job, my other hobbies, and a social life though?
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Old 2nd October 2013, 05:51 PM   #28
RNMarsh is offline RNMarsh  United States
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Thank you Giulianodes -.... this is something many people have been waiting for... a remake of Bob's circuit. Can you do the freq switching via relays (reed?) instead of rotary switches.

-Thx-RNMarsh

Last edited by RNMarsh; 2nd October 2013 at 05:55 PM.
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Old 2nd October 2013, 06:13 PM   #29
davada is offline davada  Canada
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Originally Posted by dewasiuk View Post
I have a mouthwatering desire to create a version of this with an LCD display and keypad for selecting the options all controlled by a microcontroller(interfaced with optofets for resistor selecting). It would be a surface mount version with larger components (resisters and capacitors would be no smaller than 0805 packages) so it would step up the difficulty but not be impossible. This would be a large and serious project for DIY'er but I believe I could handle it since I did graduate from an electronic engineering technology program and have job experience in surface mount soldering, PCB design with Altium (couldn't be able to afford it for personal use though), and a good interest in audio and filter fundementals. For me the largest difficulty would be in the programming required for the microcontroller, LCD, and keypad interfacing. As well as preventing high frequency clock noise from the digital portions of the project entering the analog sections.

How on earth would I be able to balance this with an 8-5 job, my other hobbies, and a social life though?

If you want to use the Microchip stuff I can help you with that.
It's probably the most supported micro controller out there and tons of stuff on the web.
Free tools and 32 bit processing. Offers a small free DPS library as well.
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Old 2nd October 2013, 06:59 PM   #30
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RNMarsh,
all switching operations are by relays. The small light green cubes that you can see in the photos, they are 9V DC micro relays.

giuliano
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