| mlloyd1 |
For those of us that don't have access to an Audio Precision set, what are folks out there doing for measuring THD, IMD, SID, whatever? Anything clever for helping to set bias points or actually measuring impacts of changing ground connections, etc?
Here are some excerpts on the topic from another thread ( the Leach amp design thread, I beleive); comments welcome (hint, hint!):
From Damon:
...As for other designs and tweaks, I can only point to Doug Self
and Randy Sloan and their books on amplifier design for hints
as to what might be possible. But we're already into a range of
low distortion that my geriatric collection of Heathkit test
equipment can't possibly measure, and I can't afford an Audio
Precision test set.
Unless I can come up with a "cheap" Audio Precision equivalent,
I'd be shooting completely in the dark with new designs. And
that's daunting....
From me (mlloyd1):
..... I wrestle with the same problem as far as distortion measurements. I usually look at distortion waveforms on my scope. I think we need a group discussion about how to work around this issue for DIYers. There was a REALLY nice project in Audio magazine (RIP) some years ago for a very serious analyzer that could be reworked with current tech (using OPA604 op amps and current buffers instead of TL071, for example) and probably have much more performance than we would need. I've also seen some writings in past issues of The Audio Amateur (now Audio Electronics) by Erno Borbely ( I think) for a distortion analyzer. There was a project in Radio Electronics some years back also. I actually made a PCB and partly built this one. In my opinion, it worked very well - it gave consistent measurements with a Audio Precision I had access to at the time. However, it was VERY tough to control the electronical noise in my DIY environment though - halogen lights, misc dimmers, hair dryers, etc. (it's tough being a married DIYer!) don't make for a clean test environment. Finally, reading through the service manual for some of the HP distortion analyzers (I think the guy that did the Audio magazine project referred to doding this also) suggest a few ideas as well. Alas, I can't seem to find my copies of this anymore :-( I can't recommend enough times that service manuals from GOOD test equipment makers like HP and Tek are EXCELLENT sources of material for study!
I'd be happy with a distortion test box that could spot check with high resolution at about 4 frequencies: 50Hz, 1KHz, 20KHz, 75KHz.
Who's game?
Nelson, what do you do when you have something to test (say an idea at home late at night) and the Audio Precision is nowhere around? Wait until later? :-)
Maybe we could even ask an analog Guru like Jim Williams at Linear Technology to design a simple, high peformance THD analyzer circuit (I single him out because I remember and oscillator circuit he designed that was claimed to have THD specs in the single digit parts per million. This is incredible!). They might already have such a design sitting around somewhere; I haven't checked their web site and app notes lately. Hmmmmm ....
from Grey:
.... Give the distortion analyzer project its own thread so people will be able to find it more easily.
I'm interested.
I'd like to reiterate--for those who haven't understood what I've said on these matters--that I'm not against low distortion, per se, just the use of massive quantities of negative feedback to achieve the distortion figures. Just don't pursue low distortion as an end in itself, as you'll usually find that once you reach a certain point sound quality suffers. But up to that point it can be a useful tool.... |
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| Joe Berry |
Michael,
To evaluate distortion, I currently use a PC-based test instrument from Pico Technologies (www.pico-tech.co.uk). Their ADC-216 has only a 166 kHz bandwidth, but its 16-bit dynamic range and Spectrum Analyzer function makes it useful for looking at distortion products. The software will also calculate THD, S/N, and other standard specs from captured waveforms. At around US $700, it doesn't look cheap until you compare with something like the AP System One.
My signal source is a simple version of the improved Wien/HP oscillator described by Jim Williams in his book "Analog Circuit Design: Art, Science, and Personalities." I plan to build a better version when I have time, but even this little single-IC unit is quite useful as it gives .003% distortion at 1kHz. This is better performance than you'll find in many commercial designs, and costs very little to make. |
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| Damon Hill |
I know some find the objective approach to measure in audio
design anathema, but I've always been design-oriented and
I have learned that my hearing isn't completely trustworthy;
I need test equipment for a reality check. But I like to think that
listening is complementary to objective analysis.
My test setup is mostly geriatric solid state Heathkit; I can measure down to around .03% THD on a good day. The THD meter has an autonull feature but it is tempermental. It is most
useful in conjuction with a scope display to visually judge the
relative amounts of noise, ripple and distortion products; I always
set amplifier bias so the crossover notch just disappears.
The IM meter has excessively high residuals and needs troubleshooting; it lacks an oscilloscope output. I found a used
Crown IMA a few years ago in a guitar shop for $30, but alas,
both of the meters are shot and I've been quoted $200 for their repair or replacement. And I still think the unit has other problems; I suspect a voltage surge and aging took a considerable toll on the poor thing. But it has some useful features like the oscilloscope output and a tracking level control so a range of power levels can be quickly measured.
Going beyond this level of performance demands not just low residuals but spectrum analysis of the distortion products; an
Audio Precision system does all of this in software with a handy
graphical display and helps to make documentation a LOT easier.
But the price of a new system is a killer, and I can't even bid on
the occasional units that appear on Ebay: I'm poor.
I know there's a company that local to me in Seattle (or thereabouts) which may have an offering similar to the AP, using
commonly available computer sound cards such as the Sound Blaster Live! (the newer Audigy should be even better). I'm not
clear on cost or how capable their system is compared to AP.
I'm convinced that the AP is the route to take for measurement
and documentation to the point that even used Sound Technology
and the HP339 don't interest me as much as they might have ten
years ago.
I could, however, continue to work with a conventional THD meter
if the residuals were an order of magnitude lower than my present setup.
I'd like to do more IM measurements, perhaps with the CCIF standard of two closely spaces HF tones, possibly swept. |
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| GRollins |
Damon's point about spectral analysis is a good one, and in my opinion could stand to be put in italics & bold print.
My take on this is that the harmonic structure of the distortion is <i>far</i> more important than the total percentage. It's widely acknowledged (even by specs-are-everything folks) that lower order harmonics are much less obnoxious than higher order. To the extent that I must, in the real world, have distortion, let it be 2nd and 3rd order rather than higher order products.
I'm also convinced that a lot of the problem is transient in nature, hence difficult to spot with the averaging techniques commonly used. Crossover distortion is a good example of this, as it's there...then it's gone. But while it's there, it's really, really ugly. Yes, it's easy enough to bias for class A and avoid the problem entirely, but then you get into heat dissipation and overall decreases in efficiency. I choose class A and AB in order to sidestep the problem, but that doesn't mean that I'm always overjoyed about the tradeoffs.
Michael, thanks for breaking this out as a separate thread. Rest assured that I'll be watching developments closely.
Grey |
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| mrothacher |
I'm currently using a Velleman PCS64i, which is a PC-based DSO. It's available for around 350.00 US$ from www.qkits.com. I've found it to be a pretty handy instrument. Velleman also has less expensive models (as low as $129) available as kits. The software includes a spectrum analyzer, which can be used for distortion measurements albeit with some post-processing. I've been thinking about writing a post processor to do THD calculations, Distortion vs. Frequency, Distortion vs. Power, etc. But, I'm not sure if there are many people using this instrument, so for now I just do the post in Microsoft Excel.
For those interested in quick and dirty distortion analysis, Nelson Pass published a great sidebar in his Mosfet Citation 12 article which explains how to use one channel of an amplifier to analyze the other.
A DSO is really a pretty indespensible instrument if you're a serious audio hobbyist. And at $129 and up the PC based models available are hard to beat and they can replace several instruments on your workbench.
Another option (if you don't need to measure THD to .001%!) is to build a shunt that could be connected to the line input of a PC sound card. There a several sound card based software FFT analyzers available as open source and freeware. Again, you're limited by the accuracy of the sound card and your function generator.
P.S.
I asked a sales guy at AP if they've ever considered building a hobbyist-level product. He laughed at me. |
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| Damon Hill |
| quote: | Originally posted by mrothacher
Another option (if you don't need to measure THD to .001%!) is to build a shunt that could be connected to the line input of a PC sound card. There a several sound card based software FFT analyzers available as open source and freeware. Again, you're limited by the accuracy of the sound card and your function generator.
I asked a sales guy at AP if they've ever considered building a hobbyist-level product. He laughed at me. [/B] |
Ah, for the good old days when Heathkit was around and providing us with mid-level test equipment at affordable prices.
Velleman is almost the only kit company left out there. At least
you can get good oscilloscopes, but otherwise signal analysis
has been the forgotten stepchild.
I think the best hope for an Audio Precision equivalent is going
to be based on the better sound cards and someone clever at
writing the necessary support software. I'm >really< impaired
when it comes to the latter. |
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| alaskanaudio |
Some of you may not be aware that there is a lot of used test equipment available and sold daily on Ebay. If you are satisfied with older equipment that works perfectly well then this is a good place to look.
I have sold quite a few items of my older equipment there and have shipped them all over the world.
Look under the Industrial section and then test equipment. Don't get side tracked in the electronics parts section.
John Fassotte
Alaskan Audio |
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| mrothacher |
The line input on most soundcards is limited to 2 or 3 volts, so you'll need to design some kind of attenuator to scale the voltage down 10 or 100 times for measuring signals at high power.
The neat thing about using a PC sound card as an FFT with a PC-based function generator is that you could sweep the generator and automate THD vs. Frequency for example. However, I'm not sure about the distortion performance of software based function generators. |
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| smoking-amp |
| Forgot to mention, when using a PC sound card for amplifier testing, make sure you attenuate the amplifier output with a 50 to 1 (or so) resistive divider before connecting to a sound card input. I smoked my old soundblaster card once when I forgot to attenuate the amplifier! |
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| mlloyd1 |
Joe:
I found my app note having the ultra low THD oscillator. It is from Linear Technology AN43 from 1990 "Bridge Circuits: Marrying Gain and Balance" by Jim Williams. I don't have my copy of the book you mentioned handy, so I can't tell if it's the same circuit. This one uses a current buffer (LT1010) for the main gain block op amp (LT1115), another op amp (LT1022) for common mode supression and another op amp (LT1006) to drive the opto coupler (VTL5C10 used as the nonlinear element for gain control. Jim claims less than 3ppm distortion for this very simple circuit.
Michael |
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| Joe Berry |
Michael,
The AN43 circuit sounds similar in overall approach. In the piece I refer to, Williams starts with a single-IC wien circuit using the Hewlett idea of a lamp as the AGC element. The circuit grows to two ICs when he replaces the lamp with a FET and IC#2 to drive it. The FET later becomes an obscure opto device (actually I think you can get them through Newark), and he adds IC#3 to buffer IC#1's output and get rid of some residual crossover distortion. Finally, IC#4 is added to cancel common mode distortion, and at that point the distortion figure drops below the AP System One's radar.
I built the single-IC version on a breadboard using an LT1028 and got .003% of mostly second harmonic at 1kHz as soon as I remembered to add local supply bypassing. Or you can omit the local bypass and get 10% at 10 MHz, your choice. :) |
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| Damon Hill |
I got that Wien bridge oscillator to work with an AD797 and
some random selection of a small incandescent lamp with
a precision decade box. The residual on my old Heathkit
IM-5258 analyzer is just under .02%, but both are twitchy
about settings and the oscillator tends to develop a
crossover notch after a while.
Thanks for the lead on the application note; made for
some interesting reading. Already has me thinking about
taking my spare IG-18 and doing something creative with it!
I wonder what could be done with a word generator and
one of those 24-bit DACs...? I have NO experience with
digital electronics, I'm ashamed to say. |
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| smoking-amp |
The low cost 24 Bit D/A sound card that I am using (M-Audio Audiophile 2496)($160) has an accuracy of better than .002%. More expensive 24 bit soundcards may be a little better but not much. The best I have seen is the Delta 1010 which has a separate shielded box outside the computer for the D/A and A/D chips and it quotes D/A better than .0015 % dist. and A/D better than .001% dist. Software is available on the Web for oscillators and analyzers to control sound cards. The one I listed in my earlier post has a sine wave generator with optional sweep and other waves available. The white noise function can be used for a clever IM distortion test mentioned in AudioXpress August 2001 page 73. Just feed the amp. with a white noise signal filtered to remove frequencies below 500 Hz or so and measure the amplifer output with a low pass filter that only passes below 500 Hz. Any low frequencies coming out of the amplifier had to be generated in the amplifier.
The best distortion analyzer still remains the simple difference scheme mentioned in my earlier post. http://www.angelfire.com/ab3/mjramp...d/golopid6.html
Its sensitive to .0001% distortion or better (depends on the Op. Amp. chip used to construct it). Could use Burr Browns OPA2604 or OPA2134 (Digi-Key $2) or can use the discrete op. amp. in the article. This approach doesn't need a high quality oscillator, can even use a function generator or live music. A design extention in the article even allows it to be used with a speaker load on the amp. instead of a dummy resistive load. The expensive commercial distortion analyzers don't even come close to this capability. Can use the soundcard software approach if you want to get a hard copy of the distortion signal or spectrum. For casual testing can just use a scope or even a headphone to listen to the distortion signal output. Its almost a crime not to put this simple circuit on the PCB for all amp. projects so one has a permanent distortion monitor for occasional tune ups. |
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| Nelson Pass |
To answer the earlier question, I keep an AP at my
bedside.
I have been trying to talk Karen into a project doing
a very inexpensive IM analyzer that can be built for a
few bucks and works great. If you email her at
help@passdiy.com, maybe she'll get on it. :)
Lacking that, the technique in the Mosfet Citation 12
article works great to check the circuit and adjust the
bias if you have just a scope and an oscillator. |
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| alaskanaudio |
Nelson mentioned the AP unit and it reminded me of some of the white papers Audio Precision has on their web site.
One of the ones that may be of interest to those measuring distortion may be the one on the "Comparison of nonlinear distortion measurement methods", by Richard C. Cabot, the vice president of the company.
It can be found here:
http://www.audioprecision.com/publi...ent_Methods.pdf
I stopped by and talked to the folks at Audio Precision in Beaverton a number of years back and found them to very friendly. They gave me a very nice tour of their facility.
John Fassotte
Alaskan Audio |
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| ALW |
It's not cheap, but SpectraLAB and a good soundcard make a very impressive measurement system.
Something like a Soundblaster live will allow THD to be measured to 5 decimal places, plus some good FFT plots for examining the spectral content.
There's a built in sweep and tone generator so providing your card is full-duplex saves a bit of space on the shelf.
There is a time-limited full function demo available, it's not cheap to buy, but possbibly much cheaper (and neater) than many s/h test equipment routes.
Bear in mind if s/h equipment fails it can get expensive!
SpectraLAB's power is only limited by the hardware you've got.
Andy. |
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| Marc |
You might want to check-out
http://www.purebits.com
Sample Champion software is their main product
It has several plug-ins available,
one of which deals with Audio Quality.
(SNR, THD, THD+N, and IMD)
Other plug-ins available deal with -
Room Acoustics
Impedance (Thiele-Small parameters)
Filter Banks and more |
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| alaskanaudio |
I obtained SpectraPlus back in 1996 from Pioneer Hills Software and also HPW Works from http://www.hpw-works.com a few years back.
These software packages along with SpectraLab are excellent for distortion and other analysis. My SpectraPlus can only handle 16 bit data. Both SpectraLab and HPW Works can handle true 24 Bit sound cards and can internally pad it up to the 32 Bit data width that a PC uses when processing 24bit audio.
If these software packages had some of the features that the AP box uses for automating measurements they would truly be exceptional values. They can already match the basic performance of a AP box when used with a very high quality sound card in both the digital and analog modes. HPW Works is especially worthy since it can handle very high sampling rates. The upper frequency limit on any sound card will be around half the sampling rate.
As mentioned, to get optimum performance from these software packages a very high quality sound card should be used. A sound blaster or built in sound option on a motherboard just won’t do. You will loose a tremendous amount of accuracy and reduce the measurement dynamic range by 40Db or more in the analog mode alone.
HPW Works could use a better human interface (tool bar). Its present layout makes it more difficult to use than SpectraPlus or SpectraLab when you first start to use the package. HansPeter (HPW Works) has however always responded very rapidly to questions that I have asked of him about his software and is very pleasant to deal with. I highly reccommend considering buying his software package if you are in the market for this kind of software.
I use the software packages I mentioned to supplement my HP test equipment when making adjustments to equipment. I retain my HP test equipment since the upper samplig rate of my present sound card is 96Kbs. This is sometimes too low to suit all my needs. I would purchase a AP box but I would gain to little to justify the expense. I can purchase a lot of very high quality high sampling rate cards with balanced line inputs for the price of a AP box. After all the AP box is basically just a high quality external sound card with similar software to those I mentioned for data processing and display.
If you are serious about making subjective measurements on equipment that you design and build I would strongly recommend any of these relatively low cost software packages. HPW Works Pro version has an especially low and very attractive price for the features and accuracy it has.
Any of these programs will allow you to start with a low quality card sound card and then upgrade to a better one later.
John Fassotte
Alaskan Audio |
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| AuroraB |
Some 10 years or so, a patent became public, by an article in WW. I think it was a BurrBrown patent, where the idea was to increase the measurement sensitivity of a circuit by appx 60 dB, by a special topology where an high precision op-amp cancelled it's own noise while amplifying the signal. This was specifically designed for increasing the sensitivity in spectral measurements.
Does this sound familiar to anyone?? |
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| mlloyd1 |
YES! I hear a distant bell in my head starting to ring.... :)
Down to the basement to look for a paper ....
Michael |
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| mlloyd1 |
OK, I found what I remember. Are you referring to the test procedure found on pages 7 and 8 of the OPA604 datasheet ?
It uses the DUT to boost the amplitude of the distortion to make the result easier to measure.
mlloyd1 |
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| AuroraB |
Yes,- I think I remember this circuit. This approach could be married to a soundcard type analyzer....
I do have the original copy stashed away somewhere, but because of a rebuild of my basement section I don't have complete tracking of all my stuff!
The idea of using a sound card for spectral and distrtion measurements is great, but I think most of us do want "a little extra" if terms of freq. range and probably sensitivity. I have been looking for 96kHz soundcards and software for some time, so this thread is very useful.
Hope it continues for a while with a lot more tips and links. |
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| mlloyd1 |
| For the folks that recommended PC sounds cards, I just wanted to point out some interesting discussion taking place in this thread |
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| R. McAnally |
| quote: | Originally posted by mrothacher
The neat thing about using a PC sound card as an FFT with a PC-based function generator is that you could sweep the generator and automate THD vs. Frequency for example. However, I'm not sure about the distortion performance of software based function generators. |
I measured a SBlive at around 0.05% THD at the output, both from internally generated tones and through the line-in jack. (the plot is shown in another thread somewhere). The distortion is mostly 2nd order.
I currently use a Clio system by Audiomatica to do FFT/freq. response/THD measurements. It is a custom ADC/DAC PC card based system with a really nice software package. It measures capacitance/inductance, plots impedance curves and derives TS paramaters of any driver (without a box!). Other cool features are an arbitrary function generator and scope. It will also do power VS thd and TIM of amplifiers.
It'll also score your car stereo with an official IASCA score sheet =)
Don't know how much it costs, since someone gave it to me... I am assuming it is quite expensive. |
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| R. McAnally |
| quote: | Originally posted by ALW
Something like a Soundblaster live will allow THD to be measured to 5 decimal places, plus some good FFT plots for examining the spectral content. |
SBlives measure in around 0.05% themselves, so it would not be possible to measure distortion much lower than that with it.... Check the high quality PC sound thread for a distortion and frequency response plot of a SBlive I measured a short time ago =) |
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| AudioFreak |
| you could measure thd lower than the soundcard input by using a preamp with gain as the input to the soundcard... altho the signal path is starting to look fairly long ... so dont expect the measurements to be very accurate. |
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| R. McAnally |
| quote: | Originally posted by AudioFreak
you could measure thd lower than the soundcard input by using a preamp with gain as the input to the soundcard... altho the signal path is starting to look fairly long ... so dont expect the measurements to be very accurate. |
Please elaborate! Have you done this successfully?
I already had the line input signal maxed out at one point (tried with and without a preamp). The SBlive exhibits at least 0.05% or worse at any input level I have tried. Also tried tones generated internally using a tone generator program without better results.
S&N ratio is in the mid 70's so noise measurements are a joke. |
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| ALW |
| quote: | | SBlives measure in around 0.05% themselves |
This may be true in loopback, but I'd suggest most of the distortion is at the output side of things (internal sig.gen.).
When fed with an external low distortion AF sig gen my SBLive measures MUCH less than this - may be I've been lucky on the three samples I tried.
Andy. |
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| rmetz |
Folks, In response to a posting about a design that was in Audio some time ago, it was in the July/August/September issues, written by Bob Cordell. The design and article were nothing short of phenominal. A friend and I have built several of them. It has full automatic phase/frequency tracking and overall measurement floor of .001%, and typically .0003% through much of the audio band. It stands up well to the best and most expensive alternatives. No,it does not use TL071s, it uses 5534s. There may be better op amps now, but you'd be hard pressed to surpass this design. I use mine often, along with an audio spectrum anayzer for IM distortion measurements.
If there is a sufficient clamoring, I can copy or .pdf or photgraph and .jpg the articles. They are a landmark in audio engineering and disclosure for the DYIers. |
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| rmetz |
| OOPS! Forgot part of the date in previous posting: It was the July/August/September >>1981<< issues of Audio. |
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| blmn |
Rmetz,
I want a copy of this article, if it's possible.
Tks in advance |
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| FEThead |
| Me three, please! |
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| tortello |
Four, with me...
Thanks:D |
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| ftorres |
May I be the fifth ? ;)
Thanks |
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| AudioFreak |
| Number 6 here ... thanks. |
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| vbd |
I'd like to be the seventh ;)
Thanks |
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| mlloyd1 |
rmetz:
It's funny you mention this; I very recently acquired the boards for this project from a friend who got them from an ad in Audio before they vanished. It seems the company that made the boards for Bob was looking to get rid of some leftovers.
I'd like to see the articles also. I think I'd like to try building this thing myself, but all I have are the boards :( .
mlloyd1 |
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| nicharis |
| me nunmber 9. Thanks |
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| rmetz |
OK, it will take me a couple days, but I will try to copy the original to a .pdf and either post it here or on my website.
Cheers for now! |
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| HarryHaller |
I got this from from Mr.Cordell:
Thanks for your interest in my THD analyzer article, and thanks for asking before republishing on the web. It turns out that another gentleman and I have been in conversation regarding a possible updating of the design and publication in a magazine, so I would not want it to be republished on the web. However, I'm interested in knowing more about what you have in mind.
Best regards,
Bob Cordell |
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| mlloyd1 |
Harry:
Was Mr. Cordell's response recent (e.g. within the last month or so)? To answer his question, he could take a look at the beginning of this thread. I was planning on building the original, since I was lucky enough to find a set of PCBs from the original article. If he's seriously thinking of redoing the design, I'd be willing to wait and see. It seems the only magazine left with a decent worldwide distribution is "Audio Engineering".
By the way, would you also ask if he's also considering updating the power amp design he published some years ago in JAES (and various MOSFET manuals) with the super high performance JFET front-end? :)
Thanks!
mlloyd1 |
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| HarryHaller |
The response was today and I am afraid that I spoke prematurely on the new article since it is tenative and I don't have a date or what magazine it is in. I would press on with the existing project. I am working on getting together a list of articles and web resources on distotion measurement A.S.A.P and will post soon. I don't believe that extremly low distortion measurements are an indication of sound quality but think that measurements down to 0.05 % are not too hard to measure and cound be useful.
Thanks,
H.H. |
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| mlloyd1 |
Folks:
I was able to track down the following info about the source for the PCBs I mentioned earlier:
Circuit Works
85 W. Sylvania Avenue
Neptune City, NJ 07753
(908) 774-1811
Set of 3 boards for Cordell THD Analyzer project was about $45
However, the invoice was dated 16Dec1992 (!!!) so the possibilities are slim. However, if you're feeling lucky, you never know what could happen......
Michael |
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| haldor |
I agree we should respect Mr. Cordell's interests. But for goodness sakes, the article in question was published 20 years ago! Is Mr. Cordell actively working on this article or is he just trying to keep his options open. I hope he can provide some indication on when and were we can expect the new article to be published. If the answer is "I don't know, whenever I get around to writing is and can get someone interested", then this is just being an obstructionist.
I imagine anyone interested in a copy of this article would also be interested in purchasing Mr. Cordell's new article when it is published, even if in the meantime they managed to get their hands on a copy of the '82 issue.
Anyone got a lead on where to buy back issues of Audio?
Phil
| quote: | Originally posted by HarryHaller
I got this from from Mr.Cordell:
Thanks for your interest in my THD analyzer article, and thanks for asking before republishing on the web. It turns out that another gentleman and I have been in conversation regarding a possible updating of the design and publication in a magazine, so I would not want it to be republished on the web. However, I'm interested in knowing more about what you have in mind.
Best regards,
Bob Cordell |
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| jackinnj |
I resurrected a Heath HD-1, new caps, better wiring (star grounding etc.) , faster meter diodes, and replaced the 50 ohm trimming potentiometer with a multi-turn variety. The darn thing is pretty acurate (probably +/-0.02%, down to about 0.03%), certainly not up to my HP339a, certainly susceptible to ground loop problems, but for $10 not bad at all. I guess if I were to gild the lily I would put in a true RMS detector!
I've toyed around with notch filters/elliptical filters/all sorts of configurations. If you are going to make a highly selective notch filter with discrete components you will spend a lot of time matching components.
Here's an important note -- if you decide to use the Linear Tech LTC1063 active low pass filters in a high quality oscillator note that they do not like to be switched mechanically and are susceptible to overload if the + and - rails do not come on at exactly the same time. I owe thanks to Linear's technical service rep who helped me through some problems implementing these filters. You must also filter the "filtered" output as there is some residual tnoise from the clocking source. (This is pointed out in their applications notes) Note further -- the filter design using the LTC1063 in Audio Amateur had the power supply connections incorrectly marked. As they say in carpentry, measure twice, cut once. Linear has some nice filter design "free-ware" on their site.
Jack |
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| hagtech |
If you want a quick and dirty solution for measuring audio distortion, just get an HP 334A from eBay. They typically go for about $100. I got my (near mint) 333A for $50. These are good down to about 0.01% on the meter. There are always plenty for sale.
Unless, of course, you want to DIY.
jh |
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| buchignani |
These HP distortion meters also respond well to basic tweaking. I replaced all the aging electrolytic caps in mine, did a little bypassing here and there, and the RMS voltmeter's range was extended from 3 to 7 MHz+, and the residual voltmeter noise is now below 14 microvolts.
They are also very stable (except perhaps at the ultimate bottom of their distortion range).
At the same time, they're pretty well useless if you are interested in fine tuning in the really low distortion domain (sub 0.02 %).
Norman |
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| capslock |
I am a little wary of anything that sits inside a computer and is powered by it. What are current soundcards that have a S/PDIF input that will accept >20 bits, preferably at 96 or 192 kHz?
TI has a nice 24 bit ADC. With a decent oscillator and S/PDIF transmitter (or even I2S directly), it would make a very good analyzer.
Eric |
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| hagtech |
Wow, I had forgotten all about the Williams version of the HP200 (wien using a lamp). So I quickly built one myself just to see. Yup - 0.01% distortion (as far down as my HP 333A goes) right off the bat. No extra filtering, just a one opamp oscillator.
I used a common 5W nightlight bulb we had on hand. They're quite easy to solder to. See schematic below for a 1kHz frequency. Adapt as you need, but this is one quick and easy project.
Now 0.01% may not be low enough for some of you high-feedback fans, but for my zero-feedback tube amplifiers, it's adequate. I measured my new "Cornet" phono preamp with this oscillator (padded down to 5mV) and got a total 1kHz distortion level of 0.02%. Not bad! :) |
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| HarryHaller |
| I built one with a AD797 and 5 series connected bulbs for a 60Hz amplifier transformer power source. Low distortion at 60Hz even. |
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| capslock |
One could actually read I2S or any other serial audio format into a PC provided the PC is fast enough to detect changes on the serial clock and the read the data line.
The last time I tried I/O port programming, I could get about 300 kHz on the parallel port and it seemed to be pretty independent of the speed of the computer used, but that was in the times of 75 MHz pentiums and 133 MHz 486s. Anybody know the polling rate of a modern GHz PC running Windoze98 or better and using Delphi?
Eric |
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| P.Lacombe |
I have in project to build a bridged tee filter, and to insert it at the input of a standard S Blaster PC card. Correctly designed and constructed, bridged tee can attenuate the fundamental by 60 dB.(Some amplification will be necessary between the filter and the SB card, depending of the level of the signal.)
S Blaster 128 can give correct analysis on a 60 dB range (or more) So, I expect possibilty of measuring distortion in the range -60 to -120 dB (or more), which is exactly that I need to measure audio amplifiers or components... cheap and effective !
Thank you for your comments/advices... :)
Regards, Pierre Lacombe. |
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| SumTingsBurning |
Anyone try Praxis? http://www.libinst.com/
I use LAUD (no longer available) to get distortion <.01% which is not great but it is easy to use. |
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| leiade |
Hi,
One idea I saw when browsing the internet run the signal to a ordinary THD analyzer wich attenuates the fundamental to -60dB feed the output to a cheap 16 bit soundcard with a freeware fft program do the math and calculate the residual I think one can get really low thd figure out of this I don't remember how but it should work.:) |
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| Kevin Graf |
On the Cordell Analyzer: I have the 3 part article and the PC boards, but this is a vary complex project and the selector switches are made out of unobtainium (at least at resonable prices ).
On the Jim Williams Oscillator: The best version uses a CdS photocell, the Vactec VTL5C10 or Clarex CLM410 both made out of unobtainium.
On the Charles Hansen Oscillator: The 5/1998 issue of Audio Electronics magazine has his article on a buildable version of the Williams oscillator.
On SpectraLab software: You can downlord a 30 day trial of the complete package. When the time expires get a new computer and to it again. |
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| Fred Dieckmann |
Right on brother Can I get a witness?
"On the Jim Williams Oscillator: The best version uses a CdS photocell, the Vactec VTL5C10 or Clarex CLM410 both made out of unobtainium."
You can make this device from an LED and CdS photocell from Radio Shack I think but will have to adjust the circuit for the different transfer function. It looks very doable and has loads of cooliosty potential.
I use the light bulb feedback Wein bridge for my clean AC power and it is very clean. More on the subject of oscillators later and
freeware FFt at:
Fred |
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| vbd |
On the Cordell analyser:
could you please post the schematics ?
I'm curious about it !
Thank you ! |
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| Kevin Graf |
The "Audio" magazine article (July, Aug, Sept 1981) is 30 pages long, of which 9 pages are schematics !
The project uses 40 IC's and 5 multi-pole, multi-section rotary switches !
One is a 5 position, 9 section 18 pole switch. |
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| mlloyd1 |
Someone posted them at one time, but was warned about copyright infringement. Mr. Cordell requested the schematics not be posted.
A local college MIGHT be a place to see if microfilm of the back issues (and therefore copies) are available that way.
Check earlier in this thread. I posted the address of the company that provided PCBs for this project a LONG time ago.
mlloyd1 |
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| vbd |
Cordell analyzer
I checked the previous posts, and the schematics were not posted, as the copyright problem arose...
Unfortunately, the Audio magazine is not well known here, in France...:bawling:
I'm contemplating the schematics published in Electronics World August 1999 by Ian Hickmann, that would make a much simpler analyzer, with a smart topology, at first glance...
Any thoughts about it ? |
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| Pjotr |
| quote: | Originally posted by Marc
You might want to check-out
http://www.purebits.com
Sample Champion software is their main product
It has several plug-ins available,
one of which deals with Audio Quality.
(SNR, THD, THD+N, and IMD)
Other plug-ins available deal with -
Room Acoustics
Impedance (Thiele-Small parameters)
Filter Banks and more |
Purebits is really good software but it cost some money. I am quite happy with it.
Also check out:
http://www.sumuller.de/audiotester/
http://www.audiotester.de/mainE.htm
For $28 it is really a bargain. With a 96 kHz soundcard you can do almost any distortion analyses needed for DIY.
:cool: |
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| sam9 |
| The tech-diy.com link posted by jackinnj has gone bad. Too bad, the image looked good. |
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| cocolino |
| quote: | | originally posted by sam9 The tech-diy.com link posted by jackinnj has gone bad. Too bad, the image looked good. |
Try it again later - if it does not work anymore and when You have a big mailbox - email me - I saved the hole article already:mafioso:
By the way building this Analyzer looks like hell of a project.
I think it`s much less hassle to buy a used Tek AA501/SG505 combo or a HP339 at ebay and at the end this might be even cheaper. |
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| jackinnj |
I moved the folder on the website. It'll be fixed in the morning.
Jack |
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| sam9 |
| Thanks for your efforts. I had wondered where such a project might be hidden. Now for some heavy reading. |
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| vbd |
Thanks a lot, jackinnj for your time !!!
This classical hard wired approach looks interesting !!!
Cheers !!!! |
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| tortello |
Thanks Jackinnj,
very interesting stuff!
Marcello |
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| AuroraB |
2 questions ...
1. Having studied the analyzer article quite extensively over the last months, - I wonder if it is possible to further improve upon it, by careful component selection, new opamps etc... the ref. .0003% is quite impressive for a DIY design, but still.......any ideas..??
2. Have any of you tried the programs referred to, like the Sumuller audiotester??
results anyone... |
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| jackinnj |
if it really has to go a lot lower, you can consider using very low noise op-amps (let me know if you need AD797's !) and hand matching the filter network resistors, more careful attention to power supply noise, more shielding -- there has been a really great series of articles in Electronics World by Bateman in which he crafted a 1kHz distortion analyzer capable of reading the wrinkles on a gnat's eyebrow.
if you go to the Analog Devices website and noodle around their application notes dealing with ultrasound, sonobuoys etc. you will pick up some valuable hints. |
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| john curl |
| Folks, you are pushing your luck. Do you have the time and money to build a first class distortion analyzer? Why not try to get a pretty good one on e-bay or somewhere else and modify it, if necessary? Trust me, it will save you both time and money, to get something that already is in a box, ground loops worked out, and has a good meter. If you look around, (and are lucky) you can get a good distortion measuring instrument for 5%-10% of new cost. The used equipment companies seem to charge very high prices for the same thing that you can find elsewhere. Just look around. :nod: |
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| Nelson Pass |
All true, but this is diyAudio.com
;) |
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| cocolino |
| quote: | All true, but this is diyAudio.com
;) |
Indeed, and exactly therefore I buy equipment second hand -
because after building my own DIY distortion-analyzer, DIY power-supplies, DIY spectrum-analyzer, DIY noise-meter, DIY curve-tracer, DIY scope (no joke, I have done this already.....well, it was a kit.... but still), DIY LCR-Meter... .......I would need a second life for my actual audio projects and....... meanwhile I`m aware that I got only one:bigeyes: ;) |
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| mrothacher |
Don't forget one of NP's favorite Sagan quotes:
'To make an apple pie, you must first create the universe':nod: |
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| sam9 |
Of course to get the most from a distortion analyzer you need a sine wave source with lower distortion and noise than the analyzer. Most diy sine generators come up a little short comparred to what people have been talking about in these post.
This leads me to the question: Has anyone tried to build the sine wave generator in the LT1115 datasheet?
If so, can you explain how it works -- it is just too many opamps "running in circles" for me to grasp clearly (or even murkily). |
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| jackinnj |
yeah, but it probably beats the little light bulb for control -- i think you could use the AD7528 Dual DAC or a digi-pot as the control element.
Linear Tech is in the biz of selling op-amps, anyway. |
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| sam9 |
One og my 401k funds may carry a few of their shares, so I'm not so worried that they asre justvtrying to sell more opamps. As the wise man said, " They ain't in it for their health, kid". More annoying is no comment about setting the 500-ohm trimpot. Whgile it obviously (I think) optimises the feedback, I wouldn't know hoew to set it without a distortion analyser that has a THD+N floor lower than the circuit. This something I don't have. It would be nice to know if there is a voltage reading somewhere that would accomplish this.
At least I'm not a complete ignoramus: I've figured out two mods to make this more practical. One is trivial; just the observation that C1&C2 need to be switchable to get a useful range. Better still a 4-ganged rotary switch to provide a set of fixed frequencies. The second one is that the VTL5C10 is nearly unabtainable unless you want to buy in volume from P-E, but you can use two VLT5C4/2'a from Allied in parallel to get very neary the same voltage/current to resistance curve. |
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| jackinnj |
pity, now Nat Semi is busy remaking themselves too.
let me know if you want a copy of the oscillator section schematic from the HP 8903B, I have it around here somewhere.
jack |
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| PT |
This is an interesting discussion.
I've seen a lot of designs for instruments like frequency meters over the years, and I'd always thought it might be a useful thing to have. Eventually I bought a secondhand Racal for £50. Overnised xtal, 200 MHz and more features than you can shake a stick at. The magazine designs would have cost that much for parts and the end result would never have been as good. They don't use overnised xtals for a start.
I've just built a low distortion double integrator loop audio oscillator from a design in a defunct British magazine called ETI.
It's amplitude control is a precision full wave rectifier and LED/LDR. The article claims 0.001% THD at 1KHz and 0.008% at 100Hz.
After much fiddling with shielding and earthing, an HP8903 shows 0.009% and 0.015%. A Marconi TF2104, which costs about £100, does <0.003% in both cases. With just a bit more fiddling/trying different op amps (presently NE5534s, TL052s in the article) I'm convinced I'll get there. Of course, I'm using a crummy dual pot, not a nice one like you find in commercial instruments.
Was building the oscillator entertaining?
Definitely, and much better value than the equivalent expense in say cinema tickets or restaurants.
Was it a cost effective way of obtaining a low THD oscillator?
No.
Of course, you can regard building test instruments as a worthwhile activity in its own right.
There's also the possibility of tweaking or regutting commercial instruments.
Conclusions:
The design and connecting the parts is just a part of the story - what you're paying for in a commercial unit is the debugging, the robustness and the very nice presentation.
I'm not tempted to try the Cordell THD analyser, but the article is a good read. Might just try the oscillator section one day.
The Linear Technology AN 43 final design is worth a go if the parts aren't too dear, and the design in AN 67 is interesting.
I'll pounce on a Marconi TF2104, or better, the HP equivalent, when I see one for a price I like.
Secondhand instruments can be a very good buy. Broken ones can be dirt cheap and easy to fix.
The age of kit companies is not over for nothing.
It would be interesting to see the circuit of the oscillator in the 8903B.
Pete. |
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| tortello |
Hello Pete and all,
I totally agree with your comments.
Refurbished instruments' prices can be very attractive, and you may find a good 8903 ( -A, -B) in a fraction of the price when new.
With my 8903B I can obtain about -94dB (0.002%) T.H.D. (+N) above almost the whole audio band, and this instrument is more complete than a simple distorsion factor meter with its oscillator.
The service manual with the electrical schematics can be downloaded from the agilent site, the file is "08903-90062V2.pdf".
Marcello |
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| peranders |
| Has anybody mentioned the excellent freeware RightMark from Russia? |
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| fmak |
| quote: | Originally posted by peranders
Has anybody mentioned the excellent freeware RightMark from Russia? |
---------------------------
Doesn't work fully on my 98SE system. |
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| peranders |
| It's really nice on XP though. Very easy to use and the support from the russian guys is excellent. Really nice guys. |
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| AuroraB |
Since I obviously was the one who kicked this thread alive again,-- maybe I should join in...
I do have an old HP339, which I haven't used since my tape recorder days,- way back when...
I do have the service manual for it, tucked away somewhere ,- too well - can't find it yet.. but I think it will be a lot harder to modify this old unit to compete with even the newer DIY ones.... am also aware of Ian Hickmans article in E&WW...
I personally think that it easier to work with a THD meter in some applications, rather than spectrum analysers, even if I do agree that in the finals, the spectrum is probably of more interest than the actual THD number.
For the cost of e.g. the Cordell analyser, - or the Hickman version, I think a lot of us has most of the components in stock anyway, after a great number of years in DIY and professional electronics.
Even if Ebay is accessible for me to, the freight cost of sending something close up to the north Pole is somewhat preventive in many cases... ( I live 69.4 deg N-16.1 E !! :) )
For the software analysers, - I think they fulfill a slightly different niche, and also has other uses, but what I asked for was peoples impressions in using the programs,. I do have some smaller freeware versions, but these are not up to measure, for very low THD work.... |
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| jackinnj |
| quote: | Originally posted by AuroraB
I personally think that it easier to work with a THD meter in some applications, rather than spectrum analysers, even if I do agree that in the finals, the spectrum is probably of more interest than the actual THD number. |
Where the spectrum analyzer is really useful is in looking at power supplies -- and line harmonics
I am still scratching my head trying to figure out how to use the FFT to measure phase margin -- the folks at Tektronix said I will have to become more proficient at MathCAD ! (you can do it automatically with their TDS7XXX series and a pulse generator.) |
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| jackinnj |
here's a Krohn-Hite 6900A on the BAY -- these are truly "hands-free" devices with auto null for both frequency selection and input -- I have a 6900B and it is much easier to work with than the HP339 I own -- of course you only have a 1kHz oscillator. The measurements of the KH and HP units match exactly FWIW.
http://cgi.ebay.com/ws/eBayISAPI.dl...gory=25423&rd=1 |
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| sam9 |
"here's a Krohn-Hite 6900A on the BAY"
Yup. Got one a while back. Just the thing for the less astute(like me) - real plug-n-play. It seems to be a bit unstable at points in the spectrum where you have to switch from one range to the other. But otherwise no problems.
You can download manuals including service at the K-H website |
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| jackinnj |
| quote: | Originally posted by sam9
"here's a Krohn-Hite 6900A on the BAY"
Yup. Got one a while back. Just the thing for the less astute(like me) - real plug-n-play. It seems to be a bit unstable at points in the spectrum where you have to switch from one range to the other. But otherwise no problems. |
If it's unstable you have to check the maintenance section of the manual and adjust the frequency discriminators.
Mine is a 6900B and it locks perfectly, but I would say that the integrator requires two or three seconds to stabilize. |
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| sam9 |
I figured I would open the service manual and work though the adustment proceedures. However, they specify having a couple of ultra low (<.001%) signal generators available. These are not easy to come by cheaply so I'm in the process of trying to build the one in the LT1115 datasheet. I should hve the parts by the end of the month. If it works I'll see what I can do with the K-H.
Meanwhile I'll look at the manual again -- maybe I can improve the adjustment in the interim. |
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| jackinnj |
| quote: | Originally posted by sam9
I figured I would open the service manual and work though the adustment proceedures. However, they specify having a couple of ultra low (<.001%) signal generators available. |
I adjusted it using the Heathkit IG18 and an HP3336B Synthesizer and HP465A amplifier -- you don't need that high precision to adjust the A.C. voltmeter, adjusting the discriminators is a matter of ramping the input at different frequencies and making sure that the comparators latch. You do need a scope for looking at the lissajous figure to adjust the balance on the oscillator.
For the precision resistors -- just make these up from standard values. |
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| sam9 |
| Thanks for the advice. I'll see what I can do. |
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| ionomolo |
| Somebody has tried to use a lock-in amplifier to get the amplitude of the harmonics instead of rejecting the fundamental? |
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| janneman |
Many years ago I build Bob Cordell's THD analyzer. Sold it when I got something else, but still miss it.
Worked beautifully, with very low residual, auto tuning, auto nulling, BW up to 100kHz IIRC. The design should be on www.cordellaudio.com. At the time, he had pcb's available, which I got. It is a fairly involved design mechanically as there are several stepped switched with multiple wafers, but as a DIY analyzer it can't be beat.
Jan Didden |
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| forr |
A 1000 Hz sine generator and an accompanying distorsion meter with sub ppm performances are described in these downloadable files :
Cyril Bateman s'site :
http://uk.geocities.com/cyrilb2@bti...ownloads_4.html
Download Capacitor Sounds series1 papers:
"CapSound1.pdf"capsound1pdf 1.1MB.
"CapSound2.pdf"capsound2.pdf 1.5MB.
"CapSound3.pdf"capsound3.pdf 0.7MB.
"CapSound4.pdf"capsound4.pdf 0.7MB.
"CapSound5.pdf"capsound5.pdf 0.7MB.
"CapSound6.pdf"capsound6.pdf 1.2MB. |
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| syn08 |
| quote: | Originally posted by ionomolo
Somebody has tried to use a lock-in amplifier to get the amplitude of the harmonics instead of rejecting the fundamental? |
I did. It's an extremely sensitive method (you can measure amplitudes of under 100nV buried in noise), but requires a dual phase model, with variable (frequency an amplitude) internal reference signal. The method is also very time consuming (at least for the old-ish Princeton Applied Research I got from EBay). Newer models may have some degree of automation, but are certainly very expensive...
Unless you have deep pockets and/or plan to measure distortions well under 1ppm, it doesn't make much sense to invest time and money in an (e.g.) Signal Recovery device that will conveniently do the job. For that kind of money you can buy a very good (and relatively new) spectrum analyzer that will serve you well for 99.99% of the audio applications. |
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