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- Опубликовано: 18 сен 2024
- Episode 1992
found this on eBay, offered as kit or PCB only
I learned you can select resolution bandwidth to display on the HD3
search eBay for 'Low Distortion Audio Range Oscillator 1KHz Sine Wave Signal Generator PCB Board'
Be a Patron: / imsaiguy
In Linear Technology Application Note AN132 - Fidelity Testing for A to D Converters, Jim Williams and Guy Hoover present a low-distortion oscillator circuit that eliminates or reduces the distortion that is present in several features of the eBay circuit. For instance, the FET that is a source of nonlinearity is replaced with a cadmium sulfide cell that is illuminated by an LED, and the LED bias source is designed to contain very low ripple from the oscillator. 🙂
EDN had a viewer circuit a couple years ago; 4 phase (0,90,180,270) rectifier from state variable with either FET or LDR - I forget who wrote it; but that reduces ripple. Green LED / Cds Thanks for AN132 👍 (same circuit?)
Nick Cornford June 18, 2024 ultra low distortion osc 1ppm (ooops, couple months ago)
AN132 is two opamp Wien with LDR -126dBc 5ppm
I looked at that app note and thought I would make one. oh. ouch. very expensive opamps in that thing. I don't need it that bad.
@@IMSAIGuy OPA2134's are ~$6
LM4562 are ~ $1.80
stuff I have laying around to try different designs... SOTA is not necessary to see how it works
25 years doin' live sound reinforcement & recording (fun and profit, mostly fun) tells me
@@IMSAIGuy Okay. I presume the authors used the very best available from Linear Technology at the time, and those remain expensive, particularly since there is very limited production volume. Fortunately, there are modern high-performing audio op amps that are much less expensive that could be substituted for A1 through A3, and the remaining op amps could be any of a wide variety of modern inexpensive parts.
@@analog_guy How do you know there are exact, or good enough, part substitutions. Sometimes it isn't possible, even using the specified parts, to get to the spec without binning.
The capacitors on the board that control the oscillation frequency are selected and paired using a digital meter. The resistors that control the oscillation frequency are a 3.3K resistor and a 200 ohm 3296 multi-turn potentiometer in series. The pairing is measured and adjusted using a digital meter to ensure the accuracy and low distortion of the output frequency.
Way more work than The Guy, will probably put into any video. It's only show and tell level, for this kind of video.
Ah, but your assumption abut using good components is the issue. The circuit was designed only to work with crappy components.
Low distortion coefficient sine wave oscillator circuit board, oscillation frequency: 1 KHz, ultra-low distortion, distortion coefficient 0.000064% (-124dB) line from "Transistor Circuit Design and Production" (Japan) written by Toru Kuroda.
where did you found this book? I can't :(
Yeah, This is a state-variable oscillator which are not known for low distortion. Wien bridge oscillators with a good amplitude control can be much better. Jim Williams discovered this in his paper “Max Wien, Bill Hewlett and a rainy Sunday afternoon”. He tried to get better and better distortion figures by using different amplitude control circuits. When he got to the J-FET he found that the control voltage modulated its channel length which limited his efforts to improve it. I also found this but did better eventually with a homemade voltage-controlled resistor made from an incandescent lamp and a Cadmium-Sulphide photocell pushed into opposite ends of piece of small tubing. I got the THD down to 0.00018% at best using a notch filter to see the residue and measured on a HP 3580A spectrum analyzer.
I purchased the same unit you have but I got the assembled one. Interesting mine have multiturn trim pots for R6 and R7. I bought it in 2023. They are using those two pots as rheostats for the R values. Using a +/- 15 volt power supply and the QA403 24-bit Audio Analyzer I measured a frequency for the sinewave of 996 Hz. The THD was -82.00 dB or 0.00794%. So I would rate this as a very good low distortion sinewave oscillator. Maybe you have a defective part or possibly an error in assembly of your unit. Or the one they build, they trim R6 and R7 for best performance. I would be happy to measure the pots for you and you maybe able to use fixed resistors or add two trim pots. Also any measurements or component values on the board, just let me know.
I use an app on my iPad to generate clean audio range sine waves on the headphone output. Since it is not referenced to earth, there is no risk of any earth loops.
I built something similar (found it in the Dutch electronics magazine "Elektuur", sometime in the '80s). It was originally designed to be adjustable from about 20 Hz to 20KHz, but I extended the range from about 1.5 Hz to 120 KHz. It has low distortion, around 0.01% at 1 KHz. The Elektuur design uses one Quad TL084.
The circuitry to control the gain was better than this design, resulting in lower harmonics.
I built one it works if all you need is a quick input signal. Fun to assemble. That’s pretty much it.
Classical Wien Bridge oscillator with incadecent lamp in feadback gives me 0.0017 % THD.
Distortions starts at -96dB. All "measurments" was made with Visual Analyser software and creative SB soundcard.
Made of nothing but the best Chineesium.
a copy cat gotcha - good use of the Keithley 🤔saves the vid for me
You need a decent pairing of capacitors and resistors in the upper part of the circuit, matching R6,R7; C1, C2; R1, R2
A guy from Czech Republic has done a lot of work on this sort of circuit; he even sells kits of his adapted design. He has a PDF of schematic and a lot of commentary as well, you should be able to find it if you search
"An ultra low-distortion oscillator with THD below -140 dB"
Part of it is building an equally decent notch-filter just to be able to see any residual spurs! A decent modern replacement OPAMP to make these kind of circuits with should be OPA1656 by Ti, it won't break the bank at least. Harder component to find is a really good linear optocoupler.
That's not considered low distortion! Even a simple twin T oscillator using an op amp can easily hit fractional three digits distortion.
One opamp Wein bridge with diode clipper ~.15% instead of lightbulb. Two opamp Wein with thermistor
@@jim9930That little r19 thermistor can still beat the pants off all but the best digital stuff even today!.
The real ultra low THD ones are designed, made and sold by Viktors on eee bay.
By the comments, eager to see Part2. ;)
FET attenuation has some distortion .
It would be interesting to measure the THD, using the same opamps but with better quality: the issue should be there.
I have my 36k$ APx 555 to design my audiophile amplifier 😊😊.
I built one as a Chinese kit and got .025%
Quick question... Are R8, R9 & VR the correct values? I can't really see the part numbers on the PCB as the part probably covers them but I think I'm seeing 10k, 10k and 0.5R. Not likely I know and maybe the colours aren't translating well (or I'm just thick! ;) Thanks... Clark
FYI: I'm looking at 8:41 & 9:33
The Ebay listing for this board claimed a distortion level of 0.0064%.
Wow...
Wrong FET or bias will ruin THD. State variable circuit with 5532's should be an order of magnitude lower.
edit: 120-150k is to ensure startup. 27 and 1k II with FET (on resistance, R4') sets the loaded Q with this circuit board. TL071 and diodes(secondary effect on distortion) are the peak detected level adjust for highest Q. R5/R4' = 3Q-1
ie proper bias for low on resistance
The bottom 'pink' 5532 is to set the common mode voltage and bias at 1/2 gate to drain voltage to reduce even harmonics
Compared to the 10% of a typical speaker, 0,3% distortion is fine.
But 0.00001% be much better
No doubt. Mind you 10% distortion is intolerable to listen to, it is the norm for car audio when used at maximum power output. At least, the audio chips I've seen that are designed for simpler automobile systems seem to be rated like that.
I think you should use a wireless mouse with the new keysight scope.
I have one on it now
What is the output frequency of the board? The harmonic distortion test should be checked according to the test frequency before testing.
The keithley measures the frequency and harmonics automatically
...and R1, R2 &R6 seem to be orange / orange / red / red / white, what's that? My novice mode / screen might be betraying me...
How to mesurment audio Amplifiers damping factor and THD