In order for 3rd order distortion to do that you'd need crazy levels of distortion and quite a dramatic rise in distortion with signal level.
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All good reason why most/ all of those doing LDR's match them. Also having no
capacitance or resistance directly in parallel that has branch to ground, and using a single gang pot, as well
as returning cathodes to a potential difference, or at the least a star ground point rather than lumping with signal ground, diminishes the issues you raise. Try it and see.
Cheers / Chris
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Pass' data shows that even if you match two LDRs perfectly e.g. at 1k then they can be so nonlinear as to not match at 200 ohms or ~10k anymore.
From the limited data available this doesn't seem to be excessive, but could still be audible with passive attenuators.
That's why I asked Ryelands about resistances of each LDR at his min/max settings. 😉
However, the OPs device has a calibration mechanism that should fix that.
From the limited data available this doesn't seem to be excessive, but could still be audible with passive attenuators.
That's why I asked Ryelands about resistances of each LDR at his min/max settings. 😉
However, the OPs device has a calibration mechanism that should fix that.
If fed from a 7805, with capacitance and resistance
in parallel with anode and cathode to ground, yes you could
expect some weird measurements wavering. Did Nelson
go to some trouble with the circuit he used, or just resort
to a 7805 ? If he used a 7805, no wonder you assert these issues.
There is nothing wrong with a 7805 contained within a far
more complex LDR circuit, however the mistake is allowing
ground of that device, to be ground, and relying on it to
accurately provide the tiny currents needed by LDR's.
Relying on a 7805 as the only device, even as as a Vref/R,
let alone as a fixed voltage device, is just asking for trouble.
Adding capacitance and resistance in parallel to somehow
then be the load, just compounds the problem, in such simple circuits.
Cheers / Chris
I have a great deal of difficulty understanding Why is it SO IMPORTANT to the technical people/experts to continue to denigrate the people who want to DIY their hifi projects in the company of this website, and to continually inform them of the depths of their ignorance and 'the error of their ways' - and, why is some unspecified level of a measured test held as the 'Holy Grail' of engineering excellence exclusively?
The people that have tried and like what these volume controls 'sound like' aren't particularly interested in any 'high levels of distortion' discussion that these devices are supposed to add to the sound, or not - they just 'sound good' in comparison to other volume controls in their hifi systems - the 'subjective' view
To the technical people, these volume controls are like a 'red rags to a bull' - it generates an inordinate emotional response similar to a religious epiphany - those misguided fools need to be educated about the 'reality of engineering practices' and the superiority of the humble pot, for example.
Now, I'm one of these 'misguided' people and I would just love to have a competent engineer/scientist carry out a series of tests on one of these volume attenuators in a real, or simulated, system similar to that done on a 'normal' hifi preamp - Harmonic content of any distortion under a variety of load/source conditions at different amounts of attenuation - an expanded test of Nelson's work done quite a few years ago - not just reiterations of the Siliconex website or Jack Walton's testing of individual devices.
[Unfortunately, I have neither the facilities, or the ability, to do this myself but I would love to see exactly what goes on with these units that have produced so much 'cyber bullying' over the years]
Interestingly, people who design and develop these attenuators aren't particularly interested in publishing their results and on the other hand, people that are critical about the devices aren't interesting in doing any tests, or even listening to the devices to get some first hand knowledge of them (and critical posts about something that they have no actual experience or first hand information about)
Post #140, a few days ago -
"LDR volume controls distort the signal more than normal arrangements" -
I have a great deal of difficulty with this, Dave - could you explain this 'normal arrangement' and again, what's this distortion that you keep talking about?
What exactly is it and what are the testing protocols to obtain the measurements?
And this extraordinary one in post #113 by our Moderator Yanager
This is a technical forum. If you want to market to people happy to accept claims with no backup if the story is good enough, there are better places to do so
[this is about the "BTF Systems modules" advertised in the Vendor's Bazaar]
The arrogance is breathtaking - how you go about alienating people and ruining a DIY website in one easy lesson
And this under a byline by Mencken (an authority on Nietzsche), a writer that also called himself a 'jackass'
The people that have tried and like what these volume controls 'sound like' aren't particularly interested in any 'high levels of distortion' discussion that these devices are supposed to add to the sound, or not - they just 'sound good' in comparison to other volume controls in their hifi systems - the 'subjective' view
To the technical people, these volume controls are like a 'red rags to a bull' - it generates an inordinate emotional response similar to a religious epiphany - those misguided fools need to be educated about the 'reality of engineering practices' and the superiority of the humble pot, for example.
Now, I'm one of these 'misguided' people and I would just love to have a competent engineer/scientist carry out a series of tests on one of these volume attenuators in a real, or simulated, system similar to that done on a 'normal' hifi preamp - Harmonic content of any distortion under a variety of load/source conditions at different amounts of attenuation - an expanded test of Nelson's work done quite a few years ago - not just reiterations of the Siliconex website or Jack Walton's testing of individual devices.
[Unfortunately, I have neither the facilities, or the ability, to do this myself but I would love to see exactly what goes on with these units that have produced so much 'cyber bullying' over the years]
Interestingly, people who design and develop these attenuators aren't particularly interested in publishing their results and on the other hand, people that are critical about the devices aren't interesting in doing any tests, or even listening to the devices to get some first hand knowledge of them (and critical posts about something that they have no actual experience or first hand information about)
Post #140, a few days ago -
"LDR volume controls distort the signal more than normal arrangements" -
I have a great deal of difficulty with this, Dave - could you explain this 'normal arrangement' and again, what's this distortion that you keep talking about?
What exactly is it and what are the testing protocols to obtain the measurements?
And this extraordinary one in post #113 by our Moderator Yanager
This is a technical forum. If you want to market to people happy to accept claims with no backup if the story is good enough, there are better places to do so
[this is about the "BTF Systems modules" advertised in the Vendor's Bazaar]
The arrogance is breathtaking - how you go about alienating people and ruining a DIY website in one easy lesson
And this under a byline by Mencken (an authority on Nietzsche), a writer that also called himself a 'jackass'
I recently measured the harmonic distortion products of my LDR attenuator unit (see attachment).
I don't have a high-end distortion analyzer, so I used my USB soundcard (Behringer U-PHORIA 202HD) with my own software (MATAA). I played a 1 kHz sine through the attenuator (input voltage about 1 V-eff, attenuatrion set to 27 dB), recorded the output from the attenuator and analyzed its spectrum. I compared this to the residual distortion produced by the soundcard itself, a fixed-resistor voltage divider (10 kOhm + 470 Ohm, 27 dB attenuation), and a potentiometer that was floating around on my bench (100 kOhm linear single gang, set to 27 dB attenuation).
The spectrum obtained from the fixed-resistor and 100 kOhm pot did not show any difference to the soundcard residual (not shown). The LDR attenuator showed somewhat higher distortion levels for the 3rd and 5th harmonics, but doesn’t show any harmonics otherwise.
This is probably not exactly the test you're looking for. But maybe you guys could think about a (small and simple!) set of measurements that would be useful -- and maybe I can do some of these.
Attachments
I do not think this is what is happening. What is happening is that this project is presented as transparent, hi-fi if you like, volume control.
People with tech knowledge are aware that due to the use of LDRs the distortion of these things is much higher than from a 'normal 'pot, or a stepped attenuator.
So why use an opto? If the answer is, 'hey, I like DIY, it doesn't sound half bad and it's an interesting project', that would be the end of it.
But it then derails because the users try to convince the techies that the opto really is (much) better than a pot etc. Which it clearly objectively is not.
So you reap what you sow.
Jan
Thanks for that mbrennwa, it's a good indication of what this mysterious distortion actually looks like in a complete working attenuator - it's 'really excellent' (sorry!) to have a clear definition of this contentious issue
From this graph, if I've got this right, there's about 0.4% H3 at 27dB attenuation and the pot has about 0.05% at same attenuation, and similar difference for H5 (a bit surprising that there no other H's).
This presumably, is Dave's order of magnitude (higher distortion) he's been muttering about for a long time (see, I can be diplomatic!)
It's reasonable to assume the various different versions of the LightSpeed vol control will exhibit this similar characteristic to some degree or other
Perhaps this is why it's such a good complement to valve gain stages and/or buffers, etc, in my systems
Could I trouble you to do similar tests at lower attenuation (10, or maybe 15dB), to see if the H3 content maintains the same relationship? (ie. lower attenuation = less H3, or not?)
I wonder if it's the series or the shunt device that's responsible for the H3 content at these attenuation levels, and if this changes in any linear fashion with the current thru the devices?
And, would a T-Pad exhibit similar figures or could we expect some H3/H5 cancelation?
From this graph, if I've got this right, there's about 0.4% H3 at 27dB attenuation and the pot has about 0.05% at same attenuation, and similar difference for H5 (a bit surprising that there no other H's).
This presumably, is Dave's order of magnitude (higher distortion) he's been muttering about for a long time (see, I can be diplomatic!)
It's reasonable to assume the various different versions of the LightSpeed vol control will exhibit this similar characteristic to some degree or other
Perhaps this is why it's such a good complement to valve gain stages and/or buffers, etc, in my systems
Could I trouble you to do similar tests at lower attenuation (10, or maybe 15dB), to see if the H3 content maintains the same relationship? (ie. lower attenuation = less H3, or not?)
I wonder if it's the series or the shunt device that's responsible for the H3 content at these attenuation levels, and if this changes in any linear fashion with the current thru the devices?
And, would a T-Pad exhibit similar figures or could we expect some H3/H5 cancelation?
Yes, but there's lots of masking and/or speaker distortion at very loud peaks. Just thinking out loud. Another one slight expansion cancels slight compression of speaker. 🙂
Are these all at exactly 1K with the plots offset? In that case note the near side bands on the 1K tone for the LDR, possibly excess noise.
Yes, the excess noise of CdS is well known in the literature it is mentioned in "Noise in Semiconductors and Photoconductors" K. M. VAN VLIET 1958 but I don't have access to the paper he mentions with the computations.
EDIT - Found it, anyone can download this it's from NASA. Excellent overview of the physics of CdS http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750002713.pdf
The take away is that an excess noise mechanism proportional to current (signal) dominates below 10kHz.
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The measurement does not show any distortion for the pot (and the fixed-resistor voltage divider). The 0.05% H2 value of the pot (and the fixed-resistor attenuator) is just the residual distortion of the soundcard. The same applies to the higher order harmonics. I should have included the soundcard residual in the plot to make this clearer, but I felt it just adds too much clutter.
Ok, I'll put that on my to-do list. But please don't hold your breath...
I believe I remember there is some information on this in the Silonex documents and also in Nelson Pass' measurements (but I might be wrong).
I could also do some measurements on the OptiVol, which is a voltage divider made up from a fixed resistor and an LDR. I'll put that on my to-do list, too.
And unlike any before you, publish the schematic of the circuit used to drive the
LED of the LDR. ... and try to use a single gang pot, and have no resistance
or capacitance directly in parallel with the LDR anode and cathode.
Happy to help with a board set, that has these essential features.
Cheers / Chris
Yes.
Yep, I also wondered what this is. There's also some "noise" between 15 and 20 kHz.
Ok. What exactly are the key factors here?
Ok. Why (just curious)?
Ok. Why (again)? I used my BTFS board in my distortion tests. I don't know the details of this circuit, but I guess it will be possible to figure this out.
That would be great! Comparing different boards would be useful. Could you just send me a complete board that I could use for a while? What are the PSU needs for this?