Try to help - a little
Dave,
Your measurements do not track what I have seen with sorted LDR. With a single LDR per pot section, a 100K pot gives 25K measured impedance. Look at the wiring, I suspect an issue. With the increased current pull the impedance of the pot should be close to 100K.
Dr H,
The 3.5 volts or so across the pot is correct. The supply voltage is 5 volts, the diode drops 1.5 volts, the pot and series resistor drop the rest. The voltage across the pot is fairly constant, the current is what changes with rotation.
With 200 ohm series resistors, a 5.06 volt supply, and 100K pots I measure 80 - 90 ohm at max current and 28 -29K at minimum. Max current is about 3.5/200 17.5 ma. Min is 3.5/100K 0.035 ma.
Try flipping the polarity on the rear wiper. Also crank the pot while measuring from input to output. If the impedance is still in the Mohm range it is bad news, need new LDR's.
Dave,
Your measurements do not track what I have seen with sorted LDR. With a single LDR per pot section, a 100K pot gives 25K measured impedance. Look at the wiring, I suspect an issue. With the increased current pull the impedance of the pot should be close to 100K.
Dr H,
The 3.5 volts or so across the pot is correct. The supply voltage is 5 volts, the diode drops 1.5 volts, the pot and series resistor drop the rest. The voltage across the pot is fairly constant, the current is what changes with rotation.
With 200 ohm series resistors, a 5.06 volt supply, and 100K pots I measure 80 - 90 ohm at max current and 28 -29K at minimum. Max current is about 3.5/200 17.5 ma. Min is 3.5/100K 0.035 ma.
Try flipping the polarity on the rear wiper. Also crank the pot while measuring from input to output. If the impedance is still in the Mohm range it is bad news, need new LDR's.
I guess I'm taking the plunge into the LDR world, as I have 20 pcs coming from Allied. Thanks to George for brining this to light and for offering such good 'tech support'.
I'm looking at a slightly unusual setup, although it's been touched on in the thread. It's a balanced active multi-amp setup, which means up to 7 channels (3-way plus sub). I'm taking it as a given that a 'conventional' series/shunt setup for 7 balanced channels is a practical impossibility due to matching. My plan is to use a single LDR per balanced channel in the shunt position, with precision series resistors in the series position. This will mean variable impedance for the stage, but I'll (eventually) be feeding UcD's with 100k input impedance, so I should be OK. (I may use a buffer in my current setup which has a lower input impedance) Furthermore, rather than a continuous pot I'm planning to use 'stepped attenuators' to adjust the LDR current, meaning that they can be matched via resistor selection. Since I'm using an all-digital PC source, I can use the LDR setup for coarse (6-10dB per step) adjustment, and use digital attenuation to fine-tune the level - I'm aiming at 3 or 4 steps of either 6dB or 10dB, since with a single LDR and say 5k series resistors it looks like 30dB is about the most attenuation I should expect.
I do have a question though - in the above setup the volume will be max when the LDR is 'off', and so it introduces a potential failure mode - if the power to the LDR's fails, they'll go high-impedance, and thus max out the volume. I don't really want to introduce relays into the picture since that would largely undermine the effort, but a NC relay that shunts the LDR and is pulled open by the same supply that powers the LDR seems to be the easiest solution.
Has anyone looked at this in their units? In the 'normal' series/shunt setup I *think* PS failure will send the unit to ~6dB attenuation which may be enough to avoid problems. Would an open relay tied to the signal path introduce any significant artifacts? I'm probably just paranoid about supply failure, but since I may have significant power tied directly to the drivers in my active setup, it would seem to be a 'better safe than sorry' situation.
I'm looking at a slightly unusual setup, although it's been touched on in the thread. It's a balanced active multi-amp setup, which means up to 7 channels (3-way plus sub). I'm taking it as a given that a 'conventional' series/shunt setup for 7 balanced channels is a practical impossibility due to matching. My plan is to use a single LDR per balanced channel in the shunt position, with precision series resistors in the series position. This will mean variable impedance for the stage, but I'll (eventually) be feeding UcD's with 100k input impedance, so I should be OK. (I may use a buffer in my current setup which has a lower input impedance) Furthermore, rather than a continuous pot I'm planning to use 'stepped attenuators' to adjust the LDR current, meaning that they can be matched via resistor selection. Since I'm using an all-digital PC source, I can use the LDR setup for coarse (6-10dB per step) adjustment, and use digital attenuation to fine-tune the level - I'm aiming at 3 or 4 steps of either 6dB or 10dB, since with a single LDR and say 5k series resistors it looks like 30dB is about the most attenuation I should expect.
I do have a question though - in the above setup the volume will be max when the LDR is 'off', and so it introduces a potential failure mode - if the power to the LDR's fails, they'll go high-impedance, and thus max out the volume. I don't really want to introduce relays into the picture since that would largely undermine the effort, but a NC relay that shunts the LDR and is pulled open by the same supply that powers the LDR seems to be the easiest solution.
Has anyone looked at this in their units? In the 'normal' series/shunt setup I *think* PS failure will send the unit to ~6dB attenuation which may be enough to avoid problems. Would an open relay tied to the signal path introduce any significant artifacts? I'm probably just paranoid about supply failure, but since I may have significant power tied directly to the drivers in my active setup, it would seem to be a 'better safe than sorry' situation.
Hi DWK123, glad your taking the plunge into building one, the questions you have, are what I've gone through in the past.
The use of just a series LDR is what started me of many moons ago and it does work very well with naturaly a shunt high quality resistor to ground, I did later find that series/shunt LDR's sounded so much better than just the series LDR, be it because of the constant impedence or what I just don't know, maybe it's that the LDR used as a Shunt sounds better than a high quality resistor, this goes against my anti voodoo thinking, so it must be the constant impedence that I prefer.
Your seven channel drive problem, I would first try everything possible to get the input impedence up to 500k a piece on your amps, if that is impossible then I would look at using high quality fet input buffers with say 500k input impedences, one I would look at are the AD825's good output drive, stable at unity gain, highly biased into class A, even though I would loathe to add these into the sigal path it may be your only option if you cannot raise the inputs of your amps to around 500K.
Cheers George
The use of just a series LDR is what started me of many moons ago and it does work very well with naturaly a shunt high quality resistor to ground, I did later find that series/shunt LDR's sounded so much better than just the series LDR, be it because of the constant impedence or what I just don't know, maybe it's that the LDR used as a Shunt sounds better than a high quality resistor, this goes against my anti voodoo thinking, so it must be the constant impedence that I prefer.
Your seven channel drive problem, I would first try everything possible to get the input impedence up to 500k a piece on your amps, if that is impossible then I would look at using high quality fet input buffers with say 500k input impedences, one I would look at are the AD825's good output drive, stable at unity gain, highly biased into class A, even though I would loathe to add these into the sigal path it may be your only option if you cannot raise the inputs of your amps to around 500K.
Cheers George
Buffer
The BUF634 is also a nice alternative to the AD. I think you'll find it adds negligible affect, provided you power it with unregulated +/-12V SLA batteries and use plenty of fast capacitance across the battery terminals and bypass caps at the IC pins. I've never heard it sound really good on regulated AC/DC.
The BUF634 is also a nice alternative to the AD. I think you'll find it adds negligible affect, provided you power it with unregulated +/-12V SLA batteries and use plenty of fast capacitance across the battery terminals and bypass caps at the IC pins. I've never heard it sound really good on regulated AC/DC.
George,
I may be missing something, but why would I need 500k input impedance? I'm not attempting to drive 7 channels in *parallel* - each one is completely independent with a separate DAC and associated output. The impedance characteristics needed shouldn't be any different than a straight conventional stereo setup as far as I can tell.
David,
I've seen the BUF634 floating around, but haven't ever looked at it in detail. I'm hoping I can get away without a buffer, but I'll look at them if it appears necessary.
I may be missing something, but why would I need 500k input impedance? I'm not attempting to drive 7 channels in *parallel* - each one is completely independent with a separate DAC and associated output. The impedance characteristics needed shouldn't be any different than a straight conventional stereo setup as far as I can tell.
David,
I've seen the BUF634 floating around, but haven't ever looked at it in detail. I'm hoping I can get away without a buffer, but I'll look at them if it appears necessary.
dwk123 said:
No I must have missed something, ok then if you are going to drive each amp with it's own Lightspeed Attenuator then each channel should see an input impedence on that amps channel of say over 80k ish and all will be fine, and then all the Lightspeeds can if you want, can be driven from the one dual gang volume control if you want to keep it simple, naturally all the gains of the amps will have to be right for your system to do this.
As for the Buf opamp, I noticed it's circuit is bipolar input, from experience this usually means output dc offset are high which need coupling caps, this is why the AD825 is good because of it's fet input and very low (couple of millivolts) dc offset so no coupling caps are needed and you can then direct couple to the amps, I found that the next worse thing to a potentiometer in the signal path is a coupling cap no matter how good the quality.
Cheers George
Ok, it's a long thread and I only read the beginning and the end, so forgive me if I managed to miss the meat of it all...
To me this thread seems faintly like commercial ad in nature, since the original poster says he is making these things commercially for profit.
On the merits of the LDR - anyone test its freq response vs resistances??
Might be fine, but anyone test it?
I have my suspicions.
Now as far as using a buffer - what's the point of using a device that needs a buffer, if the goal is minimalist to begin with?? Seems antithetical to the basic concept.
Why not a VCA if ur going that way?
Musing out loud...
I'll stick with my large silver wiping contact stepped DESCRETE L attenuators for now. Rather than pass through "n" resistors that are in a string (pot style), I use two resistors in an L network. Always two resitors, not more in the signal path. That's minimalist enough for moi. I use a 25k impedance, so that I can drive most situations satisfactorally. And I prefer 1/2watt MF resistors. Vishays would be cute as would Caddocks, but that's for you to muck with.
Relays in general are sonic death... well documented causality. (since someone mentioned them...)
For a cheap and fast way to get a remote control, seems like a plan... how good it is, that's very much another story.
Btw, better watch carefully what you use for a light source... and how do you linearize it??
_-_-bear
To me this thread seems faintly like commercial ad in nature, since the original poster says he is making these things commercially for profit.
On the merits of the LDR - anyone test its freq response vs resistances??
Might be fine, but anyone test it?
I have my suspicions.
Now as far as using a buffer - what's the point of using a device that needs a buffer, if the goal is minimalist to begin with?? Seems antithetical to the basic concept.
Why not a VCA if ur going that way?
Musing out loud...
I'll stick with my large silver wiping contact stepped DESCRETE L attenuators for now. Rather than pass through "n" resistors that are in a string (pot style), I use two resistors in an L network. Always two resitors, not more in the signal path. That's minimalist enough for moi. I use a 25k impedance, so that I can drive most situations satisfactorally. And I prefer 1/2watt MF resistors. Vishays would be cute as would Caddocks, but that's for you to muck with.
Relays in general are sonic death... well documented causality. (since someone mentioned them...)
For a cheap and fast way to get a remote control, seems like a plan... how good it is, that's very much another story.
Btw, better watch carefully what you use for a light source... and how do you linearize it??
_-_-bear
Not very commercial
Bear,
The LDR have the light source in the component. That is one of the advantages, the light source is all contained in each element.
I suspect the frequency response is very linear, guess I could run a 100K squarewave through and measure with spectrum analyzier. The FFT might show something. But since the resistive element seems fairly inert it may show nothing. And I am watching it snow in Nashville while my Scope/generator/FFT sits in Houston.
As far as commercial interests, georgehifi has given up his circuit details to allow direct copies if desired. Plus his selling price in Oz looks to be reasonable. And, if interested parties wanted one, he has never posted a link to order or for basic infomation. And this seperates his "business" from those who do post links to commercial interests.
George
Bear,
The LDR have the light source in the component. That is one of the advantages, the light source is all contained in each element.
I suspect the frequency response is very linear, guess I could run a 100K squarewave through and measure with spectrum analyzier. The FFT might show something. But since the resistive element seems fairly inert it may show nothing. And I am watching it snow in Nashville while my Scope/generator/FFT sits in Houston.
As far as commercial interests, georgehifi has given up his circuit details to allow direct copies if desired. Plus his selling price in Oz looks to be reasonable. And, if interested parties wanted one, he has never posted a link to order or for basic infomation. And this seperates his "business" from those who do post links to commercial interests.
George
Sorry, I'm going to disagree - disclosure of the circuit details or not.
Been reading forward in the thread, and he mentions his selling price, I think I saw a pix of the finished product, and of course people can reach him and me for that matter with no problem, click of the button to arrange a purchase.
If this is permitted here, I was not aware.
I can market a range of things in this manner, if it is...
_-_-bear
Been reading forward in the thread, and he mentions his selling price, I think I saw a pix of the finished product, and of course people can reach him and me for that matter with no problem, click of the button to arrange a purchase.
If this is permitted here, I was not aware.
I can market a range of things in this manner, if it is...
_-_-bear
Bear, I think you should probably read the middle of the thread. I don't speak for the moderators, but probably if you were to give complete instructions for a build of something, you'd "get away with" also mentioning that you sold it commercially for anyone who didn't want to DIY.
My personal take on it is that this thread is a useful resource and its usefulness is not affected by the mention of commercial availability.
Regards.
Aengus
My personal take on it is that this thread is a useful resource and its usefulness is not affected by the mention of commercial availability.
Regards.
Aengus
Going back in time:
It has been shown in various research, including Geddes recently, that the audibility of distortion has not so much to do with absolute level and more with the spectra of distortion. So, let's re-evaluate this statement.
Now, there are a great many DACs that either do or can be set to produce much more than 1 or 2v peak output. Commercially (I am 100% certain that) the Theta DACs all can produce ~10vpeak or better, the Monarchy DACs can be set to produce ~7vpeak or set to do more like 1-2Vpeak output. I am certain there are more that can and do produce more output.
Imho, the point of an attenuator is to attenuate signal if needed, linearly, and without adding or removing anything from the original. Regardless of the absolute value of the line level signals being dealt with.
Regarding another comment about the lack of "test equipment" to measure the distortion or lack thereof of the DUT, today it is fairly trivial to download some freeware that does nice FFT, and even with a lowly built-in sound card you get >90dB S/N, with a good 24/192 card you'll get ~>120dB. So, if you can sell them at $500 a pop, I'd expect that one could buy the requisite hardware?
With respect to "adding H2" - I expect that the addition of subtle amounts of second harmonic distortion can be a nice thing, especially with systems that may for one reason or another tend toward a somewhat "mechanical" or "hard" sound - or put another way sounding less than relaxed, natural and effortless - to use much hackneyed terminology. Nothing wrong with finding a way to do this for a solid state system that needs such a thing. But, let's not suggest that such a thing is either "transparent" nor "accurate?"
Also, it should be fairly simple, even with a scope to compare a good quality attenuator made of two MF resistors to the LDR attenuator. Just run the same signal into both, use a dual trace scope, invert one channel, ADD. The residual is the input of the LDR. Of course you can compare the input signal to the output of the MF attenuator and see what the difference is first - nil I expect.
Feel free to post the pix... ;_)
As I said, a nice way to add second harmonics is not an all bad thing, but IF that is what the LDR attenuator does, then let's get that on the table??
_-_-bear
georgehifi said:
It has been shown in various research, including Geddes recently, that the audibility of distortion has not so much to do with absolute level and more with the spectra of distortion. So, let's re-evaluate this statement.
Now, there are a great many DACs that either do or can be set to produce much more than 1 or 2v peak output. Commercially (I am 100% certain that) the Theta DACs all can produce ~10vpeak or better, the Monarchy DACs can be set to produce ~7vpeak or set to do more like 1-2Vpeak output. I am certain there are more that can and do produce more output.
Imho, the point of an attenuator is to attenuate signal if needed, linearly, and without adding or removing anything from the original. Regardless of the absolute value of the line level signals being dealt with.
Regarding another comment about the lack of "test equipment" to measure the distortion or lack thereof of the DUT, today it is fairly trivial to download some freeware that does nice FFT, and even with a lowly built-in sound card you get >90dB S/N, with a good 24/192 card you'll get ~>120dB. So, if you can sell them at $500 a pop, I'd expect that one could buy the requisite hardware?
With respect to "adding H2" - I expect that the addition of subtle amounts of second harmonic distortion can be a nice thing, especially with systems that may for one reason or another tend toward a somewhat "mechanical" or "hard" sound - or put another way sounding less than relaxed, natural and effortless - to use much hackneyed terminology. Nothing wrong with finding a way to do this for a solid state system that needs such a thing. But, let's not suggest that such a thing is either "transparent" nor "accurate?"
Also, it should be fairly simple, even with a scope to compare a good quality attenuator made of two MF resistors to the LDR attenuator. Just run the same signal into both, use a dual trace scope, invert one channel, ADD. The residual is the input of the LDR. Of course you can compare the input signal to the output of the MF attenuator and see what the difference is first - nil I expect.
Feel free to post the pix... ;_)
As I said, a nice way to add second harmonics is not an all bad thing, but IF that is what the LDR attenuator does, then let's get that on the table??
_-_-bear
I think this thread is great. George not only provided the schematic but also the source of the components used in the schematic. Anyone can DIY a unit based on the information on this thread. Anyone that want to buy from George may do so as well. If this thread is a giant commercial, I say we need more of this kind of commercial.
Commercialism
I heartily agree that anyone like George who generously discloses his design should be able to at minimum provide a pointer for those who might like to purchase a finished product. As a precedent, check out the ECdesign DIY NOS DAC thread on this forum. Over a year and many thousands of hits this thread evolves from an exchange of ideas to a $7000 finished kit now available for sale. If you don't care to buy it, build one from scratch.
Best regards,
Dave
I heartily agree that anyone like George who generously discloses his design should be able to at minimum provide a pointer for those who might like to purchase a finished product. As a precedent, check out the ECdesign DIY NOS DAC thread on this forum. Over a year and many thousands of hits this thread evolves from an exchange of ideas to a $7000 finished kit now available for sale. If you don't care to buy it, build one from scratch.
Best regards,
Dave
Sorry guys, but I'm not going to sit back and let this slander of Bears go un-noticed, this thread originally was started by st2_998 with the heading of
" Passive preamp? chapter 1 "
It was the moderators themselves after discussions with DAN the head moderator who saw fit to change it to
" Lightspeed Attenuator a new passive preamp "
you will see now the original post and heading shifted down from 1st to 2nd post. As all of the discussions on this thread are of the Lightspeed circuit.
Bear chimed in on the post 371 of this thread and is just being bear, can't see the forest through the trees, leave him alone and he'll go back into hibernation.
Maybe he's sick of seeing this thread constantly in the SOLID STATE forum where it doesn't belong.
So am I, this is why I've asked to have a separate PASSIVE PREAMP forum because of the great numbers of members who believe in passive preamps.
Please if you feel the need for a passive forum vote here under the PASSIVE PREAMP heading, it's the only way it will be achieved.
http://www.diyaudio.com/request/
Cheers George
" Passive preamp? chapter 1 "
It was the moderators themselves after discussions with DAN the head moderator who saw fit to change it to
" Lightspeed Attenuator a new passive preamp "
you will see now the original post and heading shifted down from 1st to 2nd post. As all of the discussions on this thread are of the Lightspeed circuit.
Bear chimed in on the post 371 of this thread and is just being bear, can't see the forest through the trees, leave him alone and he'll go back into hibernation.
Maybe he's sick of seeing this thread constantly in the SOLID STATE forum where it doesn't belong.
So am I, this is why I've asked to have a separate PASSIVE PREAMP forum because of the great numbers of members who believe in passive preamps.
Please if you feel the need for a passive forum vote here under the PASSIVE PREAMP heading, it's the only way it will be achieved.
http://www.diyaudio.com/request/
Cheers George
bear said:Ok, it's a long thread and I only read the beginning and the end, so forgive me if I managed to miss the meat of it all...
1 To me this thread seems faintly like commercial ad in nature, since the original poster says he is making these things commercially for profit.
2 On the merits of the LDR - anyone test its freq response vs resistances??
3 Might be fine, but anyone test it?
I have my suspicions.
4 Now as far as using a buffer - what's the point of using a device that needs a buffer, if the goal is minimalist to begin with?? Seems antithetical to the basic concept.
5I'll stick with my large silver wiping contact stepped DESCRETE L attenuators for now. Rather than pass through "n" resistors that are in a string (pot style), I use two resistors in an L network. Always two resitors, not more in the signal path. That's minimalist enough for moi. I use a 25k impedance, so that I can drive most situations satisfactorally. And I prefer 1/2watt MF resistors. Vishays would be cute as would Caddocks, but that's for you to muck with. bear
1 This has been addressed by other members
2 It's frequency response is flat from Dc to Gigahertz regardless of volume settings as long as the source and load impedences are met, that I have outlined many many times.
3 Myself and a few other have, it's fact
4 This was only a suggestion if source and load impedences could not be met as I have outlined many many times, I also am against the use of buffers if you read the thread and have said this many many times also.
5 There are now over 100 units that I know of, DIY and my own, and not one of these owners had a negative remark about the Lightspeed in fact all were very possitive, even up against some of the best preamps, also very positve against the best passive devices such as your switched silver control, expensive silver passive transformers, and a host of high quality potentiomters Alps, Bournes, Penny@Giles.
Now Bear, how many Lightspeeds did you say have you heard?
Cheers George
bear said:
Seems entirely possible to me that the LDR, by adding H2 distortion, actually reduces perceived distortion in the listener. If the ear masks distortion in a distortion spectrum where H2>H3>etc (Hn>Hn+1), then adding H2 to those nasty H3 components every SS generates might work to reduce their perceived audibility.
But that's just guessing, and the real story, in its fullest extent, probably lies outside my (and probably anyone's) ability to actually explain why two different components sound the way they do. I mean, try to get two people to agree on why two different capacitors (say, an mkp and a teflon) sound the way they do.
Off the mains, no relays, distortion-reducing disortion, wow, what a deal.
BTW, guys, I've been experimenting with the LDR power supply and can hear sonic improvements with a cleaner supply. The audibility of the supply could be easily tested, methinks, by injecting a healthy, say, 100Hz component onto the 5V supply rail, then cranking up the volume to hear if it's audible. If it is, one could reasonably guess that other frequencies on the 5V supply (noise) would be audible likewise.