Yet another LDR Volume control / preamp variant

[gamve]

Trying out another version of a passive light dependant resistor volume controller pre amp. This differs to the "Lightspeed" and the "Lighter Note" as it does not use optocouplers simply a discrete LED positioned closley to  L&R channel LDR's. This can be tuned basically by mechanically moving either the LDR of either channel or the position of the single LED. External tuning is via a trimmer for balance. What really got my interest was a novel approach to fixing the usual impedance matching issues. This design has high bandwidth video buffers on both the inputs and the outputs making impedance matching good for all sources and power amps. The only issue so far is that you need a power amp that is easy to drive to get sufficient volume. This pre was custom built for me with 3 inputs and dual outputs to feed a power amp and a set of sub woofers.
Will comment later on the sound quality as I have only had it set up with an Otari MX5050 BII-2 R2R playing a sus 1/4 track tape for tonight. Did have a quick listen with a CD the other night. Though it sounded a little hard and sharp but thats a hard judgement for a brand new piece of equipment.
Cheers
G

[ozmillsy]

interesting...... is there any online info on it,  or is it a diy thingy that has been made for you?

[gamve]

Bit of both really Oz. Let me do a bit more fiddling and tuning before I decide if it's worth sharing further.
It is clunky to use with 3-5 degrees as the full operating range of the volume pot. Having said that
the sound quality after a bit more use has potential.

[gamve]

OK, Spent the weekend listening to this thingy and I can say is that it is like a drug. The longer you listen the more you want to listen. It's quirky and not easy to use but it is the best passive type pre I have heard to date. Any of you guys want to check it out we can probably work out a loan. Might work out good for you Oz as I would love to hear the lightspeed?
Link    http://www.audiophiletalk.com/cgi-bin/yabb2/YaBB.pl?num=1254751267/0

[ozmillsy]

Graham,  is there any chance you can confirm the cost of the parts for this LDR volume control ?

[gamve]

Hi Oz,
Cost me about a grand including shipping from the US but its fully built and tested and to suit 240V. (it is not a kit)
Just remember this is nothing like the lightspeed as in it has no impedance restrictions due to buffers on both the
inputs and the outputs. The manufacturer is a dude I have had a few telephone conversations with him. His take on
the unit is what he calls redneck engineering. He has had similar circuits running for ten years or more. I've nicknamed
mine the lighthouse as the whole interior of the unit is lit up permanently by the exposed LED. Tinkerers delight as you
can move the LED and the LDR's around (discrete components) to tune sensitivity and fine tune the channel balance
(also external balance control) If your power amp is sensitive enough and your source has plenty of drive, this thing
sounds great. Where are you located? Could organize a loan.

The lighter note kit was $ 130.00 near enough if that is what you were referring too.

[ozmillsy]

Cheers,  yeah I've always felt that buffers were a good idea in the Lightspeed,  and it's definetly a good thing for us KillerDac users.   It would be fun to build 1 of those kits.  How do I source 1 of them?

Happy to send you the Lightspeed, to do your own comparison (when I get it back).

I wonder if LDR's in general impart their own kind of distortion?   Does anyone have an opinion on this?

[zenelectro]

Cheers,  yeah I've always felt that buffers were a good idea in the Lightspeed,  and it's definetly a good thing for us KillerDac users.   It would be fun to build 1 of those kits.  How do I source 1 of them?

Happy to send you the Lightspeed, to do your own comparison (when I get it back).

I wonder if LDR's in general impart their own kind of distortion?   Does anyone have an opinion on this?

Ref pics below extracted from Silonex info.

As you can see they start to fall over after about 1V rms

I have measurement capability on pc + soundcard + analyser software of around 0.0007%
but never got around to setting up an FFT measurement of LDR to see exactly what the
spectrum looks like.   

cheers

T



 

[zenelectro]

Great volume control idea:

Use an LDR shunt vol control across 1541 DAC OP resistor.   

Instant shunt vol control inside DAC.


T

[georgehifi]

Cheers,  yeah I've always felt that buffers were a good idea in the Lightspeed,  and it's definetly a good thing for us KillerDac users.   It would be fun to build 1 of those kits.  How do I source 1 of them?

Happy to send you the Lightspeed, to do your own comparison (when I get it back).

I wonder if LDR's in general impart their own kind of distortion?   Does anyone have an opinion on this?

Ref pics below extracted from Silonex info.


As you can see they start to fall over after about 1V rms

I have measurement capability on pc + soundcard + analyser software of around 0.0007%
but never got around to setting up an FFT measurement of LDR to see exactly what the
spectrum looks like.   

cheers

T



 

The graph you posted was from the Perkin Elmer Vactrols VCL5c1 and 5c2 not the Silonex NSL32SR2S brand which much better because they allow you to get to a lower much volume also, been there done that.
The posted Silonex graph shows you at typical cd levels a distortion of .1%  or less, these are 2HD (second harmonic) which are the good ones as Nelson Pass has also measured. The guys who use SET AMPS (Single ened Triodes) love their 2HD which is in the whole %'s sometimes 2 and 3 % or higher.
Cheers George   

[zenelectro]

Cheers,  yeah I've always felt that buffers were a good idea in the Lightspeed,  and it's definetly a good thing for us KillerDac users.   It would be fun to build 1 of those kits.  How do I source 1 of them?

Happy to send you the Lightspeed, to do your own comparison (when I get it back).

I wonder if LDR's in general impart their own kind of distortion?   Does anyone have an opinion on this?

Ref pics below extracted from Silonex info.


As you can see they start to fall over after about 1V rms

I have measurement capability on pc + soundcard + analyser software of around 0.0007%
but never got around to setting up an FFT measurement of LDR to see exactly what the
spectrum looks like.   

cheers

T



 

The graph you posted was from the Perkin Elmer Vactrols VCL5c1 and 5c2 not the Silonex NSL32SR2S brand which much better because they allow you to get to a lower much volume also, been there done that.
The posted Silonex graph shows you at typical cd levels a distortion of .1%  or less, these are 2HD (second harmonic) which are the good ones as Nelson Pass has also measured. The guys who use SET AMPS (Single ened Triodes) love their 2HD which is in the whole %'s sometimes 2 and 3 % or higher.
Cheers George   

Right you are George, the pdf had no reference to manufacturer and I assumed Silonex.

WRT getting enough attenuation - this is an interesting point of discussion. For a commercial product you need
a wide range of attenuation to suit everyones needs. However the very best sounding systems, such as
Stevens ref system usually uses none!

I am of the same way of thinking and use a shunt vol control by varying the DAC OP load R with a
12 way switch. Instant no pre amp!

Correct me if I'm wrong here but from memory your LDR vol control uses LDR's in both series and shunt
- hence the highish distortion figures. I'm thinking that a series R with shunt LDR would be a better
way to go transparency wise but with a more limited range of volume control.

What's your experience with this?

One of these days i'll do a spectrum measurement on a few LDR's and see what sort of nasties
lie beyond the second harmonic.

cheers

Terry




 




 

[georgehifi]

Nelson Pass using the latest AP analysers has done it I trust him more that anyone, he also kindly developed a very simple direct coupled  buffer (later to be called the B1 buffer) for the Lightspeed Attenuator so customers can use it on his low input impedance single ended class A amps  
My MK1 Lightspeed attenuator was just that years ago (series resistor and shunt LDR) but the Series LDR and Shunt LDR sounds so much better all the MK1's were all recalled and converted to MK2 status, and not one customer regretted it.
 MK2's also have these advantages they can go much lower in level, have a more stable i/o impedance and the sound was so much better, tighter lower base more delicate highs better sound staging depth and dynamics. You can read all about it on the DIY Lightspeed forum, over 3,000 posts and 1/2 a million reads,
  http://www.diyaudio.com/forums/analog-line-level/80194-lightspeed-attenuator-new-passive-preamp.html

I only wish the MK1 was as good because the expense and time is exponentially more doing the MK2

Cheers George  

[zenelectro]

Nelson Pass using the latest AP analysers has done it I trust him more that anyone, he also kindly developed a very simple direct coupled  buffer (later to be called the B1 buffer) for the Lightspeed Attenuator so customers can use it on his low input impedance single ended class A amps  
My MK1 Lightspeed attenuator was just that years ago (series resistor and shunt LDR) but the Series LDR and Shunt LDR sounds so much better all the MK1's were all recalled and converted to MK2 status, and not one customer regretted it.
 MK2's also have these advantages they can go much lower in level, have a more stable i/o impedance and the sound was so much better, tighter lower base more delicate highs better sound staging depth and dynamics. You can read all about it on the DIY Lightspeed forum, over 3,000 posts and 1/2 a million reads,
  http://www.diyaudio.com/forums/analog-line-level/80194-lightspeed-attenuator-new-passive-preamp.html

I only wish the MK1 was as good because the expense and time is exponentially more doing the MK2

Cheers George  

Thanks for your input George,

Did Nelson post the results anywhere on DIYA?

WRT Mk1 vs Mk2 - that's interesting.

I was thinking about this a bit deeper, a couple of things could be happening here -

a) There is a degree of non linearity cancellation of the series and shunt LDR. At exactly 1/2
vol I would expect it to be most linear, as they both match each other.

b) Maybe we just like listening to H2 and that nice juicy spectrum!   

If I had time or inclination, I would like to really check this is with spectrum analysis at a few different
volume attenuation levels and input levels.

cheers

Terry
 



 

[gamve]

Cheers,  yeah I've always felt that buffers were a good idea in the Lightspeed,  and it's definetly a good thing for us KillerDac users.   It would be fun to build 1 of those kits.  How do I source 1 of them?

Site http://www.buildanamp.com/

Happy to send you the Lightspeed, to do your own comparison (when I get it back).

I wonder if LDR's in general impart their own kind of distortion?   Does anyone have an opinion on this?

[ozmillsy]

Terry, you're welcome to borrow my unit, to measure (when I get it back).

[zenelectro]

Terry, you're welcome to borrow my unit, to measure (when I get it back).

Thank you.

I won't be doing it for a while though, don't want to get distracted
from current projects.

T

[ozmillsy]

I won't be doing it for a while though, don't want to get distracted
from current projects.

hehe,  no worries.

Intestesting info on the 2nd harmonic distortion.     Everything is a compromise in some way I guess.

[georgehifi]

Nelson Pass using the latest AP analysers has done it I trust him more that anyone, he also kindly developed a very simple direct coupled  buffer (later to be called the B1 buffer) for the Lightspeed Attenuator so customers can use it on his low input impedance single ended class A amps  
My MK1 Lightspeed attenuator was just that years ago (series resistor and shunt LDR) but the Series LDR and Shunt LDR sounds so much better all the MK1's were all recalled and converted to MK2 status, and not one customer regretted it.
 MK2's also have these advantages they can go much lower in level, have a more stable i/o impedance and the sound was so much better, tighter lower base more delicate highs better sound staging depth and dynamics. You can read all about it on the DIY Lightspeed forum, over 3,000 posts and 1/2 a million reads,
  http://www.diyaudio.com/forums/analog-line-level/80194-lightspeed-attenuator-new-passive-preamp.html

I only wish the MK1 was as good because the expense and time is exponentially more doing the MK2

Cheers George  

Thanks for your input George,

Did Nelson post the results anywhere on DIYA?

WRT Mk1 vs Mk2 - that's interesting.

I was thinking about this a bit deeper, a couple of things could be happening here -

a) There is a degree of non linearity cancellation of the series and shunt LDR. At exactly 1/2
vol I would expect it to be most linear, as they both match each other.

b) Maybe we just like listening to H2 and that nice juicy spectrum!   

If I had time or inclination, I would like to really check this is with spectrum analysis at a few different
volume attenuation levels and input levels.

cheers

Terry
 



 

[ozmillsy]

why do they all have a peak at 1k ?

[zenelectro]

why do they all have a peak at 1k ?

Thanks George, are these the measurements Nelson did?

Oz, 1k is the frequency of the signal being measured. It is at 0dBV which is 1V RMS.

A few observations about these measurements:

- 2nd harmonic is below 3rd and 4th is below 5th - so there is even order cancellation going on
in the circuit. The P.E. manufacturers data I showed previously indicated that the fundamental mode
of distortion is 2nd harmonic in LDR's.

- This indicates that the circuit is very likely either a) balanced or b) 2 LDR's in series/shunt configuration.
This results in distortion cancellation and even orders are the ones that cancel.

- In a 6dB attenuation series/shunt arrangement most -all- distortion should be cancelled if the 2 LDR's are closely
matched.

George, what is the circuit configuration that these measurements were done on?

Also, whilst -112dB distortion figures at 1kHz look great, I'd be much more interested in 1k versus 10k figures
and at a certain source impedance. This will tell of any distortion due to capacitance of the LDR and will give a
much better indication of how the attenuator will react to a high source impedance.

To make meaningful sense out of these very low numbers requires more information.

What this doea tell us however is that LDR's when used in the right topology can be -very- linear.

You've got me interested!

cheers

Terry