[randytsuch]

I searched, and saw at least one guy here who had one of these
http://www.diyaudio.com/forums/showthread.php?s=&threadid=80194&perpage=25&pagenumber=1

I am thinking about building one.  Right now, my SB is the only source I listen to, for "real music", and I use the digital volume control in the SB, but I am worried that I am losing information that way.

I ran across this thread at diyaudio the other day, sounds interesting.

Basically, you use a LED and an optocoupler to control the volume.  The LED brightness varies the resistance of the optocoupler. 

Seems simple, and it is supposed to sound very good.



link to picture
http://drop.io/randytsuch01/asset/cdocumentsandsettingsrtsuchiyamamydocumentsmypictureslightspeedschematicduallogpot

Here is one version I found in the long diyaudio thread.
I think the only hard part is matching opto's, to find a set that will track together well.  You have to buy a bunch of them, and hand match them for tracking, for at least a few points.

Randy

[BradJudy]

I found kits for something like this recently, let me see if I can track it down. 

IIRC, they are have particular impedance matching needs that don't make them appropriate for many uses. 

[mgalusha]

There is an article in this months audioXpress about using something similar in an AVC. The author mentioned that he found essentially no distortion in the photo resistor. Certainly an interesting idea.

[Gordy]

Greg Ball has a kit... http://www.ska-audio.com/diy/optivol.html

[BradJudy]

Greg Ball has a kit... http://www.ska-audio.com/diy/optivol.html

That's the one I was thinking of - I couldn't find it.  Thanks for the link. 

[randytsuch]

Greg Ball has a kit... http://www.ska-audio.com/diy/optivol.html

From the diythread, on the optivol

As I have stated before this was the same way (series resistor/ shunt ldr MKI) I built the first MKI Lightspeed Attenuator around some 35 years ago, and the start of the production Lightspeed Attenuator a few years back.
It works but it's performance/sound quality is considerably down on the on the (series ldr/shunt ldr MKII) system.
Bass is tighter and overhaul more transparent and dynamic on the MKII version. The MKI sounds very much like a good old school tube amp, a bit soft, thickish in the bass and bit veiled and lacking in dynamics and transparency, but comfortable.
This is why all my MKI Lightspeed Attenuators were recalled and all upgraded to MKII status, and everyone of the owners said it was a vast improvement, except for one, who said now it is not very tolerant of hard sounding equipment anymore, that to me is a plus.
The MKI (series resistor/shut ldr) has a varying input impedance presented to the source and also a varying output impedance presented to the input of the amplifier, it also has a lot louder minimum volume, all these factors contribute to the reduction in sound performance/quality, also from memory it has a lot higher second harmonic distortion to it, that may be the reason for it's veiled thicker sound.
I think back in this thread a way I posted the distortion figures from Silonex on the difference between the MKI and MKII, and for a typical 1-2 volt input from a cd player and the MKI system had far greater 2HD distortion figure to that of the MKII version.
I wish I could have still made the MKI because it was so much less time consuming to make, but it is comprimised, and that won't do.

Cheers George


This is from the "inventor" of the lightspeed.  So, the SKA version has a fixed resistor and variable ldr, will the newer lightspeed has two variable ldr's.

Randy

[randytsuch]

And here, courtesy of Nelson Pass, as an active version of the lightspeed.

Takes care of the amp's input impedance concerns.

OK, this would be the last manual version that you are
likely to see. It has all the pieces, and allows pretty complete
adjustment of input impedance and range, top to bottom.

As a 2 channel piece, LDR1 and LDR3 should be matched to
each other, and LDR2 and LDR4 should also be matched.
The match should take place around the 5 Kohm resistance
value. LDR1 and 3 do not have to match LDR2 and 4.



I'm thinking about trying the active version.

Randy

[JoshK]

I read about these some while back.  I was interested in the adaptability to multichannel applications.  It still seems that there are very few no compromise solutions to multichannel attenuation.  I am interested in multichannel for multi amping active crossover setups, not surround sound, but it could be either or. 

I was interested in the LDR method because of its reported transparency but also because it is the voltage that defines the attenuation.  So it is conceivable that one could control the voltage reference with an pot, even motorized pot for remote, and then buffer this reference to drive the voltage for each channel.  The drawback is channel matching, but it looks doable. 

As with all things, I just have too much to get done to attempt something like that.

[ondesx]

Greg Ball has a kit... http://www.ska-audio.com/diy/optivol.html

Has somebody tried one or several of these kits ?

[dwk]

I read about these some while back.  I was interested in the adaptability to multichannel applications.  It still seems that there are very few no compromise solutions to multichannel attenuation.  I am interested in multichannel for multi amping active crossover setups, not surround sound, but it could be either or. 

I was interested in the LDR method because of its reported transparency but also because it is the voltage that defines the attenuation.  So it is conceivable that one could control the voltage reference with an pot, even motorized pot for remote, and then buffer this reference to drive the voltage for each channel.  The drawback is channel matching, but it looks doable. 

The problem with multi-channel applications and conventional use of the LDRs is matching. You have to match not only against the other LDR in the pair, but now across channels as well.  You better get a big batch.

I'm still noodling on a multichannel approach, but pairing up an arduino with the LDR's is something that looks attractive. The arduino is a simplified Amtel microcontroller board, but it supports 6 input a/d channels for pots and 6 output pwm channels to control the LDRs. I figure it's a good platform for an LDR solution because you can programmatically vary the PWM output per LDR to achieve dynamic matching. the main potential problem is that with only 256 levels, it may not have enough resolution for well-matched output over a wide volume range.

My alternative idea is the basic PGA2310 setup, and while I don't doubt it would work 'well enough', it's not terribly exciting.

Like you though, it's a question of time - I think this would work but it would be tweaky/fiddly, and so it may not come together all that quickly.

[georgehifi]

Sorry this should have been posted here instead of over at "DIY Sowter/Shallco TVC vs commercial offerings"
http://www.audiocircle.com/index.php?topic=66348.0



Hi guys, saw this thread when I Googled my Lightspeed Attenuator. Just to clear up the impedance matching issue.
For a good match in a system the Lightspeed Attenuator should see:
1 A CD player or Dac that has an output impedance of less than <1kohm (1000ohms)
2 The poweramp solid state or tube should have an input impedance of more than >47kohms (47K)

When these parameters are met you will have the deepest cleanest bass, most dynamic transients, most transparent sound stage and cleanest sweetest highs, you have heard.

For those of you who wish to build you own, I finally relent and give the details to DIY in this thread.

http://www.diyaudio.com/forums/showthread.php?s=85840c28cfe5fd93fde3bc2b751c8d7d&threadid=80194&perpage=25&pagenumber=1

And for those who wish pictures and more info on my production Lightspeed Attenuator please email me with your email address and I will send it to you.

Cheers George

[BillB]

A friend of mine sent me a production lightspeed to try out. My system does not meet those specs apparently as I do not have full off and volume ramps up rather quick (too quick for my tastes).

It does sound very, very nice though.

My test rig is a Panasonic A-105 1st gen dvd player to a monarchy sm-70 pro.

[Mike B.]

I built one last year and find it the best of passives if you have the right impedance match. What it has that the transformer offerings don't is a sense of drive only available with actives. I am currently running low input impedance SS amps so have switched to a H-Cat preamp. Georgehifi is the original designer.

[georgehifi]

A friend of mine sent me a production lightspeed to try out. My system does not meet those specs apparently as I do not have full off and volume ramps up rather quick (too quick for my tastes).

It does sound very, very nice though.

My test rig is a Panasonic A-105 1st gen dvd player to a monarchy sm-70 pro.

Looking at the specs of your Panasonic A-105 1st gen dvd player & Monarchy SM-70 pro everything should have been ok.

It is true that the minimum volume is not complete zero, if it was made using the NSL32SR2S in series & shunt (my MkII) version of the Lightspeed Attenuator (and not the NSL32SR3)  then the minimum level is so low in level you almost have to put your ear up to the speaker to hear anything.
All my original MkI (shunt only) I thought I upgraded to MkII a few years ago, but I may have missed this one. If you give me the name of your friends one I can look at my records and see.
As for the ramping up too quick this also says MkI to me, as the volume curve min to max of the MkII is very close to the ideal logarithmic curve. At least you got to hear just how good it was.
If you care to build one below, is the circuit I've used now for the past few years, MKII production Lightspeed Attenuator, as you can see it is double regulated, the 1K calibration (with generator & oscilloscope) trimpot only has to go onto the louder or the two channels.
For some reason (I'm an idiot ) I cannot insert a circuit image? Maybe it's broken and I'm not an Idiot.
Got it!!!!!! man this has to be one of the most complicated forums to post a circuit on.

Cheers George 

[BillB]

Thanks George, I will make sure he sees this thread. Zero volume was more like 10-20%...background music type levels.

[georgehifi]

Thanks George, I will make sure he sees this thread. Zero volume was more like 10-20%...background music type levels.

Or your Monarchy power amp could have higher than normal amount of gain, it does use an opamp as the input stage, which is not my cup of tea, (I prefer discrete) but then it is an OPA2604 which is one of the better opamps if you have to use them.

Cheers George 

[BillB]

Yeah, I wasn't too thrilled about that part either, but it is a very good sounding amp. I had given thought about bypassing the input because I always use a preamp, but even in a pre with gain I can usually get 1/2 volume or greater when really rocking out.

It is supposed to only need 1v input though.