Lightspeed Passive Preamp

[jonclancy]

Hi All,

OK, I'd thought I'd share this gem. It's all Leo's fault, as he was the one who mentioned it in the first place!

First off, a BIG thanks to George, who has generously shared the details of his commercial pre.

The Lightspeed is an attenuator (passive preamp) that uses a Light Dependent Resistor (LDR) in series and one LDR shunted to ground per channel. The LDR package comprises one LED and one Photocell in a 6mm cube (ish). The LED intensity, and therefore the resistance, is controlled by a standard Dual 100K LOG pot and powered by a 5VDC supply.

I recieved my LDRs a few days ago and had to drop everything to lash them together. In short, the results are superb!! The project details I'll post below, and I would highly recommend a tryout if you are looking for a decent attenuator.

"But what would this have to do with headphones?" I hear you ask...

How about a drop-in upgrade to your headphone amp? This project is simple enough to do on matrix board. Mine is still point-to-point lashed up, but I have stripped out my dead Trends TA-10 amp and the case will be perfect as a stand alone attenuator.

Has anyone else tried this marvel??? I'm using a 100K Alps Blue RK-27 pot and a spare ALW Super Regulator I had in stock. I think you will get similar results with a cheapo pot and 7805 power supply...

Essential details are on the first 4 pages of the DIYA thread.

www.diyaudio.com/forums/showthread.php?s=&threadid=80194&perpage=50&pagenumber=1

Silonex Audiohm range:

www1.silonex.com/audiohm/

And the NSL-32SR2 used in the project:

www1.silonex.com/datasheets/specs/images/pdf/104057.pdf

Cheers

Jon

[Deleted]

A small word of warning.
The project requires an amplifier with a minimum input impedance of 100K ohm. Most solid state amplifiers have an input impedance considerably lower than this.
Jon may remember me mentioning that I played with a similar concept around 30 years ago, where the LED and LDR in a tube were used to give a small amount of dynamic range expansion. The ordinarily available LDRs like the ORP12 etc. are not suitable for use here, as their characteristics vary too widely between samples, and their lowest resistance is not low enough.
SandyK

[jeffc]

Hi Jon,

We might have discussed the Lightspeed concept vol control and I certainly have with Alex. Heeding Alex's warning about impedance needs, a variant has been designed for which PCBs and matched LDRs are available over at the DIYaudio Nelson Pass forum.

www.diyaudio.com/forums/showthread.php?s=&threadid=134316

www.diyaudio.com/forums/showthread.php?postid=1753639#post1753639

Looks to be one very nice piece of gear that promises something a bit better than even the fanciest attenuators.

Guys, enjoy your Easter long weekend.

cheers.. jeffc

[Deleted]

Looks to be one very nice piece of gear that promises something a bit better than even the fanciest attenuators.

Jeff
If you are going to use it only with a unity gain buffer, I tend to agree.
However, if the intention is to use it to replace the attenuator in a normal amplified preamplifier, I disagree.
There is no way that I would replace the attenuator in the SC HA,or my Class A preamp, with the LED controlled attenuator + buffer. IMHO, the performance will then become inferior to the attenuator, and in the case of the DACT2, I have doubts that it will outperform the DACT2 , although it does have the advantage of continuous volume control action. I would also like to see an indepth distortion AND frequency response comparison between both types, as well as noise figures,especially as an LDR is a semiconductor variant.
Alex

P.S.
I would be interested to see any comments from Robert on these questions.

[jeffc]

Hi Alex,

No first hand experience and you know me, I really have no idea technically where this LDR-based buffer come preamp would be most applicable, or indeed hit the mark in terms of improved SQ over a passive implementation of an attenuator or a well designed preamp with gain. Obviously impedance of the up and downstream components will play an important role in this, but in principal, removing a contact based volume control seems a worthwhile line of investigation, and the Lightspeed attenuator concept couldn't have gotten such good warps for sounding crap. Jon sounds happy enough.

That said, if there was some fail safe way of protecting speakers against inadvertent destruction due to volume spikes, digital volume supplied by software in HDD music server setups could bypass the need and attenuator or preamp altogether in most cases. That's where I'd ultimately like to head.

I use a 10K attenuator in the T-amp system fed by the sans output decoupling caps, LM4562HA opamp CS4297-based cheapo DAC and it seems to work a treat. Input impedance of the T-amp is about 20K and average max output impedance from the 10K attenuator about 1K, this seem right. If so you'd expect that it should work OK at worst.

In my lounge NAD system, the simple jfet based capless Nelson Pass B1 buffer sans input or output caps that currenty uses a cheap by OK 50K pot and 1K input and 220R output series resistors feeding the NAD C320BEE power amp section directly absolutely slays the NAD preamp, even with use of my extensively modded MF X10V3 buffer between CDP and preamp.

Take home message is that these simple low impedance implementations for 'controlling volume' can work a treat. Will the capless B1 buffer add anything over a 10K attenuator in the T-amp system, I don't know, but sure as hell I'm going to try it as I haven't read anywhere else where someone has, and I'd like to know, if simply out of curiosity. ;D

Sorry for the rave, but I'm keen to hear about the fun Jon has been having building and testing a Lightspeed.

And for what its worth, here's another potentially good solution to that 'what volume would I like to listen to now' need, sexy one at that.

diyparadise.com/web/index.php?option=com_content&task=view&id=144&Itemid=26

Just a rave, few beers helps.

cheers.. jeffc

[Deleted]

Jeff
Silicon Chip designed a fairly similar kit a little while back, but with a more comprehensive blue display. Unfortunately, neither Altronics or Jaycar supported this particular project. I would have tried one, with or without the lovely Beyonce, if they had.
Alex
P.S.
I am not surprised the B1 slays the NAD preamp. If I didn't need a little added gain for some sources , I would be trying this unity gain buffer too.

[robertkd]

these are certainly an interesting concept, having used this arrangement in audio compressor/limiters with much success but felt at the time dual gate fets to be as good but offering better characteristics, mind you this is like 20 plus years ago

I see from some of the write-ups the L type attenuator seems popular anyone thought of making it a T type i.e. series in shunt and series out ?

Also when they say low distortion what do they actually mean, I had a hunt around the site but couldn't find any specifics, noted that there is mention of expected even order harmonic distortion product but no real detail. Has any one looked at intermod distortion products with multi tone test signals?

there is always some form of semiconductor associated with gain control elements given simple metal on metal metal on conductive track or something in the solid state or dare I say trans conductance element.

They sound interesting where are they available in Aus??

Robert

[jonclancy]

Hi All,

I bounced off the forum last night, but needed some Zzzzs, so no update till now.

This is a very interesting project, because for me, in my system, it appears to be working extremely well!! I checked the D290P input Z and it was a mere 7K5 - so nowhere near the 100K or so that in theory the LS prefers. As a side note, I am using the SR2 and not SR3 part that some have tried. George specifies the SR2. Matching is an issue, and from what I gather from Uriah, it is a pretty huge amount of work to get all these parts sorted. Also, the massive variation among the parts means that getting "selected" items from the supplier is a complete waste of time and money. You're better off just getting a random batch and matching them yourself - rather than trying to rematch from a group with wide tolerances.

Quite a few people have tried them with, or are considering using, buffers. I'll be posting the Q over at DIYA as to which minor changes I would need to do to the Pumpkin to allow the LS to be used without a buffer. If a buffer is required, then Mr Pass' B1 should do the job nicely - and the artwork is provided via the Gerbers posted at PASSDIY.

I'm in the process of casing up my hardwired LS. I think I might even keep it hardwired in the main for this implementation, and go with a small board for V2. My case is drilled and the matrix board cut. Sadly, it won't be worked on for a few more days as I'm working over Easter. With the D290 back in the system (with the D290P), I can hear the differences. The D290 isn't that bad (maybe the BG caps in the AYA are finally burning in), but it doesn't have that timbre that the LS is able to reproduce.

There is much more to come with this story (even though it will take a while), as the LS really ought to be properly compared with the TVC, stepped attenuator, and even a shunt-modded pot. Either way, I don't see myself chosing any one over another, as each will hold it's own in different systems. Flexibility and choice - the name of the game.

Cheers

Jon

[jonclancy]

P.S. George is in Manly, IIRC. I believe there might be another batch coming up for sale in a couple of months - details will be over at DIYA on the Lightspeed thread.

[rbrook]

Sounds interesting,
I've ordered 16 of the SR2's so hopefully can match a few up to make at least 2 in close tolerance.

Any recommendations on the 5V supply for them or will a simple 7805 fed from a 12v wall wart suffice?

I'll put together a spare one if i can to let anyone in the grotto have a play with if they want to. (its so small postage wont be an issue)

Richard.

[robertkd]

Is it me or is the title a bit of an oxymoron given it's got active electronics

Robert

[Deleted]

Any recommendations on the 5V supply for them or will a simple 7805 fed from a 12v wall wart suffice?

Richard
It will work, but you won't get the best possible results.
A good quality shunt regulator is recommended.
Alex

[Deleted]

Apr 14, 2009 8:45:14 GMT robertkd said:

Is it me or is the title a bit of an oxymoron given it's got active electronics

Robert

Yeah, I could never work that out.

[Deleted]

More info from George about amp input impedance.

www.audiocircle.com/circles/index.php?topic=59802.msg617882#msg617882

[jonclancy]

Hi Everyone!

Here's an update. I have finally got round to casing up my LS. I had a dead Trends Audio TA-10 T Amp here, and as I wasn't going to ever find a new PCB to replace the one I murdered, it seemed sensible to press the chassis into service. I am using an ALW Super-Reg set to 5V (4.98 rock steady!), fed by a 12V wall wart. The ALWSR board needed a bit of fettling to fit, but nothing a hacksaw and file couldn't deal with! ;D

All wiring was P2P on matrix board. I used some 5-Nines (or whatever it was) silver wire with teflon sleeving for most of the wiring. Where components could be joined directly, they were.

It sounds just as good as when it was "naked" on the chopping board, but has the added convenience or two switchable inputs. It's as loud as I'd ever want it on CD, with the two D290Ps in mono configuration. Tuner output is a little lower. I just need to bung an LED in there when I can be bothered to do the maths and it's job done.

I'm pretty chuffed with the result, and I'd highly recommend this novel attenuator!!

Here are some pics:









Cheers

Jon

P.S. George calls it a Passive Attenuator....

www.diyaudio.com/forums/showthread.php?postid=1804879#post1804879

[udailey]

Hi everyone.
This is Uriah. I was doing a huge amount of matching of the LDRs for the Lightspeed. Jon really did a great job.
Wanted to address a few concerns.
The output impedance of the LSA totally depends on which LDR you happen to put out of the bag. I have had them up to 25k and as low as 2.5k for total output impedance. The MyRefC has about 60k input impedance from what I recall and thats what I am running at the moment with the LSA. Runs fine. No problem. Driving 8ohm load.
I also have driven a Nelson Pass F5 with it. PHENOMENAL.
The sound is amazing and I know no one wants to hear a bunch of descriptive words but man I have nothing more for you except to say that its like the difference of going from commercial $500 speakers to some of the finest DIY speakers you have built. The increase in sound quality is astounding and I was so thrilled with I got the LSA built and playing. Its really, for me, the ultimate attenuator.
They are extremely easy to build, but yeah the matching really is not fun. You can probably get a good match out of 25 LDRs. I just matched up 350 or so LDRs a few months ago and sold them off to guys who wanted to build it. NO COMPLAINTS lots of praise. Out of the 350 I have about 100 LDRs that are going to waste. The more you buy the better chance you have for finding a match for each one. They are available from ALLIED. Silonex nsl-32sr2s. The 3s has better distortion numbers than the 2s but will be about 25 times harder to match so I think stay away from them as the 2s is already incredible. They cost nearly $3 each but the result you get is worth the excess LDRs you wont use. Seriously is got to be tried. Your Alps, your Penny and Giles, your TVC (at least with the amps I have tried) wont beat it. To me this is the ultimate.
Yes you can use a buffer if you need to. Try not to as we want as little as possible in the signal path. I dont know why they sound better, they just do and its not subtle.
Wish you all the best and please ask questions if you need help. Also stop by George's thread on DIYA. He is trying to get a passive pre amp section going over there and the mods have told him to keep getting hits on his thread and its likely.
Also, it is passive. No active components in the signal. Just a resistor and then a shunt resistor.
Uriah

edit: got a bit carried away. Bringing me back to what I wanted to get to.. on average you get around 7-8k output impedance on these things.

[Deleted]

Uriah
Have you been able to compare it against a high quality attenuator such as the DACT2 ? Incidentally, I would warn that the use of any passive preamp will almost certainly result in HF rolloff, and be extremely sensistive to the type of, and length of the interconnects used. I imagine that the LS attenuator would be an excellent match for a dual supply rail version of the Nelson Pass B1 buffer stage. Again, not everybody can get by with a unity gain buffer directly into a Power Amplifier. Incidentally, many years ago, I had a brief play with an Expander using LDRs and LEDs.
SandyK

[Deleted]

Hi Alex,

Read the first line, George may be biased though.

www.diyaudio.com/forums/showthread.php?s=&threadid=80194

Lightspeed with JC or NP buffer:

www.diyaudio.com/forums/showthread.php?s=&threadid=134316

regards
Greg

[Deleted]

Greg
I have been aware of this thread long before RG members found it. Perhaps George from Manly in Sydney is a little biased too, after all he sells the LSAs.To function properly, it needs a much higher input impedance than available from the majority of SS amplifiers. When all is said and done, LDRs are not just variable resistors, they are a type of semiconductor. As I said , I had a play with LDRs and LEDs many years ago , using the only readily available LDR of the time , the ORP12.The concept of gain control using an LDR is not new. It has just been rediscovered.
Alex

P.S.
JeffC and myself discussed via email the possibility of using a LSA with a dual supply rail B1 buffer back on 22/11/2008

[Deleted]

Hi Alex,

Yes it's not new.

Quote from George,

"I make these LDR Volume controls ("Lightspeed Attenuator") as a source of income, I started them 30years ago, but back then the ldr's were way out tracking wise and changed their resistace curve with use, you needed 100 to match up 4, then I shelved the idea."

So he might have been doing them at the same time as you???

George has stated that the input impedance needs to be reasonably large, initially he stated 100k but has revised that to 35k.

regards
Greg

[rickcr42]

Is it me or is the title a bit of an oxymoron given it's got active electronics

tough call that because it is not "active" in the classic sense being that the only part that needs to be powered in the unbuffered version is the light element while the very phrase "active electronics" usually denotes some kind of signal manipulation in an amplifying stage be it voltage gain,power gain or impedance matching,not the case here at all

BTW-the "LDR" type attenuator controls are not new,just newly "rediscovered" as is much of what we think is new these days with the only real "newness" being we have access to not only better devices (mostly) due to better manufacturing techniques but more input via forums to get a design from the "idea" stage to "I can make a few bucks of this" stage

[Deleted]

The attached quote from Russ White of Twisted Pear Audio (Buffalo DAC) as posted in DIYAudio may be of interest :

"Re: Re: Re: Roaming Buffalo now at home Post #326
quote:
Originally posted by Spartacus



Hello Russ, have you measured this?


No, I don't have an LSA to measure, but I know do know how LDRs work. And I can read the distortion charts . LDRs add a pretty hefty amount of distortion especially once you get over 1VRMS. I for one would not be willing to take a -50db THD+N distortion figure over a -117db THD+N. Even if were all 2nd order...

The DNR of the Buf32S at -60dbfs is 127db. If you run that clean signal through a set of interconnects and then some more passives (even the best quality) it will be less. Add to that the output impedance will be higher. So the best approach in my book, is to put nothing at all between the source and the amp. Then you will have the purest possibly signal and good line driving capability.

This is just my 2C. Some people may like the sound of the LSA, but I try to keep distortion to a minimum.

Cheers!
Russ

Attachment: ldr_distortion.pdf
This has been downloaded 41 time(s).

www.diyaudio.com/forums/showthread.php?s=&threadid=143315&perpage=25&pagenumber=14

[rickcr42]

This is just my 2C. Some people may like the sound of the LSA, but I try to keep distortion to a minimum.

so one guy "says" the distortion is high because "someone else" measured it,he didn't but someone did and it seems he did not listen either,and the topology should be dismissed entirely on that basis ?

I could if I wanted to,"prove" a particular topology either sounds great or sounds like crap simply by searching out threads,posts and/or technical pages that agree with my own predisposition while totally ignoring (and failing to add to the argument I might add ) any that do not agree with that position but that only PROVES there are two sides to every argument and not any "this is it,agree or die" ultimate truth.


Is the Lightspeed a step forward ?

Well it sounds fine from my VERY LIMITED own experience (someone elses build),was highly touted as a large part of the highly regarded sound of the Melos products that used similiar devices and it seems has many admirers


www.stereophile.com/headphones/796melos/index.html


Uses the VTL3A47:

Manufacturer = EG&G Inc
Description = Light-Dependent-Resistor-Output Optocoupler
Manufacturer = EG&G Inc
Number of Input Channels = 1
Number of Output Channels = 1
Input Type = Incandesct
Photocell Type = LDR
P(D) Max.(W) Power Dissipation = 550m
V(BR)Out Min.(V)Breakdown Volt = 100
I(IN) Min.(A)Control In. Curr. = 40m
V(IN) Min.(V)Control In. Volt. = 12
r(on) Max.(ohms) On-state Res. = 40
t(off) Max. (s) Turn-off Time = 900m
t(r) Max. (s) Rise Time = 25m
Viso Max.(V) Isolation Voltage = 1.5k
Package = Axial


then there is this "similiar" application,again touted as "great" because by the designers then proven to be by the same as would be expected

www.6moons.com/audioreviews/dartzeel2/preamp.html




where problems arise as I see it is not so much what is and is not a good/best/better method but so many so called "DIYers" who are in it for the loot rather than for the hobby,something that was my main bitch with Head-Fi and why I no longer post there,something that is a problem for me with HeadWize and why I no longer post there (and @ one time I was top 5 in posts at both forums) and a problem with the "diy" Audio forum and why I NEVER post there and that is simply because everyone has an axe to grind depending on either if they have a financial stake OR having purchased a "product",even if it is a simple pcb,line up against everyone that does not agree with their findings creating a situation where I personally trust NONE of the findings of those involved and that goes for the above linked post where Mr.White being a "seller" of a product can make a comment dismissing another "seller" of a product who is in many ways the competition even if the topology is different in the volume control offerings of both.


What I think would be the strength of the Lightspeed is the fact that it can be controlled by either a varying voltage or a varying current which to my mind lends itself to having the ability to be controlled remotely which in my experience NONE of which are perfect but ALL of which are very useful for some situations in a domestic system (movie viewing,general non critical listening,etc) thus a useful path to explore there being many options,many GOOD options,but no "this is the one and only" options.

How would I go about doing the "dirty deed" of using a lightspeed ?

Simply put I would use a shunt-control topology with a buffered output and would use a "digital pot" as the voltage control element.I would NOT use the digital pot as the volume control part because in my experience they are still very "lacking" in actual SQ no matter which one is used but as a variable voltage or current source with repeatable push-button control of the levels seemingly would be ideal.


If I wanted ultimate resolution AND remote control capability I would modify my analog pots (still runing the PECs) to accept motor control then use the Jan Didden circuitry to do all the dirty work www.linearaudio.nl/JDpubs/remvol.pdf


keeps the "essence" of the device unchanged yet adds a remote control functionality-adds something,takes away nothing

as a concept the lightspeed is very cool,as competition to most of the available audio system stereo volume control options a player to reckoned with there being no actual "pot",active or otherwise in the direct signal path (shunt control remember ) but since I am first and foremost a "KISS" topology adherent I prefer to use a "simple" dual mono log taper pot arrangement for most of my audio attenuating needs though no doubt at some point I WILL cob one of these suckers together and see what all the hub bub is about because in the end I trust my own judgement over any "well i have not actually heard or measured it but this guy did and he says...." type advice every time because that is not much better than "I have no clue" as far as I can tell and even if it does not come up to "world class" levels of performance i CAN see where having a simple "lightspeed in a box" could be useful to have banging around the house

[Deleted]

hi jonclancy,

I was wondering how the Lightspeed Attentuator has proven itself now you have had a reasonable amount of time using it.

regards

[jonclancy]

Hi Greg,

It's still in the system - I'm sorting out the input selector on my TVC at the moment. I still think it's a really nice attenuator and produces clean, detailed sound that is impressive. I'd like to try a decent active pre (like my unbuilt Pumpkin kit) in the same system to compare. As I mentioned earlier, the input impedance of my amp is only a mere 7K5, so not the ideal operating conditions for the LS. Despite this, it acquits itself well.

So, bottom line, I'm still enjoying this simple but clever attenuator!!

Cheers

Jon