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Lightspeed Attenuator a new passive preamp
Lightspeed Attenuator a new passive preamp
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Old 25th May 2006, 07:13 AM   #1
georgehifi is offline georgehifi  Australia
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Default Lightspeed Attenuator a new passive preamp

I've tested and listen to just about every concievable passive potentiometer (volume control), Bournes, Alps Blue Velvet, Alps Black Beauty, Penny&Giles, Dact Switched resistors, and transformer (TVC) based volume controls.

Nothing has equaled the transparency and dynamics of sound than that of a series/ shunt LDR arrangment in the attachement, it has no contacts in the signal path, I believe that is the secret.
The only stipulation that all these passive controls including the series/shunt LDR need I believe is,
1: Low source impedence (cd player) <1kohm
2: High input impedence (amplifier) > 33kohm.

Cheers George



8-3-2015
To explain a little the 5 photo attachments.

1st: Is the basic led/ldr and how it works in the Lightspeed Attenuator, it mimics a 10kohm series shunt logarithmic potentiometer.

2nd: Is the Lightspeed Attenuator stereo version, complete circuit diagram, note how the signal goes through nothing but one soldered resistor, no sound robbing light contact of a metal wiper on a resistive track in a volume control (potentiometers) in the signal path.

3rd: Is the same but a dual mono, good for room or system imbalances, (for some reason owners of both say the dual mono has the edge sound wise) I believe it's the fact you can precisely centre the image regardless of recording or environment influences.

4th: Is the buffer Nelson Pass designed a few years back for the Lightspeed Attenuator (later to be known as the B1 Buffer) for it to be able to drive low input impedance amps of less than 33kohm.

5th: Is the $5 led tester you can get on Ebay. With this you can quad match your own set of NSL32SR2S(sorted) Silonex led/ldr's you'll need get about 15 pieces to get 2 sets of quad matched led/ldr's for two x stereo Lightspeed Attenuators. So go halves with a buddy and you can have one each.
Match the LDR resistances using a DMM, powering the LED from 1mA to 20mA, give them time as they need a few minutes to warm up.

PS: You can skimp on the cheaper NSL32SR2 but you have to buy almost twice the amount, as their low resistance is not as low and constant, they cannot get to quietest minimum volume level as the NSL32SR2S (sorted) can.

Cheers George
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Last edited by georgehifi; 17th December 2016 at 06:54 PM. Reason: Images attached
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Old 25th May 2006, 07:27 AM   #2
gfinlayson is offline gfinlayson  Scotland
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George,

I'm looking to build a high-quality passive preamp in the near future. I like the LDR idea. Can you provide any more information on the design and components used?

Thanks,

Graeme
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Old 25th May 2006, 07:50 AM   #3
georgehifi is offline georgehifi  Australia
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Use a dual 100k log or linear pot to supply the voltage to the leds, one gang of the pot increases the series ldr's led voltage while the other gang in reverse lowers the voltage on the shunt ldr's led, this way you have a constant input and output impedence of the passive, regardless of where the voltage control setting is. I believe it's the perfect passive volume control.

You can put an LRD in one end of a tube and a Led in the other end x 4, you have to sort through a few ldr's to get 4 that track together for good channel balance, the last thing you want is to have a balance control, but even that's possible if you use two single 100k led voltage pots instead of a dual.


Cheers George
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Old 25th May 2006, 01:34 PM   #4
st2_998 is offline st2_998  Italy
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Quote:
Originally posted by georgehifi

The only stipulation that all these passive controls including the series/shunt LDR need I believe is,
1: Low source impedence (cd player) <50ohms
2: High input impedence (amplifier) > 100k.
Thanks George, I'ma very bad at electronics, so I'll try to sort it out. For example, I don't remember if LDR increases or lowers resistance with increasing light.
This type of control perhaps should be put after a buffer (attenuating buffer's self-noise) but I understand that impedance matching would be wrong, isn't it?
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Old 25th May 2006, 09:42 PM   #5
georgehifi is offline georgehifi  Australia
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You should look for a LDR who's led on resistance is around 50ohm and dark resistance around 1megohm.
Also the balance control that I said would be 2 x 100k single pots for the leds is wrong, it would have to be 2 x 100k dual pots.
Also you could have a LDR in each end of a 1inch tube and the LED through a drilled hole in the middle, that way you'll only need 2x tubes for a stereo control.
There are combined Led/Ldr packages out there, but they are very expensive and you still need to match them.
And forget about using buffers before and after the LDR control, been there done that they only add their own colouration to the sound. A CD players output drive is plenty capable of driving a amp to clip, all you need with the LDR Control is to keep the interconects to the amp as short as possible, <2mts and to use ones that are <100pf per foot.

Cheers George
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Old 25th May 2006, 09:56 PM   #6
gfinlayson is offline gfinlayson  Scotland
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George, do you have any manufacturer's part numbers for the LDRs? The lowest values I can find are about 2 kohm when illuminated.
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Old 25th May 2006, 10:26 PM   #7
pinkmouse is offline pinkmouse  Europe
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Lightspeed Attenuator a new passive preamp
So the Heretical line stage isn't good enough 'eh Graeme?

Interesting topic guys, looks like a good way of controlling lots of channels for a HT setup.
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Old 25th May 2006, 11:16 PM   #8
georgehifi is offline georgehifi  Australia
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Quote:
Originally posted by gfinlayson
George, do you have any manufacturer's part numbers for the LDRs? The lowest values I can find are about 2 kohm when illuminated.

They are out there, you'll just have to do your homework to find them.
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. In the late 90's Melos bought out their flagship preamp the SHA-Gold-Reference $20kUS which used the ldr's (hailed as the best sounding preamp ever, probally because of the ldr volume), still unreliable, nearly all that were built came back to them, maybe this is what sent them broke.
But now there are ldr's which can be 100% reliable(you still have to sort them 10 to get 4 to track), you will just have to search for them like I did, then you will keep it under your shirt like I am, because if I tell everything it will effect my sale of my
"Lightspeed Attenuator" units.

Cheers George
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Old 26th May 2006, 07:27 AM   #9
gfinlayson is offline gfinlayson  Scotland
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Al,

The heretical line stage is very good indeed; transparent, detailed, very smooth. I'm just curious to compare it with a good passive. The LDR passive seems a very simple, low cost way to try it out.

I'll probably end up building a heretical line stage anyway, if you're happy to share the design.

Graeme
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Old 27th May 2006, 02:16 AM   #10
powerbecker is offline powerbecker  Germany
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Hello,
here is some usefull information about LDRs:
http://optoelectronics.perkinelmer.c...tID=VTL5C2%2f2

snip
The lowest values I can find are about 2 kohm when illuminated.

Normally with only 10 lux. With more brightness they easy get down to 50E.

George,
please can You tell how equal in which range Your stereo-attenuator works?

Regards
Heinz!
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Old 27th May 2006, 06:50 AM   #11
georgehifi is offline georgehifi  Australia
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Heinz, you did not look too hard at the Perkinelmer site, you will find one as low as 1.5ohm at it's max led current of 40ma at 10ma it's still 5ohm which plenty, mine are 50ohm (different brand) which is still great, it will not go to absoulte zero volume, but that's no problem to me as it is just a whisper.

Cheers George
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Old 27th May 2006, 08:41 AM   #12
georgehifi is offline georgehifi  Australia
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Quote:George,
please can You tell how equal in which range Your stereo-attenuator works?

Regards
Heinz!


Heinz, my attenuator after calibration is at worst +- 1db from min to max level at best I can achieve +- 1/4db from min to max level if my ldr matching is good, I use a 25 turn 500ohm pot in series with the led resistor on the strongest channel to calibrate this, it could almost be used as a balance control.

Cheers George
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Old 27th May 2006, 10:17 AM   #13
gfinlayson is offline gfinlayson  Scotland
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George,

Do you use anythig fancy for the 100k pots on the LEDs, or just standard carbon track items?


Thanks,

Graeme
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Old 27th May 2006, 12:16 PM   #14
powerbecker is offline powerbecker  Germany
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Quote:
Originally posted by georgehifi
Heinz, you did not look too hard at the Perkinelmer site, you will find one as low as 1.5ohm at it's max led current of 40ma at 10ma it's still 5ohm which plenty, mine are 50ohm (different brand) which is still great, it will not go to absoulte zero volume, but that's no problem to me as it is just a whisper.

Cheers George
George,
indeed, my "knowledge" stems from long years back experiments
with some "no name" LDR`s!
Regards
Heinz!
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Old 27th May 2006, 12:49 PM   #15
powerbecker is offline powerbecker  Germany
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snip

Heinz, my attenuator after calibration is at worst +- 1db from min to max level at best I can achieve +- 1/4db from min to max level if my ldr matching is good, I use a 25 turn 500ohm pot in series with the led resistor on the strongest channel to calibrate this, it could almost be used as a balance control.

Thank You George,
these results are very good for this simple arrangement!
But as You surely know it`s not really "hihi".
(I take the quality of a switched attenuator as a ruler.)
If I like to design such one with LDR`s I would try to insert
this passive arrangement into a servo circuit which compares in/output and steers the led`s.
A little bit simpler to do this can be the using of a high frequency
pilottone with a notch in the output.
Of course this is much more input so the LDR using lost its simplicity!
And of course one had to sell it for "$20kUS"
Regards
Heinz!
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Old 27th May 2006, 05:39 PM   #16
st2_998 is offline st2_998  Italy
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Quote:
Originally posted by powerbecker

Of course this is much more input so the LDR using lost its simplicity!
And of course one had to sell it for "$20kUS"
Probably I missed the point but I thought that LDR use had its advantage in avoiding contact and moving parts, not in being "simple".

High-frequency pilottone filtered out without spilling into audio band, i.e. by harmonics combination? Doesn't seem hi-end as well
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Old 27th May 2006, 09:35 PM   #17
georgehifi is offline georgehifi  Australia
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Well I suppose now I have to give my long winded explanation of why I believe it has beaten everything I've put up against it, here goes.

I while back a freind of mine who worked for the CSRIO (massive research company in Australia) borrowed a giga hz storage oscilloscope worth 10's of K$ and we started to do some experiments.
On all the best pots mentioned in my first post, what we found was that during fast pulsed millisecond transisents at cd levels, audio band squarewaves, then stored and reviewed frozen on the scope, the leading edge was getting corrupted sometimes ringing, this leading edge has a very fast rise time > 100's volts per microsecond, and what we found was that the wiper on all of the pots were getting bounced, if you like off their tracks during these fast transients, (it was as though the contact area of the pot had become a diode and it was trying to rectify the ac signal) if we pushed on the wiper to give it harder contact to the resistive track the corruption stopped. Naturaly this would wear out the track in no time with this pressure. This also happened to switched resistor pots and also input switches, though to a lesser degree.
Now maybe you all can see why myself and later on Mark Porzilli (of Melos fame) were so obsessed with getting rid of small area contact devices (pots&swithes) from the singal path as they give some form of transient distorion. Hence the use for us to the led/ldr arrangment(zero contact area)

And Graeme you asked

George,
Do you use anythig fancy for the 100k pots on the LEDs, or just standard carbon track items?

No nothing fancy mate, the good old $2 carbon pots will do fine, but a good super smooth regulated 5v supply for the leds is a must.

Cheers George
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Old 27th May 2006, 11:23 PM   #18
powerbecker is offline powerbecker  Germany
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snip
Probably I missed the point but I thought that LDR use had its advantage in avoiding contact and moving parts, not in being "simple".

....and how do You avoid the contacts of the connectors?

With "simple" I mean of course the difference between the use of
LDR`s alone compared with a additional servo arrangement.
Regards
Heinz
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Old 27th May 2006, 11:30 PM   #19
powerbecker is offline powerbecker  Germany
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snip
High-frequency pilottone filtered out without spilling into audio band, i.e. by harmonics combination? Doesn't seem hi-end as well

In principle I agree!
On the other hand it depends from circuit design and the frequency....about 500kHz.
Regards
Heinz
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Old 28th May 2006, 12:06 AM   #20
powerbecker is offline powerbecker  Germany
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snip George
On all the best pots mentioned in my first post, what we found was that during fast pulsed millisecond transisents at cd levels, audio band squarewaves, then stored and reviewed frozen on the scope, the leading edge was getting corrupted sometimes ringing, this leading edge has a very fast rise time > 100's volts per microsecond

How this??...after a 20kHz filter! Possible done through a internal
overload?

snip
This also happened to switched resistor pots and also input switches, though to a lesser degree.

I have never notice this effect, though I built (non audio) amps to 10MHz.
What about the contacts in the scope attenuator?

snip
if we pushed on the wiper to give it harder contact to the resistive track the corruption stopped

In this case one need stronger/cleaner contacts!

snip
Hence the use for us to the led/ldr arrangment(zero contact area)

Why not, but at the expense of less stability and more non linearity (especially with high input voltages!)

For a "hifi" attenuator I`ll prefer therefore precision resistors connected with hermetically closed relays.

Regards
Heinz
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Old 28th May 2006, 12:59 AM   #21
georgehifi is offline georgehifi  Australia
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Heinz, you mention rca conectors contact, look at the massive contact area of those compared to the pinpoint contact of a wiper in a pot, also the pressure of that contact probaly ten fold, even rca's that have been cleaned sound better for some reason, we've all done it, go figure, relays are good, larger contact, heavier pressure, yes they are better than pots, but still not as good as a soldered joint. The pulsed square waves we were using were 1k and 5k, unless you have a extremely fast storage scope magnified on the leading edge at the top corner you wo'nt see it. Just try it, as you say it's cheap enough, you will believe.

Cheers George
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Old 28th May 2006, 01:20 AM   #22
powerbecker is offline powerbecker  Germany
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snip
Heinz, you mention rca conectors contact, look at the massive contact area of those compared to the pinpoint contact of a wiper in a pot, also the pressure of that contact probaly ten fold, even rca's that have been cleaned sound better for some reason, we've all done it, go figure, relays are good, larger contact, heavier pressure, yes they are better than pots, but still not as good as a soldered joint.

George, I agree!

snip
The pulsed square waves we were using were 1k and 5k, unless you have a extremely fast storage scope magnified on the leading edge at the top corner you wo'nt see it. Just try it, as you say it's cheap enough, you will believe.

Ok when I get the time I`ll have a look at this!
Regards
Heinz
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Old 28th May 2006, 01:36 AM   #23
Terry Demol is offline Terry Demol  Australia
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Quote:
Originally posted by georgehifi
I've played with every concievable passive pot, Bournes, Alps Blue Velvet, Alps Black Beauty, Penny&Giles, Dact Switched resistors, and transformers.
Nothing has beat the sound of a series/ shunt LDR arrangment in the attachement, it has no contacts in the signal path, I believe that is the secret.
The only stipulation that all these passive controls including the series/shunt LDR need I believe is,
1: Low source impedence (cd player) <50ohms
2: High input impedence (amplifier) > 100k.

Cheers George
George,

I have done quite a bit of work with LDR's also and like them a lot.

They do however introduce a small amount of H2 which is voltage
dependant. I have not had the chance to measure a spectrum on
them however.

Also LDR's drift (deteriorate) over time, so really, a self
calibration circuit is probably a good idea for very critical
applications.


Cheers,

Terry
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Old 28th May 2006, 01:41 AM   #24
georgehifi is offline georgehifi  Australia
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Also I can not stress enough, that the source (cd player/ Dac ) must be of low impedence <50ohm and decent current drive, this rules out a few of cd players/Dacs with tube output stages.
And the input device (amplifier) impedence > 80k-100k some of my customers say they can hear a difference beween 100k and 300k for the input of the amp with the Lightspeed as the passive.
Also the interconects between the passive LDR and the amp as low capacitance as possible < 100pf per foot.

Cheers George
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Old 28th May 2006, 01:47 AM   #25
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I've attached a photo of what i use for a passive Pre-Amp. I run a few Mini Aleph's Right at the moment i'm using the bozos Pre-amp Built from Twisted Pearl Audio. In some cases i can say that just running my cd player into a volume ( passive Pre-Amp ) and into a set of aleph's It sounds way better more controlled. I have tried DACT switches and didn't like them Over time they fail and sound horrible. BrianGt had a 150$ on in his gain-clone and when i was using it it sounded like HARSH ****! a wasted 150$ on a dact EW. Right now in this cheap YES cheap passive is a peter daniel volume pot it's a Alps. Im happy with it till i get more money to buy a way better one.

Do we really need a pre-amp to run these amps ? I was told we do but i just proved to my self that we don't. NOW if you need lots of output and hi volumes you might need a pre-amp. I have found running the alephs in Se mode they are pretty loud running a passive Pre-Amp.

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