Lightspeed Attenuator a new passive preamp

[jkeny]

Could an LDR not substitute for a pot in a circuit - given the surrounding circuitry? i know it is much more complicated than a simple pot but there are situations where one needs a high quality variable resistor (is this not what it is?)

 

[Bigun]

my 2c: I think there are few cases where you put a pot into the signal path, most times pots have little impact on sonic quality. But as a servo control method this could be really neat - so long as it's a slow servo you need such as controlling the current through an LTP to achieve small dc offset at the output.

 

[jkeny]

Yes this has been done already on the Krill amp see attached

 

[ptysinn]

Re: Potential Divider

Comparing any serial/shunt fixed/LDR combination (MKI / MKII) it would be usefull to post the concrete values of the compared resistances (fixed and LDR-resistance): Serial, shunt and input imp. of the amp.

Without doing this, I think further serial/shunt-discussions are senseless...

To get the "same" at every volume step, you need constant input impedance at every step. That is basically only possible with pot. dividers: serial-LDR and shunt-LDR (Georges MKII). If you want exact impedances at every step, there is much work on it, but you don't have to match any LDR, you have to match the resistances (i.e. in the voltage circuit):

- specify the impedance (i.e. 7kOhm) of the whole pot. divider
- specify the input impedance of the used amp
- specify the necessary steps in -dB
- calculate the serial and shunt values for every step with these 3 values
- adjust these pot. dividers at voltage side (stereo: 4 values each step) the way you like (i.e. stepped attentuator...) - measurement at resistance side ...

Regards
Thomas

This is exactly what I wanted to point out in the previous post. Without those parameters given, it is hard to conclude that which configuration sounds better using different parts. So far the concensus is mkII sounds the best if mkII and cd/amp impedance are matched up well.

Regards,
Philip

 

[jkeny]

Gareth,
But does it open up possibilities where a conventional pot would not be considered for instance in a MKII LDR the signal is passing through the LDR resistor - might there be a reason for wanting to vary this, as is being suggested in a MK1.5 version?

 

[ptysinn]

The first time I bought 12 pcs which gave me 4 well matched LDRs. The remaining 8 pcs are perfectly used for signal switching. So you have per channel 2 LDR for about 5$ which are far superior in relation to any kind of relay!

So, for about 30 $ you'll get the best sounding preamp (I dislike the word preamp regarding to a vol control; in Germany is a better word for it: Vorstufe ~ pre stage) you can imagine. And I have heard so many of them...

Cool! I will use the unmatched LDR to build a switch for selecting input.

What current did you feed those switch LDR when turn it on?
Did you just disconnect the power when switching it off? Any crosstalk problem?

Regards,

Philip

 

[georgehifi]

Still have no clew as to what all this is about, other than a a very fine attenuator

Are there any pictures anywhere

I believe this is the first time I have posted it up, there you go, it's not exactly eye candy, but then neither is the price.

Cheers George

 

[tinitus]

Expect to hear from me, I think it cute and just right

If there is light in in the blue/red thing it will be cool

 

[tinitus]

This ligtspeed attenuator is obviously a very good foundation fore a sweet purist "preamp"
And excells over the ordinary passive option

I wonder how much will be lost when combined with an active buffer or gain stage
Will other more common options maybe be "back on track" again

Or has it been compared with a TVC/autoformer
Again I wonder whether those may loose its benefits when combined with active buffer or gain stage

I hope you know what Im asking

 

[Bearman]

Here's a possible idea for another use for the unmatched LDRs - could one be used as a high quality variable resistor in place of a potentiometer in a circuit?

That was my thought using the DS1809 digital pot as the shunt resistor. It has a nonvolatile memory so it could be adjusted and retain position when powered off.

???? just a thought.

 

[TJF]

The first time I bought 12 pcs which gave me 4 well matched LDRs. The remaining 8 pcs are perfectly used for signal switching. So you have per channel 2 LDR for about 5$ which are far superior in relation to any kind of relay!

@ Tolu

Hi Thomas,

how did you do that? Any Post-Link?

Thank you!
Regards
Thomas

That was my thought using the DS1809 digital pot as the shunt resistor. It has a nonvolatile memory so it could be adjusted and retain position when powered off.

@ Bearman

Hi,

if I understand you the right way: Look at the maximum current of such circuits with digital pots (about 5 mA...)

Regards
Thomas

 

[gianxdiy]

WOW, 82 pages read like it was my favourite book!
Georgehifi, i thank you for your work, and for all the details explained here. Thanks also to all the contributors.
I was serching a SS attenuator since i started many years ago to study music that involved me in hifi-diy passion and related (private) studies. High efficiency and tube lover, always hated those crappy potenziometers, never matched, never good sounding and always source of noise.
You solved my 25years war against pots, poly-switches, relays, IC's, etc...
I will build my LSA and post the pictures, i will do the version with single log pot and 4 32SR2S.
I'm sure it will be a great and final improvement, re-giving me the joy to turn that pot again!

P.S.
I can buy them from RS (20pz) in italy and match them by myself...but can I please ask if someone (i don't know, maybe Panelhead) has matched R2S (2+2) to sell? Obviously paying the right cost for the seletion.

Thanks to you all, see you soon
Gianx

 

[georgehifi]

Time to do some recuriting for our own "Passive Preamp" forum, or maybe that should read "Passive Attenuation" forum, please vote here to get us out of the (yuck) Solid State forum.

http://www.diyaudio.com/request/


Cheers George

 

[ptysinn]

Success story..

Hi,

I think it is about time to get a row call for all the successful story using LSA (either self made or bought from George).

1) What is your current equipment or setup?
2) If you are self-made, can you measure the impedance of your LS at your listening level.
3) A attached picture of your successful LSA is great too.

I am sure a lot of great LSA out there.

Let me start mine first:

1) CD Player: Sonic Frontier SFCD1
Amp: Rowland Model 5
Speaker: Self-modified Sea Excel Tweeter and 6.5" excel Woofer 2-way bookself speaker.
Cabling: Self-made WE single core 28awg RCA interconnect, self-made WE wire speaker cable. Powercord Zu cable. Self-made Balanced isolated transformer.

2) My lightspeed is about 4Kohm. Followed George's design completely.

3) Mine is still not in the box yet...so I will attach picture later...still finding a nice box for it.

 

[tinitus]

This ligtspeed attenuator is obviously a very good foundation fore a sweet purist "preamp"

And excells over any other pure passive option

Still no answer to my question

Wont the lightspeed work just like any other attenuator when combined with a buffer or gainstage?
Apart from not depending on stepped switches etc

Seems to me it has its obvious advantage mostly when used as passive, where any other fails

Except maybe TVC/autoformers attenuators, though they are flawed by the step switches, and number of steps

Or is it because I dont understand the design


But fact is that many high quality attenuators around here easily cost the same or more than the finished product offered by George
So I could very well imagine to use it with say a Pass B1 buffer or Aikido
Or simply just ad a simple source switch module
Not really sure how to do it

 

[agent.5]

Cool! I will use the unmatched LDR to build a switch for selecting input.

What current did you feed those switch LDR when turn it on?
Did you just disconnect the power when switching it off? Any crosstalk problem?

Regards,

Philip

Seconded the request and maybe even a schematic. Don't you need to match the LDRs somehow? Otherwise, wouldn't you get left-right inbalance?

 

[georgehifi]

Seems to me it has its obvious advantage mostly when used as passive, where any other fails

That's half right, when you add a buffer you narrow the playing field between the different passives, still the Lightspeed wins out, but the margin has narrowed, "remember what I keep saying the best buffer is NO buffer" The sound is only as good as the weakest link in the system. Take out one of the weak links and you have broadened the playing field.

Wouldn't it be nice to drive the speakers directly from a dac's current output with volume done in the digital. No problem the a current output dac would love to see an 8 ohm speaker load trouble is the speaker would have to be a 1000db odd efficiency.

Cheers George

 

[tinitus]

Thanks, I know what you are talking about, but sometimes I seem to forget

Seems we also need new some lightspeed speakers

btw
I once removed a small source toggle switch from a tube pre
I didnt doubt fore a second that sound improved so much that I couldnt live with switch in the curcuit
I know it could be related to a microphonic tube, I really dont know fore sure
Or the toggle switch could have been better quality

Oh, is the optocoupler below similar to the lightspeed principle?

 

[georgehifi]

Oh, is the optocoupler below similar to the lightspeed principle?

This is a very expensive way to go, and I have no idea if it is MkI or MkII in configuration, if it's MkI it's a ripoff

Cheers George

 

[soongsc]

This is a very expensive way to go, and I have no idea if it is MkI or MkII in configuration, if it's MkI it's a ripoff

Cheers George

What's the difference between MKI and MKII?