Lightspeed Attenuator a new passive preamp

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R

redmaster

Member
Joined 2011
udailey said:
I understand what you are saying now. I wonder if we did something like this with a .1% SS amp and a .1% tube amp if we would hear the warmth of tubes left over.
Uriah
Click to expand...
YES!
it's amazing idea - null audio, you can hear what device 'added', after nulling, you will get possibly some 'extra' distortions in upper freq.
generally you could check audio and be 100% sure
just noticed you mentioned amplifier - I don't know if speaker outputs of amp could be checked this way...
 
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gmphadte

Member
Joined 2003
AndrewT said:
use a double pole double throw (dpdt) relay and use the other changeover contacts to reduce the currents through the LEDs.
This could be used to protect the LED/LDR from long term deterioration if left in the max or min volume by accident.

Invoke the Mute at power off. Use the Play to delay signal transmission at start up.

Now that extra contact in the signal route has a benefit.
Click to expand...

There r other features I designed in the circuit that r not mentioned.

Preventing thump when changing input selector and preventing speaker relay arcing.

Not necessary. Put a resistor of your choice across the series (used) contact. U have to drive the shunt LED hard otherwise mute will not be effective.

Enjoy

Gajanan Phadte
 
U

udailey

Member
Joined 2008
picasaweb.google.com/udailey/MyRef#
Hi gmphadte,
Wow, I wish I understood logic circuits. It is something that I do want to learn one of these years because it can replace the need for programming in some situations and can reduce circuit size in other situations.
For mute I use two more LDRs and a mosfet based optocoupler inbetween them.
It goes like this
Signal - LDR - photomos to ground (normally closed) - LDR - output
So when we apply power to the circuit the LDRs go to low resistance and the photomos turns on and goes open circuit so that we dont have any signal going to ground. There is no click or pop of course.
Its a simple idea but I managed to complicate it a bit :)
Here is a link to my Picasa album showing the implementation.
Of course the sound improvement is not huge but it is better than straight relays/contact switchers.
Uriah
https://picasaweb.google.com/udailey/InputSelectorBuildGuide?authuser=0&feat=directlink
 
O

oenboek

Member
Joined 2005
onebuckmusic.com
redmaster said:
Is there any LDR user who has ideal (say 0.5dB) difference between channels?
In my case it's not just 1 channel is louder - it's also inconsistent - changing from R 5dB louder to L 5 dB louder.
George, is it possible with LDR to have it equal less then 1db?
thanks for input
Click to expand...
I made a design where I can do an autocalibration of the resistance of each LDR. See here : http://www.diyaudio.com/forums/analog-line-level/182294-sylonex-arduino-preamp.html. And it has an LDR in the feedback loop of the DAC to compensate the nonlinearity and temperature drift. I guess it allows for nearly perfect linearity between channels, but I've never measured this.
I admit that it's a lot more complicated then the standard version, and for sure it has not better sound, that is impossible. But it was fun to make and design, and it allows me to play with input resistance etc. It's still working fine, and I'm still working from time to time on the Arduino side of the game.
I'm convinced that the sound of the optocoupler pot is one of the best available. I have it together with a salas DCB1 and a Hypex amp. This combination is really sounding great, the impact of the music is huge. I doubt I will ever move to something different.
 
georgehifi

georgehifi

Member
Joined 2003
lightspeedattenuator.com
oenboek said:
I made a design where I can do an autocalibration of the resistance of each LDR. See here : http://www.diyaudio.com/forums/analog-line-level/182294-sylonex-arduino-preamp.html. And it has an LDR in the feedback loop of the DAC to compensate the nonlinearity and temperature drift. I guess it allows for nearly perfect linearity between channels, but I've never measured this.
I admit that it's a lot more complicated then the standard version, and for sure it has not better sound, that is impossible. But it was fun to make and design, and it allows me to play with input resistance etc. It's still working fine, and I'm still working from time to time on the Arduino side of the game.
I'm convinced that the sound of the optocoupler pot is one of the best available. I have it together with a salas DCB1 and a Hypex amp. This combination is really sounding great, the impact of the music is huge. I doubt I will ever move to something different.
Click to expand...



Hi Oenboek, I did watch your auto cal development closely, as I went down that path with a designer/friend of mine also years ago, and I think I may have posted on your thread, that while with unmatched ldr's we did get the channels very closely matched with a similar auto cal circuit, the problem was the I/O impedances were vastly different for each channel at all differing levels, and this effected to tonal quality of left vs right, so while the gains were correct it was as though we had two different speakers of the same efficiency.

Cheers George
 
O

oenboek

Member
Joined 2005
onebuckmusic.com
georgehifi said:
Hi Oenboek, I did watch your auto cal development closely, as I went down that path with a designer/friend of mine also years ago, and I think I may have posted on your thread, that while with unmatched ldr's we did get the channels very closely matched with a similar auto cal circuit, the problem was the I/O impedances were vastly different for each channel at all differing levels, and this effected to tonal quality of left vs right, so while the gains were correct it was as though we had two different speakers of the same efficiency.
Cheers George
Click to expand...
Thanks for replying. I do an online calibration of the resistance of each LDR individually. This guarantees that each of the resistances is OK and perfectly linear. Therefor the total impedance is also controlled and stable, as the gain is. It could be easier to calibrate gain only, but I wanted to have control of the impedance as well. As you state, this is as important as the gain especially between left and right channels. The additional advantage is that I can adapt automatically the impedance for each different source. I haven't programmed this yet, and I agree it goes too far, but it's possible.
 
Tolu

Tolu

Member
Joined 2007
Hallo Lightspeed fans,

I have a problem with my Lightspeed. One Optocoupler is damaged. The one of the right channel gives a variety of 30 to 19000 Ohm and the damaged one gives with the same voltage a range from 62 to 159 Ohm.

Does anyone of you have one 32SR2S in C or D grade left over? Please send me an email!

Thanks a lot!

Thomas
 
georgehifi

georgehifi

Member
Joined 2003
lightspeedattenuator.com
Tolu said:
Hallo Lightspeed fans,

I have a problem with my Lightspeed. One Optocoupler is damaged. The one of the right channel gives a variety of 30 to 19000 Ohm and the damaged one gives with the same voltage a range from 62 to 159 Ohm.

Does anyone of you have one 32SR2S in C or D grade left over? Please send me an email!

Thanks a lot!

Thomas
Click to expand...

This is typical of damage from shorted output with higher than normal signal input, esspecially from audio sig gen's.

Cheers George
 
D

dpac996

Member
Joined 2006
orders from Buildanamp.com

Hello all. I have been following this thread / treatise for some time now. Many thanks to George Stantscheff for posting the design. I plan on building one and have purchased 4 matched ldrs as advertised from build an amp website. I was wondering what is typical for turn around/ shipping?

I plan to use the ldrs as the volume controlled followed by a modified buffer based on buf03 (mil spec version). My amplifier presents with a nominal 22k ohm Zin. The buffer currently uses a high quality 25k Noble pot and sounds quite excellent as is but im curious about replacing the noble with the ldr. What is the verdict on placing a bypass cap around the led? One last question: worst case wiper current (trimpot set to minimum resistance, led s fully on) is (5V-0.7V)/ (100//100) = 83mA, for a 100k ohm (control pot).
Im not sure of wiper resistance in conventional carbon pots, but 83 ma seems very high. What are folks using for 100k ohm pots: 2watt? I see quite a few photos of standard quarter watt jobs ...

Thanks
Dave
 
A

AndrewT

R.I.P.
Joined 2004
Trim pots are rated in W or mW.
But to use the information contained in that tiny bit of data one must convert the power to an equivalent full track current.

eg.
a 400mW 2k pot/trimmer can pass (I=sqrt[P/R]) ~14mA. No part of the track nor wiper can pass more than 14mA if one wants to stay within the manufacturers power rating.

Doing the same for 100k 2W trim pot, Imax = ~4.4mA

If you want to get good reliability from your trim pot then de-rate to either half power (divide Imax by sqrt[2]) or de-rate to half Imax (25% of max power).
 
D

dpac996

Member
Joined 2006
Andrew: you understand then that the circuit as posted will fry conventional 100k ohm carbon pots if one cranks it to full volume or to full closed. I think an option to better handle the led current (just instead of a wiper) would be to use an adjustable voltage regulator; one could easily whip up an adjust circuit with trim for channel balance.
 
D

dpac996

Member
Joined 2006
Hi Andrew,
I must have missed your comments its challenging to sift through 10
00's of posts on my DROIDX.
The EE in me cringes when I see a regular old pot used to control voltage ( negates a precision regulator) to any load. There are potentially tens of mA passing through the wipers at various pot positions. I understand the kiss principle is the modus operandi of the lightspeed but sometimes too simple is not the right solution. Its kind of odd that most pot manufacturers don't list the max wiper current.
 
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