also see messages here
http://www.diyaudio.com/forums/anal...uator-new-passive-preamp-435.html#post2847577
Make sure to put the lightspeed in front of, not after, the gain element.
Until you get the buffer built, you can try a fixed 20k to 40K resistor in front of the series LED/LDR. This can also be a LDR with a pot to fix the R.
Hi Florian, first off an additional series LDR will add more distortions and reduce the amount of transparency of sound, it is detectable in the circuit when doing an A/B with a single series one.
I don't know why you are using an active preamp after the Lightspeed maybe for better impedance matching, is it because you have a low input impedance poweramp? If so and you want to keep it, then make it unity gain this also will reduce your min volume problem.
The other way you could reduce the gain is to use LDR's with a lower "on" impedance, the NSL32SR2S quad matched that I use in the production Lightspeed Attenuator has this, at 40ohms "on" value this is far lower than you may be getting now without pushing them to their mA limit and cooking them.
Cheers George
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Hey George,
To keep things in perspective, I didn´t use a high gain pre after the lightspeed. It is just that I wanted to use the lightspeed in my preamp because it is known to be a good volume control. I know now, that this is maybe a bad combination, and consequently plan to use it with a unity gain buffer, where it is apparently better suited. I have been listening today with my low gain alternative preamp, and it is definitely better sounding than the alps blue fitted before.
Thanks for the tip with the alternative LDRs, but I guess it won´t be needed.
To keep things in perspective, I didn´t use a high gain pre after the lightspeed. It is just that I wanted to use the lightspeed in my preamp because it is known to be a good volume control. I know now, that this is maybe a bad combination, and consequently plan to use it with a unity gain buffer, where it is apparently better suited. I have been listening today with my low gain alternative preamp, and it is definitely better sounding than the alps blue fitted before.
Thanks for the tip with the alternative LDRs, but I guess it won´t be needed.
Hi kumori and anyone similarly interested in actually measuring equipment rather than guessing
Can you measure with the product switched on, but unloaded ie without input or output cables Shunt and Series resistance for each channel ideally at 27 or more settings, so we can see the Left and Right channel resistance figures please.
Now write down each measurement and add each Left figure this provides an aggregate for the left - a true indication of the products resistance on the Left ,without the volume control moving at all .do the same for the Right Now very slowly move the attenuator and measure each channel Shunt and series to show Left and Right channels for each of the settings ( starting L Shunt ) suggested below. Its painstaking work but will really show us if it is a good volume control, or something else.
If you publish the figures for each volume setting i have provided suggested left start point ohms for each below and publish back here in this format;
L Shunt =
L Series =
aggregate= Ohms
R Shunt =
R Series =
aggregate= Ohms
Result for each volume setting ie Difference Left to Right = OHMS
Can I suggest (Reference Start point Shunt L ) At beginning then 75R then 165R followed by 350R 421R 532R 645R 748R 853R 959R 1K 1.19k, 148K 2k 2.5k 3K, 3.5k 4K, 4.5K 5k 6K 7.5K 9K 10K 12K 15K 40K 105K and at end
I look forward to those figures.
Cheers / Chris
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You have me confused here. Are you suggesting that linearity is the sole criterion of attenuator quality?
If so, it seems you're the one who's guessing. If not, what is the purpose of the 'painstaking' work you want people to do? IOW, what are you trying to prove?
One of those £20 Valab devices (good value BTW) will outperform any LDR design in that regard. Sound quality? Hmmmm.
When you get a LDR design also managing good channel balance within approx 400 ohms each channel through the range where normal listening is done, you will not want any other attenuator. Accurate channel balance is amazing to listen to, and is an extremely good indicator of valid design with LDR's My post encourages someone to measure this product as it is supplied with channel balance, If no one can, I rest my case and will be happy to continue my life believing that it is very inaccurate,
Cheers / Chris
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You'll have to forgive my seeming to neglect your posts but I don't visit here that often; if you could link to "elsewhere", I'll catch up with them with the care I'm sure they deserve. (I've skimmed posts on that now apparently moribund thread, "Light Dependant Resistor Current Control" but didn't find the core argument very convincing.)
Thanks for your concern but, no, I understand reasonably well how a volume control works though I understand less well why you should want to be offensive.
Thanks to Chris for making his point clearer though a bit less pedagogy and a bit more by way of his own data might be helpful as "If no one can, I rest my case" is, as I'm sure you've spotted, a non sequitur. Besides, I don't listen to "accurate channel balance" and am not sure how one could though that might be because I'm not sure what exactly is meant by the term in this context.
So you are accepting inaccuracy with channel balance as being acceptable ?
I didn't think I would see the day when inaccurate channel balance was thought as OK in the forums. Oh dear ! we are slipping. If it was a power amplifier would it be OK then too ?. We would see manufacturers slipping to sub standards in no time 20% resistors on the left and 5% on the right that will do. Recording engineers could happily make mistakes. etc etc, Even the Queen previously in glorious stereo could happily wander off to one side of the balcony and no one would notice ! The Blumein pair could be forgotten... need I continue.
Accurate channel balance with LDR's is the the figure to strive toward, its the difference between a average LDR product and an exceptional one capable of accurate reproduction.
Cheers / Chris
Sorry, my remarks were intended for Chris Daly. I assume you (Ryelands) know how a volume pot works, and therefore what is the correct criterion for channel balance. Most people on here do. Chris may be an exception, but it is difficult to pin him down.Ryelands said:
In case it is not obvious to readers, Chris Daly may have an (undeclared?) interest in criticising LDR volume controls which he, rightly or wrongly, perceives to be rivals to his own peculiar 'design'.
I am not a great fan of LDR controls, but if I were in the market for one I would look for one which is designed by someone who understands voltage dividers (and op-amps etc.) and so can give a coherent explanation of how his circuit works. Given that prerequisite, a little gentle criticism of rivals would then be acceptable. I will say no more, as I don't want to try the mods' patience.
I am simply asking someone to find both their multimeter and an hour to measure this LDR product... is that too much to ask. ? Post after post avoiding proper measurement of this product, is not a viable answer.
Cheers / Chris
You already have a Lightspeed, and have measured it? Why insist other people do your work?
I think Chris should just go to a DartZeel forum and ask those big spenders to measure their channel balance.
I do not have this product, and therefore I have not measured its Left/ Right channel differences, but invite someone who owns one to do so.
Cheers / Chris
Yes, strange this, post after post,lots of varied and wonderful excuses, but no measurements ?
Cheers / Chris