Lightspeed Attenuator a new passive preamp

[ondesx]

Originally Posted by soongsc

I was wrong, it's only 1/3 of the story. Even for the correct current, the resistance value may not be the same from device to device. If you get the resistance values to match at one point, they may not match at other points.

This is EXACTLY why I explained that you MUST determine THE CORRECT current FOR EACH attenuation STEP and obviously for ALL LDRs !... These measures must be done either manually or automatically, then they are STORED and lastly USED to reach the correct resistor values, i. e. the required target attenuation factor !...

This is ALL the story. No more, no less.

Then you may have an impedance issue. But, this is mostly addressed by the use of a "good" buffer...

[soongsc]

Not really, but I do not care to elaborate since you seem to insist that you are correct. Good luck.

Man, the boss always tries to summarize the opinion of others, but usually get things messed up. Over budget, late delivery, ....

[georgehifi]

Let's get this right, and stop all the ** being told here, feeding the same varying currents to each led segment does not guarantee the LDR portions will all have the same resistantaces at those currents, so getting the currents the same is only a third of the problem, other third is the matching resistances at those currents, the last third, this is to be done over a 5 x spread of 1mA to 20mA to each led.

Cheers George

[soongsc]

George, all this have been mentioned throughout the thread. But it's always good to have someone pull it together. Sorry about it, but the way "ondesx" puts it really reminds me of a boss or two other colleagues of mine had. Those were the fun old days.

[kerux]

Originally Posted by udailey

Kerux,
Nice job. Got pics?
Was at a friends house yesterday. He had been a DJ for a jazz station in Louisiana for several years back in the 80's. I've not met anyone with that many records/cds. "3000 more records out in the garage" he says. Jeeeeez!
Anyway with a LS in his system for the first time his words were "are those MY speakers?" pretty much what his wife said. The difference was huge. Fun times.
He had a Melos pre in there when I arrived which also sounded incredible, not equally incredible but I am predjudiced towards LDRs of course, but it was quite a different sound. I wouldnt mind trying that pre in my own system from time to time.
Uriah

I don't have any pics since the lightspeed and the shunt are in plastic radio shack boxes with not very precise holes. It is on my todo list to make nicer boxes and clean up the wiring between the two boxes.

[kerux]

I did buy one of dvb-projekts lightspeed board but I overheated the ldr on one channel and I had a hard time with unsoldering the ldr so the board is sitting unused at the moment. I ended up making it with perfboard and using 2 pin terminal strips for the led/ldrs to avoid having to solder these parts. The terminal strips sound fine to me. I use them for the audio connection in my simple se. I have seen dvb-projekts shunt regulator but I thought that was for high voltages only. Quanqhao boards have one 1.5-5volt connection which is what I used.

[georgehifi]

In the production Lightspeed Attenuator I use 2 x 4 pin DIL opamp sockets side by side so all ldr's are close/touching each other, this alows me to insert the ldr's "NSL32SR2S's" without soldering or cutting their legs, then I calibrate with an audio signal generator and dual trace scope at all levels. Then once I am sure they are all close matched, I solder each leg with heat sinks quickly with a fine very hot tip "into the sockets". Then calibrate them again once cool. Then pot the whole lot together with hard potting wax, wait for all to cool again and then check the calibration one more time for good measure.

Cheers George

[Marra]

I connected up my version of the Lightspeed this morning. I am impressed.
Thank you George for making your design available to we diyers; also thanks to Oliver for the pcb's and other who have posted with their findings.
Back to some listening

[ondesx]

Originally Posted by georgehifi

Let's get this right, and stop all the ** being told here, feeding the same varying currents to each led segment does not guarantee the LDR portions will all have the same resistantaces at those currents, so getting the currents the same is only a third of the problem, other third is the matching resistances at those currents, the last third, this is to be done over a 5 x spread of 1mA to 20mA to each led.

Cheers George

OK George since a couple of years ago you helped me, it's now my turn to help you... At least to clearly understand what I did to improve the idea of the LS MkII.

To be clear, I'll explain for one channel single-ended LS MkII. The attenuation Factor is noted Fi, then the series resistor of LDR is noted Rsi ohms obtained with Isi micro-amps of current and for the LDR shunt the values are respectively noted Rshi and Ishi. Of course, each measure is done with all the required attention (long time and temperature controlled, etc.). This will take some time to be done. As you would probably agree, for any given attenuation value Fi, you'll have a combination of the 2 currents for the LDRs. Right ? Then suppose by one mean or another you stored elsewhere these 2 values. Now, imagine you do the same for ALL the attenuation steps and obviously you store ALL these "doublets".

Afterwards, if you need a F attenuation value, you just have to feed the LDRs with the "adapted" currents, i. e. those you already stored elsewhere !

In fact, this is the theory, and obviously the values of the resistors in LDR will vary to some degree... This is why a new "calibration" is required from time to time (i. e. check the correct currents).

The final step of this improvement will obviously be to monitor the resistor value in real time and feed the LDRs with the required currents for a given attenuation value. Not an easy task indeed...

[udailey]

Originally Posted by georgehifi

In the production Lightspeed Attenuator I use 2 x 4 pin DIL opamp sockets side by side so all ldr's are close/touching each other, this alows me to insert the ldr's "NSL32SR2S's" without soldering or cutting their legs, then I calibrate with an audio signal generator and dual trace scope at all levels. Then once I am sure they are all close matched, I solder each leg with heat sinks quickly with a fine very hot tip "into the sockets". Then calibrate them again once cool. Then pot the whole lot together with hard potting wax, wait for all to cool again and then check the calibration one more time for good measure.

Cheers George

I would only add that I think using a liquid solder flux is a fantastic way to improve your chances of keeping the LDRs safe. I have found differences before and after soldering and I attribute it to overheating the LDRs which is why using some sort of tweezer/heatsink during soldering is a great idea. If you add liquid flux the soldering happens very very fast. Can be acheived in less than 3 seconds. Flux the pads.. heat the pad with the iron.. with the iron on the pad apply the solder to the iron... roll the iron over to the LDR lead... as SOON as the solder jumps to the lead remove the iron immediately. This is all you need to do to keep your hard matched LDRs matching once in the circuit.
If for some reason you need to remove the LDRs then use solder flux again. This is EVEN IF you have a rosin core solder. Also use a good copper mesh wick.
So the removal procedure would go like this: Visualize what you are going to do.. Pray.. Now apply flux to the joint as well as to the wick.. Lay the wick on the joint.. Heat the wick directly above the joint with a decent bit of downward pressure to the iron onto the joint.. when solder starts to flow you will keep the iron where it is while you drag the wick across the joint underneath the iron but remember to keep the iron in place. The wick is moving between joint and iron and it is sucking up more solder. This should take about 5 seconds.
One you have remove all or almost all of the solder you will probably still have a film of solder that is holding the LDR to the pad. You heat directly and pull the LDR out of its position on the board then clean up the board.
Good luck and really its not that tough you just have to prepare and have in your mind the procedure you will follow, then follow through exactly in this manner and you will keep your LDRs from burning.
Uriah