panelhead
The matching has to be there, but the very close matching requirement can be a bit relaxed by using the added preset. The LDRs of a stereo lightspeed attenuator should be closely following -series for series and shunt for shunt.
If the stereo pairs match better, then u can use them in series with a single linear pot. Mind that u will have to increase the voltage by one LED drop or 1.8 volts.
edit:so I bought 74 no name LDRs and I wish I can find atleast 8 tolerable ones to use in two amps as volume controls.
Enjoy
Gajanan Phadte
The matching has to be there, but the very close matching requirement can be a bit relaxed by using the added preset. The LDRs of a stereo lightspeed attenuator should be closely following -series for series and shunt for shunt.
If the stereo pairs match better, then u can use them in series with a single linear pot. Mind that u will have to increase the voltage by one LED drop or 1.8 volts.
edit:so I bought 74 no name LDRs and I wish I can find atleast 8 tolerable ones to use in two amps as volume controls.
Enjoy
Gajanan Phadte
jkeny wrote
The preset in series with 100ohms resistor has to be adjusted for getting equal value on both the LDRs. That means this preset increases the resistance of the LDR when its light source(LED) is fully bright. For making the light source fully bright, u will have to make the pot value zero ohms. This is explained by George earlier.
After adjusting the 1K preset, remove the mismatch adjust preset.
Measure the LDR of the darkest source. Now turn the pot to other end and measure the second LDR.
Now add a high value preset (1Meg) in the branch of the higher resistance LDR.
If 1Meg makes the resistance lower then the other one, put a series 1meg, and continue... and adjust the preset to match both values.
Gajanan Phadte
The preset in series with 100ohms resistor has to be adjusted for getting equal value on both the LDRs. That means this preset increases the resistance of the LDR when its light source(LED) is fully bright. For making the light source fully bright, u will have to make the pot value zero ohms. This is explained by George earlier.
After adjusting the 1K preset, remove the mismatch adjust preset.
Measure the LDR of the darkest source. Now turn the pot to other end and measure the second LDR.
Now add a high value preset (1Meg) in the branch of the higher resistance LDR.
If 1Meg makes the resistance lower then the other one, put a series 1meg, and continue... and adjust the preset to match both values.
Gajanan Phadte
Better with a diagram...
Remove PR2 if installed.
Set PR1 to zero ohms.
Set pot to CW.
Measure LDR of OC1.(say 63 ohms)
Set pot to CCW
Measure LDR of OC2.(say 60 ohms)
Now adjust PR1 to make LDR of OC1 to 60 ohms, with pot at CW.
With Pot in CW measure resistance of OC2 LDR (say 8Kohms)
With Pot in CCW measure resistance of OC1 LDR (say 7Kohms)
PR2 to be installed in the higher value LDR branch
Adjust PR2 to make OC2 LDR to 7Kohms with pot in CW
Finished.
Gajanan Phadte
Remove PR2 if installed.
Set PR1 to zero ohms.
Set pot to CW.
Measure LDR of OC1.(say 63 ohms)
Set pot to CCW
Measure LDR of OC2.(say 60 ohms)
Now adjust PR1 to make LDR of OC1 to 60 ohms, with pot at CW.
With Pot in CW measure resistance of OC2 LDR (say 8Kohms)
With Pot in CCW measure resistance of OC1 LDR (say 7Kohms)
PR2 to be installed in the higher value LDR branch
Adjust PR2 to make OC2 LDR to 7Kohms with pot in CW
Finished.
Gajanan Phadte
vph-2020
I doubt even the best matched stay same total everywhere on the pot. I have no way to check. Can't buy them here.
Also that is the case with dual potentiometers. Try to get a better match and better use the sea saw circuit with 100K linear pot. Don't go 11K and 30K as in one of my posts.
Clear your memory of their imperfections and just install them in the amp and listen. If u find a pitfall then go further... and further...
Moreover, the volume controls are rarely turned.
Gajanan Phadte
I doubt even the best matched stay same total everywhere on the pot. I have no way to check. Can't buy them here.
Also that is the case with dual potentiometers. Try to get a better match and better use the sea saw circuit with 100K linear pot. Don't go 11K and 30K as in one of my posts.
Clear your memory of their imperfections and just install them in the amp and listen. If u find a pitfall then go further... and further...
Moreover, the volume controls are rarely turned.
Gajanan Phadte
You guys are turning this into a nightmare.
There is no easy way out, you must match the two (one each channel) series ldr's together, and then the two shunt (one each channel) together.
When matching the series ldr's sub the shunts with a 10k resistor and when matching the shunt ldr's sub the series with a 10k resistor, a scope and audio generator is handy when doing it this way, or just a steady 1k sine wave from a cd or audio generator and a DMM will do.
And they should be matched at 9 o'clock, 12 o'clock and 3 o'clock on the volume control. Then once done, fine tune with the trimpot, that is only on the channel with the most gain.
Cheers George
There is no easy way out, you must match the two (one each channel) series ldr's together, and then the two shunt (one each channel) together.
When matching the series ldr's sub the shunts with a 10k resistor and when matching the shunt ldr's sub the series with a 10k resistor, a scope and audio generator is handy when doing it this way, or just a steady 1k sine wave from a cd or audio generator and a DMM will do.
And they should be matched at 9 o'clock, 12 o'clock and 3 o'clock on the volume control. Then once done, fine tune with the trimpot, that is only on the channel with the most gain.
Cheers George
In-Circuit Matching Proposal
Here's how I propose matching the LDRs in-circuit:
Please refer to the attached schematic...
1. With the 5Vdc PS off set the control pot (single) at 50% by measuring R from each side terminal to the wiper terminal.
2. Turn on 5Vdc supply and measure R across shunt and series resistors of LDR on both Channels.
3. Insert 1k trimmer where the shunt R is lower (in this case the Right Ch) and adjust to match with the Left Ch shunt R.
4. Do step 3 with the series Rs.
5. You can try the above procedure on the 9, 12 and 3 o'clock positions.
Here's how I propose matching the LDRs in-circuit:
Please refer to the attached schematic...
1. With the 5Vdc PS off set the control pot (single) at 50% by measuring R from each side terminal to the wiper terminal.
2. Turn on 5Vdc supply and measure R across shunt and series resistors of LDR on both Channels.
3. Insert 1k trimmer where the shunt R is lower (in this case the Right Ch) and adjust to match with the Left Ch shunt R.
4. Do step 3 with the series Rs.
5. You can try the above procedure on the 9, 12 and 3 o'clock positions.
second report on my lightspeed
I have had my lightspeed for about 3 weeks now and I have done various mods to the adjoining equipment and the lightspeed itself so I will tell you what I have found.
My cd player now has a 20 ohm output impedance and I removed the electrolytic o/p cap so I now have about 1V feeding into the LDR's. I put 3.9uF cap on the o/p of the LDR's and this is feeding into my amp with approx 100k i/p impedance.
This group of mods brought more clarity and definition - all good stuff. The only draw back is that if I turn the volume control quickly the speakers leap forward or backward so some care is needed if you adopt this arrangement.
The next group of mods is all to do with power supply and power supply noise. For me this is the area that offers the most radical improvement in quality.
First some philosophy.
for me, one of the biggest barriers to great sound is Power supply noise - both both audioband and ultrasonic. Anything that can be done to reduce this reaps good reward. If RF noise is present ( and most power supplies generate it ) it seems to get everywhere and ruins the sound in the process and I do not regard the "earths" in circuits as being immune for this RF pollution.
back to practice.
First step I took was to remove the power line inside the box from the back socket and power the regulator with directly from a 9V alkaline battery. I you have already tried a 12V lead acid battery I would suggest you try an alkaline - they have the reputation of being much quieter and I have found that LA batteries need parallel capacitors to give half decent results.
The alkaline battery sounded really great - more transparency and detail and also very smooth - but of course this solution becomes tiresome and expensive with time....
So I built a VERY quiet power supply and fed it in to the regulator board. It has CRCRLC plus snubbers at all stages. This sound very nice also - very smooth but seemed to loose some dynamics / detail - just a tiny bit - but noticeable.
final stage in next post ...
I have had my lightspeed for about 3 weeks now and I have done various mods to the adjoining equipment and the lightspeed itself so I will tell you what I have found.
My cd player now has a 20 ohm output impedance and I removed the electrolytic o/p cap so I now have about 1V feeding into the LDR's. I put 3.9uF cap on the o/p of the LDR's and this is feeding into my amp with approx 100k i/p impedance.
This group of mods brought more clarity and definition - all good stuff. The only draw back is that if I turn the volume control quickly the speakers leap forward or backward so some care is needed if you adopt this arrangement.
The next group of mods is all to do with power supply and power supply noise. For me this is the area that offers the most radical improvement in quality.
First some philosophy.
for me, one of the biggest barriers to great sound is Power supply noise - both both audioband and ultrasonic. Anything that can be done to reduce this reaps good reward. If RF noise is present ( and most power supplies generate it ) it seems to get everywhere and ruins the sound in the process and I do not regard the "earths" in circuits as being immune for this RF pollution.
back to practice.
First step I took was to remove the power line inside the box from the back socket and power the regulator with directly from a 9V alkaline battery. I you have already tried a 12V lead acid battery I would suggest you try an alkaline - they have the reputation of being much quieter and I have found that LA batteries need parallel capacitors to give half decent results.
The alkaline battery sounded really great - more transparency and detail and also very smooth - but of course this solution becomes tiresome and expensive with time....
So I built a VERY quiet power supply and fed it in to the regulator board. It has CRCRLC plus snubbers at all stages. This sound very nice also - very smooth but seemed to loose some dynamics / detail - just a tiny bit - but noticeable.
final stage in next post ...
The Ultimate Mod ???
For a few days I had been thinking that we are missing a really great opportunity with this design.
That is to completely isolate the LED circuit from the LDR circuit. This will mean that any stray noise coming from the power supply OR regulator will have absolutely no physical conductive path to the - i/p, LDR, o/p - side of the circuit.
This is the last mod I have done and I do not think that this design can get any better than this ( but I would love to be proved wrong ! )
With this mod the clarity, transparency and ultimate refined quality is better than anything else that I tried.
Now the only connection between the LED circuit and the LDR circuit is light.
Sorting this out requires some thought - I did not even want the potentiometer attached to the box - which at present is carrying the earth return signals.
On George's original design this mod involves breaking the earth tracks under each LDR and moving the regulator output ground wire from the terminal block close to the rear of the box and reconnecting it to the two zero volt connection point between each of the LED pairs.
Many thanks for this design George - I hope you, and all here, enjoy the results of these suggested mods as much as I am enjoying them.
cheers
mike
For a few days I had been thinking that we are missing a really great opportunity with this design.
That is to completely isolate the LED circuit from the LDR circuit. This will mean that any stray noise coming from the power supply OR regulator will have absolutely no physical conductive path to the - i/p, LDR, o/p - side of the circuit.
This is the last mod I have done and I do not think that this design can get any better than this ( but I would love to be proved wrong ! )
With this mod the clarity, transparency and ultimate refined quality is better than anything else that I tried.
Now the only connection between the LED circuit and the LDR circuit is light.
Sorting this out requires some thought - I did not even want the potentiometer attached to the box - which at present is carrying the earth return signals.
On George's original design this mod involves breaking the earth tracks under each LDR and moving the regulator output ground wire from the terminal block close to the rear of the box and reconnecting it to the two zero volt connection point between each of the LED pairs.
Many thanks for this design George - I hope you, and all here, enjoy the results of these suggested mods as much as I am enjoying them.
cheers
mike
Re: second report on my lightspeed
To cure this problem, I would put a 10uf polyprop at the output of cd player and direct couple the Lightspeed to the amp, this will cure two of your problems, no dc on the LDR's and no woofer flapping if you turn up the volume too quick.
And my calcs with the Lightspeed at 7k input you will have a -3db at 2hz which is fine. Certainly doesn't look what I sent you a couple of weeks ago any more.
Cheers George
mikelm said:
To cure this problem, I would put a 10uf polyprop at the output of cd player and direct couple the Lightspeed to the amp, this will cure two of your problems, no dc on the LDR's and no woofer flapping if you turn up the volume too quick.
And my calcs with the Lightspeed at 7k input you will have a -3db at 2hz which is fine. Certainly doesn't look what I sent you a couple of weeks ago any more.
Cheers George
George - I am just listening some more - this isolation mod is really the business - much clearer that just with an alkaline battery.
It may be that with this isolation mod, even a cheap PSU sounds fine. I will check it out and report later
10uF in CD player ? - yes but I could not squeeze a cap like that inside my player without some more work.
Now back to some music ..
It may be that with this isolation mod, even a cheap PSU sounds fine. I will check it out and report later
10uF in CD player ? - yes but I could not squeeze a cap like that inside my player without some more work.
Now back to some music ..