Lightspeed Attenuator a new passive preamp

[gmphadte]

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

[gmphadte]

jkeny wrote

Thanks Gajanan,
Can you say somehing about how these three pots are used to balance the system. Just to put some figures on these Rs - the two Rs just before pins 1 are 100ohm?. What value for the three pots?

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

[gmphadte]

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

[gmphadte]

correction
Now adjust PR1 to make LDR of OC2 to 63 ohms, with pot at CW.

[jkeny]

Thanks Gajanan for taking the time to explain this - it's now clear.

Now to buy 8 of these and throw away 4 - just seems so wasteful!


John

[gmphadte]

Throw them all at my address, if at all.

[vph-2020]

Hi Gajanan

Correct me if I am wrong, the preset is just work at two extreme point, CW and CCW.

Between CW and CCW, the two mismatch LDRs still mismatch, or it's too complicated for me


Thanks

[gmphadte]

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

[Will]

Originally posted by gmphadte
correction
Now adjust PR1 to make LDR of OC2 to 63 ohms, with pot at CW.

I don't get this. With pot at CW shouldn't the resistance of OC2 be 7-8Kohm instead of 63 ohms?

[gmphadte]

will
UR correct, there is a mistake.

correction again
Now adjust PR1 to make LDR of OC1 to 63 ohms, with pot at CW.

Gajanan phadte