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Old 12th March 2007, 09:03 AM   #1
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Default Lightspeed Tone Control...

Lightspeed for Tone

Suppose u want
-a pot of 100K in the Baxandall tone circuit
-a volume control of 22K or 150K
-donít have the required amp input and/or source output impedance
-7K pot will not do

-and donít want to match
Remember it is very difficult to match or get two vactrols for making high value LDR resistance

Decide the values of the stepped attenuator/potentiometer u want to make.
There are so many on DIYAUDIO

Method...
Number your vactrols
Make a table for vactrol No. 1
Use the calibration circuit

Table columns are
R-LDR, R-LED
60 , 108 ohms
and so on...

And make a calibration table for each vactrol.
Repeat for other vactrols

Now assemble a 10 stepped attenuator by using 4pole 10way switch of any cheap quality and connect the R-LED as per the calibration.
Connect the LDRs as needed in your application circuit

Remember: use exact same supply for clibrartion as u would use for the application. The same components would be the best and will retain the R-LDRvalues.

Enjoy

Gajanan Phadte

Edit: the applicatiom example has a mistake and was the quickest thing I had at hand, to explain the use.
Attached Files
File Type: pdf lightspeed tone.pdf (32.4 KB, 473 views)
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Old 15th May 2007, 12:44 AM   #2
jkeny is offline jkeny  Ireland
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Hey Gajanan,
I just came accross your post and as I was thinking along the same lines myself I was interested.

Basically, I thought why try to match these optocouplers (with the expense of buying 10 for two matched pairs & the waste of discarding the rest) - just buy two and use a pot to control the gross volume (remote controlled) and a second pot (remote controlled) to act as fine balance between channels - done by ear from the listening position.

Now, I see your stepped attenuator with different value resistors used to balance the optos. I don't understand your PDF though - can you explain?

John
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Old 15th May 2007, 04:46 AM   #3
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The idea is given to use any non-matching optocouplers.

Take a tone control circuit of your choice.
Look at one channel, and one pot amongst the two e.g. Bass
For simplifying, let use consider the pot to be linear.

Let us assume the pot to be 50K for convenience.
The flat position i.e. in the middle position, the pot forms 25K and 25K, the wiper in the middle.
For making this position your LDRs should be illuminated by R-LED that corresponds to 25K of LDR resistance for both LDRs forming the control.
Remember, one pot is made from two optos.

The example above is for flat position and for total Bass boost and total Bass cut, one of the LDR should be illuminated to be of Zero ohms which is not possible as LDR do not reach to zero ohms. So The LDR should be the minimum resistance possible meaning its LED should be driven to full brightness. This corresponds to minimum R-LED.

Gajanan Phadte
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Old 15th May 2007, 08:27 AM   #4
jkeny is offline jkeny  Ireland
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Ah yes Gajanan ,
I was still thinking of a vol control and forgot it was a tone control circuit. Doh!

Thanks again - have you built one - how well does it work?

John
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Old 16th May 2007, 03:45 AM   #5
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I have built one for the volume control and is much better than anything I can buy here. However the tone control circuit will be equally good except for the second harmonics, which is an issue of these devices at high resistance.

Gajanan phadte
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Old 16th May 2007, 11:06 PM   #6
jkeny is offline jkeny  Ireland
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Final question Gajanan ,
Did you use the unmatched Silonex parts or the Perkin-elmer vactrol parts?

John
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Old 17th May 2007, 01:45 AM   #7
Leolabs is offline Leolabs  Malaysia
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Can Silonex be used???
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Old 17th May 2007, 03:48 AM   #8
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I bought about 60 LDRs, the total available in the local retail shop and categorised them and made my own matching opto-couplers. U cannot buy any of those here. even if u order them from other foreign suppliers, u will run out of your patience and intrerest due to a big delay in arrival of the parts.
Customs...u can't believe them here, u may be bankrupt LOL!!!

The idea is for non matched parts...
Anything can be used....

Edit: my need for the idea was due to non-availability of the opto-couplers of any kind

Gajanan Phadte
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Old 17th May 2007, 08:03 AM   #9
jkeny is offline jkeny  Ireland
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What brand of LDRs did you find with a low enough on resistance ie about 50ohm which is what is needed to get a low enough vol when turned off?

When you say you measured LDRs to create matched optocouplers - how close was the matching - do you have the equivalent of the matched Silonex parts?

John
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Old 17th May 2007, 09:46 AM   #10
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They are local make and carry no brand and damn cheap.
They were not tightly matched(upto the first decimal point of K)
The life of the LDR reduces if u use bright light for illumination.
I wanted a higher life expectancy for LED and LDR. My LDRs showed about 150 ohms with LED current of 20mA, the max I prefered. The volume with 150 ohms is low enough.

If u want to make one, use the cal method above and u can track them to whatever tolerance u wish.

U just have to buy one or two small-ones and try.

Gajanan Phadte
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