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Old 8th January 2012, 01:17 AM   #4371
soongsc is offline soongsc  Taiwan
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Having read about various volume control topology and it's gain/position curves published in Wireless World, it seems that linear gain/position is best? Since LDRs already have a near log nature, using linear pots in the Lightspeed already acts like a normal log pot volume control which provide quite linear gain/position relation, but also dependent on the input impedance.
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Old 8th January 2012, 02:29 AM   #4372
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Quote:
Originally Posted by soongsc View Post
Having read about various volume control topology and it's gain/position curves published in Wireless World, it seems that linear gain/position is best? Since LDRs already have a near log nature, using linear pots in the Lightspeed already acts like a normal log pot volume control which provide quite linear gain/position relation, but also dependent on the input impedance.
Tried that all back a few years ago, you need both the log of the pot and the gain slope of the led/ldr's to get a good smooth progressive feel from down low to mid position. If you use a linear it comes on way to fast from minimum to 10 o'clock position and then there's not much after 12. But then this is with the NSL32SR2S quad matched, It will be different for the NSL32SR2 or the NSL32SR3.

Cheers George
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Old 8th January 2012, 06:13 AM   #4373
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Originally Posted by georgehifi View Post
As you may/may not know the pot has nothing what so ever to do with the LDR's or the incoming or outgoing music signal, and with this design the sound quality has always come first.
All it's there for is to control the amount voltage going to the LED's in the simplest, cheapest most convenient manner, if you have a simpler way I'm all ears, KISS.
I was thinking you might like using a digital potentiometer to control the LDRs instead of a mechanical potentiometer. It would also allow you to easily incorporate a remote control.
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Old 8th January 2012, 06:58 AM   #4374
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Originally Posted by Wayne Parham View Post
I was thinking you might like using a digital potentiometer to control the LDRs instead of a mechanical potentiometer. It would also allow you to easily incorporate a remote control.
Thanks for the offer in your PM Wayne, I have looked at this remote path many a time, as it is the production Lightspeed Attenuator has a very very high reliability factor, the only ones that have come back for repair are ones that have been damaged by customers doing weird things, like hooking up car batteries reverse polarity, or being fed massive sine wave voltages on test benches with low impedance loads on the output cooking the ldr's, or just trying to mod them and stuffing things up.
The thing that has always given me concern about going the remote path is the quality of the hand held remotes that are available, and they are not from reliable suppliers (here today gone tomorrow) I have yet to find one cheap enough that would even last a quarter the distance of the Lightspeed Attenuator as it is. And the cost would treble from what it is now, motorized or digital pot, receiver, quality remote, more power supplies, and a bigger chassis to house it all in. Yet the sound quality would remain exactly the same for treble the price? Like I said I firmly believe in the KISS principal in this case.

Cheers George
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Old 8th January 2012, 11:47 AM   #4375
AndrewT is offline AndrewT  Scotland
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Originally Posted by georgehifi View Post
it's the NSL32SR2S I'm refering to because it has a much lower "R on" resistance than the other two,
Accepted, lower "R on" lowers the minimum volume obtainable.
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Old 8th January 2012, 01:44 PM   #4376
maximus is offline maximus  Scotland
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Wayne,

The VCCS (voltage controlled current source) module, originally designed for my own system requirements, uses a digital potentiometer for LED current control and it has additional modules available for remote control. The range of operation can be scaled by resistor selection of R1 to R4 in the current sources to suit the required range of volume control for a given system efficiency. It can be operated by push button controls, either on the preamp chassis or on the remote handset, and provides balance control as well as volume control.

The VCCS module and optional remote control modules were offered as a group buy early in this thread and are still available.

Regards
Paul
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Old 8th January 2012, 04:40 PM   #4377
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Yes, that's more what I was thinking. It is actually much more reliable than a potentiometer. An RDAC will last a lifetime, whereas a potentiometer will probably not. Wipers get dirty and substrates crack. Besides, the RDAC approach allows volume control to be implemented with a true log sequence instead of the crude approximation as implemented on mechanical pots. And of course, there is the ability to incorporate remotes.

Uriah Dailey (BuildAnAmp.com) has chips like this for his version of the LDR attenuator, so you can reach out to him if interested.

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Last edited by Wayne Parham; 8th January 2012 at 04:43 PM.
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Old 11th January 2012, 08:27 PM   #4378
BFNY is offline BFNY  United States
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Originally Posted by udailey View Post
Your amp:
" Line-level input sensitivity is 150mV which may lead to rather low settings of the volume control for some source units"
From Review Marantz PM66 KI amplifier (January 1998) - DutchAudioClassics.nl
I think if you put in the extra series LDRs you will be good.
Uriah
Perhapd I'm missing something here. Wouldn't an obvious solution be to add a resistor, say 20K to 40K, in front of the audio input to each series LDR ?

Is the reason this is not suggested, because the LDR "sounds better" than a resistor?
If even a 40K resistor is used, for a 2V rms source, this would still give 400mV rms out at full volume, which for a 150mV sensistive amp should be plenty. In the low end, it would make the control adjust to about -65dB, versus -50 now, and probably be much better.
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Old 11th January 2012, 08:42 PM   #4379
BFNY is offline BFNY  United States
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I would also add, that if that amp puts out 50W into 8ohms with an input of 150mV rms, it has a voltage gain of about 42dB, about 16 dB more than the commonly seen amp gain of 26dB.

By adding 40k in front of the series LDR, you are reducing (i.e. shifting) the sensitivity of the whole attenuator range down 15dB, and you will make it work similar to a "normal" gain amp.
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Old 11th January 2012, 09:22 PM   #4380
udailey is offline udailey  United States
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I did suggest that. perhaps a LDR attenuator in front of the lightspeed. It wouldnt require matching if it just divides voltage. So you would just use trimmers. Total added cost would only be around 12-15usd and would fix the problem.
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