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godfrey 27th March 2012 01:30 AM

LDR attenuators - Better balance with DC bias
 
Hi all

At the risk of being shot......
I think it may be useful to add a bit of DC bias to the photocells handling the audio signal.

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The idea is that the photocell's resistance is the same at DC and audio frequencies, so by measuring the DC voltages you can find out exactly what the attenuation at audio frequencies is.

More importantly, it becomes fairly easy to implement a servo which compares the DC voltages of the left and right channels, and adjusts the LED currents to give (almost) perfect channel balance, without any need for trimmers or matching of parts.

If DC offset is considered undesirable, one can use a symmetrical bridge type circuit to cancel any DC at the audio input and output.

OTOH, in AC coupled circuits where DC offset isn't a problem, there's another advantage to be had; Nelson Pass's measurements showed the distortion to be mainly 3'rd harmonic, but with suitable DC bias the distortion characteristic can be changed to being predominantly 2'nd harmonic, which some people may prefer.

Also, according to PerkinElmer:
Quote:

The minimum distortion or threshold distortion shown
in Figure 3 is a second harmonic of the fundamental frequency. The
actual source of this distortion is unknown, but may be due to some
type of crossover nonlinearity at the original of the I-V curve of the
photocell.
If there is indeed some kind of zero-crossing distortion in their devices, it can be avoided by biasing the device away from zero.

Cheers - Godfrey

gootee 27th March 2012 02:03 AM

I was going to suggest DC, but mainly to use it to have the ability to automatically adjust the resistances and therefore the attenuation WITHOUT MATCHED LDRs. The channel balance would just be a natural side-effect of perfect resistance settings.

1543 27th March 2012 08:37 AM

A few days ago I had same idea if it would be possible to use DC in a LDR circiut and have possibility of resistance measurements in operating conditions.
This would allow to adjust resistance fine enough to use LDRs in a balanced circuit. The 2nd harmonic distortion of LDR should be canceled out in balanced circiut.
If it would be possible also, as you state, to change 3rd harmonic by DC bias to be predominantly 2nd harmonic most of LDR induced distortion could be eliminated.
1543

DF96 27th March 2012 10:28 AM

Quote:

Originally Posted by godfrey
The idea is that the photocell's resistance is the same at DC and audio frequencies

Is this known to be the case? LDRs introduce distortion. Therefore they are (to some extent) non-linear. Therefore their AC and DC resistance will not be the same, although it may be close. Is it close enough that perfect DC balance implies good enough AC balance?

SY 27th March 2012 11:50 AM

Quote:

Originally Posted by DF96 (Post 2961892)
Is this known to be the case? LDRs introduce distortion. Therefore they are (to some extent) non-linear. Therefore their AC and DC resistance will not be the same...

Is that indeed the case? If the current versus voltage characteristic were (say) linear plus a small square law term, the device would be nonlinear, but could have identical AC and DC resistances. All it needs is sufficiently low reactance (stray cap or inductance).

godfrey 27th March 2012 12:17 PM

2 Attachment(s)
@DF96: Close enough, I think. The distortion would have to be pretty horrific to get even 1% difference.

Below are a couple of "simplest possible" circuits that I'm thinking about to start with.

In the first one, the DC voltage at X is directly proportional to the gain (i.e. 1 / attenuation) of the circuit. For control purposes, it will either have to be filtered or compared with a reference and integrated. If an output buffer is used, that could be configured as a 2'nd order VCVS filter to minimize the VLF output when the user twiddles the volume knob.

In the second circuit, The DC voltage at X is also proportional to gain, but there will be minimal signal voltage imposed (if the DC offset is zero). The DC voltage at Y can be monitored (via an integrator) by a second servo to set the DC offset to zero.

I take back what I said about the servos being easy. One awkward bit is choosing time constants due to the LDRs having very different rise and fall times, both of which vary wildly with attenuation level.

There's also the question of whether the control system should simply be linear or if it would be better e.g. to take the log of the measured voltage (to get a control proportional to dB), and use an exponential function for the LED current to compensate. Maybe some overall non-linearity is called for? Nelson's measurements suffest that, but looking at some datasheets, it seems to depend on the selected device.

And then there's dynamic range vs noise and topology......
Much head-scratching ahead.

DF96 27th March 2012 12:35 PM

Quote:

Originally Posted by SY
f the current versus voltage characteristic were (say) linear plus a small square law term, the device would be nonlinear, but could have identical AC and DC resistances.

I understand DC resistance to be V/I and AC resistance to be dV/dI. A non-linear curve could have these equal at certain points, but in general they will be different. If it is only weakly non-linear then the difference will be small enough to ignore.

AndrewT 27th March 2012 01:27 PM

I wonder if this is in any way similar to the Member who is using a micro to control the LED side to get the required audio outputs from the LDR side?

gootee 27th March 2012 09:09 PM

It could be done with a micro, and memory for the measured LDR curves. But this way it could be done with no memory, or processor, which could also be accomplished by using pilot tones that were outside of the audio range.

I think it might be difficult to filter out the DC, well-enough, especially since music usually skews one way or the other, for a short time, continuously, meaning the filter time constant might have to be too long, in order to ignore those meanderings.

godfrey 28th March 2012 12:34 AM

Quote:

Originally Posted by gootee (Post 2962641)
....could also be accomplished by using pilot tones that were outside of the audio range.

I think that would be a lot easier, but may struggle for audiophile approval.


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