Variable Resistor question.

[Julian22]

Hi there!

Could somone please explain why a LDR (in let's say an opto compressor) wouldn't be able to control the passing Audio just by its own varying resistance, without the use of a second fixed resistor (in a voltage divider configuration)?

I understand the concept of voltage dividers, making it possible to get a variable Voltage out of a fixed Voltage source, but in this case the voltage is already varying.

Cheers!

/Jules

 

[sismofyt]

You don't need a second resistor. Just think about it, the LDR shunts the signal to ground. At least they do in my comps :wink:

 

[gyraf]

Jules,

Welcome to "The Lab"..!

The resistor divider scheme is applied to ensure that you have predictable no-GR gain.

The LDR's vary quite a lot in on-resistance, which leaves you with not-quite-predictable gain through the unit when no gain-reduction occurs.

And we don't like that..

On the other hand, for things like noisegates or clickless switches, LDR's are often used as series-pass devices. But then more like an on/off device.

Jakob E.

 

[Julian22]

Thanks Jakob...I get it=)

So is it the same in the case of a JFET circuit? Because of what I've heard LDR's and JFET's both operate as VCR's in compressors.

So does this relate to JFET's as well? Is the fixed resistor only there to make sure you get a predictable no-GR gain, or is there something more to it?

Cheers!

Julian

 

[PRR]

> why a LDR (in let's say an opto compressor) wouldn't be able to control the passing Audio just by its own varying resistance, without the use of a second fixed resistor (in a voltage divider configuration)?

Draw it, as a complete system, surrounded by amplifiers.

First assume that amplifers have low output impedance and high input impedance. This is generally true.

In the top plan, the output amplifier gets exactly the same voltage that the input amplifier puts out. The LDR just makes the input amplifier work harder to feed the LDR. With practical chip-opamp amplifiers, when the LDR goes below about 500 ohms the amplifier will clip on loud sections, but when not clipping the gain is unchanged. The LDR would have to go below 1 ohm to affect gain, and in this case it IS a "2 resistor voltage divider", working against the ~1 ohm output resistance of the opamp. And LDRs don't go that low.

In the second case: the output amp will get full signal until the LDR resistance rises higher than the output amplifier's input impedance. The naked opamp input impedance may be over 100 MegOhms, and LDRs are very slow to reach 100M.

> I understand the concept of voltage dividers, making it possible to get a variable Voltage out of a fixed Voltage source, but in this case the voltage is already varying.

Makes no difference. We are trying to change gain, not set a voltage (though in a complete Limiter, we vary gain to keep the peak voltages to a set level).

For the most common LDRs, for controlling audio, the LDR's impedance should work between 1K and 100K. It is too hard to get them down below a few Kohms, and they are very slow to rise above 1 Mohms. In the LA2, the designer picked about a 30K threshold value. This is much higher than normal pro audio source impedances (under 600 ohms, maybe 10K with input transformer) so there is a 30K series resistor (actually a network: 68K series plus two 100K shunt). For maximum gain, the LDR is dark, rises above 100K, gain is 100K/(30K+100K)= 0.77 or higher. For very low gain, we light the LDR to 1K, gain is 1K/(30K+1K)= 0.03

> the LDR shunts the signal to ground. At least they do in my comps

Always with a series resistor.

 

[sismofyt]

Yup, I thought he meant a resistor in series with the LDR, both being parallel.

 

[Julian22]

Hey all!

Wow...I didn't expect all this=)

About Simofyt's last reply...I think that was what I meant...a fixed resistor in series with the LDR (in a voltage divider) circuit. Where does that parallel thing come in???


To PRR:::::

So if I got you right, the voltage divider configuration (a second fixed resisor in series) is mostly about impendance matching then?

But what about what Gyraf said: that the second resistor is applied to ensure a more predictable no-GR gain? Any comments on that?

Another source also told me that the main reason is to be able to achieve a more dramatic dB cut, when a LDR on its own wouldn't be able to offer a lot of dB cut. Any comments on that?

And finally, could also this also apply to JFET VCR's? Does a JFET have to be a part of a votlage divider of the same reasons???

Thank you!

/Rob

 

[PRR]

Show me how it could work without a voltage divider.

Or: what is the sound of one variable resistor flapping?

 

[Julian22]

Thanks PRR and you all!

I did some reading about JFET, used as attenuators in compressors, and as far as I understood it they work in pretty much the same way as LDR;s, with variable resistance.

Please correct me if I'm wrong, but this is how I got it:

An AC voltage is applied at the gate. The Vg is the i/p voltage, controlling the Id, flowing through the channel, and makes it fluctuate (sort of like the plate current in a tube?). The same AC change is imposed to the Vds, which is sent to the JFET attenuator o/p. (But what happens to the Id?)

When the DC bias is made more positive, the effective resistance of the channel decreases and the Vo decreases as well, bringing down the o/p.

Please let me know if I've got it right...

Many thanks

/Julian

 

[Julian22]

:grin: :shock: [/img]

 

[PRR]

> controlling the Id, flowing through the channel

When used this way, there is no static current flow.

The bidirectional audio voltage appears across the FET channel. A bidirectional audio current flows. The ratio of voltage and this current can be controlled by gate voltage.

While we usually think of amplifiying devices as unidirectional, the FET is symetrical for very-small voltages both sides of zero volts on the channel.

 

[Julian22]

Hi all!

PRR wrote:

Quote:

The bidirectional audio voltage appears across the FET channel. A bidirectional audio current flows. The ratio of voltage and this current can be controlled by gate voltage.

When you say "gate voltage", do you mean the audio Voltage (AC) or the CV voltage (DC)? So do you mean that it is the CV that changes this "ratio of voltage and the current" by going more nengative or positive? Why is this ratio relevant?

I feel pretty comfortable with the theory and operation of the JFET as a normal amplifier, but I find it really hard to understand the relation between Vgs, Id, Vds and R(JFET) when used as a VCR.

I find it all pretty confusing, especially when some people say that the Vgs doesn't really change neither Vds nor Id, just the relation between them, while others say that an increased gate voltage decreases Id. I think what is confusing is to know if they mean the audio voltage or the CV, when talking about the Vgs.

Does, for example the audio Voltage (Vgs) affect the overall resistance of the channel at all, or is this just the CV?

In the output characterisics graphs, when showing different values of Vgs, are these values of the CV (DC voltage) or the Audio voltage (AC)?

PRR (or anyone=)...I would highly appreciate if you could come up with one of your super clear and mind blowing explinations=) I read the one you wrote about Vari-mu tubes...great stuff!=)

Please!

What I would like to know is what happens when a audio voltage is applied to the JFET, with the CV voltage constant. How does this change in audio voltage affect the Vds? What happens with Id? And is the change in audio signal going to affect the channel resistance? Or is the channel resistance related to changes in the DC control voltage?


And is the Vgs in the output characterisics graphs demonstrating the CV or the audio voltage?

Thank you!!!

Jules [/b]
Jules

 

[jhaible]

Just a few remarks about LDRs and JFETs in voltage dividers:

LDRs: You put the LDR in the "thru" path and the fixed resistor as a shunt if you want to have fast turn-on (and not so fast turn-off). Typical application: Noise Gate.

You connect the LDR as a shunt (with a fixed resistor in the "thru" path) if you want to have fast Gain reduction (and not so fast return to original gain). Typical application: Compressor.

Using two LDRs, with complemantary control signals, is possible, too, and can make a voltage divider (or cross-fader!) where turn-on and turn-off are both pretty fast. The thing will be slightly slower somewhere in midrange of attenuation or crossfading, though.

FETs: They have a "linear range" (at low DS voltages, see below), where the FET just behaves like a resistor. (For higher DS voltage, it behaves more like a current source.) Without going into deep maths, I think it's best to see these as two completely different functions. (In reality, there's some transition, of course.)
So we have a VR resistor, just like when we use a LDR.
Big Problem: *Low* voltages! You must divide down your input signal a lot in order to be in the range where a FET works as VC resistor. And this means you must amplify it a lot afterwards. You need an amp with superb SNR for this. There are tricks to increase the linear range of a FET (by feeding parts of the signal voltage to the CV / Gate path), but even then you're working with very small voltages.
The only way to get usefull results out of this (IMO) is a configuration a la Urei 1178, where the Compressor action keeps the signal voltage across the FET quite constant, i.e. you always work at a fixed threshold internally, and adjust the "outer" threshold by using different input- and output gain settings.
Also, a FET normally has one pin (the source) sharing audio signal and CV signal, so ju must be carefull not to feed CV transients into your signal path. There are special parts called "opto FETs", which get rid of that problem. But I have never tested them. They aren't cheap, either.

JH.