LDRs as pots?

[Gregg]

How about redesigning a tube circuit to work with regular digital pots that run off +/-15V? I'm sure with some thought you could come up with a circuit that covered tones similar to classic amps, but never applied more than 30V p-p of signal across any one pot.

At some point I was considering this idea as well but then it's not going to be completely analog.
I guess a 30V digital pot will handle the signal after the first gain stage without problem but then we're getting to the tone stack. One of the solutions /the most straighforward I think/ is to take the signal from a voltage divider and proceed to the now digital TS. I see two problems here: 1/ A voltage divider after the TS doesn't sound the same as a directly coupled to the CF TS. 2/ 1M digital pots are not very common and have low frequency response /6kHz/.
What I was thinking is loading the CF with a preset /flat/ TS to decrease the signal level and to provide the CF loading in question. Then follows the digtal TS. Аt some point however the project is getting too digital which makes it a different project /I have a working DIY JMP-1 already.../.

[Don Symes]

The circuit posted by Don is like one that was posted on the old Ampage as a pdf file. I don't remember who posted it but I have a copy. The date on the copy I have is Jan6, 2005. It's actually a brilliant circuit. To make it into a pot I would remove the ground from the lower end of the programmable pot and run it to an identical circuit. Leave the wiper of the programmable pot grounded. Now the second circuit controls it's LDR to mimic the resistance of the bottom portion of the programmable pot. Hook the LDR's from the two circuits to form a voltage divider and you have a pot.

While I can't speak to the distortion of LDR's in current production, I know that the ones used in the Tektronix AA501 distortion analyzer had very low distortion. I'm pretty sure they were Vactrols.

Thanks, Thud - that date makes me think that was my posting. The thing about grounding the pot's wiper is a slick way of simplifying the 2-section application - that knocks almost 1.50USD out of the BOM cost of a _pair_ of photopots, and every bit of margin helps make the kit less of a pipe dream.

Oh - left implied in your improvement is that the now-freed-up end of the digipot gets another 10k pullup and the rest of the circuit is duplicated (including the scaling resistor) to make the other half of the pot.

[loudthud]

One problem I thought of is that many digital pots have some resistance (up to 1K) in series with the wiper. This suggests that you would like the highest ratio of end-to-end resistance vs wiper resistance to minimize the error. Not really a problem, just change the pull up resistor to match the digital pot. One way to compensate for the wiper resistance might be a fixed resistor in series with the LDR on feedback side.

[Don Symes]

One problem I thought of is that many digital pots have some resistance (up to 1K) in series with the wiper. This suggests that you would like the highest ratio of end-to-end resistance vs wiper resistance to minimize the error. Not really a problem, just change the pull up resistor to match the digital pot. One way to compensate for the wiper resistance might be a fixed resistor in series with the LDR on feedback side.

Both good approaches - but the bigger digipot value doesn't have to be re-scaled for each scaling resistor value like the compensating series resistor probably would.

... no, wait - with a modern op-amp's bias/input currents, the voltage dropped across that wiper series resistor is going to matter very little ... isn't it? Not able to concentrate adequately right now. The lore is that a lower-impedance source is lower noise ... but I don't remember why, or if it matters to a circuit intended to have a BW limit of a few Hz.

[unclejed613c]

you would use the LDR as the lower half of a voltage divider, or invert the input voltage to one of the LED's and wire the outputs in series as a voltage divider

[Don Symes]

you would use the LDR as the lower half of a voltage divider, or invert the input voltage to one of the LED's and wire the outputs in series as a voltage divider

Wire two output photcell elements in series and use the junction between them as the photopot's wiper.

Ground the digipot wiper as Loudthud suggests, and use that other digipot terminal to drive a duplicate LDR/scaling resistor/op-amp circuit.

No inversion is required as the circuit's job is to use its half of a digipot to scale a percentage of the pot's end-to-end resistance to reflect a percentage of the scaling resistor's value in the LDR. Since you're effectively measuring the resistance in each half of the digipot with a separate scaling/LDR circuit, and the respecive values move in concert - and are already inversely proportional to each other - the inversion is already done.

[Gregg]

Obviously VTL5C4/2 is not good for this application, VTL5C3/2 currently is not available /Allied, Farnell/ except for ridiculous prices which makes VTL5C2/2 the only plausible choice at this time.

[Don Symes]

Have you looked for Clairex or Hamamatsu parts?

Try googling Photocoupler (then ignore all the logic/transistor-output parts)

[Gregg]

Yes, I looked but I couldn't find anything close to that especially dual cell photoresistor optocouplers.

[Don Symes]

Yes, I looked but I couldn't find anything close to that especially dual cell photoresistor optocouplers.

Of course.

I wonder if R.G.Keen has an LDR-matching jig for his UniVibe-style stompboxes?

Anybody seen R.G. around here lately?

[Gregg]

It looks like Perkin Elmer have established a monopoly over the dual cell LDR but good people from Eastern Europe have few things in mind /including a dual cell LDR/:

http://www.tesla-blatna.cz/en/produc...ronic-elements

Scroll down the page and check out the PDF files.

Unfortunately these parts are nowhere to be found yet.

[max_lwedge]

Does anybody have comparisons or A/B experience between the Vactrols and the Silonex OIs, ie., NSL-32, etc. Any preferences to one over the other in specific applications? I've used both satisfactorily in tube amps as switches and shunts, but never anything between on and off and no close comparisions.

[Gregg]

I was looking at Mesa'a patent drawings and I can't figure out how the "wiper" terminal is supposed to be such if it's actually connected to ground?

[Gregg]

Excuse me, may be I'm dumb or blind /or both/ but I can't figure out how Mesa's device is supposed to work if two of the "pot" terminals are connected to ground.

I wonder if R.G.Keen has an LDR-matching jig for his UniVibe-style stompboxes?

Maybe this is the way to go - get some photo cells /for example from the VT900 series/ and build your own 4 terminal dual cell optocoupler.

[Steve Conner]

The circuit makes no sense with the bottoms of all four LDRs grounded. I think it's a mistake or deliberate obfuscation by Mesa. The two left-hand cells should probably be grounded but the right-hand ones should float.