LDRs as pots?

[Gregg]

Hello,

I read somewhere that some of the dual cell Vactrols /the VTL5Cx series optoisolators/ can be used instead of mechanical potentiometers /by controling the LED brightness/. I can figure out how to do it with two separate LDRs. Wouldn't however the resistance of both halves of the dual LDR increase and decrease simultaneously?

[Tom Phillips]

Originally Posted by Gregg

Hello,

Wouldn't however the resistance of both halves of the dual LDR increase and decrease simultaneously?

Yes. There is only one light source inside so a dual element unit cant be used as a pot. If you are interested, there is an application note showing a dual unit in a circuit at http://optoelectronics.perkinelmer.c...on%20notes.pdf It's at the very end of the article.

Tom

[Gregg]

Thank you for the link but I don't see how it can be useful in a tube amp.

[Tom Phillips]

Right. I didn't mean to imply it would be useful in a tube amp. Just an informational link to give a clue about how the dual LDR part is applied in a circuit.
Cheers,
Tom

[Steve Conner]

I once helped design a DJ mixer that used a pair of dual vactrols in its crossfader. Each vactrol acted as a dual gang variable resistor to attenuate one stereo signal, and the crossfader was wired up to dim the LED in one vactrol while it brightened the other. I think that might be the kind of application you were thinking of.

If you want to use vactrols as a replica of a pot, you are absolutely right that you need two separate units and some kind of clever control circuit that dims one while brightening the other. In a pretty complex non-linear way, if you want it to act like a real pot, keeping its total resistance constant while moving its "tap". I guess you could use two dual units, and use the unused section of each for feedback to a control circuit.

I heard rumours that the Mesa Boogie Triaxis used LDR "Pots" like these to give it its programmability, though I don't know how they did the linearisation. Randall Smith probably patented it so you can go and look it up and figure out a way round it. (Or just rip it off if you're feeling brave.)

[Gregg]

Triaxis used LDR "Pots" like these to give it its programmability

I heard that too. I'm looking for something like this because I'm planning to build a programmable all tube preamp. Unfortunately the current production digital pots specs are not very suitable for applications where large voltage swing is present.

[Rick Erickson]

How about motorized faders or pots?. All you need is a circuit to control their position, and it's cool to watch them move by themselves.

RE

[Gregg]

This is interesting but how about when switching between different presets?

[Rick Erickson]

That's the fun of design - you have to sort through the options and find the best solution for yourself. Sometimes you even have to spend time and money on things that don't work. It's all part of the process.

RE

[Gregg]

I think I'll stick to the LDRs. Maybe create a logarytmic table in Excel, take say 20 positions /from -10 to +10 for example or from 1 to 20/, then set the values manually via regular pots to ADC and memorize them.

[Enzo]

To replace a volume control, could you not:

From the signal source - plate cap perhaps - run a series resistor to the following stage - grid perhaps - and also from that point an LDR to ground. The two form a voltage divider, and the variable resistance of the LDR makes the two work more or less as a pot would. Maybe?

[Gregg]

Do you mean something like Mesa style volume control? Using two Vactrols for Gain, Volume and Treble seems to me like a better idea. Bass, Mid and Presence can go with single LDR.

[carl gigun]

Enzo's suggestion of one LDR and one resistor as a voltage divider between stages is exactly how old tube compressors like the LA-2A accomplished their gain control. In their case they had the dry signal drive the light so it would automatically "turn the volume down" when a hot signal came along.

I guess storing presets is a separate problem that needs to be tackled whether you use one or two LDRs.

[Tobacco Soup]

This is a great thread! I had a similar idea about a programmable & MIDI controllable tube preamp of my own design. Storing parameters and their control/access can be accomplished with a microcontroller and EEPROM. What the data means, and what it does is the fun part. An 8-bit resolution can give you 256 increments, which is far more of an 'analog' interface than the ADA/Triaxis broad brushstrokes. Anyways, you've given me a lot of food for thought with the LDR idea, and I thank you! A lot of the other options I had considered weren't applicable to tube circuits, so I was mulling-over the possibility of controlling tubes with transistors and the like.

[Gregg]

The question /at least for me/ is how to control the LED current /brightness/? I did some search and it looks like LED drivers from different companies are available, some of them programmable, but are they suitable for a project like this? For example:

http://focus.ti.com/paramsearch/docs...ODE_STRY_PGE_T

http://para.maxim-ic.com/cache/en/results/4959.html


Any other ideas?

[Tobacco Soup]

Most of the drivers, I'm assuming, provide the logic and/or buffers for driving an LED array to provide a desired order of active segments. My experience with microcontrollers and non-tube amp ccts. is extremely limited. However, you can use a D-A converter (or even digital resistors that are controlled) from the microcontroller to control the brightness/intensity of an LED through other ccts.

[Gregg]

As you can expect somebody has already figured it out /at least this is one way to do it/. Check out page 2:

http://home.comcast.net/~sbernardi/e...echo_delay.pdf

I guess RV1 and R33 can be omitted.

[novosibir]

Originally Posted by Steve Conner

I heard rumours that the Mesa Boogie Triaxis used LDR "Pots" like these to give it its programmability, though I don't know how they did the linearisation.

It's a flock of about 22 or 24 of that VTL5C9 in those Preamps IIRC. And it's a pure nightmare, when they become older and are loosing their specs - and you try to find the 2 or 3 causer.

Recently I've told an owner, that I either swap all inside, or none - another way I can't offer. He went off.

Larry

[Gregg]

I wish there was an easier solution for that but I can't figure it out at this time. Obviously digital pots are not an option. Maybe that's why you don't see many all tube programmable preamps these days.
Any other ideas are more than welcome.

[Tobacco Soup]

IIRC, transistors can be configured to multiply (and/or 'transfer') impedance/resistance. So, my guess is that if that is so than a cct. can be made from a stout enough transistor to handle what a tube needs, and serve as an intermediate device between the tube and the fragile digital resistors. I believe that a transistor can also be used as a passive device, and perhaps the behavior of the passive transistor can be manipulated by another cct. Just guessing here, because I don't know a lick about transistors.

[Don Symes]

The triaxis uses TWO dual vactrols to replace a pot (I studied the patent, but my library is still down so I don't have the patent number handy).

Each pot is replaced by a 10-tap voltage divider with the values in the stack selected to replicate the upper/lower resisitances in the required pot. An analog multiplexer selects a tap in each stack, which then feeds a voltage into an op-amp. The op-amp drives the LED in a dual vactrol, and uses one of the resistor elements in its feedback loop. The other resistive element is the upper or lower pot element in the tube side of the circuit. Each potentiometer-connected pot (fender treble control) requires a PAIR of these lashups to act as upper and lower segments of the pot. A rheostat-connected pot (fender mid control) only needs one of these.

The patent covers using a mux-selected voltage divider tap, so using a DAC should not infringe.

I did some work to make a generic DAC-controlled version of these that would be scale with a resistor of the end-to-end value of the pot - again, my library is down. A 12-bit DAC would give you enough resolution to do linear, 10% and 20% audio and reverse audio tapers, and dual 12-bit DACs are easy to find.

Email me at das_float@yahoo.com to remind me to dig for my old files when I get my library going again.

Hope this helps!

[Gregg]

Did you mean something similar to the programmable gain /fig.10/:

http://optoelectronics.perkinelmer.c...oisolatorB.pdf

[Don Symes]

Originally Posted by Gregg

Did you mean something similar to the programmable gain /fig.10/:

http://optoelectronics.perkinelmer.c...oisolatorB.pdf

For the controller side, yes. The differences are that the Vactrols have dual INDEPENDENT LDR elements (4 resistor terminals) and that the _controlled_ side resistor gets hooked up like half a pot.

[Gregg]

Which part number exactly do you mean because I don't see any that coresponds to your description /I see only 3 terminal ones/?

[Don Symes]

Originally Posted by Gregg

Which part number exactly do you mean because I don't see any that coresponds to your description /I see only 3 terminal ones/?

Oh - yeah - This was the project killer. Perkin-Elmer only does the 3-terminal ones these days. Mesa went to a Japanese company for their parts ... it will take me a while to remember the name ... I found their US rep, too - again remembering the name will take some doing.

I'm supposed to get my library back tomorrow. Just hope that stuff was in there.

I thought about rolling my own - getting and matching CdS cells and packaging pairs with LEDs, but I'm not mechanical enough to guaratee matched illumination over two cells. Using 2 independent LED/photocell units with the LEDs in series looked like an answer, but the LED datasheets I looked at showed a 3x variability in light output for a given current, so I'd have to match those as well.

Doable, but a gigantic PITA.

Hope this helps!

[Gregg]

Thanks for sharing these details with us. I think the way to go is still using Vactrols because of their predictability but introducing a programmable current source to control them. There are some pretty simple schematics over the Net how to implement it.

[Don Symes]

Mesa's US Patent number is 5,208,548 and is a bit less than clear.

Mesa's supplier for the dual-cell vactrols is Hamamatsu.

I'll continue to hunt for the circuit I designed - it was reasonably slick. The end-to-end pot value was programmed by sticking that resistance in an input divider - the op-amp would then drive the LED to produce the same resistance on the photocells, one of which was in the op-amp's feedback divider. A linear digital pot was the other side of the input divider. Since any practical application would only need about 20 settings, picking out a set of points to imitate any taper was pretty easy.

I hope I find one of the drawings, that would be so much easier to explain.

As always - hope this helps!

[Steve Conner]

Hi Don,

I understand how the circuit works (kind of like I thought it did...) It should be easy enough to convert it to voltage programming with a DAC, and I don't mind helping out with that.

I never bothered experimenting with this stuff in my own amps. It's just such a P.I.T.A. compared to a $1 pot, a knob with a numbered scale, and a brain (or notebook and pen) to remember what settings you like. If I really wanted a tube amp with a patch memory, I'd be trying to source a set of the motorised pots that Fender use in the Cyber-Twin.

I don't though, because I spend all day at work designing digital electronics and writing firmware, and tube amps are a great antidote. I try to avoid any component that has more than 9 pins or is smaller than a Jolly Rancher

[Don Symes]

While I both sympathise and empathize, Steve, I am by nature a Feature Creep. There's also this pipe dream to turn this into a kind of kit to bring programmability to any amp.

The scaling resistor sets your end-to-end pot value. The controlling computer has to have a table of values to get you the taper you want out of the 256 available digipot values. The op-amp's frequency response has to be seriously degraded to just a few Hz or it's gonna oscillate something awful. It takes one of these to make a rheostat (variable resistor) or a pair to make a potentiometer (variable divider). And you still gotta match your own photocells and/or LEDs.



Good luck!

[Tobacco Soup]

Thanks for posting that!

It'll take me awhile to understand it, but it's a great start for me!!!

[Gregg]

This also looks like a good idea :

http://www.elecdesign.com/Files/29/9944/Figure_01.gif

I liked the idea for a kit or why not a presoldered board /no way I'm soldering TSSOP packages!/. I was thinking more of a /relatively/ universal tube preamp - the ten or twenty most famous guitar preamps in one. Imagine a tube amp with six knobs only but capable of much more than that.

[Don Symes]

Originally Posted by Gregg

This also looks like a good idea :

http://www.elecdesign.com/Files/29/9944/Figure_01.gif

I liked the idea for a kit or why not a presoldered board /no way I'm soldering TSSOP packages!/. I was thinking more of a /relatively/ universal tube preamp - the ten or twenty most famous guitar preamps in one. Imagine a tube amp with six knobs only but capable of much more than that.

The difficulty is that the LED current/brightness relationship isn't real linear and neither is the CdS cell's illumination/resistance (AFAIK). What you need is a simple, predictable way to replicate a known rotation of a pot of known value and taper - that way you can keep the user interface's 'feel'. Using an op-amp to 'servo' the resistor value in a photocoupler is the closest you're going to get.

The universal preamp and/or amplifier - I called my project ToneLego and was gonna house it in an old Hot-Swap Drive Array chassis - each module a preamp, with a couple of low-power power amps in the back. All I can say is: Don't start down that path until you have a couple of solid builds under your belt - the complexity is factorial, not exponential.

Hope this helps!

[Steve Conner]

Gregg, all I can say on the subject of SMT homebrewing is http://scopeboy.com/elec/misc/prototype.jpg so there

I think Don's circuit is probably the best basic building block to use for a system like this. It could probably be simplified somewhat (especially if the op-amp has enough beans to drive the LED directly) and adapted to take a voltage input from a DAC instead of a resistance from a digital pot. Driving the LED directly would make the thing a lot more stable too, since the MOSFET adds gain in the loop and the R-C network on its gate adds more phase lag. That was probably part of the reason for the low-frequency oscillations.

Another cool method for computer gain control is the multiplying DAC, which acts somewhat like a pot. If you only needed 16 steps, say, you could make a high voltage multiplying DAC using four LDRs and a R-2R ladder network. The LDRs are just used as on/off switches, so they don't need to be specified so tightly.

If you needed a variable resistor, you could use four binary weighted resistors and four LDRs to short them in the right combinations.

P.S. I love the ToneLego thing! A Seymour Duncan Convertible for the internet generation

[Ray Ivers]

Steve,

all I can say on the subject of SMT homebrewing is http://scopeboy.com/elec/misc/prototype.jpg so there

I couldn't have said it better myself.

Ray

[Gregg]

It could probably be simplified somewhat (especially if the op-amp has enough beans to drive the LED directly)

Are you talking about an opamp like this:

http://www.analog.com/en/prod/0%2C28...8532%2C00.html