LDRs as pots?

Steve Conner · 12-02-2006, 01:28 PM

No, that's overkill. LEDs only need about 20mA max, so most op-amps can probably drive one.

Gregg · 12-02-2006, 02:31 PM

Then maybe NE5532 would be a good contender?

Don Symes · 12-04-2006, 08:18 PM

Quote:

Originally Posted by 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.

Hey, thanks! I hadn't really thought about an op-amp with an output that would be happy at 20mA. Slowing the response adequately was the problem that was eating me up.

Anybody up to designing a stable discrete diff amp that doesn't need a ton of trimming?

Quote:

Originally Posted by Steve Conner

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.

Two points here - the Mesa patent uses what amounts to an audio-taper resistor ladder, so that implementation might infringe enough to cost a mint in defending a civil suit. Also, the R-2R ladder really only works for linear pots, and at least half the pots we need are audio tapers of various slopes.

Quote:

Originally Posted by Steve Conner

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

Viso drawings are available - as of abandonment a couple of years ago.

Tiago · 12-08-2006, 11:46 PM

Hi all!
Some time ago I did some research on how to digitally control a potentiometer.
LDR's (optocouplers) look like a perfect approach, but IMHO they are not.
Mesa Boogie, ENGL, and even ADA 20 years ago used them one their programmable preamps.
Mesa Boogie use lot's and lots of circuitry to control them (as you can see on the triaxis schematics), maybe the aren't aware that there is new technology out there which would simplify it. I don't have access to ENGL's preamp schematics, but by reading their manual, it seems that they use a lot less circuitry by using micro-controllers.
What I've seen about them is that their specs will change a LOT with temperature changes. That's why these manufacturers use some king of feedback to control them.

I think that where they place a Optocoupler to control something (gain, bass, volume, etc...) they will place another identical optocoupler close to this one so that the two are subject of the same climatic conditions and the second one will just be there for feedback purposes. As ENGL's says in the manual, for temperatures above 50ºC theres no reliability on these controls.
And we might think that a Optocoupler is "transparent", but they are not. As you can see in some datasheets, their produce some distortion, and this dirstortion percentage increases as signal leves increases. Altough, guitar preamps are mostly distortion generators, so we might get away with a tiny bit of it from the LDR's circuitry.
So, in order to control them we could use microcontrollers(I would't imagine my life without them....). PIC's for example have very cool built-in features, like a built in USART, very usefull for MIDI communication, several built in ADC's, very usefull for the feedback, a built in PWN, usefull to control the Led fo the Optocouplers if you dont want to use a DAC. Not to mention Flash, ROM and RAM memory, and the option to use a bootloader! Very usefull for debugging! By the way, features like PWM, are not restricted to the hardware capabilities. If we need more, it can be implemented by software.

Altough, in the end, I prefered the Soldano approach, using motorized potenciometers...in better words, normal potentiometers attached to stepper using heatshrink tubing.
Signal integrity is the best. Maybe it has some mechanical stress...but as far as my experience go with stepper motors..I never saw one going bad.

But if I had to chose between non-mechanical approaches, I would chose the Marshall JMP1 approach, simply controlling the signal using digital pots. I had one JMP1 and I liked its sound (except the OD2 channel).

Just my 2 cents....My heater is on, so its 5 cents

Ray Ivers · 12-09-2006, 12:47 AM

Tiago,

Do you (or anyone else) know of a good, easy-to-read schematic of a motorized-pot setup?

Ray

Tiago · 12-10-2006, 05:30 AM

Yes I know, but it's in my head and my breadboard...
But I will show some of the resources that helped me to figure it out.
This application note (http://www.st.com/stonline/products/...re/an/1679.pdf) from ST Microelectronics, shows what you need to know to drive stepper motors and it includes a schematic of the worldwide famous L298 and L297 combo. I also have to mention that I did not used the L297/L298 combo, but it would be much more simpler if you use it. I used only the ucontroller (PIC 16F876) and the L298, which is the motor driver chip. By the way, I used a unipolar motor.
Using only the L298, you will have to deal with the connecting / disconnecting order of the windings of the motors. But using the L297 you just have to say were you want it to rotate, if would be half or full step...etc(check the app note).
To know the resistance of the potentiometer, we have to use a dual ganged potentiometer. One for the part we want (Gain, presence, etc), another for the position feedback. For feedback, we just have to connect the potentiometer to an A/D converter pin of the PIC. (I connected one of the pot "legs" to Vcc,another to ground, and center one to the PIC). I also have to mention that for the higher resistances found in most tube amps, the center pin of the potentiometer should be buffered (just put an opamp there).
For the hardware, I think that's all. Now it just needs to be programmed so that the motor rotates until the feedback value is the one we want (I also advice for some tolerance there). The pot then, could be also used as an encoder.

To me, the electronic part isn't the worst, the worst is to tie the motor to the potentiometer while having the shaft available to the front panel of a amp or preamp...

To motors don't need to be high-speed high-torque million dolar ones. Size 17 (I dont know what this value really means or which units is it) 200 steps to start are great, ebay is full of them. For my tests I used a small motor from a hold printer (a 48step) and it was ok, but just "ok", not great.
The potentiometer I used was an ALPHA dual gang (you can rotate the shaft with an allen key from the opposite side of the shaft).

Sorry for my broken English, if you can't figure it out by this tips, tell me of a good and free schematic capture program and I would draw it to you.

EDIT:
I've just remembered that some time ago, I founded "Diy motorized volume control" project on the net. Here it is: http://home1.stofanet.dk/hvaba/stepp...e/stepper.html
In the middle of the page there is a very good schematic, just like you want. The L293 controls the motor, and the Rservo is the feedback source.

Don Symes · 12-13-2006, 02:18 AM

A (probably) overlooked advantage of the servo-pot idea is that the pot connected to the 'setting' knob and the pot that reads the servo/stepper position are the two that have to match. The 'controlled' pot (that actually operates the amp) can be whatever value/taper is required by the circuit. All you really need is a fraction-of-rotation to sense and apply.

A multi-channel ADC to read the knobs and the feedback pots and a multi-channel stepper controller tied to something like this gets you cooking.

Gregg · 12-13-2006, 10:17 PM

For various reasons I definately don't like the idea of motorized pots and stuff. If this is the alternative I'd prefer to skip it altogether or better wait for a 100V digital pot or something.

loudthud · 12-14-2006, 01:20 AM

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.

Steve Conner · 12-15-2006, 12:32 PM

I bet money there will never be a 100V digital pot chip. The IC maker would need to move his digital pot design to a whole other semiconductor process, and he wouldn't invest that time and effort unless he thought there was a big market.

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. It may involve throwing away a few dB of headroom here and there, but does that really matter? A case in point is a passive tone stack which may have 15-20dB of loss anyway: you could make the tone stack out of digital pots and put a potential divider ahead of it (or ahead of the cathode follower driving it a la Marshall, unless you think the heavily driven CF adds to the tone) and just make up the extra 6dB or 9dB or whatever, in the following stages.

I've managed a few reasonable hybrid designs, my favourite is the "Toaster" 2-band parametric EQ, a cathode-follower-driven tone stack that went on a very long trip and ended up with 10 op-amps bolted onto it. The way I designed it, the tubes are guaranteed to always clip before the op-amps do, therefore all of the knobs in this circuit could be replaced with a mixture of digital pots and multiplying DACs, with no other mods needed.

Gregg · 12-15-2006, 10:53 PM

Quote:

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 · 12-16-2006, 11:56 PM

Quote:

Originally Posted by loudthud

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 · 12-19-2006, 03:01 AM

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 · 12-19-2006, 04:27 AM

Quote:

Originally Posted by 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.

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 · 12-20-2006, 03:06 AM

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 · 12-20-2006, 04:52 PM

Quote:

Originally Posted by 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

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 · 12-23-2006, 04:47 PM

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 · 12-23-2006, 06:07 PM

Have you looked for Clairex or Hamamatsu parts?

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

Gregg · 12-23-2006, 07:49 PM

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

Don Symes · 12-23-2006, 09:20 PM

Quote:

Originally Posted by Gregg

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 · 12-24-2006, 12:44 AM

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 · 01-03-2007, 08:05 PM

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 · 01-12-2007, 10:22 PM

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 · 01-16-2007, 03:58 PM

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.

Quote:

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 · 01-16-2007, 04:09 PM

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.

Gregg · 01-16-2007, 04:33 PM

You're right - they use 4 terminal LDRs /I guess custom made/for their tone stack:

tbryanh · 09-30-2007, 06:37 PM

Quote:

Originally Posted by 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 don't think the "pot" terminals are connected to ground. Only the cathodes of the LEDs inside the LDRs are connected to ground.

Look at the attached Vactrol. There is no internal connection between the resistor elements and the LED.

As a side note, the Vactrol attachment is a little confusing. The arrow in the center of the resistor element makes the resistor look like a potentiometer. It is not a potenetiometer. The arrow only indicates a connection point, not a wiper that is controlled by the LED.

Gregg · 09-30-2007, 10:14 PM

I think you should observe the schematic more carefully.
There's no internal connection between the LED and the center tapped photocell - they are connected together externally.
The Vactrol is not a pot by itself it's just a LDR with a center tapped photocell. To make a taper pot out of it you'll still need a 4 terminal /2 independent photocells/ LDR which obviously is not available except in 1000+ quantities but can be made at home out of readily available parts online.
Here's my test version of it:

tbryanh · 09-30-2007, 11:25 PM

Does anybody have the schematic for the Mesa TX4 D board pictured above?

It is the board with the LDR pots and op amp drivers.

http://www.tubefreak.com/triaxis.htm seems to have all the diagrams except that one.

Thanks

tbryanh · 09-30-2007, 11:54 PM

Quote:

Originally Posted by Gregg

I think you should observe the schematic more carefully.

To make a taper pot out of it you'll still need a 4 terminal /2 independent photocells/ LDR which obviously is not available except in 1000+ quantities but can be made at home out of readily available parts online.

Yeah, your right, I needed to look at the schematic a little closer. Yes, it does appear that the cells need to be isolated from each other. A centertapped cell such as the Vactrol or the one in Mesa's patent will not work.

Attached is the one from Mesa's patent. I touched it up a little bit to show the connections as I think they should be.

For the upper LDR, the bottom of cell 1 is connected to ground, but the bottom of cell 2 is not.

Likewise, for the lower LDR, the bottom of cell 3 is connected to ground, but the bottom of cell 4 is not.

tbryanh · 10-01-2007, 12:00 AM

Quote:

Originally Posted by Gregg

Here's my test version of it

If you get a chance to post a schematic of your test unit with a parts list, that would be cool.

loudthud · 10-01-2007, 04:21 AM

It looks to me like item 25 is connected backwards.

Gregg · 10-01-2007, 10:29 PM

My test unit is based on the hand drawn schematic posted by Don Symes back in the thread /page 3, answer #29/. The changes I made:

1/ I'm using LM324 because it's capable of supplying large currents.
2/ Because of 1/ the LDR's LED is connected to the opamp output only through a 100 Ohm resistor.
3/ The whole schematic is doubled /as in Mesa's patent/ and the 10k pot doesn't connect to ground but its wiper is connected to ground instead /as suggested by another forum member/. The other terminal goes to the "mirror" side of the schematic.

The trimpots on my picture are the scaling resitors which let you set the LDR pot value.

I don't have the "damping" filter because I don't have any idea how to determine those values.

My LDR pot has been only "dry" tested so far. It worked but I didn't "listen" to it in a real circuit.

tbryanh · 10-02-2007, 03:04 AM

Quote:

Originally Posted by loudthud

It looks to me like item 25 is connected backwards.

Yeah, it does appear that it is shown backwards.

Here is a touched up version.

tbryanh · 10-02-2007, 03:06 AM

Quote:

Originally Posted by loudthud

It looks to me like item 25 is connected backwards.

Yeah, it does appear that it is shown backwards.

Here is a URL to a touched up version to indicate that.

http://subdomain.freeservers.com/schematics/LDR_Pot_Touchup2.jpg

The Manage Attachments function doesn't seem to work right now for AMPAGE.

12-10-2006, 05:30 AM	#41
Tiago Member Join Date: Jul 2006 Posts: 37	Yes I know, but it's in my head and my breadboard... But I will show some of the resources that helped me to figure it out. This application note (http://www.st.com/stonline/products/...re/an/1679.pdf) from ST Microelectronics, shows what you need to know to drive stepper motors and it includes a schematic of the worldwide famous L298 and L297 combo. I also have to mention that I did not used the L297/L298 combo, but it would be much more simpler if you use it. I used only the ucontroller (PIC 16F876) and the L298, which is the motor driver chip. By the way, I used a unipolar motor. Using only the L298, you will have to deal with the connecting / disconnecting order of the windings of the motors. But using the L297 you just have to say were you want it to rotate, if would be half or full step...etc(check the app note). To know the resistance of the potentiometer, we have to use a dual ganged potentiometer. One for the part we want (Gain, presence, etc), another for the position feedback. For feedback, we just have to connect the potentiometer to an A/D converter pin of the PIC. (I connected one of the pot "legs" to Vcc,another to ground, and center one to the PIC). I also have to mention that for the higher resistances found in most tube amps, the center pin of the potentiometer should be buffered (just put an opamp there). For the hardware, I think that's all. Now it just needs to be programmed so that the motor rotates until the feedback value is the one we want (I also advice for some tolerance there). The pot then, could be also used as an encoder. To me, the electronic part isn't the worst, the worst is to tie the motor to the potentiometer while having the shaft available to the front panel of a amp or preamp... To motors don't need to be high-speed high-torque million dolar ones. Size 17 (I dont know what this value really means or which units is it) 200 steps to start are great, ebay is full of them. For my tests I used a small motor from a hold printer (a 48step) and it was ok, but just "ok", not great. The potentiometer I used was an ALPHA dual gang (you can rotate the shaft with an allen key from the opposite side of the shaft). Sorry for my broken English, if you can't figure it out by this tips, tell me of a good and free schematic capture program and I would draw it to you. EDIT: I've just remembered that some time ago, I founded "Diy motorized volume control" project on the net. Here it is: http://home1.stofanet.dk/hvaba/stepp...e/stepper.html In the middle of the page there is a very good schematic, just like you want. The L293 controls the motor, and the Rservo is the feedback source. Last edited by Tiago; 12-10-2006 at 05:47 AM. Reason: Founded a schematic of a MotorPot Setup!!

12-15-2006, 12:32 PM	#45
Steve Conner Supporting Member Join Date: May 2006 Location: Glasgow, Scotland Posts: 2,413	I bet money there will never be a 100V digital pot chip. The IC maker would need to move his digital pot design to a whole other semiconductor process, and he wouldn't invest that time and effort unless he thought there was a big market. 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. It may involve throwing away a few dB of headroom here and there, but does that really matter? A case in point is a passive tone stack which may have 15-20dB of loss anyway: you could make the tone stack out of digital pots and put a potential divider ahead of it (or ahead of the cathode follower driving it a la Marshall, unless you think the heavily driven CF adds to the tone) and just make up the extra 6dB or 9dB or whatever, in the following stages. I've managed a few reasonable hybrid designs, my favourite is the "Toaster" 2-band parametric EQ, a cathode-follower-driven tone stack that went on a very long trip and ended up with 10 op-amps bolted onto it. The way I designed it, the tubes are guaranteed to always clip before the op-amps do, therefore all of the knobs in this circuit could be replaced with a mixture of digital pots and multiplying DACs, with no other mods needed. Last edited by Steve Conner; 12-15-2006 at 12:44 PM.

09-30-2007, 10:14 PM	#63
Gregg Senior Member Join Date: May 2006 Posts: 206	I think you should observe the schematic more carefully. There's no internal connection between the LED and the center tapped photocell - they are connected together externally. The Vactrol is not a pot by itself it's just a LDR with a center tapped photocell. To make a taper pot out of it you'll still need a 4 terminal /2 independent photocells/ LDR which obviously is not available except in 1000+ quantities but can be made at home out of readily available parts online. Here's my test version of it: Last edited by Gregg; 09-30-2007 at 10:50 PM.

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12-02-2006, 01:28 PM	#36
Steve Conner Supporting Member Join Date: May 2006 Location: Glasgow, Scotland Posts: 2,413	No, that's overkill. LEDs only need about 20mA max, so most op-amps can probably drive one.

12-02-2006, 02:31 PM	#37
Gregg Senior Member Join Date: May 2006 Posts: 206	Then maybe NE5532 would be a good contender?

12-08-2006, 11:46 PM	#39
Tiago Member Join Date: Jul 2006 Posts: 37	Hi all! Some time ago I did some research on how to digitally control a potentiometer. LDR's (optocouplers) look like a perfect approach, but IMHO they are not. Mesa Boogie, ENGL, and even ADA 20 years ago used them one their programmable preamps. Mesa Boogie use lot's and lots of circuitry to control them (as you can see on the triaxis schematics), maybe the aren't aware that there is new technology out there which would simplify it. I don't have access to ENGL's preamp schematics, but by reading their manual, it seems that they use a lot less circuitry by using micro-controllers. What I've seen about them is that their specs will change a LOT with temperature changes. That's why these manufacturers use some king of feedback to control them. I think that where they place a Optocoupler to control something (gain, bass, volume, etc...) they will place another identical optocoupler close to this one so that the two are subject of the same climatic conditions and the second one will just be there for feedback purposes. As ENGL's says in the manual, for temperatures above 50ºC theres no reliability on these controls. And we might think that a Optocoupler is "transparent", but they are not. As you can see in some datasheets, their produce some distortion, and this dirstortion percentage increases as signal leves increases. Altough, guitar preamps are mostly distortion generators, so we might get away with a tiny bit of it from the LDR's circuitry. So, in order to control them we could use microcontrollers(I would't imagine my life without them....). PIC's for example have very cool built-in features, like a built in USART, very usefull for MIDI communication, several built in ADC's, very usefull for the feedback, a built in PWN, usefull to control the Led fo the Optocouplers if you dont want to use a DAC. Not to mention Flash, ROM and RAM memory, and the option to use a bootloader! Very usefull for debugging! By the way, features like PWM, are not restricted to the hardware capabilities. If we need more, it can be implemented by software. Altough, in the end, I prefered the Soldano approach, using motorized potenciometers...in better words, normal potentiometers attached to stepper using heatshrink tubing. Signal integrity is the best. Maybe it has some mechanical stress...but as far as my experience go with stepper motors..I never saw one going bad. But if I had to chose between non-mechanical approaches, I would chose the Marshall JMP1 approach, simply controlling the signal using digital pots. I had one JMP1 and I liked its sound (except the OD2 channel). Just my 2 cents....My heater is on, so its 5 cents

12-09-2006, 12:47 AM	#40
Ray Ivers Lifetime Member Join Date: Jun 2006 Location: Connecticut Posts: 442	Tiago, Do you (or anyone else) know of a good, easy-to-read schematic of a motorized-pot setup? Ray

12-13-2006, 02:18 AM	#42
Don Symes Senior Member Join Date: May 2006 Posts: 145	A (probably) overlooked advantage of the servo-pot idea is that the pot connected to the 'setting' knob and the pot that reads the servo/stepper position are the two that have to match. The 'controlled' pot (that actually operates the amp) can be whatever value/taper is required by the circuit. All you really need is a fraction-of-rotation to sense and apply. A multi-channel ADC to read the knobs and the feedback pots and a multi-channel stepper controller tied to something like this gets you cooking.

12-13-2006, 10:17 PM	#43
Gregg Senior Member Join Date: May 2006 Posts: 206	For various reasons I definately don't like the idea of motorized pots and stuff. If this is the alternative I'd prefer to skip it altogether or better wait for a 100V digital pot or something.

12-14-2006, 01:20 AM	#44
loudthud Supporting Member Join Date: May 2006 Location: Texas USA Posts: 796	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.

12-19-2006, 03:01 AM	#48
loudthud Supporting Member Join Date: May 2006 Location: Texas USA Posts: 796	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.

12-20-2006, 03:06 AM	#50
unclejed613c Junior Member Join Date: Dec 2006 Location: colorado Posts: 3	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

12-23-2006, 04:47 PM	#52
Gregg Senior Member Join Date: May 2006 Posts: 206	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.

12-23-2006, 06:07 PM	#53
Don Symes Senior Member Join Date: May 2006 Posts: 145	Have you looked for Clairex or Hamamatsu parts? Try googling Photocoupler (then ignore all the logic/transistor-output parts)

12-23-2006, 07:49 PM	#54
Gregg Senior Member Join Date: May 2006 Posts: 206	Yes, I looked but I couldn't find anything close to that especially dual cell photoresistor optocouplers.

12-24-2006, 12:44 AM	#56
Gregg Senior Member Join Date: May 2006 Posts: 206	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.

01-03-2007, 08:05 PM	#57
max_lwedge Junior Member Join Date: Dec 2006 Posts: 2	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.

01-12-2007, 10:22 PM	#58
Gregg Senior Member Join Date: May 2006 Posts: 206	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?

01-16-2007, 04:09 PM	#60
Steve Conner Supporting Member Join Date: May 2006 Location: Glasgow, Scotland Posts: 2,413	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.

01-16-2007, 04:33 PM	#61
Gregg Senior Member Join Date: May 2006 Posts: 206	You're right - they use 4 terminal LDRs /I guess custom made/for their tone stack:

09-30-2007, 11:25 PM	#64
tbryanh Senior Member Join Date: Apr 2007 Posts: 403	Does anybody have the schematic for the Mesa TX4 D board pictured above? It is the board with the LDR pots and op amp drivers. http://www.tubefreak.com/triaxis.htm seems to have all the diagrams except that one. Thanks

10-01-2007, 04:21 AM	#67
loudthud Supporting Member Join Date: May 2006 Location: Texas USA Posts: 796	It looks to me like item 25 is connected backwards.

10-01-2007, 10:29 PM	#68
Gregg Senior Member Join Date: May 2006 Posts: 206	My test unit is based on the hand drawn schematic posted by Don Symes back in the thread /page 3, answer #29/. The changes I made: 1/ I'm using LM324 because it's capable of supplying large currents. 2/ Because of 1/ the LDR's LED is connected to the opamp output only through a 100 Ohm resistor. 3/ The whole schematic is doubled /as in Mesa's patent/ and the 10k pot doesn't connect to ground but its wiper is connected to ground instead /as suggested by another forum member/. The other terminal goes to the "mirror" side of the schematic. The trimpots on my picture are the scaling resitors which let you set the LDR pot value. I don't have the "damping" filter because I don't have any idea how to determine those values. My LDR pot has been only "dry" tested so far. It worked but I didn't "listen" to it in a real circuit.

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