LDRs as pots?

[Don Moose]

Just curious, how many of these things do you estimate there will be a demand for? Who would be the customer for them?

Looks like about ... ten customers - most of them in this thread. A bit of marketing might boost that up to a couple of hundred - how many customers do CAE have?

I'm back to thinking about working on a kit of some sort.

[Gregg]

Maybe we're back to that discussion whether the demand determines the supply or the opposite. As you know both are true.
What is more important you can DIY them for your own purposes. If you're planning a production that's another story.
If projects including descent control unit are available maybe more people will look into it I guess.

[Tiago]

Still no Scope measurements?
I've seen this datasheet:
http://optoelectronics.perkinelmer.c...oisolatorB.pdf
If you look in the page 10 of the PDF document, which is page 36 of the full datasheet, there are graphics of the distortion vs. voltage across the cell vs. resistance.
If found that with a VTL5C2:
1%Distortion at 1.4 VRMS across the cell with 30K resistance;
Distortion will increase with voltage increase;
Distortion will increase with resistance increase;

This is what is keeping me away from these Vactrols and any other optoisolator around.
Can someone post results of some experiment in a tube preamp with a Oscilloscope or Spectrum Analyser so we can have more to work with?

[tbryanh]

Yeah, the distortion appears to be high. This is probably the odd-harmonic type of distortion. Too much of that kind of distortion and you have that nasty transistor distortion sound.

Anyway, I am thinking that vactrols might not be needed. The Mesa circuit selects a different point along a string of resistors to adjust the "wiper" of the LDR Pot. The switching is done with digital gates. The digital gates are easy to connect to the address lines of a small microprocessor controller that can interpret messages from a midi port.

Bottom line is, volume pots, tone pots, etc. could be replaced by resistor strings rather than by LDR Pots. Mechanical relays (or decent solid state relays [if such beasts exists]) could then choose which point along the string the “wiper” is set to. This setup would be similar to the setup that replaces volume pots, tone pots, etc. with rotary switches. (Such as is found in some high-end audio equipment.)

The primaries of the relays could then be connected to digital gates and on to a microprocessor or whatever else you have in mind.

I think this is the route I might take. The distortion was always an issue in the back of my mind, and the LDR Pots might distort bad. If that is the case, I won’t touch them.

As far as I know, the Triaxis is a brilliant amp. It has a big following, and is the only amp of its kind. Hate Randall Smith as much as you want, but you must give credit where credit is due. There is a big complaint about the amp though and that is the farty distortion it can put out unexpectedly. This is probably mostly due to the op amp in the active filter circuit that is in the last stage of the Triaxis, but I wouldn’t be surprised if the LDR Pots distort unexpectedly too.

I am pretty sure I am going to go the route of replacing volume pots, tone pots, etc. with resistor strings in a programmable “rotary switch” type of setup. No distortion here.

[Don Moose]

Bottom line is, volume pots, tone pots, etc. could be replaced by resistor strings rather than by LDR Pots. Mechanical relays (or decent solid state relays [if such beasts exists]) could then choose which point along the string the “wiper” is set to. This setup would be similar to the setup that replaces volume pots, tone pots, etc. with rotary switches. (Such as is found in some high-end audio equipment.)

And since the resistor string always has a DC ground, that end wouldn't pop. The wiper side ... keep a 2M2 grid leak resistor on the 'wiper' end of the relay array and you should be OK.

Now you just have to cram 16 relays per control into the chassis ... and supply coil current for 1.5x to 2x the number of controls (keeps the selected relays closed while you close another). That's 16 relays if you want one-contact-per-step. You could use DPDT relays in a binary tree, but then you wind up with floating contacts (pop city).

A conceptually good, technically solid approach that (IMO) winds up snagged on the feasibility thing.

... anyone know a photochemistry/physics guy who might have something other than Cadmium Sulfide in mind?

Hope this helps!

[tbryanh]

Now you just have to cram 16 relays per control into the chassis ... and supply coil current for 1.5x to 2x the number of controls (keeps the selected relays closed while you close another). That's 16 relays if you want one-contact-per-step. You could use DPDT relays in a binary tree, but then you wind up with floating contacts (pop city).

I'm not quite following this. It seems that if there are 8 taps in the resistor string, then 8 SPST relays are needed.

As far as the popping goes, the amp could have a relay that squelches the amp during the switching period much in the sam way that old Public Address amps squelched the amp during the time that a mic was keyed up.

I haven't tried anything yet, but I am thinking things are far simplier than you indicate and much more possible than you indicate.

From what I get your saying this approach is not possible.

It might be the only reason Measa took the route they did is because it was cheaper.

[Enzo]

Perhaps instead of relays, you could use a row of JFETs as switches, or even transistor arrays in IC form for even more compact.

[tbryanh]

It might be the only reason *Measa* took the route they did is because it was cheaper.

Mesa Boogie

[Enzo]

If you are looking for pop mutes, PV has used a little circuit they call CLAMP on their schemos. Most of their recent era tube am ps have it. It is a little triac shunting the signal and is triggered momentarly when you hit the channel switch. I think it would be simpler to adapt that than using yet another relay

[tbryanh]

If you are looking for pop mutes, PV has used a little circuit they call CLAMP on their schemos. Most of their recent era tube am ps have it. It is a little triac shunting the signal and is triggered momentarly when you hit the channel switch. I think it would be simpler to adapt that than using yet another relay

Well, there you have it. Thanks Ezno.

[tbryanh]

So if you want a 100K linear taper pot with 12 possible settings, for example, you put 10 10K resistors in series, use 12 SPST relays to select which setting you want for the “wiper,” and use the PV-style CLAMP Ezno mentioned to mute the pops during switching. No Distortion. Simple circuit, and IT WORKS.

[tbryanh]

The bottom line is, the Mesa approach was probably the cheapest, not the best.

[Enzo]

The clamp thing works well enough, but that brief moment of silence bugs some guys. They refer to it as a "delay" when switching channels. It might be REAL annoying if it mutes every time you adjust a control. If course a relay doing it would likely have the same result. Best to engineer the pop out of it.

[tbryanh]

The clamp thing works well enough, but that brief moment of silence bugs some guys. They refer to it as a "delay" when switching channels. It might be REAL annoying if it mutes every time you adjust a control. If course a relay doing it would likely have the same result. Best to engineer the pop out of it.

You got a point there. I've never used a Triaxis, but I am assuming there is very little delay when switching to a new setting.

How would you engineer out the pop?

[tbryanh]

Is it necessary to use LDR Pots to avoid delay when changing settings?

The delay that would occur if a PV-style clamp circuit was used to prevent the popping sound from being heard if mechanical relays were used instead of LDR Pots?