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Voltage controlled amplifier

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Jimbo

New Member
hello all,

I am looking for a low-cost and "not-that-accurate" voltage controlled amplifier for one of my revamped projects. Sounds easy? That's what I tought about a week ago :D (after picking up this hobby again after 7 years or so).

Ages ago I built a 12-channel fully analog light-control unit including a bunch of chasers, strobes (ugh, square wave generators really) and audio controlled flash-channels. Works like a charm, but it was missing one part which at the time was deemed unnecessary (read: I had bugger all idea how to make it): a 'master fader'. I.e. a sliding potmeter controlling the maximum output voltage of all channels simultaniously. I really wanted to add this feature now, only to find myself stuck

What I have now is a 'analog signal bus', containing 12 different input signals (waveforms, chasers, etc) ranging between 0V (light completely off) and 10V (light fully on). What I obviously want to do is to simultaniously 'dim' these 12 signals by using a control voltage which I will derive from the master fader. The signal bus is then fed to the preset masters and then onto the presets, but that part already exsists and needs no change (it would be dumb anyway, as I have the flexiblity to add as many channels as I want if I 'dim' the inputs rather than the output channels directly).

This sounded like a voltage controlled amplifier to me.

My first idea was opamps, so out came the college books, only to find nothing... I did remember about OTAs, or Operational Transconductance Amplifiers, which are current controlled opamps. I want voltage control, but that is easily converted. I even found a schematic - on Vego.nl (Dutch) - doing EXACLTY what I wanted to do, based around an CA3080, but I had to scrap this idea for the dumbest of reasons: I can't find these bloody OTAs (or others at a reasonable price) anywhere! (Me no shoppy online. Me scary weasel).

So out with the opamps, in with the digital potmeters. That is, until I google searched on how they work. I was hoping to find a 'databus' in which I could latch the x-bit digital potmeter slider position, but apparently these little buggers work pulse counter controlled, or even worse, by I2C. Still possible of course, but sounded like major overkill - and expensive too - and way out of my league so I didn't put much more thought in that idea either.

Final idea was a PWM controlled sampling circuit. Generate a 0-10V sawtooth signal (with one 'vertical' edge, dunno if they call that sawtooth in English) at a reasonable frequency, let's say 100 times that of wat the eye can see (5kHz or so), compare this to the 0-10V DC tapped off the master fader, and use the resulting "PWM" blockwave to drive 12 electronic switches which 'sample' the 12 signal bus channels. The higher the master fader slider, the wider the sample pulses, and the bigger the output voltage. Afterwards some levelling/integration would be required to level the output to 'semi-DC' again. Advantage of this system is that I can really get 0 and 10V out of the switches by simply finetuning the opamp comparator (and by adding some milivolts of offset to make sure the switch is permanenty open or closed at both extremes).

That looked good enough on paper. Then I tried to build it (laugh as hard as you wish about what follows, but don't put it in writing :D). I ran across the following problems:

a) I can't generate a sawtooth anymore. Sigh (I can do triangles though :) - but they don't have the required vertical edge). Couldn't find one on Google either. What I tried so far: I had an integrator (integrating 10V DC) based around an opamp, which a short circuit transistor in parallel with the cap (to get the vert edge), the transistor base being driven by an astable with extreme short duty cycle. Not only does that sound like overkill, it WAS, since I couldn't get it to work. So, how does one generate a sawtooth at say 5kHz with an amplitude of 10V?

b) I don't really have a clue yet on how to 'integrate' or level out my sampled signals. You'd think another basic integrator (well, the word "integrator" itself said what I wanted it to do anyway :) ), but I couldn't figure out the RC constant to get an acceptable "flat" output (for testing purposes I was trying to integrate outputs from 555 square waves with varying duty cycle).

Of course, if you have an alternative to my 'voltage controlled amplifier', I'd love to hear that too. Knowing my luck I'm probably forgetting about the most basic of circuits :D... Hey, it HAS been 7 years :)...
 

Roff

Well-Known Member
1. Have you looked at LM13700 for your VCA? They were available from multiple vendors less than a year ago.

2. You shouldn't need the "vertical edge" for a PWM generator. Think about it - the duty cycle of the triangle wave is unimportant.

3. If you really want to use PWM, you'll probably need multi-pole lowpass filters. Search Google for "switched capacitor filter(s)" and "Sallen and Key" for some of many alternatives.

4. The digital pot was the first thing that came to my mind. It still seems to me like your best bet. You can control them with an up/down switch(es) or with a pot and an A/D converter. Loading them will take some additional hardware, but it seems like it would be much simpler than either of the other alternatives.
 

Jimbo

New Member
OK, an update (Late as ever :D):

I knew I was missing something obvious when making the sawtooth. Thanks Ron for pointing it out: it indeed does not require a sharp edge, any triangle would do, making the solution a heck of a lot easier. That being said, I completely failed to average out the "samples" that the prototype was making. Sampling itself worked like a charm, but in the filter I always ended up with some capacitor charging up to supply voltage. The S&K stuff was too complex (both for me personally as well as costwise :)). So out with PWM-idea.

In the meantime I managed to find some CA3080s (a whopping number of two...), and after some finetuning of the Vego schematic - which used obsolete transistors for current source etc - managed to get something to work. It isn't 100% linear as I had hoped (*), but it is good enough for my application: dimming lights with slider pots.

Thank God I didn't have to take a second look at those digipots :D! (Me scary weasel, remember ;)?).

Jimbo.


(*) In my solution, "dimming" an input voltage is actually is fully linear with the control voltage, even beyond the accuracy of my Fluke, but only for 'full' input voltages of 10V. If the input voltage is any lower, 'dimming' happens a bit faster making the output hit 0V a bit before control voltage hits 0V. The lower the input voltage, the bigger the problem. Since this is about dimming lights - which isn't exact science to begin with - with sliders that are 'tuned' while you're looking at the lights and where something like 10% difference isn't even noticeable most of the time, this solution suits my needs more than enough.

Keep in mind that the presets will always go to full 100% - which delivers the voltage for which linearity is best (10V) - while the new master will be used for finetuning overall brightness. Also, the area where non-linearity is worse, i.e. at very low voltages (or slider positions) on both the presets and the master, the lights are not on yet to begin with, so the problem only shows on a voltage meter :).
 
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