Voltage Controlled Signal Amplitude

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CutRock

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Joined 2010
#1
Hi all -

First, as a preface, I have a very limited EE background. So, if possible, a simple solution would be much preferred. Along those same lines, as I'm not familiar with much of the terminology surrounding audio electronics, this may be a very straightforward question - so my apologies if this is something I could have looked up quite easily if I knew of the right keywords.

For background, I've struck an interest in analog synthesis and have started researching schematics for the different components I'll eventually build. To keep my oscillators as versatile as possible, I'd like to be capable of selecting which waveform to output from a rotary switch, but also be able to run that selection into other VCO's. To accomplish this, I believe the easiest mechanism would be to control the signal using a voltage which can be run to each VCO to mirror the selection.

Essentially, this is the functionality I'm looking for:
KTXnI.png


The effect can be discrete - either having the signal unaltered or completely off, and doesn't need to be able to output the signal at continuous levels of attenuation.

Thanks all.
 
#2
If you really want what the picture shows, you could easily implement this with an analog switch. 15 volts may be a little high for newer devices, but you could use a lower voltage signal at the input and then amplify it at the output.
 
#3
Your picture does show just a simple switching arrangement whereby the output is shut off as the control voltage changes its logic level.

As sawreyrw mentions, an analogue switch IC would do this. Many types available. If you could put up with lower levels you could use the common CMOS 4016/4066 IC's to experiment with.

JFET's are another option and can work well if applied correctly.

One thing though when switching any "audio" type waveform. If the switching occurs part way through a cycle (not at the zero cross point), then you will get a click/pop/thump in the output as you are effectively switching when the waveform is at some "equivalent" DC value. I'm not sure if there are specialist switching IC's available to cater for that.
 
#4
CutRock said:
For background, I've struck an interest in analog synthesis and have started researching schematics for the different components I'll eventually build. To keep my oscillators as versatile as possible, I'd like to be capable of selecting which waveform to output from a rotary switch, but also be able to run that selection into other VCO's. To accomplish this, I believe the easiest mechanism would be to control the signal using a voltage which can be run to each VCO to mirror the selection.

Essentially, this is the functionality I'm looking for:
KTXnI.png


The effect can be discrete - either having the signal unaltered or completely off, and doesn't need to be able to output the signal at continuous levels of attenuation.

Thanks all.
Click to expand...
What you need is a VCA (Voltage Controled Amplifier)
Loads of diy stuff out there. Start http://www.musicfromouterspace.com/analogsynth/DLLVCA001/DLLVCA001.html
 
#10
Thanks all,

sawreyrw said:
If you really want what the picture shows, you could easily implement this with an analog switch. 15 volts may be a little high for newer devices, but you could use a lower voltage signal at the input and then amplify it at the output.
Click to expand...

sawreyrw - This looks like a very straightforward and cheap solution. If possible, I'd prefer not to have to adjust the amplitude of the waves before and after, but if I can't get anything else working as is, this is probably the route I'll take.

Mooly said:
JFET's are another option and can work well if applied correctly.
Click to expand...

Thanks Mooly - would you mind providing a reference, resource or search term that I can look into for how to accomplish this? Also, what do I have to consider to get this option to be 'applied correctly?'

Dirk95100 said:
What you need is a VCA (Voltage Controled Amplifier)
Click to expand...

HighTec said:
You can also look at how Ecler has implemented the VCA THAT1280 in their PAM4100 and PAM6100 amplifiers.
Click to expand...

Thanks Dirk & HighTec - Although I'm sure VCA's would accomplish what I'm looking for, the VCA IC's (THAT2180) seem a little expensive for what I need to accomplish and a diy VCA would probably be quite a bit more than what I need for only a discrete on/off switch. Nevertheless, I appreciate your suggestions.
 
#12
Instead of on/off switching, what variable level of music?

For example, having a POT or other circuit that adjusts voltage within a certain range to make amplifier go from 0-100% output

This would allow control of multiple channels with different signals (L&R stereo for example) but using only one signal? Any good IC's or circuits that do that without adding much noise?
 
#13
Mooly said:
For the JFET switching I'm going to point you to my amplifier and the relevant threads.

Post #201
http://www.diyaudio.com/forums/solid-state/119151-my-mosfet-amplifier-designed-music-11.html
Click to expand...

Thanks for your help, Mooly - I found that schematic very useful. I don't have all my gear with me to test it out, so I ran some simulations in LTSpice and it seems to behave fairly well. Here's a screenshot of the circuit I set up where "Tri_Out" is a simple +-15V triangle wave and "Tri_CV" is the voltage to regulate the output.

4KytY.png


As the regulating voltage ("Tri_CV") was increased, the output waveform approached the original waveform, showing almost exact character once "Tri_CV" equaled the amplitude of the "Tri_Out" wave.

bebNo.png


Furthermore, as "Tri_CV" approaches the negative of the amplitude of "Tri_Out", or -15V, the output waveform approached a constant 0V.

SWTsf.png


However, this shows slight peaks once per oscillation. Below -15V this isn't an issue, but it would make my life much easier if I could keep everything on the same +15V, 0V, -15V spectrum. Is there any simple way to do this?

As an aside, I changed the 22k resistor in the original schematic to a 1k resistor because it gave a better waveform in the 'open' state and got rid of the series capacitor because it was offsetting my waveform. If you have any suggestions to changes or explanations as to why my changes are detrimental, I'd be glad to hear them.

Thanks again for all your help.
 
#15
There should be some circuits for synthesizers (Moog and such) online if you search. They used lots of circuit block similar to what you want. Also, look up multiplier chips and four quadrant multiplier. You input a signal and a control voltage/signal and the two get multiplied for the output. Zero times signal equals zero. One times signal equals signal, and you can do anything in between. You can also modulate or mix one signal with another. Search modulator and mixer.
 
#17
gmphadte said:
OTAs might be cheap.
Click to expand...

Thanks for the suggestion Gajanan. I looked up some OTA's (assuming that means Operational Transconductance Amplifiers) on digikey and found that they can be found as low as ~$2 for a dual IC. Although I'm sure this would work well for my application, I'd like to do this in as cheap a means as possible. Judging by the circuit suggested by Mooly, I should be able to do what I want with more general (and hopefully much cheaper) components. I'll need ~9 of these circuits per oscillator (5 waveforms, 2 suboscillators and soft and hard sync) and each one will need some manner of selecting which waveforms to mix into a final output from that output. To accommodate this, I'd like to keep it relatively cheap.


Conrad Hoffman said:
There should be some circuits for synthesizers (Moog and such) online if you search. They used lots of circuit block similar to what you want.
Click to expand...

Perhaps my google'ing skills aren't quite up to par, but I wasn't able to find any schematics for what I was looking for. I've seen quite a few schematics for VCA's, but they are often excessively complicated for what I need. I only need a discrete on/off switch for the signal, and adding the functionality to have the waveform attenuated seems to add a lot of bulk to the circuit that I simply don't need for this application.

Are there any keywords that I should be using to describe this application during my searches? It seems all I can find now is VCA's.


Michael Bean said:
How about MOSFET switches? This one is two SPST normally open, LED controlled switches: http://www.irf.com/product-info/datasheets/data/pvt312.pdf
And this one has one normally open SPST, and one normally closed SPST LED controlled switches: http://www.clare.com/home/pdfs.nsf/0/A659676C22064E7A85256A2C00678724/$file/Pba150.pdf
They aren't real cheap, but simple and easy to use.
Click to expand...

Thanks for the suggestion, Mike. I didn't give any price criteria in my original post, so I should have specified that I'm looking for something rather simple and cheap. It seems Mooly has had the best suggestion, providing a circuit which seems to accomplish almost exactly what I needed without any specialty IC's. This approach seems to be the most cost effective thus far.


Thanks again for all the suggestions.

If anyone has a means of shutting off the waveform completely within the -15V, 0V, +15V spectrum that my circuits currently use (using the circuit inspired by Mooly's post in my previous reply), then that option would probably be ideal.
 
#18
CutRock said:
Thanks for the suggestion Gajanan. I looked up some OTA's (assuming that means Operational Transconductance Amplifiers) on digikey and found that they can be found as low as ~$2 for a dual IC. Although I'm sure this would work well for my application, I'd like to do this in as cheap a means as possible. Judging by the circuit suggested by Mooly, I should be able to do what I want with more general (and hopefully much cheaper) components. I'll need ~9 of these circuits per oscillator (5 waveforms, 2 suboscillators and soft and hard sync) and each one will need some manner of selecting which waveforms to mix into a final output from that output. To accommodate this, I'd like to keep it relatively cheap.




Perhaps my google'ing skills aren't quite up to par, but I wasn't able to find any schematics for what I was looking for. I've seen quite a few schematics for VCA's, but they are often excessively complicated for what I need. I only need a discrete on/off switch for the signal, and adding the functionality to have the waveform attenuated seems to add a lot of bulk to the circuit that I simply don't need for this application.

Are there any keywords that I should be using to describe this application during my searches? It seems all I can find now is VCA's.




Thanks for the suggestion, Mike. I didn't give any price criteria in my original post, so I should have specified that I'm looking for something rather simple and cheap. It seems Mooly has had the best suggestion, providing a circuit which seems to accomplish almost exactly what I needed without any specialty IC's. This approach seems to be the most cost effective thus far.


Thanks again for all the suggestions.

If anyone has a means of shutting off the waveform completely within the -15V, 0V, +15V spectrum that my circuits currently use (using the circuit inspired by Mooly's post in my previous reply), then that option would probably be ideal.
Click to expand...

Did you look at an integrated switch like those of the DG300 family? Seems the easiest, least complecated way to do what you need.
You can use a pair of switches, a NO to gnd and a NC in series with the signal.

jan didden
 
#20
If you just want on and off, an analog switch is hard to beat. You can do it with an FET too, but the analog switch will out perform it with far less work. I'm not up on diode rings, but I think it can also be done by switching the bias on and off of a diode ring using a single transistor. Jim Williams did lots of things with diode rings- they look like a bridge, but the polarity is different.
 
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