Attack Release AR circuit to VCA problem

[george2525]

Hello

I am trying to make a music synthesiser and this is the AR part of the circuit. My values may not be very precise as its just a multisim version at present but I think the general idea seems ok.

Everything before U4A is just my power supply and Key A is to simulate a gate.

The output shown goes from about -4.01V to + 3.07V . Im not sure why it is not symmetrical but this is not a major concern right now.

The problem is that I dont know how to handle the negative output with regards to controlling a VCA at the output. I have attached a random Jfet here but ideally I want to connect a LM13700 OTA here. Should I be biasing the last op amp to raise the negative voltage to ground here?

Im realy confused about how to link this circuit to some kind of VCA that I assume should function in a range above ground. I say this because in my experience using synths the AR/ADSR plots always seem to be above or equal to ground.

I hope my question makes sense.

Thanks for any help

 

[Harald Kapp]

The output shown goes from about -4.01V to + 3.07V . Im not sure why it is not symmetrical but this is not a major concern right now.

Even though not a major concern: The LM324 may not be the best suited opamp for this purpose as its positive output is well below the positive supply (see Voh in the datasheet). Typical is Voh=Vcc-2V, worst case is Voh = Vcc-4V!

The problem is that I dont know how to handle the negative output with regards to controlling a VCA at the output.

So I take it the VCA operates from a positive input voltage (0 V ... Vmax) only? Then why do you generate a negative voltage in the first place (U4a)? The same circuit will work with U4 powered from a single 9 V supply (negative supply = 0 V).

When you use a double throw switch (SPDT) for the"key", you can get rid of U4a and the associated components completely. Just connect the common of the switch to D1/D2, connect NO (normally open) to Vcc, NC (normally closed) to 0 V and you're all set.

Put "adsr circuit" into your favorite search engine's entry box to find lots of circuits that include decay and sustain elements in addition to your attack and release options.

 

[george2525]

Yes if I use ground for the V- of U4A it does work as a 0 - pos Voltage output. by this I mean supply V+ = 4.5v and V- = 0V

"So I take it the VCA operates from a positive input voltage (0 V ... Vmax) only? Then why do you generate a negative voltage in the first place (U4a)? The same circuit will work with U4 powered from a single 9 V supply (negative supply = 0 V)."

Im very inexperienced with split supplies from batteries and using negative voltages to tie loads to. If I use 9v as V+ then I would have to use another battery so that I can keep the virtual ground here. this gound (between +/- 4.5V) is to be used for the whole instrument and so I assume using V+ = 9V would mean ill have to somehow reference the output to -4.5V which is like having another ground reference point in the same circuit no?

My assumption is that all output of all modules should be referenced to the same point (the ground of the split supply)

Unfortunately my previous experience was all based on everything going to ground and almost no negative voltages at all, other than for op amp supplies.

I will experiment further and try to clear up what im talking about with an example.

 

[Harald Kapp]

If I use 9v as V+ then I would have to use another battery so that I can keep the virtual ground here.

Why do you need a virtual ground at all? When the output of your attack/decay generator needs to be positive only, there is no need for a negative supply to the LM324. The negative power supply pin can be tied directly to ground (0 V of the battery).

The LM324 is expressly suitable for single supply operation and the output can reach as low as a few millivolts above V- (or 0 V in single supply operation) which should be good enough considering almost 600 mV voltage drop across the diodes.
Plus the positive supply being +9 V in this mode gives you more headroom for positive output voltages under load (see Voh max in the datasheet).

Unless you need a negative supply for other circuits that are not shown in above schematic, that is.

 

[george2525]

yes the rest of the circuit includes an oscillator and LFO which oscillates about virtual ground.

I do not have a complete schematic yet as I am working on modules seperately for the moment

the output does not have to be big as I plan to make the output a simple line out that can connect to an external amplifier (or Hi fi etc)

 

[george2525]

a brief list of parts

keyboard control - connected to gates
main oscillator - sine/square/triangle
voltage controlled resistors to adjust pitch of main oscillator incorporating R2R ladder
AR (shown)
voltage controlled filter (LP)

thats the basic stuff that has to all run from ideally one 9V battery. I could use 2 but would rather not

 

[george2525]

"Even though not a major concern: The LM324 may not be the best suited opamp for this purpose as its positive output is well below the positive supply (see Voh in the datasheet). Typical is Voh=Vcc-2V, worst case is Voh = Vcc-4V!"

Ive been looking at other options. Are you or anybody else able to recommend an op amp that can be powered by 9v and has a greater output (goes closer to the rails)?

 

[Harald Kapp]

Look for rail-to-rail opamps using your favorite search engine or the search page of your favorite manufacturer. Be aware that rail-to-rail comes in different falvors:

• rail-to-rail inputs, doesn't necessarily mean the output is rail-to-rail, too.
• rail-to-rail output, doesn't necessarily mean the inputs are rail-to-rail, too.
• rail-to-rail inputs and outputs.

The datasheet will state this clearly in the technical data tables, often it is not so clear from the overview on the first page of the datasheet.

Have you considered my idea of not using an opamp at all?

 

[george2525]

Have you considered my idea of not using an opamp at all?[/QUOTE]

Yes I tried it. seems to work just fine. Good idea, thanks