Problems with volume circuit

"Did some voltage testing across C4 and it is indeed changing when approaching the antenna, but it maxes out around -17 millivolts, would that be enough to use with a VCA? "

Nope, way, way too low..

I would expect with a correctly operating circuit for you to be seeing changes of volts - perhaps -3V minimum (as in , from say -1V to -4V).

I suspect your volume oscillator is not running - that when it was running, it caused a -ve potential on C4 which shunted the VFO and silenced the theremin, which was why it "wasnt working"..

"If so, do you have any schematics or advices for building a VCA?"

You need a voltage change, you need to buffer / level shift this voltage change to suit your VCA... At this time, any advice on VCA is pointless.. I would look at a H11F1 opto-FET as it is probably the easiest to interface with, well - bodges like this ;-) ... But you will need at least an op amp to get the control levels right... So I would be thinking about a dual RR opamp driving a transistor current sink controlling current through H11F1 LED, the H11F1 Fet strapped in the audio path, the spare op amp 1/2 being used to buffer this and provide line level...

But we are talking about new/total redesign of the volume stuff - Which is why I presented a transistor possible "fix" on the original -

IMO, if the simple fix doesnt work, turn it into a pitch-only theremin and perhaps build a completely seperate volume module using the EPE-2008  design.

As in, get rid of the volume stuff on your present board and just finish it as a pitch-only..

Then build a seperate volume board complete with antenna and its circuitry, and VCA with audio in and out..

Then put these together (volume board may need re-tuning if its oscillayor interferes with the pitch oscillators)

But first its probably worth giving the extra transistor a try - IF you can get the volume oscillator / filter to work and get a reasonable voltage change across C4. I advise disabling the pitch oscillators while you do this, otherwise confusion is likely.. With the pitch oscillators disabled, any change you se across C4 must be the result of the volume oscillator... With the pitch oscillators running, there is the possibility that what you measure is due to coupling from these oscillators.

Fred.

Do you have a frequency counter and/or oscilloscope?

It is essential to determine if the volume oscillator is running, and that the signal from this is coupling to its filter in a sensible way - For example, if its running, but Q5 is not correctly driving L3, then this could cause what you are seeing.. Also, if the tuning of the oscillator is miles away from the resonant frequency of L3 and its filter components, you would get the small change only....

You could verify the "VCA" function by removing D1 and applying a DC voltage on C4 - A 1k potentiometer across a 9V battery with its +Ve connected to GND with its wiper connected to C4 will allow you to set a voltage from 0V to -9V.. If the "VCA" function is working, adjusting this voltage should change the volume - If you note the voltages (as in, at what voltage the theremin is silent) this will give you an idea about the kind of voltage you need to see across C4 from the volume circuit.

Fred.

It probably is really worth doing the above before anything else.. Get the pitch section working, then remove D1 and supply an adjustable voltage to C4 ... If this changes the volume (without changing pitch sunstantially), great - If it doesnt, well there really is no point trying to debug the volume oscillator and filter - debugging this will probably not be easy, and if the "VCA" doesnt work, it will probably be pointless.

Just a quick word about the tuning diagram I gave..

The peak (resonance) of the filter is the point at which the highest voltage will be seen across C4... But as this voltage is rectified by D1, the voltage on C4 will be most negetive at this point.

The more -ve the voltage on C4, the more Q2 will conduct - and the more Q2 conducts, the more the signal will be shunted / attenuated...

So the "peak" of the resonance actually corresponds to the lowest volume level (!)

Whereas with every sensible theremin volume control, the peak represents maximum volume!

The ears response to sound level is log - down at low levels our hearing is more sensitive to changes than at high levels.. So one wants control to change slowly at the low volume end, and rapidly at the high volume end..

The resonant curve provides a measure of this function, because change at the peak (or close to it) is more rapid than the change as one gets further away from the resonant point..

With this "theremin" the area of most rapid change is used to control the low volume, and the area of least rapid change used to control the high volume (IF one has volume increasing as the hand moves away)..

Its a bit like putting an anti-log potentiometer in a volume control! - it will snap to near maximum sound level an the first few mm, and have almost no perceptible change in volume for the remainder of the field.

Hello!

I really hate not understanding what is happening! Here's the deal; just as when the volume circuit was connected to the pitch, disconnecting the tap on L2 solves my problems.

With the tap connected, there was only a change in 17 millivolts when approaching the antenna. With the tap left unconnected, the voltage changes from 0 to -3 volts when approaching the antenna. How can this be? Even though it works like this, it feels like I've just assembled an IKEA bookshelf and now have half the screws left ;)

P.S. I do not have access to an oscilloscope unfortunately.

"it feels like I've just assembled an IKEA bookshelf and now have half the screws left ;)"

Sorry, I know the feeling.. Dont have the answer :-(  .... If the pitch oscillators are disabled, then there can be no confusion...... But if they are still running, there could be great confusion - because the pitch oscillator could be behaving like a transmitter, and the volume "oscillator" with its tap disconnected behaving like a reciever...... just one example of the kind of possibilities.

If you disable the pitch oscillators and get the same results, I will accept that there are some screws left over ;-)

Fred

ps .. Just remember - if it can happen, it DOES happen! ;-)  Oh, perhaps "infrequently" - but just think about the improbabilities leading to where you are now ... Quite honestly, the most profound of these is that you managed to get anything to run at all, given the limited data available to you and the obvious incompetence of the design of this theremin!

Just been thinking about the hypothesis of the pitch antennas radiation being picked up by the volume "oscillator" .. This could "work!" ;-)

Changes of hand position over the volume antenna should not significantly affect the pitch - But changes in pitch will certainly affect the volume!

Dont worry too much about the electrolytics ESR.. Just get something cheap and slap it in! - If you leave enough space to add another capacitor, you can do that later.. if you need to.

Whilst important, there are many factors which will come into play - the layout is one of these.. You can get a capacitor with the lowest ESL and pay a fortune for this, but if the inductance of the track connecting this capacitor is high, you will have wasted your money! The frequencies involved are not high enough to warrant too much worrying!

;-)

Fred.

Added > Sorry for the delay in posting this - this is my 3rd attempt.. "Trouble in Ether" .. Managed to post on other threads, but every time I got to this one it bombed! ..

Anyway, got there now it seems! ;-)

Hi there again!

Quickly built a simple VCA using a LM13700 I had avaliable. Used the schmatic from the datasheet.

I must have missed something, because it's not working as I want it to. Here's the schematic:

http://imageshack.us/photo/my-images/441/voltagecontrolledamp.jpg/

First of all, since there are both V- and GND, I connected both to ground.

The grounding below the Vin, does it mean I should connect the "input terminal" to ground, because that doesn't make any sense to me.

Are there any more modifications I need to do to this schematic, or should I throw it in the bin and start over with another one?

Philip

Sorry Philip, but I dont think I can help you..

You need to at least be able to download a data sheet and understand how things work before you start on something as complex as playing with transconductance amplifiers and interfacing these to circuits like you are trying to do.. To help, I would need to run a long on-line course in electronics for you.

"First of all, since there are both V- and GND, I connected both to ground."

"The grounding below the Vin, does it mean I should connect the "input terminal" to ground, because that doesn't make any sense to me."

If you had downloaded a data sheet, you would have seen that the LM13700 runs on split supplies.. -V is - with respect to GND, GND is most often (almost always) mid way between +V supply and -V supply.

And no, connecting -V to GND will not work.. If you have a single supply, you need to create a mid supply using at least 2 resistors and a capacitor - but ideally an active stable reference.. Have you ever built a simple inverting opamp or any other opamp circuit? - If you have, you will have needed to do exactly the same kind of thing.. The LM13700 is a special form of opamp with controllable transconductance - if you havent played with basic opamps enough to understand what the +V -V and GND voltages need to be, the LM13700 is WAY out of your depth!

The symbol connected to Rin is the symbol for a voltage source or signal.. It means that the signal goes into RIN, and is referenced (usually goes above and below, centered on) to GND. Rin IS NOT connected to GND!

The reason I went to the bother of drawing up modifications to your last, was because I fully foresaw all sorts of difficulty ahead if you tried to fit any "conventional" VCA to the circuit.. The best hope (IMO) of salvaging something from that pile of junk was to adapt it.

You have no hope - Throw the stuff away... You cannot just pick up circuit blocks from the internet and plug them together without understanding anything about them - its a pointless waste of time...

Buy a Silicon Chip kit. You dont know enough to debug or adapt crap circuits and make them work - sorry.. If you want to learn electronics, the S/C is a reasonable place to start (perhaps a bit advanced for a complete beginner - but you got some oscillators to run, so you are probably a bit advanced from level 0)

Fred.

This is the best collection of opamp circuits I have found.. With EVERY opamp (unless otherwise stated) there is a +V and -V supply to the opamp, and GND is mid way between these two supplies.. For example, +V = 12V above GND, -V = 12V below GND.. The opamp power supplies are often not shown, it is assumed that everyone knows GND is mid-supply.

Basic, Fundamental, Absolute rule for ALL electronics construction / design / development.. Do not use any active component unless you have its data sheet and / or fully understand it!

Just noticed " Used the schmatic from the datasheet." and I am mystified - The data sheet is highly descriptive, and I cannot see how, if you read it, you could have been confused.. Perhaps,like me, you are just having a really bad day! ;-)..

Please forgive my gruffness - I just got some legal papers through the door heralding the start of a long divorce battle... Im too old for this shit, and my kids are too young for it.

Please forgive my gruffness -

Wizards should be gruff -Glad you're back!