Help with understanding Audio/CV and AC/DC

[Captain Biscuits]

Hi

Having built a WSG, Mini Synth, 10 step and Wall Wart power supply I’m setting out to build a modular synth based, to start with at least, on MFOS modules.

I’m making up blocks of 3 modules on A4 metal boards which lets me do the panel printing and laminating cheaply and easily. I’ve just completed my first block - three MFOS VCOs.

Having completed them I’m faced with a problem which I guess will still be around even when I’ve added many more modules - the desire to attenuate outputs, mix outputs and route multiple outputs into one.

I’m learning as I go so I’ve got a few rather naïve questions if anyone has the time to clarify a few points and offer some advice.

For now, to be able to hear all three oscillators at once and to get the level down to something that isn’t too loud through my amp I’ve just wired four 1/4” jack sockets together and am inputting into 3 of them and using the fourth to output. Then I’m just attenuating the output as Ray suggests here http://www.musicfromouterspace.com/analogsynth/oddsandends.html#POTDIVIDER

Advice wise I’d be interested to know what people think would be a useful set of functions for a multiple module given that I’m planning for it to take up a full A4 size block of space and would like it to be adaptable so I can experiment with patches as I add modules. At the moment I’m thinking of a passive 4 input mixer, 3 or 4 simple attenuators (each just In, Pot and Out) 2 or 3 groups of 4 sockets wired as I’ve described above, Ray’s active mixer (with the headphone amp) and the MFOS CV distributor.

So does that make any sense to those of you who have a lot more experience of modulars than I do? Any advice of what I might add in or miss out would be very welcome.

Now for the naïve questions! On Ray’s pot divider page he makes a distinction between AC Signal and DC Input which has thrown me a bit. I’ve been a bit puzzled about the degree of overlap between Control Voltages and Audio Signals which seem to be interchangeable much of the time but not all of the time. On the MFOS pages there is an Ultra Simple Mono Mixer and a Dual 4 Input DC Mixer. They are clearly rather different and I’m struggling to understand why.

Can anyone explain to a simple but not stupid person what is going on here, how I can make a functional multiple module and where I could go horribly wrong if I start slinging all of my outputs and inputs in together?

Many thanks in advance to anyone who can help throw some light on all of this!

All the best,

Ian

[prinsen]

Can't offer you much advise, sorry. But just wanted to comment this:

[Captain Biscuits]

Thanks Prinsen,
That's exactly what I'm struggling with. The difference between the two mixers is small but important and I don't understand why. I thought I was looking for a mixer - now I'm looking at whether I need an AC mixer, a DC mixer or both and that's puzzling me.
You call the AC mixer "Audio" and the DC "Control Voltage" and it is that difference, between Audio and CV, that I'm struggling with.
I'm just hoping someone can help me to understand.
Many thanks
Ian

[blue hell]

In general a wave will have an AC and a DC component, or a varying part summed with a constant part. When you shift down (or up) the graph of the varying (AC) part such that equal areas of that graph would be above and below the zero line the DC component is reduced to zero, and the amount you shifted the the wave by is it's DC component.

Now an AC mixer, or better said an AC couple mixer, will do that shifting process such that the resulting DC component will be zero.

For audio the DC component has no relevant information and usually it's even in the way and that is why it gets stripped off with AC coupling.

For control signals however DC usually is relevant, as a DC voltage going up could mean that an oscillator will put out a higher pitched voltage. At the same time you might want to modulate that pitch to get vibrato - this can be done by using a DC mixer.

Hope this clears it up a bit.

[prinsen]

[Captain Biscuits]

Thanks to you both for your helpful replies.

I'm gradually getting much clearer (I think!).

As I understand it all of my oscillators are AC oscillating equally above and below zero V wheras the output from an ADSR generator will be a rising and falling DC voltage (and will all be at or above zero V). I'm trying to think of another control which would be DC and can only think of keyboards, gates and triggers (I guess triggers can be either). Are there more that I'm missing?

I've just seen Ray's new toy and was interested in his double mixer from one TL074 on the kludge board which would do me nicely. It will be my first foray into building on stripboard so should be interesting. I'll let you know how it goes! I don't have an 074 around but I'm hoping an 072 will do the trick. Lets see what sort of mess I can make of an innocent piece of stripboard!

Thanks again for the help

All the best,

Ian

[blue hell]

You get the idea there alright

TL074 has four opamps TL072 has two, the opmaps themselves are identical, just a packaging difference.

[varice]

[Don Erskine]

I intend to get several MFOS VCOs. They look very good on paper.

In an analogue Synth, there is no clear distinction between audio and control voltages. Below a certain frequency, (around 20Hz) you can't hear things, but a pulse will still contain harmonics that will be audible. You can't hear DC.

For example, an LFO at sub-audio frequencies might be used as a CV for Frequency Modulation (vibrato) or Pulse Width Modulation in a VCO. If the LFO frequency is increased, the same LFO is an audible oscillator.

Control voltage signals from things like ADSRs are usually positive going; if used to control a VCA, the VCA will have maximum attenuation when the CV is 0v. Some VCA designs will accept negative control voltages; in such cases a negative voltage control signal will attenuate or amplify and INVERT the audio signal.

Audio in a synth is typically up to 10 volts peak-to-peak (+/-5v DC). Control voltages are typically in the same range. So a Control Voltage varying over 10v used as a CV into a 1v/Octave VCO will sweep the VCO through 10 octaves.

Gates and triggers are usually positive-going pulses; going above about +2.5v triggers a device or turns it on (gates it).

There is no reason why a gate signal cannot be used as a Control Voltage as well, and often is. Conversely, a square wave (in fact, almost any wave) output from an LFO can be used to trigger an ADSR. When you realise that ANY voltage source can be potentially used as a control or gate voltage, you are able to do a lot more with a synthesiser.

There are reasons why you might want to mix DC with an oscillator waveform - for example, to set the clipping level in a waveshaper.

Synthesiser modules are designed so that no harm can be done by inputting these voltages into any input, though the results may not always be musical.

The important point is that an analogue VC synth is normally DC-coupled throughout. It's only when you feed the output signal into an external amp that you certainly want to filter out the DC and the audio amp will have a filtering capacitor on it's input to do that.

On rare occasions a large capacitor may be used in series on an audio input, to remove DC and low frequencies. In a synth, DC is more normally removed by offsetting it (mixing it) with DC of the opposite polarity. E.G. If a keyboard outputs +2v, it will CV a VCO to sound 2 octaves above the pitch it sounds at 0v. If -1v is mixed with this keyboard CV, the VCO will drop 1 octave, to 1 octave above the pitch it sounds at 0v.

Synth audio levels are much higher than in 'normal' audio, which is typically around 6-700mV RMS. (0dBm) in a studio. A synth will need to be attenuated before it is fed into ordinary audio equipment.

The outputs of the MFOS VCO have 1K resistors. These are there to protect the circuit if the outputs are short circuited. It sounds like you are hard-wiring these outputs together - A DC mixer is what you require to do the job properly. As the outputs are each potentially +/-5v, you need to attenuate the output or it will be distorted. Using an AC mixer on the output wouldn't do any harm though.

[Captain Biscuits]

Many thanks for all of your replies. Now I know (as I should have guessed) that I want ALL of the mixers, attenuators and patches that I can get!
I also have a much better idea of WHY I want each of them.

I’ve managed to pack a fair bit into my A4 sized board and the 51 1/4 inch jack sockets and 21 pots should cover most of my mixing and patching needs for a fair while to come. By the time I need more I really should have worked out what it is that I’m after!

Attached is what I’m going with. 2 DC mixers, 2 AC mixers, 2 little two input passive mixers, six attenuators, a couple of four way patches and a CV distributor.

Just need to get it made now. There's a Wogglebug, Cornucopia and S&H on the production line waiting to get finished first.

Cheers

Ian

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[jordroid]

If you are building either of the MFOS filters (which you should because they sound great) they each have a three input AC coupled mixer built in, pretty handy. You are right that you will want lots of mixing capability, Ray tends to include attenuators and mixers in convenient places in his modules, which reduces the need for dedicated mixers, but you will probably still be happy to have them.

Cheers.

[Rojo]

From this description, AC couplers sound like high pass filters. If that's how they behave, what might an AC coupler's response look like across low frequencies (say 0.1 Hz - 100 Hz)?

[Don Erskine]

[blue hell]

[Captain Biscuits]

Well, despite having only the scantest knowledge of what I'm doing I've managed to stripboard Ray's "Ultra Simple Mono Mixer" which I'm pretty chuffed with.
The only issue I had was with adjusting the gain. I had to seriously reduce the resistance of the feedback resistors. The headphone amp ones I took down from 100K to 4.7K before I got well out of headphone-crushingly-loud territory.
If I did it again I think I'd be tempted to use a couple of 20K pots to adjust the gain.
Dizzy with success I'll have a go at the AC mixers when I next get the chance.
Now I just need many many more patch cables