[darren adcock]

In my MFOS make; anaolgue synth book it says that to turn a signal mixer into a control voltage mixer I just need to bypass the caps on the each channels line inputs. Is this correct? Seems far to easy. If it is the case, what is about the capacitors thatwon't allow the Control voltages to be mixed? The mixer circuit is similar to the one below, however it runs off -12v/+12v.

 

[kellys_eye]

Capacitors block the flow of DC.

 

[darren adcock]

Ahh yes ok. So does this mean the capacitors use in this fashion with audio signals is to keep DC noise out of the circuit/signal path?

 

[kellys_eye]

Not DC 'noise' but any DC 'component' that may take the peaks of the actual signal beyond the clipping level of the input.

 

[darren adcock]

thanks.

 

[darren adcock]

My signal generator has a steady 3.4 volts regardless of frequency (I'm not sure if that's what's meant to be the case, but it was a cheap digital kit). How would I calculate how to reduce the DC to a level i needed, as I often worry that inserting it into line ins to check the modules will be damaging the circuit, or giving me readings that are misleading?

 

[kellys_eye]

My signal generator has a steady 3.4 volts

I presume you're referring to the DC level at the output? If so, that's why capacitors are fitted in audio circuit inputs - it blocks the DC part and only allows the 'AC' part of the signal to pass.

Since signal generators have to cover a multitude of functions then the better models actually have a variable DC-offset capability.

If your signal generator signal output is 3.4V peak-to-peak then you need some form of variable attenuator on the output to reduce it to the levels that audio system require.

This is often a problem with 'cheap' signal generators - their inability to set an accurate output level AND their lack of steady output impedance (usually 600Ω).

 

[darren adcock]

Ok Attenuating wil be easily done. I have an analogue farnel sig genrator in storage, it was one of the first bits of kit I bought but didn't understand it what so ever. I'll dig that out on Monday and get it set up. It looks similar to this.

 

[Audioguru]

You said the signal generator has a steady output of 3.4V regardless of the frequency. Is it 3.4V AC or 3.4V DC?
You said that you want the mixer to work with "control voltages". If the control voltages are DC then why are you using an AC signal generator?

 

[darren adcock]

The sig gen i use is a tiny digital one. It gives me low frequencies to high I mainly keep it in sine wave mode. The voltage is 3.4v measured in dc mode with my DM.

 

[davenn]

I mainly keep it in sine wave mode. The voltage is 3.4v measured in dc mode with my DM.

that doesn't sound right .... there shouldn't be any DC output when in sine output mode
(unless the generator is faulty)

 

[Audioguru]

When you say "control voltage" do you mean the "modulating signal"?

 

[darren adcock]

I suspect it's faulty, it was a fiver from china, not that means that makes it crap. Just's always been a bit tempremental. I'll learn to use the farnell one tomorrow and look at the small digi one at a later date.

 

[darren adcock]

When you say "control voltage" do you mean the "modulating signal"?

I'm not sure what the difference is?

 

[darren adcock]

A control voltage would be a voltages taken from a keyboard/ribbon controller? a modulating signal would be a wave form out?

 

[Audioguru]

A control voltage would be a fixed or slowly changing DC voltage from a pot or a switch. A modulating AC waveform would quickly vary the tone (wah, wah, wah) or amplitude (burst, burst, burst) or frequency (like a siren).