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DC Offset On VCO build

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DC Offset On VCO build

Postby woodnut » Fri Mar 05, 2021 4:31 pm

Hello folks,

I just built the "Super Simple Oscillator" VCO circuit and I am measuring around 9 volts of DC offset on the output. Its based around a BC548 used in reverse avalanche mode. I'm currently feeding it 18 volts using two 9 volt batteries.

It is oscillating and appears to be working. I have no idea if that is normal for this circuit but should I be looking to employ output coupling caps before I plug this into a mixing desk? I'm a bit unsure how safe it is to feed this into my mixer.

As a side note what sort of values of cap would be in the ballpark for coupling caps to remove the DC offset?

Many thanks in advance
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Re: DC Offset On VCO build

Postby James Perrett » Fri Mar 05, 2021 4:45 pm

A mixing desk designed for phantom power should already include DC blocking capacitors but it would be a good idea to add one to be on the safe side. I'd go for around 1uF given that you've got a 100k resistor in series already.
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Re: DC Offset On VCO build

Postby Hugh Robjohns » Fri Mar 05, 2021 4:55 pm

woodnut wrote:I just built the "Super Simple Oscillator" VCO circuit...

I assume you're talking about the design on this website: https://www.lookmumnocomputer.com/simplest-oscillator

...and I am measuring around 9 volts of DC offset on the output.

If it's this circuit (below), then yes, of course you would! The audio output is DC-coupled.

Image

I have no idea if that is normal for this circuit but should I be looking to employ output coupling caps before I plug this into a mixing desk?

That would be wise, yes! Most console inputs will have DC blocking capacitors anyway, but a 9V offset is a lot and it's never a good idea to have DC floating around on audio connections!

As a side note what sort of values of cap would be in the ballpark for coupling caps to remove the DC offset?

I'd strap in a small electrolytic, with the positive side facing the oscillator... Ideally, you should calculate the value depending on the circuit (and destination) impedances and the lowest frequency you need to pass... Something in the 1 to 10 microfarad (uF) range should suffice.
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Re: DC Offset On VCO build

Postby woodnut » Fri Mar 05, 2021 5:10 pm

Hi folks,

Thanks for the super quick replies that's great. Yes, it is indeed that very circuit!

I will try a few different values between 1 to 10 microfarad as recommended and see what happens. Should I also be incorporating a bleed resistor to ground from the output cap? Any recommendations for vlaues there?

Many thanks!
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Re: DC Offset On VCO build

Postby ef37a » Fri Mar 05, 2021 6:43 pm

woodnut wrote:Hi folks,

Thanks for the super quick replies that's great. Yes, it is indeed that very circuit!

I will try a few different values between 1 to 10 microfarad as recommended and see what happens. Should I also be incorporating a bleed resistor to ground from the output cap? Any recommendations for vlaues there?

Many thanks!

Yes, you should fit a drain resistor post cap but they are usually around 22-47k and even 100k is going to drop the signal by ~6dB. Since the offset is handily 1/2 Vcc why not stick an emitter follower in there? A BC184L with a 10k emitter resistor will give you a pretty low output resistance. Still need a capacitor in series with 100 Ohms then a 22k drain R.

Tell us the lowest frequency and someone can calculate the capacitor value.

Dave.
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Re: DC Offset On VCO build

Postby Martin Walker » Fri Mar 05, 2021 10:08 pm

ef37a wrote:
woodnut wrote:Tell us the lowest frequency and someone can calculate the capacitor value.

Although James and Hugh have already suggested something suitable - my rule of thumb was always 1uF into 100K, which gives a -3dB point of 1.6Hz.


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Re: DC Offset On VCO build

Postby Hugh Robjohns » Fri Mar 05, 2021 10:27 pm

It wont apply in this case, but I have in the back of my mind a recommendation from Doug Self (Soundcraft designer, amongst other things) about calculating the capacitor size to ensure the AC voltage across it is kept below a certain level so that distortion remains inaudible. I'll dig out the details ... unless Dave gets there first! :-)
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Re: DC Offset On VCO build

Postby ef37a » Fri Mar 05, 2021 11:59 pm

Hugh Robjohns wrote:It wont apply in this case, but I have in the back of my mind a recommendation from Doug Self (Soundcraft designer, amongst other things) about calculating the capacitor size to ensure the AC voltage across it is kept below a certain level so that distortion remains inaudible. I'll dig out the details ... unless Dave gets there first! :-)
Certainly does not apply here Hugh! Surely that is a "relaxation oscillator" ? (I remember Thyristor timebases, just) and produces a sawtooth?

Self does indeed show that if you allow more than about 80mV to appear across an electrolytic capacitor the distortion becomes measurable but it is still very low. Once you have calculated the value for the low, 3dB point he reccomends increasing the value at least fivefold.

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