[sdiy] removing dc offset

[Scott Gravenhorst]

"gregory zifcak" <zifcak at hotmail.com> wrote:
>yeah,
>i'm pretty sure this is what's happening. in order to generate an opposite 
>offset i'd have to add a negative supply, right? i was hoping for a simpler 
>fix. i guess maybe i'll just high pass filter the thump out and live without 
>the bass for now.
>thanks for all the suggestions,
>greg

Well, there's always the old opamp circuit that makes a voltage at 1/2 VCC. 
That voltage can then be referenced as "ground" and creates the effect of
having +4.5V/-4.5V.  However, using that will require a schematic diagram of
the device and changing how the inputs and outputs are grounded.

Harry could be right about the device conserving battery power by going into a
standby mode after the sound is finished playing.  And the only way to fix
that is to create an anti-thump signal to add in.

>
>
>>I got the impression that this is not your case... it seems like the thump 
>>is
>>there
>>everytime you make the sound. This can be really hard to get rid of unless 
>>you
>>know
>>exactly when the sound will occur.
>>
>>This can happen with some cheap circuits where the power is OFF whenever a
>>sound is not being produced...then the unit 'one shots' when triggered. So 
>>the
>>voltage might go from zero to 1/2 the supply with an AC wave riding 
>>it...and
>>back to zero.
>>
>>If you can predict when the sound starts, you can generate an opposite DC 
>>offset
>>
>>(dynamic) that will largely cancel the one from your circuit.
>>
>>It will NOT be a simple toy anymore...
>>
>>I had a similar problem with my guitar synth design. When there are no 
>>strings
>>playing, the waveforms do not go to zero, but to the peak value. This make 
>>a
>>huge thump in the VCF. Because I know when this will happen, I force the 
>>waves
>>to
>>zero (they are still at the peak but I am clamping them to ground).  You 
>>could
>>do similar...
>>
>>H^) harry
>>
>>gregory zifcak wrote:
>>
>> > how would this work? can you explain it a little more?
>> > thanks,
>> > greg
>> >
>> > >
>> > >Ah, so you are worried about the thump, i.e. the sudden step in dc 
>>voltage,
>> > >not the dc itself.
>> > >
>> > >Perhaps a relay in series with the output would help, which is turned 
>>on
>> > >delayed?
>> > >
>> > >Ingo
>> > >
>>
>
>

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