last time i promise was: [sdiy] 5v ADV Bass / AD EG Question.

[Scott Gravenhorst]

In most cases (and I think this one qualifies), the virtual ground you create
is to be used wherever you have a ground symbol in the original schematic,
assuming the original is a dual supply design.  All the virtual ground circuit
does is create a stiffened reference point halfway between the power supply
rails.  All audio signals should be referenced to virt. ground, both for
output and input.  All internal grounds, such as grounding the + input of an
opamp are also virtual grounds.  The rails still go to the IC pins they always
did.  This will work because a dual supply schematic will differentiate
between ground, V+ and V-.  Don't confuse V- (or in your case PSU zero volts
sometimes referred to as "ground" when operating as a true single supply) with
the virtual ground.  This is very important.  The only reason you need the
virtual ground is because you don't have a dual supply.  The virtual ground
creates this as a reference to fool the circuit into operating off of a single
supply.  In reality, however, the presence of the virtual ground and it's
proper application turns your single supply into a dual supply.

"Bill Berzinskas" <wberzinskas at nc.rr.com> wrote:
>now that i think of it..   i'm using the VCO from the lmX58 family.. which 
>uses a virtual ground as well. . would this have an effect on the VCA?
>
>----- Original Message ----- 
>From: "Nils Pipenbrinck" <np at inverse-entertainment.de>
>To: <WBerzinskas at nc.rr.com>; "Synth-DIY" <synth-diy at dropmix.xs4all.nl>
>Sent: Friday, March 18, 2005 8:24 AM
>Subject: Re: last time i promise was: [sdiy] 5v ADV Bass / AD EG Question.
>
>
>> WBerzinskas at nc.rr.com wrote:
>>
>>>Hello Jay..  Thanks for the info... schematic - 
>>>http://midwest-analog.com/bass.html
>>>
>>>
>> I just had a short view at the VCA, and spotted some things that look 
>> wrong:
>>
>> First off you reference the signal at the inputs to GND. Don't connect the 
>> 680ohm resistors to ground but to the virtual ground instread (2.5V in 
>> your case.).
>>
>> Then you should adjust the voltage divider (100k / 680ohm into pin2). The 
>> input of the 3080 OTA likes to see a signal difference of something 
>> between 75 and 150 mV at it's input terminals. (it's a matter of taste how 
>> much distortion you want, the higher the input, the more distortion you'll 
>> get). Also out of the blue, the impendance of the voltage divider looks 
>> higher than nessesary. I would choose a value of 22k for R21 and pick R1 
>> and R2 that you'll see a 100mV difference at the inputs.
>>
>> Last thing, and most probably the cause of all troubles: The output of the 
>> OTA is a current output. You won't see a usable voltage at the output 
>> until the current can flow, and I'm not sure if the opamp at the output 
>> will do this job. You need a resistance between the output and the virtual 
>> ground to turn the current into a voltage.
>>
>> Nils
>>
>>
>>
>> 
>
>

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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