[sdiy] 5v ADV Bass / AD EG Question.

[Scott Gravenhorst]

Another way to achieve this is by using an opamp to create a virtual
ground.  Essentially, this is an opamp, output connected to - input,
+ input gets a voltage from an equal resistor voltage divider from v+
to PSU zero volts.  The output of the opamp will then be a virtual 
ground from which V+ looks like +2.5 volts and from which PSU zero 
volts looks like -2.5 volts.  Now you can treat the circuit as if you 
have a +/- 2.5 volt supply.  Inputs and outputs are referenced to the 
virtual ground, not PSU zero volts.  The opamp, as well as the OTA is
powered from +5 volts and 0 volts.  This technique would work for low
current applications.

<jays at aracnet.com> wrote:
>If it is between gnd and +5 then you will probably have to bias 
>the input to ~+2.5 volts. Basically V+/2. You'll have to be 
>careful since the 3080 has a fairly low input voltage for low 
>distortion so bias become critical. 
>
>Also be careful. If the Iabc pin gets to much current it can fry 
>the CA3080. So at least in prototyping the circuit add a current 
>limiting resistor. 
>
>Crude calculation:
>V=IR, R = V/I, V=5, I=.001 (1ma), R=4.7K or 5.1K should work.
>
>Jay S.
>
>
>
>Bill Berzinskas <wberzinskas at nc.rr.com> wrote :
>
>> well.. i breadboarded this this morning..  and 
>> most everything seems to work..  except the vca..  any thing 
>> specific i need to do to get this 3080 vca working on 5v?   
>> 
>>  
>> 
>>   ----- Original Message ----- 
>>   From: 
>>   Bill 
>>   Berzinskas 
>>   To: Jay Schwichtenberg ; synth-diy at dropmix.xs4all.nl 
>>   
>>   Sent: Thursday, March 17, 2005 11:16 
>>   AM
>>   Subject: Re: [sdiy] 5v ADV Bass / AD EG 
>>   Question.
>>   
>>   preciate that jay..   good 
>>   article..  it'll prob take me a bit to get through the math tho.. lol 
>>   ..  :-)    time to break out my math book from last 
>>   semester.. 
>>    
>>   
>>     ----- Original Message ----- 
>>     From: 
>>     Jay 
>>     Schwichtenberg 
>>     To: Bill Berzinskas ; synth-diy at dropmix.xs4all.nl 
>>     
>>     Sent: Thursday, March 17, 2005 11:10 
>>     AM
>>     Subject: RE: [sdiy] 5v ADV Bass / AD EG 
>>     Question.
>>     
>>     Bill,
>>      
>>     2)
>>      
>>     Another issue is that the attack, decay and release 
>>     pots on envelope generators are typically log pots, a non-linear scale. 
>>     So if the digital pots you use are not log they may not give you the timing 
>>     resolution that you are use to. One techinque to do that is buffer the pot 
>>     and add a resistor. Picking the right resistor and placing it between the 
>>     wiper and the other ends of the pot (assuming using pot as attenuator 
>> between signal and gnd) can give you log or expo curves. Try 
>http://www.geofex.com/Article_Folders/potsecrets/potscret.htm for 
>that > info. >   > Good luck. > Jay S. > > -----Original 
>Message-----From: > owner-synth-diy at dropmix.xs4all.nl > 
>[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Bill > 
>BerzinskasSent: Thursday, March 17, 2005 7:36 AMTo: > 
>synth-diy at dropmix.xs4all.nlSubject: [sdiy] 5v ADV Bass / AD EG > 
>Question. > Hello everybody..  just woke up to a > crazy 
>little burst of snow here in NC..  wierd..   > > 
>  > Anyhow..   As you probably know i'm > modding 
>the bejezzus out of my adv-bass..   In my quest to make 
>> it digitally controlled, i've hit a few snags..  so maybe 
>you can > help me out.. >   > 1) I am converting it to 
>digital > control..  mostly thru digital pots..   
>a few things with > DAC..  so.. for the digital pots, i have 
>to use 5v input, and rather > then use way more opamps then i 
>want to deal with, i would like to make it > operate on 5v 
>possibly with a bipolar or just 12v output stage if > 
>necessary..   So.. >   > I've done some crude 
>experiments with the > Trigger section just to see how it reacts 
>to switching from +/-12v to just > +5v..    I've 
>tried with tl082s..  and i just can't > get the trigger 
>ratio right.. and it always triggers.. so.. since i broke > down 
>and got a few lm324s, i tried those.. and it seems to be working 
>> correctly..   i then went on and built the impact 
>part using the > digital potentiometers..   all good so 
>far..   if i > just switch all my opamps in the design, 
>will it carry through the rest of > the circuit properly? > 
>  > 2)  I plan to use a digital > potentiometer to 
>control the decay time..   but the highest > value i 
>can come up with is 100k.. so i'm thinking i can change the > 
>capacitor and get about the same decay time?   any > 
>recommendations for a value? >   > thanks guys!  :-) > 
>--billie >   
>
>

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