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

[Scott Gravenhorst]

As far as I know, you shouldn't have to change anything with the EG, it
probably already expects to send current to the OTA negative rail which
didn't change by adding the virtual ground.  That affects only a solution for
the input biasing.

"Bill Berzinskas" <wberzinskas at nc.rr.com> wrote:
>actually.. i guess it's not really the EG i'm talking about..  but rather 
>the 2 transistors setup to feed curremt to the VCA..    that's what i 
>meant..  is this part affected by the switch to +5V with the virtual gnd on 
>the inputs?   it would seem to me that by changing the operating voltage 
>range, the Iabc range would be affected as well?  or am i way off..
>
>----- Original Message ----- 
>From: "Scott Gravenhorst" <music.maker at gte.net>
>To: <synth-diy at dropmix.xs4all.nl>
>Sent: Thursday, March 17, 2005 6:06 PM
>Subject: Re: last time i promise was: [sdiy] 5v ADV Bass / AD EG Question.
>
>
>> I'm not a 3080 expert, but as I understand it, the Iabc pin carries a 
>> current
>> which flows into the OTA's negative rail, not ground.  A current source
>> circuit is normally used to supply Iabc, but a resistor from some voltage
>> more positive than the negative rail to the Iabc pin will cause an Iabc
>> current.  A resistor is a poor current source, however.
>>
>> "Bill Berzinskas" <wberzinskas at nc.rr.com> wrote:
>>
>>>last time.. i promise..   got everything straight pretty much.. 1 last
>>>problem..
>>>
>>>the EG that is on the schematic won't move the vca at all..   i'm thinking
>>>it has something to do with the transistors going into OTA pin 5..    do
>>>some of these resistors need to change to suit the 5v design?     any
>>>voltage change i see on this pin is very small.. a resistor to V+ and it
>>>opens up completely..   but, eg, no open..
>>>
>>>heres the schem to..
>>>----- Original Message ----- 
>>>From: "Scott Gravenhorst" <music.maker at gte.net>
>>>To: <synth-diy at dropmix.xs4all.nl>
>>>Sent: Thursday, March 17, 2005 5:24 PM
>>>Subject: Re: [sdiy] 5v ADV Bass / AD EG Question.
>>>
>>>
>>>> "Bill Berzinskas" <wberzinskas at nc.rr.com> wrote:
>>>>>just finished trying this.. any no go.. any suggestions?
>>>>>
>>>>>i am getting 2.5v out from the opamp i added..   and i have both inputs
>>>>>connected to this virtual gnd via 780 ohm resistors..    i am seeing a
>>>>>change in pin5 when i send Envelope to it..   i've tried a few other
>>>>>chips..
>>>>>still no go.. wheee
>>>>
>>>> The external input should also connect to at least one OTA input through
>>>> some
>>>> larger value resistor.  Are you seeing 2.5 volts from 5v to virt.gnd
>>>> and -2.5
>>>> volts from 0v to virt.gnd?
>>>>
>>>>>
>>>>>----- Original Message ----- 
>>>>>From: "Scott Gravenhorst" <music.maker at gte.net>
>>>>>To: <synth-diy at dropmix.xs4all.nl>
>>>>>Sent: Thursday, March 17, 2005 4:34 PM
>>>>>Subject: Re: [sdiy] 5v ADV Bass / AD EG Question.
>>>>>
>>>>>
>>>>>> "Bill Berzinskas" <wberzinskas at nc.rr.com> wrote:
>>>>>>>i think i get the jist of this..    just to clarify..
>>>>>>>
>>>>>>>the ota will still be running on 0v and GND..   but where the inputs
>>>>>>>have
>>>>>>>a
>>>>>>>resistor going to ground, this is where i would hook up to virtual
>>>>>>>ground?
>>>>>>>and everything else would be the same?
>>>>>>
>>>>>> Yes, the OTA is powered by +5v and 0 volts.  Biasing of the OTA is 
>>>>>> then
>>>>>> identical to any dual supply set up.  This means that you need only 
>>>>>> the
>>>>>> one
>>>>>> +V/2 reference.  OTA circuits that I've seen all have a large resistor
>>>>>> (like
>>>>>> 100K) from signal input to OTA input, then there's a small resistor
>>>>>> (like
>>>>>> 1K)
>>>>>> from OTA input to virtual ground.  This automatically biases the OTA
>>>>>> input.
>>>>>> If one OTA input is unused, it should be sent to virtual ground 
>>>>>> through
>>>>>> a
>>>>>> resistor of a size equal to the small resistor on the other input.
>>>>>>
>>>>>> I forgot to mention that an LM324 should work for the opamp needed, 
>>>>>> spec
>>>>>> says
>>>>>> PSU down to 3 volts.
>>>>>>
>>>>>>>
>>>>>>>----- Original Message ----- 
>>>>>>>From: "Scott Gravenhorst" <music.maker at gte.net>
>>>>>>>To: <synth-diy at dropmix.xs4all.nl>
>>>>>>>Sent: Thursday, March 17, 2005 4:12 PM
>>>>>>>Subject: Re: [sdiy] 5v ADV Bass / AD EG Question.
>>>>>>>
>>>>>>>
>>>>>>>> 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 >
>>
>>
>> ---------------------------------------------------------
>> - Where merit is not rewarded, excellence fades.
>> - Hydrogen is pointless without solar.
>> - What good are laws that only lawyers understand?
>> - The media's credibility should always be questioned.
>> - The only good terrorist is a dead terrorist.
>> - Governments do nothing well, save collect taxes.
>>
>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>> -- Linux Rex         | RedWebMail by RedStarWare
>> -- FatMan: home1.gte.net/res0658s/fatman/
>> -- NonFatMan: home1.gte.net/res0658s/electronics/
>> -- Autodidactic Master of Arcane and Hidden Knowledge.
>>
>> 
>
>

---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.