last time i promise was: [sdiy] 5v ADV Bass / AD EG Question.
Bill Berzinskas
wberzinskas at nc.rr.com
Fri Mar 18 01:52:56 CET 2005
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 >
>
>
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