last time i promise was: [sdiy] 5v ADV Bass / AD EG Question.
Scott Gravenhorst
music.maker at gte.net
Fri Mar 18 00:06:00 CET 2005
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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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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-- Autodidactic Master of Arcane and Hidden Knowledge.
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