[sdiy] 4069 / 4049 VCA

[Scott Gravenhorst]

harry <harrybissell at prodigy.net> wrote:
>Here's what I recall...
>
>The 4049 vca originated from Popular Electronics some time (ages) ago.
>The connection uses only one transistor of each inverter pair...

Tomg's cookbook Hex VCA connects Vdd and Vss to ground.  So it
is attempting to use both transistors.  I don't know if it
really makes that much difference given the transistor asymmetry.

>All 4049 are UB... there is no buffered version.

Ummm yes there is.  I have a National data sheet that says so.
There may not have been a B version at the time of that article,
but there certainly is now.

I have a tube of RCA 4049AE.  I wonder which kind they are...
RCA's website doesn't own up to it.  Is it safe to check from
Vdd to one of the gates with a DVM?  If so, at low ohms setting?

>The input diodes are different in a 4049, there is no diode to the
>positive
>rail. The 4069 has these diodes and will not work when you ground the
>positive
>rail.

Well, it will work with the caveat that the gate signal must be
between ground and the absolute value of one diode drop 
(Vdd & Vss grounded).  Not sure how useful that might be.

But this brings up an important point about the 4069.  Magnus
put up an (untested) design of a 4069 VC-ADSR that stands far
less chance of working now.  It uses the grounded Vss/Vdd 
technique.  I'm not sure how well it could work when the input
voltages can't rise above a diode drop.  Perhaps the idea
of an _all_ 4069 synth is not quite practical.  I have strayed
from that anyway in my building since I have used 4007 and 4027
as well as standard opamps (the linear V-I converter).  At this 
point, my project is a "Mostly CMOS" synth, with the idea
that many of the 4xxx parts are still common and inexpensive.

>BTW I've done a little simulation of the 4069 VCO... I think you can
>capacitivly
>couple a pulse from the "middle" of the schmitt trigger and cancel the
>reset spike
>to a great extent. What I can't simulate is does it screw up the tuning
>?
>
>I'll send you (Scott) a schematic of this later if you want to try it on
>your setup.
>It may add enough complexity to make the thing not as attractive...
>
>You also may not be able to haer the difference.

That's my guess.  I think it's more bothersome to look at on
a scope than to hear.  I like the sound of the VCO, just as it
is, but I'm willing to tinker on it if it doesn't get so wierd
that a 7555 would be a lot more simple.

I would like to see the schematic.

>H^) harry
>
>Scott Gravenhorst wrote:
>
>> I have a copy of a schematic from TomG's cookbook portraying
>> a hex VCA made from a 4049.  Unfortunately, it is labeled simply
>> '4049'.  I am assuming that it is the UB part because it runs in
>> linear mode.
>>
>> I learned (right here!) that the 4049UB does not employ symmetrical
>> transistors as the 4069UB does.
>>
>> Because of the asymmetry, I assume I would have to adjust some
>> resistor values (and also the pinout) so that I can use the 4069UB.
>>
>> A couple of questions:
>>
>> 1) Does the asymmetry of the 4049ub actually provide a benefit?
>>    (I do realise the original intent was that the 4049 would
>>    do CMOS to TTL/DTL conversion).  My question refers to a
>>    benefit that might be realised in the VCA application.
>>
>> .. if not,
>>
>> 2) Has anyone already designed a 4069UB based VCA (so I don't
>>    need to figure out why a wheel has to be round again) ?

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