[sdiy] 4069 / 4049 VCA

[harry]

Hi Scott (and lurkers)

Scott Gravenhorst wrote:

> 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.

Actually just the opposite... the P-channel trannies are reverse biased and
out of the
circuit. Its like getting 6 N-channel trannies by themselves...

>
>
> >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 will check that out.  Its gotta be weird...

>
>
> 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 A series is faster and more noise immune than the B series. For your use
I'd say the A and UB are equivalent.

the B series (buffered) have the most difference in multi input gates... not
an issue here.

> >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.

The concept might work with the 4049 instead...

>
>
> >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.

soon...soon...

H^) harry

>
>
> >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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