questions from a newbie (Op Amps again) getting long!

[Harry Bissell]

Tim Daugard wrote:

> I'm not trying to pick a fight. You geniuses have been far too quiet.
> I'm try to make you work to further my education ;-)
> Us lowly techs need all the help we can get.

No fighting... everybody has there own reasons for making choices...
Others choose different... (that's why its called 'choice')

>
>
> Tim
>
> Harry Bissell replied:
>
> >>>Gain Bandwith Product... bigger is sometimes better
>
> >> TI argues against this point. For audio applications smaller is better.
>
> >might involve phase shift in the band of interest... can you stand that ???
>
> How inportant is avoiding phase shift? (Other than 180 degrees and
> oscillation)

IMHO... avaoiding phase shift is not very important. A synth is not "audiophile"
gear
we are generating and not reproducing sound.. The phase shift might be important
if
you are trying a "trick" like maling a high pass filter by invering and summing
the original
audio with a LPF output... then the phase difference (even a little) kills the
cancellation
out. If making a notch... the notch depth will suffer... But for most stuff you
won't notice
a gradual phase shift... (IMHO flamesuit on !!!)

>
>
> >A phaser I designed using a (yuck) LM324 and upgraded to a TL084,
> oscillated
>
> Hey, I like the LM324 :-) Cheap limited, band width and generally anything
> I can get to work with a 324 will work with a TL074. My bench right now is
> heavy in LM324, TL074, and NE5534.

In my case the TL084 didn't sub for the 324 until I added 100pF caps across the
negative feedback resistor (kill the HF gain a little)

>
>
> I've been using the quads for everything. If I only need two of the op amps,
> I build the circuit twice and make the module a dual. I'm waiting for a
> design
> that uses 3 op amps so that I can justify a module with a free inverter to
> patch into.

I use some quads... but some circuits can suffer with quads. There is a little
interaction
between them because they are on the same die... My hex fuzz used quads... and
the clipping stage "bleeds" into the straight audio path a little (makes the
clean audio a little
funky (not good funky... funky like crossover distortion in a 324 :^). If I had
used separate op-amps (and good layout and decoupling techniques...) it would
have been
totally silent.  This is a worst case... but if you have a dual filter sharing a
quad... and one side is self-resonating... and the other is maybe a bandpass
near the frequency of the first... they will pick each other up. My latest
guitar to squarewave (a PV-1 front end...) project uses mostly duals... so
stages are isolated in separate opamps.

>
>
> >>ultrasonically because of the higher gain at high frequencies... Drove the
> dog
> >>nuts !!!  I almost left it that way TO drive the dog nuts... I hated that
> dog
>
> It's good to drive dogs nuts. Can I have the design? Lets see feed that into
> the guitarist and into a high power tweeter, maybe I can drive the big dogs
> next door nuts.
>
> >Many vintage synth circuits will not prosper with a faster op-amp than
>
> Which circuits are sensitive to this? Filters, VCOs?

Yes etc... everyone has a favorite horror story. If I recall JH has related a
couple from
time to time...

>
>
> >was that you NEED to decide on which specs are appropriate, and use
> >that op-amp.  As I said... the 741 is still useful for me.
>
> And this is where we started. What specs are most important?

And we go around... what specs are appropriate for the function you are trying
to
perform... Try making a long time constant integrator (that might be an LFO or
ENV GEN) then use an op-amp without very low offset voltage and bias current...
and it will
"run for the rail" every chance it gets...  The same part in a AC coupled preamp
might be wonderful...

>
>
> I have been buying:
>
>     LM324 - critical spec:  price

You get what you pay for.... but you're right if price is an issue the 324
deserves a serious
look!

>
>     TL074 -  critical spec:  got some free from TI and they worked well
>                    in a guitar foot pedal, kept buying.

A fav of mine... or the TL081,2,4... very little difference that i notice (but
same price..)

>
>     NE5534- selected especially for a microphone preamp:
>                    spec: 1) noise 2) supply voltage

But if you need a HIGH impedance preamp this is not a #1 choice... You will lose
most
of the noise advantage in high value input resistors (like higher than 5K or
so...) for LOW
impedance (like 600ohm) no problem.... If you're not trying to battery power the
poor thing... these are HUNGRY chips (you get nothing for free...)

>
>
> >"phase reversal" which is from nasty (not good nasty) to fatal !!!
>
> I have a scope. I don't think I have seen the dreaded phase reversal - what
> does it look like?

Lets say you exceed the common mode voltage range for the op-amp(in a positive
direction) . The well behaved
741 will just clip, nice and flat, and will "stick" to the positive rail just a
little after the signal comes back into range...  But try that with an OP-77
(cause this one fvcked me over good) and the output will go first to the
positive rail and clip... but as the overdrive increases it will sudddenly run
to the NEGATIVE rail and stay there instead... so you are increasing positive
and it is giving you negative....

For me this caused sudden and unexpected lock- up in a servo heater control...
damaging the heating element... which was a 18" length of 20 ga. solid PLATINUM
wire... wrapped around a hand blown quartz tube... The result caused the metal
to be sputtered
permanentlt on the glass... kind of like a Tiffany Jack Frost creation...  very
pretty but VERY expensive. Thats why I'm so sensitive to "phase reversal"

The fix is either..."don't go there"  (limit the circuit within proper operating
conditions) or use a part that has known and predictable behavior...

BTW most agree that phase reversal sounds like SH!T.

>
>
> >The LM324 (yuck) has a crossover notch in the output response
> >that can be severe, especially on a bipolar supply.... so with a
> >small signal it is a HUGE distortion....
>
> I agree with this totally my log amp amplified the signals near the
> reference
> point with lots of gain. The LM324 is not useable for small audio signals.
> However for oscillators (LFOs and such) and control signals it has been
> okay.
>
> > >Output Current... Can it drive the load you will be using ???
> >
> > I have never looked at this spec. Every thing I've done so far has had a
> > reasonable input impediance. I hooked the output of an LM324 to a 10Hz low
> > pass filter today and it seems to be working okay - I'm not sure what the
> > impediance is - I forgot to check - should be on the order of 100 to 200
> > ohms.
>
> The LM324 (yuck) has a nasty trait... which is why I look at output drive
> current. It has
> different source and sink currents... could be a problem in symmetrical
>
> circuits (like limiters) or if you are driving a capacitive load.  Again...
> you
> got to watch your @ss here.
>
> > >Output Short Circuit current protect... will it blow up if you shory it
> > >out... This can be real important in a modular synth, where you cannot
> > >predict what will be connected...
>
> I agree completely. I design so that any connection from the from of the
> modules can be connected directly to either supply rail without damage.
> I also have three different supply standards at the moment (I know,
> not optimum) 5 V for logic based devices. +12V for devices using
> old disk drive power supplies and +/- 15V  for devices that
> I've built a supply for. I prefer that nothing be able to blow up a
> neighbor.
>
> >> So far I have hung a 1K resistor off all outputs to avoid problems with
> >> this. What is everyone else doing?
>
> > thats what I do... but sometimes I want to drive a capacitive load as
> >fast as I can (like peak detector...) and I can use an op-amp with output
> >current limit...and allow the output to go into current limit on the fast
> transient.
>
> Where I need the straight of the op amps I have been using a second
> op amp as a buffer or better yet using a transistor as an emmitter follower.
> The emitter follower can get the output down to 0 V on a single supply
> system. I loose some of the top end - I can only go up to Vcc - 1 V.
> This is usually not much of a problem as control voltages don't need to go
> to the positive rail.
>
> >So I would get a linear response instead of an RC response.
> >With careful design you can get around the output current problem
> >(scale up impedances, etc...) but sometimes cheap and
>
> I started a few years ago with tube amps. Output impediances were
> always critical. I created a spreadsheet to model outputs and inputs
> so I just optimize impediances on everything. I guess it comes from
> starting in RF.
>
> >The 1K resistor shows up in series with the input of the next stage... with
> >100K summing inputs that would be 1% error... not too good for a CV.
>
> I haven't built complex filters or VCOs yet. My oscillators are long peices
> of wire on a board in the hand of a guitarist and in my hand ( a bassist)
> as long as the control voltage changes the output in the way desired we're
> happy. As you said sometimes quick and dirty is good.
>
> >I think its time for an OPAMP primer here... whaddya think???
>
> I liked the idea of a FAQ. An op amp primer is good - some of this
> discussion may be beyond primer level.

I'm thinking of going over a typical data sheet, and explaining in simple terms
what
each spec is and why oyu should or shouldn't care...

H^)

>
>
> THANKS H^)  harry (and all)
>
>  Tim Daugard