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

[Tim Daugard]

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.

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)

>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.

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.

>>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?

>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?

I have been buying:

    LM324 - critical spec:  price
    TL074 -  critical spec:  got some free from TI and they worked well
                   in a guitar foot pedal, kept buying.
    NE5534- selected especially for a microphone preamp:
                   spec: 1) noise 2) supply voltage

>"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?

>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.

THANKS H^)  harry (and all)

 Tim Daugard