[sdiy] Op amp slew in audio circuits - sims vs. real life

[rburnett at richieburnett.co.uk]

High-speed op-amps are also typically a lot more finicky about things 
like layout and supply decoupling.  Modern current-feedback op-amps that 
Roman and Neil mentioned are amazing things when you need high gain and 
bandwidth into the 10's or 100's of MHz, but if you don't get the right 
type of decoupling capacitors close enough to the chip or you put 
ground-plane under the wrong areas, your amplifier can easily become a 
VHF oscillator!

There's a lot to be said for choosing an op-amp that is just good enough 
for the job, and no better.

-Richie,

On 2014-04-09 11:24, Roman Sowa wrote:
> Couldn't agree with you more.
> For most stuff I use less than 1V/us slew rate opamps, not only to
> prevent unwanted RF radiation, but also because there is plenty of
> models to choose from when you go down to that level at specifying SR,
> and they usually have better Vos, lower supply current, and so on.
> 
> High slew rate, which in synth stuff is about 10V/us is my choice for
> saw core VCO, few fancy wave shapers and I can't image anything else.
> I used to put my beloved AD818/817 everywhere long time ago, for
> example in 40MSPS sampler (telecom stuff, not synth related). That
> tiny bugger jumps at 500V/us (350 for 817) and still is not by far the
> fastest one.
> 
> Roman
> 
> 
> W dniu 2014-04-08 21:15, Tom Wiltshire pisze:
>> I've used genuine comparator chips sometimes when required (DCO
>> designs), but mostly for the open-drain output it has to be said,
>> rather than any clever high speed malarkey.
>> 
>> Other that than, no fancy op-amps. I go cheap-n-cheerful for op-amps.
>> The modern ones are good enough it never matters, at least to someone
>> with my standards. Call it "character" and be done with it. If you've
>> read as many 1970's stomp box schematics as I have, you'll think
>> we're living in component wonderland as it is. Who could possibly ask
>> for more??! Better than the 4558? How is that possible?!
>> 
>> Mostly I regard a limited slew rate as a benefit rather than a
>> problem. Who wants high-speed sharp edges chucking RF interference
>> all over their circuit anyway? Much better to keep everything slowed
>> down and audio frequency…but then I blew my hearing listening to
>> Hendrix when I was a teenager, and I don't care above 13KHz, and only
>> design stuff to work to 20KHz, like they did back in the 70s. This
>> idea that we should all have every detail to 25KHz is fairly new. I
>> don't think it's necessarily an improvement.
>> 
>> Hoping I win you over to my out-dated mode of working, Tom
>> 
>> 
>> On 8 Apr 2014, at 13:19, Justin Owen <juzowen at gmail.com> wrote:
>> 
>>> A general question...
>>> 
>>> I'm assuming most of us throw the TL072 or similar at most of our
>>> audio circuit op-amp requirements right?
>>> 
>>> Can any of you give some examples of where you specified an op amp
>>> with better/faster slew specs because standard op-amps weren't fast
>>> enough?
>>> 
>>> Asking because I'm working on a pulse output circuit (frequency up
>>> to 20KHz) that I'd like to shave every last µSec out of in order to
>>> get the sharpest response - but I'd also like a reality check on
>>> how much to worry about the slew and slight imperfections I'm
>>> seeing in sim (using Universal Op Amp 2) and how that will
>>> translate to real life.
>>> 
>>> Yes, at some point I will breadboard/protoboard it - just be good
>>> to get a general consensus before that.
>>> 
>>> Ta!
>>> 
>>> - J
>>> 
>>> 
>>> 
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