From: META000 <META@UNB.CA>
Subject: Re: Onboard preamps????
Organization: The University of New Brunswick
Date: Fri, 2 Oct 1992 20:55:53 GMT

In article <1992Oct1.191336.24316@col.hp.com> mikem@col.hp.com (Mike McTigue) writes:
>osminer@spot.Colorado.EDU (OSMINER MATTHEW COREY) writes:
>> Well as always the onboard preamps just don't have the slew rate
>> to stick with those cruisin licks.  I must agree that an onboard
>> FET is much better and cleaner (not to mention a little more powerful)
>> than the OPamp circuits.  About the only semidecent thing you're
>> gonna get out of a opamp IC is active tone controls and even these
>> aren't the greatest.  I have a schematic for a really good
>> amplifier in general ( if I can find it).  Anybody interested?  and can
>> anybody read PSPICE notation?
>>
>> laters..
>>
>
>Whoaaaa! Not enough slew rate?? Well, I built a preamp for both
>my guitars awhile back and didn't even check the slew rate because
>the signals comming out of a pickup are not very large. I used a
>LT1012 micropower opamp. Lets make some worst case assumptions and
>see what kind of signal and slew rates are needed.
>
>1. Assume the highest frequency signal is 10khz. This is probably
>   way overkill since most guitar amps/speakers have a bandwidth
>   of only 7khz or so.
>2. Assume that this 10khz signal is at the full amplitude of the
>   pickups output. This would never really be the case since the
>   10khz signal would really be a harmonic of a lower frequency
>   fundamental and would be much lower amplitude, but lets just
>   use this number knowing that is assumes the worst case.
>
>If this 10 khz sine wave is comming out of the pickup, then the
>equation looks like
>
>                  A * sin(wt)   where A=amplitude and w=2*pi*f
>
>The equation for the slew rate is the derivative of this equation
>versus time or
>
>                  A * w * cos(wt)
>
>The max slew rate would be where cos(wt) is = to 1 , so this is
>
>                  A * 2 * pi * f  =  max slew rate
>
>The slew rate for a LT1012 opamp is spec'd at .1V/uS. So calculating
>backward to get what A would have to be to hit the max slew rate
>
>                  A = (.1V/uS) / (2 * pi * 10khz) = 1.59 Volts !!!!!
>
>I don't believe a pickup puts our anywhere near this kind of voltage
>level. I did some measurements once on my strat and the output was
>more on the order of 10's of mV, maybe 100mV on a hard transient.
>Still plenty of headroom, even if hotter humbuckers are used.
>
>Of course this analysis assumes no gain in the opamp preamp, but I
>really don't want any. I just want a buffer amp to drive the cable
>capcitance going to the amp. However, from looking at the above,
>if you do want so gain, there still is plenty of headroom. Plus,
>the LT1012 is a pretty whimpy opamp. I'm sure there are other opamps
>that consume low power that have much more slew rate.
>
>One final note. I've had my strat for years and I was not able to hear
>any degradation in the sound quality when I installed the preamp.
>In fact, it now is even better because when the volume on the guitar
>is turned down, I don't lose the high end response as all passive
>controls in guitars will do (yes you can hookup a capcitor from the
>pickup to the wiper on the pot, but this is a kludge whick now give
>you a uneven frequency response versus volume setting).
>
>Let me know if anyone sees a flaw in my thinking on the opamp circuit,
>but I'm willing to bet that it will not slew rate limit when buffering
>a pickup signal unless it is designed with lots of gain.
>
>
>
>
>                  Mike M
>
>.
>.
Although the math is nice to have worked out, the lesson is simple:
"slew rate" is a measure of the capacity of a device to produce
voltage swing per unit of time.  If you need lots of voltage, but
don't have demands for wide bandwidth, OR, if you require large
bandwidth, but don't need much in the way of voltage swing, then
devices with very modest slew rate will do very nicely thank you.
In the case of op amps used for guitar pre-amps, neither the voltage
swing required, nor the bandwidth are an issue - pickups have a very
modest bandwidth (10khz on a good day with the wind blowing), and
most pre-amps don't provide much gain.

Now, if we were talking about op-amps to use in output amps for CD
players, THERE's where slew rate makes a difference!