LM308N vs...

[DavenPaget]

Just a thought - if the slewrate is the big thing, an LM324 might be worth a look. A 301 is closer to an 071 than a 308. Hang on, a 741 is also slowish, and not popular for a RAT if I recall correctly. Thus proving there's more than just the slewrate....

Why ? Because the slewrate can be adjustable to liking . The limited slewrate by the common 30pf cap causes a ghostly sound in the background , that's what the RAT is famous for .

[seedlings]

What's a 'famous song' featuring the ProCo Rat?

CHAD

[DavenPaget]

What's a 'famous song' featuring the ProCo Rat?

CHAD

Don't know .

[ashcat_lt]

Holy parallel threads! 

The lm324 will swing about 1.2 volts further in the negative direction than positive.  The clipping from the opamp will be asymmetrical.  I don't know if this is a noticeable difference, really.  The two rat based circuits I've built around lm324 sound pretty cool.  Don't know about any "ghost sound", though.

[Mark Hammer]

As a matter of fact, if you look at the Boscorelli book, the Distort-o-matic IV exploits that property of the 324 by deliberately cascading four sections of it as a clipping element.  How on earth four unity-gain inverting op-amps give you clipping is beyond me, but then Boscorelli notes that it won't be an especially strong effect. 

[StephenGiles]

Just a thought - if the slewrate is the big thing, an LM324 might be worth a look. A 301 is closer to an 071 than a 308. Hang on, a 741 is also slowish, and not popular for a RAT if I recall correctly. Thus proving there's more than just the slewrate....

I had a spare 324 amp when building a VCF a few years back, so I used it for a Rat which sounded almost identical to my 308 version.

[StampWah]

Hi All ! I'm looking for dual version of 308. Any thoughs?

[PRR]

Welcome.

There never was a dual LM308A.

[Rob Strand]

This idea isn't so well known.

[Rob Strand]

324

358 is the dual version.

[amptramp]

All of us rejoiced when the LM308 came out.  Now we had an op amp with less than 0.5 mV input offset voltage and low drift as well.  We could do DC-coupled circuitry like absolute value circuits without needing offset adjustments and that was a good thing because offset adjustment is possible but not really catered for.  But not much of this is important in stompbox circuitry where AC-coupling is the norm.  But there were a few things you had to take into account in a design:

1. There were back-to-back diodes between the inputs so a circuit that pulled the inputs to different voltages had to have a resistor on the input to avoid burning these diodes (really, diode-connected transistors).  This is necessary because the input transistors are superbeta types with a very thin base.  The hfe might be around 1000.

2. Input common mode range was within one volt of the rail in either direction.  Not quite rail-to-rail but close.

3. The output would swing to within 1 volt of the rail with a load of 1.4 mA.

4. The supply current was 0.3 mA to a max of 0.8 mA.  Not bad and better than a lot of op amps.

5. Bias current would usually be 1.5 nA.  More than FET amplifiers but much less than other contemporary bipolars.

6. Don't skimp on the compensation.  For audio, feedforward compensation is better because it gives you more bandwidth but the standard compensation (both versions) works and keeps the bandwidth down if that is what you want.  I once had a circuit that required 100 pF compensation to ground and it still oscillated.  The problem?  The capacitor actually measured 91 pF.  A true 100 pF and above fixed it.

7. If you rely on an uncharacterized property (like gain-bandwidth), you will get different results for every device which goes against why you use op amps in the first place.

[PRR]

LM308 had lower bias current than contemporary JFET inputs at high temperature. Some folks need it hot.

The lingering 'feature' of the LM308 is its poor speed which softens audio clipping. There's a billion other ways to do this. But its use in some Classic Pedals ensures a market long after it could have been forgotten.

[rankot]

And what is real slew rate of LM308? I have two datasheets and none of them mention it.

Looking at open loop freq. response for LM301, LM308 and LM324 gives almost the same result (55dB@1k) with 30pF compensation cap.

UA776 could be a good candidate (slew rate is 0.8V/ms), but datasheed doesn't contain open-loop freq. resp.

[Rob Strand]

And what is real slew rate of LM308? I have two datasheets and none of them mention it.

The value in my head is 0.3V/us [Edit: maybe 0.03Vus] maybe for C=100pF?.   However it depends on the compensation cap.  There a lot of info scattered in the National Semiconductor Linear Applications Handbook.   You can derive it from the Large Signal Response graph  SR  = 2* pi * f_max * Vpk_max /1e6 [V/uS]; use Vpk not Vpk-pk.

One of the reasons for having uncompensated opamps is you can tweak the compensation cap to get more speed.   If you have a high gain circuit you don't need such a large compensation cap for stability (against feedback loop oscillation).  So that means you can get a higher gain-bandwidth and higher slew-rate.    Many designs just use the unity gain value.    IIRC, Philips/NXP NE5534 datasheet and associated applications notes had some nice info about that.

----------------
LM108/LM108A Slew-rate calculation from "Large Signal Response" graph

For C = 30pF,
Vpk_max = 14.3Vpk,
f_max   = 1.4kHz
SR = 2 * pi * Vpk_max * f_max / 1e3 = 2 * pi * 14.3 * 1.4e3 / 1e6 = 0.13 V /uS

For C = 100pF estimate SR = (30p/100p) * SR(30pF) = (30/100) * 0.13 = 0.038 V/uS

[PRR]

> And what is real slew rate of LM308? I have two datasheets and none of them mention it.

Read the pictures. "Voltage Follower Pulse Response".

[Rob Strand]

Read the pictures. "Voltage Follower Pulse Response".

Actually that method will match the SR value quoted in most datasheets.