Author Topic: Reliable Source for LM308s?  (Read 1514 times)

chromesphere

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Re: Reliable Source for LM308s?
« Reply #45 on: June 13, 2018, 11:39:04 PM »
Mainly ICs and Transistors.  Things that have a bit of worth. J201's, BBDS, etc.  Germanium transistors are usually ok.
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Philtre

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Re: Reliable Source for LM308s?
« Reply #46 on: June 13, 2018, 11:42:38 PM »
Banzai in Germany sell LM308s. How would one know upfront if these are genuine?

http://www.banzaimusic.com/LM308N.html

Adam_MD

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Re: Reliable Source for LM308s?
« Reply #47 on: June 14, 2018, 01:53:56 AM »
Banzai in Germany sell LM308s. How would one know upfront if these are genuine?

http://www.banzaimusic.com/LM308N.html

They’ll be real if they actually have them.  The problem with Banzai is they are either fantastic or fantastically useless.  Basically if you submit an order and they have everything you’ll get it in about a week from Germany to the uk.  If, however, one of the items is actually out of stock (though was probably showing as in stock when you ordered) they’ll hold the entire shipment and not contact you until you harass them by email 3 or 4 times then they’ll ship the parts they do have and either offer a refund on the outstanding one or ask if you want to wait and ship it separately. 

That’s been my experience. 

bsoncini

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Re: Reliable Source for LM308s?
« Reply #48 on: June 14, 2018, 10:26:47 AM »
I have to go here often for work. I can pick you one up if u want. http://www.stquentin-radio.com/?page=info_produit&info=5052&color=3&id=0&act=0

It's pretty sure they are legit. I've bought a few from them. €4 plus maybe another €2 for shipping.

Philtre

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Re: Reliable Source for LM308s?
« Reply #49 on: June 14, 2018, 10:37:01 AM »
I have to go here often for work. I can pick you one up if u want. http://www.stquentin-radio.com/?page=info_produit&info=5052&color=3&id=0&act=0

It's pretty sure they are legit. I've bought a few from them. €4 plus maybe another €2 for shipping.

Many thanks for the offer. But I'm going to stick with OP07s. :-)

bsoncini

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Re: Reliable Source for LM308s?
« Reply #50 on: June 14, 2018, 03:47:37 PM »
No problem. Let me know if you change your mind.

I've been playing in a group as a duo recently. Me on guitar and a drummer. I've been using this pedal a lot to have some more bass.

Philtre

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Re: Reliable Source for LM308s?
« Reply #51 on: June 15, 2018, 01:26:54 AM »
I've been researching LM308 vs OP07.

The tech sheet for the OP07 has pins 1 & 8 as an offset nulling option where it seems a variable resistor can be connected. Yet, on various forums discussing the ins and outs of OP07 vs LM308 some people swear that adding a compensation cap on 1 & 8 on the OP07 makes a difference to the sound (example - https://www.thegearpage.net/board/index.php?threads/rat-op-amp-chips.1326447/)

What do the experts think?

reddesert

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Re: Reliable Source for LM308s?
« Reply #52 on: June 16, 2018, 01:37:20 PM »
I mistrust thegearpage as a source for technical information, generally (although I don't read it often and may be a grump).

The LM308 is a really old op-amp. Back then, fabrication techniques made it hard to put an internal capacitor on the chip. So op-amps like the LM308 relied on an external compensation capacitor. The point of this cap is to avoid oscillations at high frequencies (above the audio band) that would make the op-amp unstable. For complex reasons, the amplifier has a phase shift that changes with frequency, and at the frequency where it is 180 degrees out of phase, the negative feedback would change to positive and cause oscillation. The compensation cap rolls off this behavior.

Later op-amps like the 741 and OP07 were made with a small capacitor on the chip, so the external compensation is not necessary and the pins are used for offset nulling (which is AFAIK something to use for precision DC differential amplifiers and irrelevant for audio circuits). If you compare the schematics of the LM308 and OP07 in the gearpage thread, the OP07 has a small cap C1 from V+ to the collectors of Q9 and Q10, while the LM308 has no capacitor on-chip.

For more information about the history behind this, look at section 4 of the History of Op-Amps book chapter at http://www.analog.com/media/en/training-seminars/design-handbooks/Op-Amp-Applications/SectionH.pdf. By the way, Sections 1-3 will happily lead you down a rabbit-hole of reading about early vacuum-tube and discrete op-amps. Here's a screen-cap of a footnote about the internal compensation capacitor:



I don't think adding a cap between the offset null pins of an OP07 or similar is going to do anything to modify the high frequency behavior, nor is it necessary.

However, I've not been convinced that the LM308's low slew rate is what gives it the mojo sound. Its slew rate is still quite adequate for audio signals.
« Last Edit: June 16, 2018, 01:39:14 PM by reddesert »

Philtre

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Re: Reliable Source for LM308s?
« Reply #53 on: June 16, 2018, 01:44:02 PM »
Thanks for that info, redderset, very informative. :-)

For my own part I've tried an OP07 and an LM741 and, to me ears, there's very little in it. I might try a LM301 at some point but I'm fast coming to the conclusion that it's just not worth the bother fretting over this, and just bung in an OP07 without a compensation cap and be done with it. I believe that modern Rats use them.

Boba7

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Re: Reliable Source for LM308s?
« Reply #54 on: June 17, 2018, 04:41:10 PM »
Very interesting thread. When I swapped an lm308 for an Op07 a few months ago I was surprised that I could barely hear a difference. And I’m pretty sure I took out the 30pf with the OP07 and it made no difference either.