From what I have seen they run $8-$12 a piece and are not found in most places (like Small Bear).
Is there a good alternative?
Demand - 10 years ago I couldnt figure out why anyone would bother to build or own a rat without the LM308. The chip had gone obsolete but I was able to buy them handily for $1 a pop IIRC. Never had a rat put into my hands for repair or mods that left the bench without a socket and the venerable LM308 in place.
Nowadays supplies have began to dry up and the pedal mod/boutique build craze have driven prices up. Consider the history of "vintage" JRC4558 chips and you will see that what was once a cheap and plentiful part used in tons of products is a buzzword magic mojo tone ingredient that players will spend good $$ for. Anyone sitting on a cache would be insane to not take advantage of this and trickle them out on ebay at 1000% mark up.
Unfortunately in this circuit, I am of the opinion that nothing sounds quite "right" except for the LM308. This is a hotly debated topic and many have claimed that there is no discernible difference between chips, especially when the compensation cap is adjusted for the chip. There may be some truth to the latter but I have spent countless hours with this circuit swapping chips and always came back to the LM308. The way the distortion breaks up with this chip past one o'clock on the dial is something special.
As far as subs many single channel opamps can be tried. OPA134, NE5534, and LM301 all sound good. Current production Rats use OP07 I believe. I would recommend installing a socket and socketing the compensation cap as well to play around. For myself, the last rat I built got a LM308 metal can from Small Bear. I figured if I was going to have to pay magic unicorn prices for a chip I may as well go all out - it was $7 or so I believe.
It would depend on what you want to use them for. And if you want to use them for a Rat, it would depend on what aspects of a Rat appeal to you. As for cost, they ceased production some time ago. Why? Because the gain-bandwidth product was poor, and better op-amps were developed.
At max gain/drive, the Rat circuit asks for gains into the thousands; way more than the poor chip is capable of, especially with a 9V supply. Because of the very limited gain-bandwidth product (i.e., how much gain can be provided at any given frequency, sometimes called open-loop gain), it attenuates the upper mids and highs (i.e., it simply can't provide the gain at those frequencies) in a way that works for the pedal.
This thread - https://www.seymourduncan.com/forum/showthread.php?163048-The-LM308-OpAmp-and-the-difference-it-makes-in-the-ProCo-Rat-2 - suggests that precious little difference results from use of a different op-amp. If you're stuck, I'd suggest use of a similarly "lousy", but cheaper and more available, op-amp - a 741. NOte that the LM308 is externally compensated by that cap between pins 1 and 8. The 741 requires no such compensation.
Quote from: GreySuits on June 16, 2019, 03:45:34 PM
Is there a good alternative?
This LM301
http://smallbear-electronics.mybigcommerce.com/op-amps-2/?sort=featured&page=2
is inexpensive, sounds good and actually Is an LM301. I add that last comment because the scarcity of many vintage chips has resulted in a flood of re-marked garbage in the broker and eBay markets.
Here is the gain-bandwidth product from datasheets for the LM308 and LM301. Both can be compensated in the same way, but if I'm reading it right, the 301 has a little more gain available for mids and highs than the 308. Close, though. Whether that is audible in the context of a Rat circuit is a whole other question I can't answer.
(https://i.imgur.com/BGEvxge.gif)
You can use LM108's as well. They're the mil-spec version of the 308. Or, the Russian K140UD14. I've used both, and they work quite well. I've sourced them from Ebay for between $3 to $5 each (not Chinese vendors).
I was feeling compulsive and picked up a couple of the metal can Russian 140UD14 deals on ebay just now. Im curious to hear them, Ive never come across this part before. Hows the sound/similarities to LM308?
Quote from: digi2t on June 16, 2019, 05:50:50 PM
You can use LM108's as well. They're the mil-spec version of the 308. Or, the Russian K140UD14. I've used both, and they work quite well. I've sourced them from Ebay for between $3 to $5 each (not Chinese vendors).
QuoteThis LM301
http://smallbear-electronics.mybigcommerce.com/op-amps-2/?sort=featured&page=2
is inexpensive, sounds good and actually Is an LM301. I add that last comment because the scarcity of many vintage chips has resulted in a flood of re-marked garbage in the broker and eBay markets.
You might even be able to use a larger compensation cap to reduce the difference in sound. The Rat has LM308 + 30pF across pins 1 and 8. A LM301 + 100pF across pins 1 and 8 might come close to original. You would obviously do better by hand tweaking the cap but that means having an LM308.
Quote from: digi2t on June 16, 2019, 05:50:50 PM
You can use LM108's as well. They're the mil-spec version of the 308. Or, the Russian K140UD14. I've used both, and they work quite well. I've sourced them from Ebay for between $3 to $5 each (not Chinese vendors).
Also LM208 if you can find them. 108, 208, 308 are all the same chip meeting tighter tolerances. I once saw 208s right beside 308s for less than half the price.
Quote from: miketbass on June 16, 2019, 07:46:39 PM
I was feeling compulsive and picked up a couple of the metal can Russian 140UD14 deals on ebay just now. Im curious to hear them, Ive never come across this part before. Hows the sound/similarities to LM308?
Quote from: digi2t on June 16, 2019, 05:50:50 PM
You can use LM108's as well. They're the mil-spec version of the 308. Or, the Russian K140UD14. I've used both, and they work quite well. I've sourced them from Ebay for between $3 to $5 each (not Chinese vendors).
I've only tried the K140UD14A, which is (on paper) the equivalent to the LM108/208. The "B" suffix has similar spec's to the 308.
As for sound, as it is with these things, it's pretty much subjective. In my best hippie poetry; the Russian version is a scant more ragged, less focused, but with tinges of harmonic content in different frequency ranges than the 108/308. Of course, the surrounding cast of characters (guitar, cables, amp) will play a role too, but all things being equal, it really comes down to wish cork you prefer to sniff. Either way, you'll soon have the opportunity to crack that sucker yourself, and have your own sniff session.
What about a TL070 wit fitting cap?
Late Rats have OP07
From a perspective of having the same pinout and using external compensation, it will "work". To the extent that you view the "sound" of a Rat to stem largely from the clipping diodes, or from the op-amp properties AND clipping diodes, you'll get what you want...or maybe not. The TL070 is a much higher quality op-amp than a 308.
But, see here: https://www.diystompboxes.com/smfforum/index.php?topic=113437.0
Quote from: Mark Hammer on June 17, 2019, 12:54:48 PM
From a perspective of having the same pinout and using external compensation, it will "work". To the extent that you view the "sound" of a Rat to stem largely from the clipping diodes, or from the op-amp properties AND clipping diodes, you'll get what you want...or maybe not. The TL070 is a much higher quality op-amp than a 308.
But, see here: https://www.diystompboxes.com/smfforum/index.php?topic=113437.0
or here ;)
https://www.diystompboxes.com/smfforum/index.php?topic=55234.0
Quote from: Steben on June 17, 2019, 12:36:25 PM
What about a TL070 wit fitting cap?
Can you find TL070s? I've not been able.
I think it is fair to assume that if the old Rats didn't use the LM308, it would cost a fraction of what it does these days. Luckily there are indeed alternatives. I have done a little experimentation with the most popular alternatives, which resulted in my personal ranking (from best to worst):
LM301
LM308
NE5334
OP07
The LM301 and LM308 are so close that to my ear it makes no real difference unless you compare them directly in the same circuit. And even then I can only hear it if I take out the clipping diodes (best Rat mod ever in my opinion but that is beside the point). The LM301 is also slightly less noisy. The NE5334 needs a significantly larger compensation cap and is NOT PIN COMPATIBLE with the LM308. 60-100pF are a good starting point. It sounds very different and is much less noisy than the LM308 or LM301. Really a matter of taste. For high-gain sounds with the diodes engaged, I actually like the NE5334 best. Why anyone at Proco would think it might be a good idea to use an OP07 is beyond me. The compensation cap does nothing on this chip (see datasheet, although the new Rats seem to still have the 30pF cap, correct me if I'm wrong) and it sounds plain awful with the diodes disengaged. Again, a matter of personal taste but I really don't like it. With the diodes engaged it sounds kind of similar to the LM308 but definitely less so than the LM301. I did not even bother to try the TL070 because of how terrible I find the clipping of the TL072. Or any JFET input opamp for that matter.
Cheers and happy experimenting,
Andy
EDIT: I just remembered that I reported on my Rat experimentations here:
https://www.diystompboxes.com/smfforum/index.php?topic=121671.0;topicseen
And once we're at it: try the Big Muff clipping mod, it's delightful!
I wish I could find it, but there was a portion of an interview with the fellow who came up with the Rat in Art Thompson's "Stompbox" book, reprinted in an old issue of Guitar Player magazine. I have about 20+ years' worth of the mag in the basement and am disinclined to thumb through them all at the moment. The fellow notes that he mistakenly used a smaller ground-leg value than he intended to, turning the gain waaaayyyy up, and was pleasantly by this "woooo" undertone. As I understand it, that results from the 308's limited slew rate and lower gain-bandwidth product. Now, depending on how much you value the "woooo", a 308 may be essential...or not.
The LM308 was designed for low input offset voltage and low bias current and was the first of the op amps to use super beta transistors for the input stage. The noise performance was not great - it never gets better than 30 nV/(Hz)0.5. The frequency response was "old school" with limitations on audio gain at moderate gains. We used a number of them where gain accuracy at DC and low (audio) frequencies was important and input impedances were higher than other contemporary op amps could handle. The loss of gain at high audio frequencies works well with some effects.
On the good side, it operates between ±2V and ±20V rails, so that works well with the stuff we build. The input common mode range goes from 1 to 1.5 volts under the rails and that is better than a TL071.
Not many people used them in their heyday and they were never cheap. We could afford them in military products but we knew other better stuff was coming, so it had a "best before" date that accounts for limited production.
My limited experience: I've used NOS metal can Motorola LM308AH, and they sound quite different than the DIP chip. Not necessarily better or worse, but very clearly different. Later I picked up some new(?) National Semiconductor metal can LM308AH (are those no longer made?) and they sound exactly like the DIP version. Small Bear has some NOS Fairchilds, I'm curious how those sound.
OT: I once had motorola "MC" 1741 and 1747 (14pin dual "741") and they also sounded a bit different to other makes of 741 (that more or less sound similar).
But yeah; 308's are so expensive exactly because they are overpriced!
Get that? ha ha
How about trying one of the discrete op-amp designs floating around to Rat ciruit and then tweaking that discrete op-amp to get somethin interesting?
Quote from: Mark Hammer on June 17, 2019, 04:52:40 PM
I wish I could find it, but there was a portion of an interview with the fellow who came up with the Rat in Art Thompson's "Stompbox" book,
That story is also on this website :
http://thermionic-studios.com/node/80
(starts half way the page)
Quote from: j_flanders on June 19, 2019, 07:22:46 AM
That story is also on this website :
http://thermionic-studios.com/node/80
(starts half way the page)
"5.) The Rat uses a "Darlington Pair" of transistors for "pop-free" function when switching the pedal on-and-off"
?
Quote"5.) The Rat uses a "Darlington Pair" of transistors for "pop-free" function when switching the pedal on-and-off"
?
[There's no BJT's: See replies 23, 24, 25]
It was a scheme used on some RATs to get an LED function without a 3-pole switch. IIRC, it was on the RAT2 and some others.
It's not pop free it adds a small amount of pop due to a small DC voltage. The smaller the source impedance the lower the DC voltage and the lower the pop.
The left hand circuit is the Rat circuit. The right hand circuit is the "Transistian Bypass" I did in 2002. The DC current on the Transistian bypass is much lower than the Rat and reduces the pop quite a bit.
(https://i.postimg.cc/Y4b4CNg7/transistian-bypass-ckt.gif) (https://postimg.cc/Y4b4CNg7)
(https://i.postimg.cc/4HhnPr04/transistian-bypass-connect.gif) (https://postimg.cc/4HhnPr04)
RG's Millennium bypass has less current and less pop.
(The Millennium pre-dates the Transistian. The aim of the Transistian was to be a better BJT type ckt and not something to compete with the Millennium which uses JFET.)
RAT2s have never had any BJTs let alone as a darlington - only JFETs (plus the op-amp). So its bypass was millennium style (I think the millennium was inspired by the RAT bypass, predated at least). I *think* you are saying JFETs can't make a darlington pair, which is what I thought, but please correct me if I am wrong - this doesn't count as a darlington pair I don't think:
(https://i.postimg.cc/dDHVhdsF/Multi-RAT.png) (https://postimg.cc/dDHVhdsF)
QuoteRAT2s have never had any BJTs let alone as a darlington - only JFETs (plus the op-amp). So its bypass was millennium style (I think the millennium was inspired by the RAT bypass, predated at least).
That's quite possible. It could have been the Turbo Rat, or the whole idea of a Darlington in a Rat could be wrong. Somewhere in history there were schematics showing Darlingtons [That schematic was in fact labelled Turbo Rat]. I don't know if I ever verified that to be true - it's a long time ago [I have a feeling not, as my trace has a "?" against that part, so I suspect I could not see the part number.]
I'm not the only one to think this because RG's Millenium page also shows a "Rat bypass" with a Darlington,
http://www.geofex.com/Article_Folders/Millenium/millen.htm
It's quite possible a dodgy trace distorted reality back then - it's not the first time with this stuff. I know there's a few guys out there who put in the hours documenting all the Rat versions. They had web pages up at some point. Those are the guys who would probably know sure (maybe you are one of those guys ;D)
QuoteI *think* you are saying JFETs can't make a darlington pair, which is what I thought, but please correct me if I am wrong - this doesn't count as a darlington pair I don't think:
No. I think the problem is distorted history. (Technically the term Darlington only applies to BJTs, but that's just dogamatic BS!)
FWIW, I do have a trace of a Rat 2 done in 2007. It's for a 90's "Rev L" unit with pcb 1986 and it shows the JFET with the 10M pull-up. So that would support your case.
Edit: Added some qualifications in [] above.
There's no doubt about the fact that any RAT (Turbo included) that had a status LED and DPDT bypass switching used the FET. This was only ever on the RAT2 and Turbo (maybe early YDR but don't think so). No darlingtons ever. Where the darlington idea came from is the only mystery. Possibly some kind of prototype or common (commercial?) mod - who knows.
QuoteThere's no doubt about the fact that any RAT (Turbo included) that had a status LED and DPDT bypass switching used the FET. This was only ever on the RAT2 and Turbo (maybe early YDR but don't think so). No darlingtons ever. Where the darlington idea came from is the only mystery. Possibly some kind of prototype or common (commercial?) mod - who knows.
I'm taking your word for it.
I believe the Turbo Rat schematic with the Darlingtons came from GFR. I think the site was Plate-To-Plate. It was early on in stompbox internet history - perhaps early 2003.
(https://i.postimg.cc/PJ84mjpD/myth-busted.jpg)
Another thing I noticed about GFR's schematic was a 100pF compensation cap (pins 1 and 8 on the opamp). From your MultiRat schematic it appears the cap was always 30pF or 33pF.
I've seen a few "build anomalies". I suspect that at times QA was not especially thorough at least with part values, and possibly parts organization as well. I also suspect they substitute parts for something close when they run out. My friend's Vintage RAT had BAT41 clipping diodes in the glass TO-35 package - looked just like 4148s - even shared "41" on the side. Maybe they ran out of 30p caps so used 100p, or somebody put 100p in the 30p bin. The "Soulsonic" RAT2 schematic shows 100R for the Vin resistor. An occasional deviation from the "stock" RAT circuit isn't all that rare IMO. I think the one that I find the most interesting is a blackface small-box RAT box with a RAT2 circuit board without the LED circuitry - I've seen a number of those on eBay. I think they must have run out of original RAT PCBs before they ran out of blackface boxes. Waste not want not.
I have an LM307. I wonder how it will work compared to the LM308?
QuoteI've seen a few "build anomalies". I suspect that at times QA was not especially thorough at least with part values, and possibly parts organization as well. I also suspect they substitute parts for something close when they run out. My friend's Vintage RAT had BAT41 clipping diodes in the glass TO-35 package - looked just like 4148s - even shared "41" on the side. Maybe they ran out of 30p caps so used 100p, or somebody put 100p in the 30p bin.
Interesting stuff. The diode one is pretty crazy. Hard to know how this stuff gets in. I suppose there's mild motivation to "do the wrong thing" with the 30pF if it's out of stock since they already use the 100pF. I did a bit of a scan over some PCBs. One thing I can see is the 30pF often looks different to the 100pF; sometimes a different size/style package, other times it has a black top indicating NPO. The opamps were LM308N and LM308AN, typically Nat Semi and sometimes Motorola.
QuoteI have an LM307. I wonder how it will work compared to the LM308?
To me the internal circuit of the LM307 looks more like an LM301A but some aspects of performance look like the LM308. I didn't study the data sheets too much matching the fine points though. Then I found this which saved me thinking about it too much:
It pretty much says the LM307 is an LM301A with a 30pF compensation cap,
https://www.analog.com/media/en/technical-documentation/data-sheets/lm101a.pdf
ROB, thanks for the quick response! I will try to put in PAT LM307.
Hi to all!
I'm planning to built a Rat and I bought an LM308H metal can. Which is the differences between this and the LM308AH version?
Thanks
Herr Strand! I'm looking at your transistian bypass and liking it; it seems like a perfect candidate for a no-frills cost-efficient smd module. With "c" group trannies like BC858C, 848C it would be possible to improve performance ("poppiness" ?) even better: quick sim shows that with a R1 at 6M8 it would be possible to obtain a 250uV (0.25mV) "pop" into a nominal 100k Rout (aka "pot"); and something like a 0.5mA current for the LED (not much, but could work).
If a darlington is used as the NPN, you can have all the output current you need for the LED; and with a 10M resistor at R1, you're sitting at 150-180uV of "pop" into 100k nominal Rout.
If I add a nominal 1K/1uF source Z to the nominal Rout, a sim shows transitional states so smoothed out it's in single-digit uVolts...
neat
QuoteI'm looking at your transistian bypass and liking it; it seems like a perfect candidate for a no-frills cost-efficient smd module.
It does work quite well. It is much better than the RAT circuit. I remember at the time some guys were having trouble getting JFETs so that BJT circuit was like a better wheel to the BJT RAT.
QuoteWith "c" group trannies like BC858C, 848C it would be possible to improve performance ("poppiness" ?)
Definitely will help the cause. Every step you take to get the DC voltage drop down helps. For 100k it's OK but for 10k it's really good. So if you can push for a factor of 10 reduction anyway you can it's definitely worthwhile.
QuoteIf a darlington is used as the NPN, you can have all the output current you need for the LED; and with a 10M resistor at R1, you're sitting at 150-180uV of "pop" into 100k nominal Rout.
Yes the Darlington does work. I might have mentioned that in my old posts back in 2002.
A MOSFET for Q2 will also help. It kind of goes against a BJT solution but N-channel MOSFETs are more readily available than they were 20 years ago.
IIRC, RG had an outright P-channel MOSFET solution for the Millennium idea. P-channel MOSFETs might not be easily obtainable by everyone.
I got that totally wrong; must have been thinking of the reverse polarity ckt. RG's MOSFET version uses an N-channel MOSFET. So given it gets good performance already it's probably silly changing Q2 on the Transistian to a MOSFET. Not sure how the MOSFET capacitances factor in.
QuoteI add a nominal 1K/1uF source Z to the nominal Rout, a sim shows transitional states so smoothed out it's in single-digit uVolts...
I have a feeling the cap size affects the audibility of the pop. If you use a tiny cap the output cap changes quickly but if you use a large cap the output changes slowly and that hides the pop. I think that's why I have the 10uF in my second pic. [Edit: A massive cap will still produce a pop.]
It's up to you which way you go. There's room to squeeze a bit more out of it.
Quote from: mickeybellinello on June 21, 2019, 08:04:23 AM
Hi to all!
I'm planning to built a Rat and I bought an LM308H metal can. Which is the differences between this and the LM308AH version?
Thanks
Same chip, but typically the 308A has tighter tolerances than the 308 for some specs like input offset voltage - but that may not be true for all manufacturers. Check the datasheets.
Rob - if I thought a mosfet has merit in your topology, I'd say it already. The good part imho is that the second bjt (or darl. or sziklai-even better) keeps the first bjt Vce at maximally 1 or 2Vbe above the LED voltage at all times, making it around 2-3V for the "simple" LEDS (R G Y), or a bit more for the "fancy" colors. This of course minimizes the spill of bjt parasitics into precious audio lines... a mosfet would completelly negate this aspect.
Second, although say a 2N7002 is cheap, two 848's are cheaper...
Third, you can use the lowest grade/cheapest npns for the darlington config and still get all the oomph you need.
Quotekeeps the first bjt Vce at maximally 1 or 2Vbe above the LED voltage at all times, making it around 2-3V for the "simple" LEDS (R G Y), or a bit more for the "fancy" colors.
There was some deliberate motivation for doing that. By keeping the Q2's base at the lowest possible voltage the base feed resistor can be high has possible so when Q1 pulls Q2's base to ground Q1's collector current is as low as it possibly could be. The whole motivating being to keep the Q1's base current low and reduce the pop. It also helps a bit when the battery voltage drops.
QuoteThird, you can use the lowest grade/cheapest npns for the darlington config and still get all the oomph you need.
I remember thinking at the time three transistors was a bit too complicated for an LED driver circuit! However, with SMD parts being small and cheap it's not a big deal. At the moment the circuit works fine but with a factor of 10 or more improvement it would take away the pop risk altogether (at least from a practical perspective).
I mentioned smd could be made a small and effective module; with 3-bjt/2-resistor layout (or 3-resistor if combining the large values). With possibly quite acceptable (or even respectable) performance.
But don't get tempted to "R&D for free on the Internet", heh heh.
Quote from: GGBB on June 22, 2019, 12:37:51 AM
Quote from: mickeybellinello on June 21, 2019, 08:04:23 AM
Hi to all!
I'm planning to built a Rat and I bought an LM308H metal can. Which is the differences between this and the LM308AH version?
Thanks
Same chip, but typically the 308A has tighter tolerances than the 308 for some specs like input offset voltage - but that may not be true for all manufacturers. Check the datasheets.
Here I Am!
Thanks so so much!