Why are LM308s so expensive and kind of hard to find?

[j_flanders]

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)

[GGBB]

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"

?

[Rob Strand]

"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.




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.)

[GGBB]

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:

[Rob Strand]

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).

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  )

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:

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.

[GGBB]

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.

[Rob Strand]

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.

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.



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.

[GGBB]

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.

[Stasss]

 I have an LM307. I wonder how it will work compared to the LM308?

[Rob Strand]

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.

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.

[Rob Strand]

I 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

[Stasss]

ROB, thanks for the quick response! I will try to put in PAT LM307.

[mickeybellinello]

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

[bool]

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

[Rob 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.

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.

With "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.

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.

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.

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...

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.

[GGBB]

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.

[bool]

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.

[Rob Strand]

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.

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.

Third, 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).

[bool]

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.

[mickeybellinello]

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!