LM308 in Tubescreamer build

[PBE6]

Has anyone tried using an LM308 in a Tubescreamer build? I've been asked to make a TS variant, and I have a bunch of 4558 opamps lying around, but since the "magic" of the 4558 probably has to do with its lousy slew rate, I was wondering if anyone had tried it with the perennial slew rate slacker king of opamps?

[Mike Burgundy]

Go for it and let us know!

[Mark Hammer]

I can't see any advantage.  The 308 is "magic" in a Rat because of the bandwidth limitations at the sorts of outrageous gain settings the Rat produces.  The TS hits max gain at x118. 

I suppose if it might sound a tad different, the way that 1458s, TL072s, and other chips would.  BUt I wouldn't expect anything startling.

Do keep in mind that a) it won't plug into any existing socketed board because it's a single op-amp not a dual, and that b) you'll need to figure out what sort of compensating cap value you want/need to use.

[PBE6]

This will be a build from scratch. I guess I'll have to breadboard it first to see.

Another reason I suspect it might sound interesting is that the feedback diodes will mix in the original signal with the diode hash. If the LM308 clipping sounds cool on its own, it might be an interesting flavour alternative to the 4558. Of course, it might just sound really dull, won't know until I try I s'pose.

The compensating cap will be interesting. Just looking at the datasheet I see the lower cap value is 3pF, is this a hard lower limit or can I leave it out altogether for the brightest sound?

EDIT: also, is there any issue with putting two caps in parallel, with one of those caps in series with a variable resistor, to make a semi-tunable faux-cap compensator?

[Gus]

The only time I can tell a differences with changing opamps in a STOCK TS circuit is when the tone is full treble and the gain is all the way up AND even then it is a very small difference, who uses a TS like that?  I can only hear it solo in a quiet place and sometimes I can't pick it out, now live with a band or on a recording I don't think anyone could hear the opamp change.

Now EQ change can be heard in TS. 

[bool]

Some poeple dig the rusty old NE5532 in a TS. But that thing eats batteries quite a bit faster. So if you have the box powerd by an adaptor, try a 5532.

For the feedback cap, I'd try progressively 33pF, 22pF and a 10pF.

For the smaller cap values I'm sure you will want a low-noise chip to try out just how "open" you can go because going small wil un-mask the self-noise gain of the circuit as well as amplify the incoming noises (at the higher "gain" settings).

[matlevo12]

Hey everyone.
I stumbled upon a "old" psk boss clone, which turns out to be a rat with asymetrical soft clipping, with buffered bypass... or a sd-1 using a LM308, if you'd prefer.

I tried to find some other pedals like this one, but I couldn't, does anyone know about some already existing overdrives using lm308 / rat with only feedback diodes clipping?

Thanks!

Here are some pics, and a schem from what I traced :

[matlevo12]

I just found out about the Proco Brat / Roadkill pedal, and it's about the same one then the psk sod-2 from my last message, except that it has soft and hard clipping diodes.

[fowl]

I just found out about the Proco Brat / Roadkill pedal, and it's about the same one then the psk sod-2 from my last message, except that it has soft and hard clipping diodes.

The schematic you posted has soft and hard clipping diodes.

edit: I see now that the hard clipping diodes were left out of the actual pedal.

How does that thing sound BTW?

[matlevo12]

How does that thing sound BTW?

Pretty good actually, it sits somewhere between a "clean rat" and a turbo, it's quite interesting.

It really rounds up the angles of it, in my opinion the "clean rat" doesn't sound that good, it's too "farty".

It's like a really dirty overdrive with bass like a rat.
I'm going to try and use this kdf of clipping on one of the modes of a "double rat" project.

[iainpunk]

after my current run of pedals, im gonna build a rat since i never finished a rat pedal. i will keep this topology in mind

im using a different IC than the venerable 308... since i dont think the slew rate is low enough on the lm308, im just using a modern quad opamp, using the normal gain stage, but with soft clipping, and replacing the filter control with a slewrate limiter circuit, then followed by a hard clipper. this order of clipping is so i have the slew limiter before hard clipping, just like in a normal rat. the reason i want to do both hard and soft is because you really don't want to send the slew rate limiting circuit in to clipping, since that oscillates and sometimes decides to keep oscillating regardless of input.


cheers

[mateusborges]

Heya,

it's not that different but I really loved the JRC4560DX chips in the 2 TSs I built. I tried about 5-6 chips, including the original vintage and the BB ones and I liked the 4560 the most. I still have 4 here so I can send you one if you can't find.

Cheers!