L.A. Boutique Screamer Question

DougH · January 06, 2009, 01:00:12 PM

http://ramalhais.com/files/amp/L.A.BoutiqueScreamer.PDF

I saw a nice demo of the BB Preamp on Youtube and got curious about this one. What is going on with that second stage?

I simmed the 2nd stage and the op amp portion and surrounding components appear to do nothing. I'm getting the same freq response from the LPF on the input as I do from the output of the opamp. And there's a slight attenuation of a db or so. (?!?)

oskar · January 06, 2009, 01:33:58 PM

Apart from the inputs wrongly marked on the first stage I see nothing special. Are you using switchercad?
It's allowed me to do pretty "unorthodox"

things with ground and DC sources without warning.

DougH · January 06, 2009, 02:45:12 PM

Just to be clear I'm referring to the 2nd op amp stage, with the 1k fdbk resistor, 19k/1k (what's up with that?) going to input and 220n/220 to ground.

It doesn't make any sense and the sim shows it having no effect. I'm using LTspice. BTW, the 10k going to 4.5v should be unnecessary since it is direct (dc) coupled to the previous stage.

Johan · January 06, 2009, 02:50:10 PM

it looks to me like a tiny amount of low freq. POSITIVE feedback...(below 130hz?) perhaps an attempt at controlled resonance( without being tuned to one specific frequency)?

johan

oskar · January 06, 2009, 02:53:54 PM

Filters isn't my thing and especially not this species with signal on both + and - input. I only see an ordinary tubescreamer filter stage with the two resistors substituting the pot for a fixed response and I don't know what that response is supposed to be. Sorry!
I've actually never touched a tubescreamer.

I like the schematics though...

DougH · January 06, 2009, 02:55:31 PM

Omigod, I see it now...

It's a hardwired TS tone control...

Evidently this thing was designed by taking a TS and slapping a baxendall (and gain stg) on the output. Then they swapped out the drive pot for a 1M for more gain...

Okay, mystery solved. Thanks.

edit: Looks like we posted at the same time!

Johan · January 06, 2009, 02:57:47 PM

..ok...feel stoopid now...

DougH · January 06, 2009, 03:01:31 PM

You're not alone...

I thought for a minute this might be something actually new and different. Funny how many things come back to this circuit.

The bax does make it sound nice though, at least from what I can hear in the vids.

oskar · January 06, 2009, 03:14:08 PM

I'm running LTspiceIV right now. Pretty addictive stuff.

Maybe I'll look into this filter too while I'm at it. I have a hard time trusting the outcome of the simulations though.
I don't trust them models...

DougH · January 06, 2009, 03:17:53 PM

Actually it works fine. If you change the 19k/1k to other values it works like a TS tone control. I just didn't understand that that's what it was. There must have been something about the rolled down TS tone control sound they liked.

mike_a · January 07, 2009, 11:07:46 AM

While we're at it, I have another question:

Why is the Level pot connected to 4.5v instead of ground?
It has coupling caps both before it and after it, so the signal should be centered around 0v.

JHS · January 07, 2009, 11:48:40 AM

Kusi altered the circuit a bit, not much. He used a std. pot for vol., not the double pot that's in the BB and dropped the half connected between the 3nd and the 3rd stage.

JHS

DougH · January 07, 2009, 12:42:08 PM

Quote from: mike_a on January 07, 2009, 11:07:46 AM
While we're at it, I have another question:

Why is the Level pot connected to 4.5v instead of ground?
It has coupling caps both before it and after it, so the signal should be centered around 0v.

Yeah, that looks like it should be grounded.

Ben N · January 07, 2009, 12:56:44 PM

Quote from: DougH on January 06, 2009, 03:17:53 PMThere must have been something about the rolled down TS tone control sound they liked.

OK, but is there anything about a fixed TS filter that can't be done just as well passively, especially with the active Bax following? I hate adding stuff (stages, complexity) for no purpose.

mike_a · January 07, 2009, 01:15:12 PM

Quote from: JHS on January 07, 2009, 11:48:40 AM
Kusi altered the circuit a bit, not much. He used a std. pot for vol., not the double pot that's in the BB and dropped the half connected between the 3nd and the 3rd stage.

JHS

I didn't understand exactly what you mean.
Do you have the original schem?

DougH · January 07, 2009, 01:18:37 PM

QuoteOK, but is there anything about a fixed TS filter that can't be done just as well passively, especially with the active Bax following? I hate adding stuff (stages, complexity) for no purpose.

Well... I didn't want to get into picking it apart but that was one thing about it that struck me as kind of "hacky"... It seems like whoever designed it plugged in their TS with the tone control set low, than plugged that into a breadboard with a gain stage and baxendall tone control on it. Voila, another "booteek" design...

If you look at the freq response of that stage, no, it doesn't appear there's anything "special" about it that couldn't be produced passively. But since another op amp gain stage was added maybe it was simpler to use another dual and implement a "fixed" tone control instead of really thinking about it. Whatever...

JHS · January 08, 2009, 06:09:45 AM

@mike_a
You should ask KUSI if he's willing to share the schem of the X-BB.

JHS

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L.A. Boutique Screamer Question