Xotic - BB Preamp  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.

Postby analogguru » 16 Dec 2007, 17:56

Na, ist die Lust an einer Pokerrunde wieder zurückgekehrt ? :wink: :lol:
Danke für Deine Beiträge der letzten Zeit.

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Postby gtrwrks » 16 Dec 2007, 19:05

sosodef wrote:I have one of these BB preamps. I was never too too impressed with it. The main reasons are:

Its overly compressed. It doesn't respond well with dynamics of your playing/picking. It never really does that great of a boost a bit too dirty still. Lastly the gain becomes a bit too loose sounding as you get past say 10 oclock.

I have had thoughts of pulling out the heatgun and trying to make it a better pedal. Is this pedal ready for mods?


Yes - ready for modifications.

I really dig this pedal and have been using it live for the past 2 + yrs. Yes, a high-brow TS-type (at least in basic sound, no sparring with Paul C on this intended), but a very good friend to a Strat/Tele for cats who like the TS-type OD.

I started out by decompressing mine via clipping diode tweaks - currently digging 1N4001 against 1N4001/1N34 - this is fairly sym (in terms of what it could be), but is nice. I've tried lots of things here and most of my customers tend to like/keep 1N4001 against 1N4001/1N4148; some choose the SD-1 setup of a trio of 4148s.

The gain pot is 1M - I've lowered this to 500K for one cat - still not immensely helpful as far as obtaining a range from no gain/low gain/more gain. As much as I love and use this box, to my ears this circuit goes from 'clean boost' to 'crappy sounding od' to 'creamy od'. Only the two outer extremes work for me.

The input cap is .022 and I generally dislike this in OD boxes as it cuts too much bass (translating to thinned mids as well) - in all TS-types (and in the AC Booster) I like a .047uF or .056uF for the input cap (this is extremely helpful in the AC to add some subtle yet helpful mids). In the BB Preamp, the .047 added far too much wooly/woofy lows - really got shitty for me/my ears/my rig/et al. Some cats wanting maximum THUMP might like this tweak - too much for me. I sent mine back to .022, but might try .033 at some point.

The couple of 1uF NPs I swapped the electros out for SMF - no discernable difference to my stage-hardened ears.

Since Xotic is very crafty with the sandpaper and the ICs, I have no idea what those might be - but for most cats, I install a BB OPA2134PA for the secondary/tone stack IC and a socket for playa's in the drive section; do this for them in the AC as well.

I really dig the fact that this circuit acts as a completely clean boost with the drive @ zero. I've used this basically as an EQ on a couple of recordings - for fattening a 335-type.

Another usable tweak in the AC is to install a mini-toggle with diff clippers - ala' Grandlaff - but, I like to set it up with 'AC' on one side and 'RC' on the other for max versatility. I like 8 (4 opposed pair) of 4148s in the 'RC' side of the switch. Since the AC has a 500K gain pot and a stock RC has a 250K gain pot, the extra pair of diodes help to keep it in the realm of a std RC. 100 Ohm output res instead of the 470 stock.

For RC tweaks I like to add another pair of diodes to the 6 that come stock (FB Loop) for even less of a tendency for dirt - at this level of forward voltage, the clippers sound really hard/harsh to my ears - the less of that the better. The input cap is .047 stock on this one; 100 ohm output res instead of the 470 stock.

The new AC+ from Xotic has a clean boost on one side which I ignorantly take to be the RC circuit - a feature it has is a push/push switch that allows you to select between 'hard' and 'soft' compression (or some such - see their site for accurate nomenclature) - for several cats I've installed a mini-toggle to achieve this same thing - using 4148s for the hard side and 1N34s for the soft side.

I really like these 3 pedals and use them all in my live rig - but some basic tweaks can make each of them work/sound better.
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Postby gtrwrks » 19 Dec 2007, 00:27

...
Last edited by gtrwrks on 03 Jan 2008, 04:32, edited 1 time in total.
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Postby gtrwrks » 19 Dec 2007, 00:29

Strange that the software won't let me type k u s i's name... ?? Is this a curse word in some language I'm not familiar with?
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Postby marshmellow » 19 Dec 2007, 00:58

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Postby The Pup » 30 Dec 2007, 07:55

Thanks for the schematic links.

I actually own an Xotic BB...and it will soon be going up for sale or trade. It just never curled my toes.
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Postby PotentPotables » 02 Jan 2008, 19:16

Hey guys, neat forum here. KU$I, I've seen you over at DIYstompboxes, and you're a solid contributor. Thanks for your generosity.

I have a question about the schematics you have posted for the B.B. Preamp and the LA Boutique Screamer. What are the values of the resistors designated "200E" and "47E?" Just 200 ohms? 47x10^1 ohms? I've never seen this notation before and I had trouble searching for it. Thanks.
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Postby analogguru » 02 Jan 2008, 19:23

"E" is used in german countries to describe "Ohm". It is derived from the german word "Einheiten" which means: units.

"E" is used instead of the Omega-symbol to avoid confusion with 0 - which may occur especially on bad copies... e.g. "390" is this now 39 Ohm or 390 ? 39E instead gives a clear answer....

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Postby kusi » 02 Jan 2008, 19:25

hi,

yes, this mean 47 Ohm and 220 Ohm.

regards, kusi
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Postby PotentPotables » 02 Jan 2008, 20:11

Thanks for the very quick replies, gentlemen!
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Postby fuzzlab » 03 Jan 2008, 01:27

Hi. Someone else noticed that the schematic has a "little" mistake?
Is totally wrong to connect the feedback net (gain pot and diodes) from the output of the first opamp to the "+" input.
So... the inputs are mislabeled (interchange "-" and "+").
So... this is not an inverting clipper, just an non inverting TS type.
Please be careful, cause if you dont make corrections, this baby will oscillate like crazy (or, perhaps it could sound like a nasty fuzz).

For this message the author fuzzlab has received thanks:
ne0 (23 Jan 2013, 09:47)
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Postby snail » 03 Jan 2008, 12:09

my bad :oops: I must be getting paranoid!
Last edited by snail on 03 Jan 2008, 17:51, edited 1 time in total.
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Postby soulsonic » 03 Jan 2008, 12:22

Oh, fuzzlab is correct. Of course it is possible to have clipper and stuff on an inverting amp, but the circuit is clearly arranged as a non-inverter.
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Postby kusi » 03 Jan 2008, 12:33

k**i wrote:the L.A. Boutique-Screamer is a little bit modified...

here you find the original BB:
http://forum.musikding.de/cpg/displayim ... 4&pos=-424

(+ and - inputs of the gainstage are in wrong direction... ;)


regards, ksui


:wink:
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Postby analogguru » 03 Jan 2008, 14:08

Hmmm....on the picture of a degooped unit I have it looks that the diodes are connected to the -input, what would mean, + and - input are interchanged on the schematic.

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Postby kusi » 03 Jan 2008, 15:51

analogguru wrote:Hmmm....on the picture of a degooped unit I have it looks that the diodes are connected to the -input, what would mean, + and - input are interchanged on the schematic.

analogguru



hi ag, this is the thing i mean... sorry, my english is not very good... :roll:

should i correct the schematic?

regards, kusi
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Postby analogguru » 03 Jan 2008, 16:04

should i correct the schematic?


If you want to make the people happier.....then yes.

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Postby mibu » 03 Jan 2008, 18:44

hi guys! happy and joyfull new year!

I have done today some mods to my original BB Preamp. mounted a DPDT mini-switch with whom I can select the original diodes or the two 1N4001 recommended by gtrwrks in this thread (man, you are right - the 4001's sound the best if you wish to get rid of part of the compression).

an alternative (provided by Toshio himself - the maker of BB) is to use two 1N4148 diodes in series on each branch, to get some more gain and better S/N ratio...
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Postby gtrwrks » 04 Jan 2008, 02:39

Very cool, mibu - I do dig the 1N400X in this circuit.
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Postby Alex C » 11 Feb 2008, 05:44

Thanks for your efforts in this thread, gentlemen. Here's a build report for the Boutique Screamer.

-Alex
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