Bluesbreaker Questions

[swinginguitar]

Just perfboarded a BB clone. Schematic here: http://www.generalguitargadgets.com/pdf/ggg_mbb_sc2.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

Questions:

1) how would I slightly increase the gain (without destroying the character of the circuit)?

2) this pedal is very bright - are there any known mods for this? I assume I could tweak R12, C8, or C9...

3) On that subject, can someone explain the tone control on this to me? I recognize R12 and C9 as a LPF, but how does it interact with R9, R16, and C8?

[Mark Hammer]

Take a look at the "Aristocrat" project over at the Madbean site ( http://www.madbeanpedals.com/projects/index.html ).  It is his project for a clone of the Analogman King of Tone.  The KOT is essentially an improved Bluesbreaker.  It is easily made using any existing layout for a Bluesbreaker.  There will be some additional offboard components, but nothing that necessarily compromises the build quality.  I find that project to be less shrill than a Bluesbreaker, though admittedly one person's "shrill" is another's "cuts through the mix nicely"

Since Analog Mike is a helpful and generous member here, it is only fair to recommend purchase of a KOT from him.  The current model is a dual channel unit in a compact package, which may be trickier for some folks here to build.  Still, the madbean schematic will give you some ideas about how to tame your BB.

[amptramp]

Check out the funny series connection of C6-R8-C7-R7 in the schematic.  It just seems like someone was going to put a switch between the C6 end of R15 and the R8 end of C7.  This would help.  But the most interesting thing to experiment with would be C3.  Increasing it would boost low frequency gain.  There is lots to play around with here.

[Mark Hammer]

That's actually an error in the GGG drawing that needs correcting.  There is but ONE cap and resistor between the output side of the gain pot and pin 6 of IC1b.

The principle design differences between the BB and KOT revolve around the values of R2/C2/R3/C3.  These provide differential gain for different portions of the spectrum, though not as radical or instense as what happens in the Proco Rat, that also uses a divide-and-conquer approach.  The KOT aims for less gain and a gentler emphasis of the high end.  It also uses a larger value of C4 to roll off a little more top end and tame the fizzies.

[swinginguitar]

That's actually an error in the GGG drawing that needs correcting.  There is but ONE cap and resistor between the output side of the gain pot and pin 6 of IC1b.

The principle design differences between the BB and KOT revolve around the values of R2/C2/R3/C3.  These provide differential gain for different portions of the spectrum, though not as radical or instense as what happens in the Proco Rat, that also uses a divide-and-conquer approach.  The KOT aims for less gain and a gentler emphasis of the high end.  It also uses a larger value of C4 to roll off a little more top end and tame the fizzies.

Can you give me some more detail/background on the so-called divide-and-conquer approach?

[Gus]

The .22ufs and  4.7ks are left over from the stock blues breaker trace the switch grounds the center of the 4 parts as a kind of mute.

This is not a high gain effect it is made more to "push" a turned up tube amp IMO a more useful effect than a TS

You can make the network a 10K and .1uf OR if you do want more gain use one .22uf and one 4.7k.  Close to same RC

Or you can do a search of this and other forums.

[Mark Hammer]

Can you give me some more detail/background on the so-called divide-and-conquer approach?

Sure.  There are two paths to ground from the inverting pin of that first op-amp.  One is 4k7 and the other is 3k3. The gain of the op-amp is set by the ratio of [pot-value + 4k7]/4k7 and [pot-value+3k3]/3k3.  The 4k7 path goes through a .01uf cap to ground.  The 3k3 path goes through an additional cap, resulting in a higher rolloff point.  This ends up providing more gain for the top end than for the bottom end.

In a sense, this is the same overall strategy the TS uses, except that it rolls off the low end before applying a uniform gain to the entire signal.  In the BB, no real filtering is done before the initial gain stage, but that stage applies gain differentially to different parts of the spectrum.

IN the Rat, there are also two gain-setting paths, except that they result in a LOT more gain being applied to one part than the other, and a lot more gain overall.

[swinginguitar]

Great info thanks!

Looks like not only does the GGG schema have 2 CR pairs (a 2nd order HPF?), but on the PCB image they are reversed on the 2nd pair - it goes C - R - R - C

So not only is the schema wrong, but the PCB doesn't even match it...


...and i see conflicting info out there - some schemas show it as R - C, and others C - R

<Austin Powers voice> so which is it, baby?

[Mark Hammer]

The order of R-C vs C-R doesn't matter.  It is the total input resistance to the second op-amp that matters, and how that resistance is distributed is largely moot.  Same way you'll see some circuits where the gain of a non-inverting op-amp stage is set by a pot and fixed resistor connected between the inverting pin and ground or Vref.  It could be resistor/cap/pot, cap/resistor/pot, pot/cap/resistor, or whatever; same effect in all cases.

If you look for other schematics of the Bluesbreaker on-line, you'll see that the C/R/C/R thing is an error in the GGG drawing.

The BB is a cute circuit in that the one pot sets the gain of two stages simultaneously.  One leg of the gai pot sets the feedback resistance for stage 1.  As that resistance gets bigger, stage 1 gain increases.  The other leg of the pot sets the gain for stage 2, and as that resistance gets smaller the gain of stage 2 increases.  It's an elegant strategy that, by virtue of how gain is multiplactive, can cover a very broad gain range quite easily.  The only downside is that the output of the circuit is inverted, relative to the phase of the input signal.  No really biggie unless you attempt to use it in parallel with something else and ignore the phase inversion.

[swinginguitar]

Mark - so am I correct then that that RC network isn't a LPF/HPF since the output isn't take off the junction with one side going to ground? More of like a frequency dependent resistor since it's in series? Will removing one pair affect the tone much?

To tame the high end, it sounds like I'm looking more at how the tone is shaped in the feedback loop of the op amp moreso than the tone control circuit itself?

[Mark Hammer]

No, you are correct in that the low-end rolloff will be set by the joint product of the input cap and input resistance.  But in this instance, the effect is negligible.  At maximum gain, when the total input resistance to stage 2 is 4k7 and all 100k of the pot in in the feedback path of stage 1), the bass rolloff starts around 154hz.  As gain is decreased, that rolloff moves downward, so like I say nothing to really ponder.