Query, RE: Maestro FRB-1 Full Range Booster

[Mark Hammer]

Curiosity got the better of me, so I'm finishing up a clone of a Maestro FRB-1 at the moment (I know, I know, I have to get back to the Hyperflange   , and yes Alan, I will get to the FSH).  I haven't found any especially informative demos of ithe unit.  You can see pics of it here: http://tonemachines.blogspot.ca/2011/11/maestro-full-range-booster-frb-1-1972.html  a YOutube of one here: http://www.youtube.com/watch?v=Mp7zS4gb4SA  and the schematic below.


Here is my question.  The unit has individual switches - whether toggle in clones, or paddle in the original - that provide for either a 100k or 570k (100k+470k) input resistance to the op-amp for each band/range,

My hope was to produce a sort of melange of the FRB-1 and the Craig Anderton Peakmaker project that appeared in Electronic Musician years ago.  The Peakmaker used an array of 5 bandpass filters, and a highpass filter section, to mimic different pickup resonances.  Rather than using a graphic EQ to boost and cut, one would simply select/deselect the different combinations of ranges via toggles.  I should note that a) the Peakmaker was intended as a "voice-shaping" tool for guitar, and not any sort of resonant fuzz, like the FRB-1, and b) Craig tended not to be terribly attentive to switch-popping during that era.

The FRB-1's switches let you set each band for higher (input R = 100k) or lower (input R = 570k) gain, to provide resonant boosts , either with, or without clipping.  It also includes a quad-ganged switch to set all 4 bands/ranges to high-gain at once as a "power boost".  But what it does NOT do is cancel any of the 4 bands, in a manner that the Peakmaker did.

What I am planning to do (and have pretty much wired it up to do atthe moment) is select between high and low gain, or cancelling, for each band.  This is easily done with an on-off-on SPDT toggle.  I don't have any particular need for a "blower switch" to make all 4 high-gain at once.  Unlike the original, I'll have a stompswitch to turn the fuzz on and off, and a master bypass stompswitch.  It will all fit comfortably in a 1590BB, and I'm using a 7662 charge pump to go from 9V to the required 18V.

Here is what I want to know.  I am guessing that I should probably expect switch-popping from the toggles if I go from having a switch open (middle position) to one of the side positions (low/high gain, closed).  I'm trying to figure out how I could prevent that popping.  What comes to mind is to have a much higher-value resistance follow the 100k and 470k, such that each band is always connected to the input of the op-amp, even if that connection results in negative gain.

So, for example, if each bandpass filter's output saw 100k, then 470k, then an additional 2M2, that would give the filter section a maximum input resistance to the op-amp of roughly 2M7, such that, even in "fuzz" mode, the op-amp would have a gain of just over 0.5 (i.e., dropped by a half).  That's not complete removal, but it would certainly be much further back in the mix.  Three different settings per band gives a lot of variety.

Does this sound reasonable, or is there a simpler way that I'm missing?

And incidentally, I can easily figure out the gain of the op-amp stage, but I have no idea what the gain of the preceding stages are.  Clearly an op-amp gain of 15x is not enough to produce "fuzz", so I'm assuming there is some gain in those earlier stages that multiplies with the gain of IC1 to ratchet the signal amplitude up to where it will clip with the diode pair.

[PRR]

> higher-value resistance follow the 100k and 470k, such that each band is always connected to the input of the op-amp

Seems OK. 10Meg may be fine unless your caps have been out in the rain.

> what the gain of the preceding stages

As you say: 15 max, and less because the EQ is all bass-cut and treb-cut, so maybe 10 or 5 mid-bump.

Another gain of 1,500K/100K in the opamp is gain of 15.

5*15 is gain of 75. 20mV times 75 is 1,500mV or 1.5V. Diodes limit this to 0.2V or 0.6V. It clips soft guitar (and hammers loud guitar).

BTW, this topology is the Blonder-Tongue Audio Baton, trannyized.
http://www.endino.com/photo9.html
http://www.endino.com/pics/BTFlyer1a.gif
http://www.endino.com/pics/BTSchem1.gif
http://www.endino.com/pics/BTReview.gif
Patent: https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US2983876.pdf
Ben is still around: http://www.bentongue.com/bht/bht.html

[kaycee]

I built one from that schematic on vero. It basically sounds like any single transistor booster with varied input caps. As you throw the switches the sound gets fuller but not that louder. The nice surprise on this one is the fuzz section, which I keep meaning to build a stand alone version.

[Mark Hammer]

What I'm having a hard time understanding is how it yields ANY fuzz, given the seeming low gain. 

[PRR]

Gain near 75, with part-Volt clipping, should distort guitar.

"5*15 is gain of 75. 20mV times 75 is 1,500mV or 1.5V. Diodes limit this to 0.2V or 0.6V. It clips soft guitar (and hammers loud guitar)."

Yes, other fuzzes have higher gain. I suspect this is beating the dead horse, but sometimes that's what you want.

Also the emitter follower on front is poorly matched to its several loads, so strong guitar may also clip in-front.

If it is not enough for you, it would be trivial to re-rig the follower as a gain stage. An opamp might be even easier and allow pretty insane gain.

[Mark Hammer]

I don't really need insane gain, but if I'm going to introduce gaping holes in the frequency response of the signal sent to the clipping stage, then I should probably beef up the input signal a little.  That could be with an external booster (and lord knows I have no shortage of those), although forum member Alain Parent recently (and kindly) bestowed upon me one of the teeny LPB-1 clones has made, built onto the leads of a 12mm Alpha pot.  I could simply make that the pre-buffer input stage.  It doesn't take up any more room than the pot itself, so there is no need to add anything to the FRB-1 board.

[Mark Hammer]

Wake uuuuuupp.  Wake uuuuuppp.  Time to get ready for school.

Sometimes, it takes a while to finish things.  I finally got this one done today.  I ended up using on-off-on toggles for each filter section, such that a given band could not only be boosted but completely cancelled out, simply by lifting its connection to the mixing node.  In some respects, it might be better to replace the toggles with a 100k fixed resistor, in series with a 1M pot (variable resistor) for each band.  That complicates things, but begins to approximate having a 4-band EQ pedal ahead of an overdrive.  Note that it provides no cut, as such, only selective boost of a given band.

The other thing I did was replace the 1M5 feedback resistor in the op-amp with 820k in series with a 1M pot, to get variable gain/drive on the overdrive section.  I also installed a stompswitch to select between overdrive and normal/unclipped booster.

Finally, I found the clipping tone a little too harsh, so I put a 22pf cap in parallel with the diode clipping pair.  In total, two stompswitches, two pots, and four 3-way toggles, built into a 1590BB.

Is it worth building?  Not exactly earth-shattering, but the idea of a more-or-less 4-band EQ that can be turned into an overdrive at the stomp of a switch has its uses.

[ElectricDruid]

Hey, I don't care how it sounds...it feels good to finish it though, doesn't it?!?

T.

[Mark Hammer]

Tell me about it!