AMZ Buffered Mini-Booster (module) not boosting "clean"

[Hotcreme]

Hello friends,

I just received the assembled, buffered Mini-Boost module from AMZ (http://www.muzique.com/tech/module1.htm) and wired it up. Works fine as a boost but not for my purposes.

My intended build was a high-pass filter with make-up gain. For the HPF, I'm running the signal to a 1MΩ reverse log pot, with a .001μF cap jumping the input and wiper (output) lugs. This obviously creates a dramatic cut, but it's exactly the sound I'm looking for, especially with humbuckers. I've tried the same RC ratio with a 500kΩ pot, but it didn't win out over the 1M's responsiveness. For makeup gain, I'm running the pot's output into the Mini-Boost's input, powering with 9V supply or battery.

My issue is that the Mini-Boost is distorting with just about any guitar signal fed to it. I primarily intend to use this with humbuckers or bass to trim the fat before feeding the next pedals in line. I've tried the "Drive" mod suggested in the instructions included from AMZ, but this has the effect of reducing the resulting input to a point where the circuit's noise floor is just about unusable. I've also tried rolling the volume back on my guitars, but the results are similar. Putting the HPF section of my build at the end of the circuit instead of the beginning results in even less headroom. Searching the forum gives suggestions that I can only apply if I built the circuit on veroboard or could swap parts. I could cut traces if absolutely necessary but only as a last resort. The thing kicks ass for an overdrive, but that's not why I bought it.

I could try using a 12 or 18V supply, but I feel like the description of the mini-boost says it gives you plenty of clean headroom, and that I'm not doing something right on my end. However, the mini-boost still exhibits this similar distortion whether or not the HPF pot is in the circuit or not. I just want "clean" boost—and only a couple of decibels to bring it to unity when the HPF cuts perceived volume.

Is my only option for clean boost ditching the Mini-Boost and getting an (unbuffered) MOSFET Boost?

Thanks for all your help,
Hakeem

[ubersam]

First, the schematic: http://www.muzique.com/amz/miniboo.gif

You can try running it on 12v or 18v but only if the caps are rated accordingly. You can try disconnection C6 by cutting the trace between that and the bottom JFET source. That might not be enough, though. You might need to increase the size of the source resistor. Or, if the JFETs are J201s, swap them out for a 2N5457 or a J305. Seeing as the components on the module are surface mount, you might not want to (I wouldn't).

You might want to build an active HPF instead and keep the booster as is.

[Hotcreme]

Thanks ubersam! I should have linked the schematic to begin with. The buffered version is a bit different as C3 isn't mounted (gives more gain if added) and R5 is completely omitted. Here's a link to the buffered schem: http://www.muzique.com/tech/mini000.gif

I'll give 12V/18V a try since the caps are rated high enough. If I want to increase the source resistor, how would I go about that if it's surface mount? I assume that's R1 in that schematic? I'm still in college for electronics but haven't got to transistor circuits just yet.

Also, could my 1MΩ pot used in the HPF be loading the input/buffer? It seems like if anything it would reduce the voltage hitting the transistors, not pushing it against the rails for it to be distorting. Just a thought.

An active HPF is a great idea; this just seemed an easier build because I couldn't screw up an assembled module, whereas I'd probably screw something up with an active HPF build or struggle sourcing parts. If you've got one in mind, I'd love to see it.

Thanks again!

[ubersam]

The buffered version is a bit different as C3 isn't mounted (gives more gain if added) and R5 is completely omitted.

That's cool, one less thing to worry about.

If I want to increase the source resistor, how would I go about that if it's surface mount? I assume that's R1 in that schematic?

The source resistor is R3 in the schematic you linked to (R5 in the one I linked to). R1 in your schem is the pulldown resistor, sometimes referred to as the "anti-pop" resistor. To increase the value R3, you'd have to remove it and replace it with another of higher value. Not so easy if you're not equipped or experienced to work with smds. I've mangled traces trying to remove a couple of smd resistors once.

Also, could my 1MΩ pot used in the HPF be loading the input/buffer? It seems like if anything it would reduce the voltage hitting the transistors, not pushing it against the rails for it to be distorting. Just a thought.

I'm not sure how you have the pot wired up. If you can put up a schematic, that will give us a better idea of what's going on. But for the sake of discussion, if the 1M pot is in series with C1, before R1, the pot will form a voltage divider with parallel value of R1 & R2, cutting the voltage. But, you have the cap shunted across lug 1 & 2 passing all frequencies above the cut-off. That might be big enough to clip the gate of the jfet if its V_GS-OFF is small enough. The J201, for example, can have a V_GS-OFF as low as 0.5~.07v.

An active HPF is a great idea; this just seemed an easier build because I couldn't screw up an assembled module, whereas I'd probably screw something up with an active HPF build or struggle sourcing parts. If you've got one in mind, I'd love to see it.

Thanks again!

Yeah, I hear ya. My first diy was a fuzz built spider web style, where the components leads are soldered directly to each other, parts from a surplus store. It kinda worked.... as an AM radio. So for my next project, I decided to buy a pre-fab board from BYOC. That worked better. Eventually, I learned to create layouts for pad-per-hole & pcb.

An active HPF can be as simple as a RC filter at the non-inverting input of an opamp set as buffer or an amplifier, like this: http://www.electronics-tutorials.ws/filter/filter_6.html - the cut-off frequency there is set by the R/C and can only be changed by swapping out components.

If you want the cut-off freq. adjustable, it gets a bit more complicated. Like this: http://www.eeweb.com/blog/circuit_projects/20hz-to-200hz-variable-high-pass-filter - it looks interesting, though. I've been playing with it on LTSpice. I'll see about posting the schem & the curves when I get home.

[PRR]

This is really a shelf filter, not a simple hi-pass.

I suspect that Mini-Boost has a LOT of gain.

You really need little or no gain, only buffering.

Since the product you got is SMD, modification is difficult.

You want a buffer with very high input impedance, and gain adjustable from near unity up to maybe 4 (since a Rev-Audio taper works for you).

AMZ Mosfet Booster may be a better choice. AMZ Opamp Multipurpose project can certainly be made to work.

[ubersam]

Here is the link to the schematic I've been playing with in LTSpice: https://www.dropbox.com/sh/c5lptn3fqz8vxi3/AADsN-FjdHYBv9RmaNRVPrw4a?dl=0

The cutoff is adjustable from about 19Hz to 325Hz. R2 & R13 are dual ganged 100K pot. Rev. Log taper seems to work best but so far I've not been able to find in a PC mount version.

[PRR]

> cutoff is adjustable from about

That's a steep (2-pole) low-cut.

Hotcreme said "running the signal to a 1MO reverse log pot, with a .001µF cap jumping the input and wiper (output) lugs." This is a shallow shelf.

BUT we ass-ume there is a guitar in front of that. And 0.001uFd hanging off a guitar pickup's inductance is a radical load. So I figured that way also.



Green is pot+C driven by perfect source. Shelf.

Red is guitar, pot, C. Slope-up from 100Hz to 1KHz, then *major* slope to a 3KHz peak.

18dB rise from guitar gut-body to string-zing is likely to cause nasty overload in most any amplifier.

[ubersam]

That topology reminds me of a volume control with a treble bypass cap.

I copied your schem and did a few more sims: https://www.dropbox.com/sh/3zsjugoomt4xpmr/AABcVZBhz_dLsJXMRCLYCBuza?dl=0

The first two are the same as yours. The third one, I added a sim of the guitar volume & tone control hanging off the pickup. The fourth, I added a buffer at the end.

[amz-fx]

Hakeem,

The module is made with genuine Fairchild jfets, which have a LOT of gain.  If you input a strong signal it will clip on the peaks and not be as clean as you may like.

There are a couple of solutions: add an input Drive control to trim down the signal level. This doesn't seem to be working in your particular application.

Power the module with 18v. It is rated for that voltage so it will not be a problem. This should make it clean enough.

As others have noted, the high pass design of your project is going to emphasize the upper range and any noise present.

If you need more help, you can email me via this page: http://www.muzique.com/email.htm

Best regards, Jack