Mini-Boost with lowest amount of gain?

[brefmint]

OK, I have now build 3 AMZ Mini-Boosters. I love them. I would now like to build a Mini-booster circuit for acoustic instruments. I have a friend that plays fiddle. He uses pedals. I think he's using a Microamp for a boost now. He hates it. I don't think he would dig the Mosfet Boost either. He wants something that fattens up his tone in the low -mids. I believe he would love the Min-Boost if I could clean up the grit for him.

I have read that if you omit the source capacitor (C3) and raise the value of R3 to 4.7k-10k it will clean up the gain. Has anyone done this? I'm just wondering if this mod would be clean enough or is there a better route.

Thanks,
Brefmint

[petemoore]

  Strato-blaster...or other Jfet, low cost for the board population.
  Anything else with 1 Jfet...that's what I'd think to try first..
  For the Mini-booster, Low Gain MPF102's...just working from schememories...large source resistor w/no bypass cap, perhaps voicing options, pregain.

[brefmint]

Thanks for the input. Is there a vero layout of the strato-boost or any 1 jfet boost? I'd love to give it a shot. I'm sure I already have the parts laying around.

[snoof]

if you copy the gain control arrangement of the stratoblaster, and graft it onto the MB, you can have adjustable gain.  Search the gallery for single jfet boost layouts...

[petemoore]

  The best reads I've found on miniboosters is at GEO, look for  "Mu"...amp.
   Single Jfet circuit board populating is slightly less expensive in various ways, if softer distortion than Mu Amp is desired, that's where I'd start

[Mark Hammer]

I can't see any particular improvement using the MOSFET instead of the Microamp.  Both are intended to provide as flat a frequency response as possible, and neither contain any deliberate tone-shaping components, other than the usual DC-blocking stuff that also takes some lower bass out.  I strongly doubt the bass that is removed is pertinent to a fiddle, though, unless your friend plays a VERY big fiddle.  The stock Micro-amp provides gains of just over 1x to just under 20x, with plenty of adjustability in between.  The MOSFET booster is sort of in the same ball park.

The Mini-Booster is likely a bit of overkill, but I take it that there is a desire to get some teeth to the sound and add some lower harmonic content.  The big question is "How much?".  Given that the fiddle probably uses a bridge-piezo pickup, it's difficult to speculate whether the fiddle would provide the sort of input signal that the Mini-Booster is expecting to ring its doorbell.  Part of that is low end and the manner in which that pushes the JFETs into some modest clipping, and the other part is overall signal level.  My guess is that the Micro-amp will still be needed for driving any additional pedal and that your friend may well wish to modify the Micro-amp to help in that task.

How?  If the 56k feedback resistor in the Micro-amp is changed to, say 220k, the max gain will be hiked up to around 100x, which is likely better for overdriving subsequent stages.  With the stock 47pf feedback cap, that introduces a treble rolloff starting around 15khz, which is kind of high if one is interested in adding harmonic content.  If the 47pf cap is changed for 100pf or even 120pf, that will help to tame the highs by moving that rolloff downwards.

Conceivably, those changes might allow your friend to evoke the sort of amp tone he wants.

[brefmint]

Thanks for the input Mark. He is definatley looking to add a bit of "teeth" and some lower harmonics. Nothing over the top. Mainly just to give it some thickness. I thought of the Mini-Boost circuit because I have run My Fat-Boost (w/ the gain rolled off) with my acoustic guitar. It worked nicely. Sounds like he wouldn't be pushing the MB very hard so It may sound fine stock w/ an 18v adapter for more clean headroom.

I just found a vero layout for this DOD 201 Jfet clean boost. It looks pretty cool. Anyone used it? http://www.aronnelson.com/gallery/v/DRAGONFLY-LAYOUTS_0/album14/album143/DOD_201_JFET_PREAMP_VERO_001.gif.html

[Dragonfly]

If you eliminate the bypass cap on the minibooster it will lower the gain quite a bit. sounds excellent with 2N5457's

[brefmint]

Is that the same as the source cap (C3)?

[snoof]

the cap you want to change is on the source to grnd of the bottom fet.  Or see my post above for adjustable gain.

[brefmint]

Thanks to all. I wish I was seasoned enough to graph the gain control snoof. It's a little over my head I believe. I'm working with Orman's  PCB.

[amz-fx]

On the pcb version, leave out C3 and increase the value of R3, to as much as 10k.

It will be quite clean, but you can make it even more so by increasing the voltage supply to 18v with two batteries in series or a power supply.

More advanced info: http://www.muzique.com/amproj.htm

regards, Jack

[brefmint]

Thanks Jack! I'll give it a shot.