Author Topic: Yet Another Mini-Booster Derivative (YAMBD)... asking for ideas.  (Read 1164 times)

earthtonesaudio

I've been messing around with a variation of the Mini-Booster.  I'm following R.G.'s advice (bottom schematic on this page: http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm ) and did the bit with the noiseless biasing and the 1k resistor between the top and bottom FETs.  I also added a 10k pot from the source of the bottom FET to ground, and put a cap on the wiper for a not-too-crackley gain control.

The one change I made (and it's so obvious I'm sure it's been done before) is to change the bottom FET to a MOSFET.  I just don't have that many JFETs in my parts bin, so I substituted in the MOSFET.  The only other change is to move the gate resistor from ground to the Vref bias point.  I also experimented with bootstrapping the first stage with a 10n capacitor, to get more sparkle. 

It's pretty good as a dirty booster, but I'm a freak about output signal being in-phase with the original signal, so I added another identical stage.  Now I have two heavily modded Mini-Boosters in series, each with variable gain.  The tone is super-bright (and sounds great driving my amp), but when I max out the gain it clips rather harshly.

Any ideas on how to keep the clipping soft?  Or, what shems should I refer to to get ideas?

Thanks,
Alex.

alanlan

Re: Yet Another Mini-Booster Derivative (YAMBD)... asking for ideas.
« Reply #1 on: August 03, 2008, 05:32:43 PM »
You could add a simple unity gain inverting stage before the mini-booster.  I think cascading 2 mini-boosters is a bit OTT but then again, who am I to say?

earthtonesaudio

Re: Yet Another Mini-Booster Derivative (YAMBD)... asking for ideas.
« Reply #2 on: August 15, 2008, 12:45:35 PM »
The two cascaded SRPP amps was indeed over the top.  So I decided to try and get gain without inverting the signal in the first place.

I went in a completely different direction and decided to use a common gate buffer for voltage gain, and addressed the impedance mismatches with a source follower up front and a SRPP-style follower on the end (again taken from here: http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm ...thanks R.G.).  I haven't done anything close to a scientific test, but my first impression is that the gain and distortion is comparable to the SRPP, but the output is in-phase with the input.  Score!

Well, that was my goal all along, so now all that's left is twiddling parts values.