How do I configure the mini-booster as a buffer?

[jakehop]

Dear all,

I've grown very fond of using the minibooster as a buffer. However, I feel that the volume-potentiometer at the output is, for a lack of a better word, stealing some of my tone. How do I set this circuit up to have unity gain, without using a resistor or pot at the end?

Or does anyone know a buffer that has that full sound of the mini-booster? I tried the single-FET buffer and the opamp-buffers on Jack Orman's site, but they didn't quite have the tonal qualities of the mini-booster.

Thanks in advance!

[John Lyons]

The Minibooster is a gain circuit with a good amount of volume boost and a little bit of overdrive.
A buffer is just buffering your guitar or pedal chain against losses from capacitence and impedance with no gain boost.
A buffer has a gain of 1 usually (unity gain).

So these are two different things.
Generally the only sound difference when using a buffer is that the high end will be restored, the signal will be brighter.

John

[Jered]

  Kind of unsure what your asking but, maybe try a JFET source follower after the MB.

[liquids]

I'm still a rookie, so someone correct me if I am wrong, but I believe that if you increase the value of the source resistor (R5 as I'm looking at it), that will lower the gain of the FET stage...also, I thought that eliminating the capacitor on the source (C6) would do the same thing...

Don Till's FET preamp is different, but this is where I got that concept:
http://www.till.com/articles/GuitarPreamp/

If you haven't tried the above simple preamp/buffer circuit and messed with values, it may prove useful to you as well, or something to compare the mini-booster too for JFET signal buffering.  This preamp is fairly low gain as-is (2x or so), so he recommended putting a 10uF cap in there (like the mini-booster has) at the FET source to ADD gain...so in reverse, I imagine removing that cap from the mini-booster would reduce the gain.  Also, why not toy with the value of it and see what happens--I think once you get below 1uF or .1uF it will affect frequency as well as gain.   Again toy with R5's value -- I've seen 10k trimmers here, and higher values will equal less gain, I believe, so maybe try at 4.7 k and see what changes, and go from there....

Once you reduce the gain the circuit produces, this should mean you will be able to "open up" the volume control, sending less signal to ground, and hence less of what you are describing as 'tone loss,' probably.   If you get the gain level to where you want it (at or near unity), you may be able to eliminate the volume knob altogether, if so desired. 

Another possibly useful tweak would be to put an ear-selected small value (pf range) mica/ceramic capacitor across the volume knob, to get some of the high end through to the "other side" of the volume knob when the volume knob is set lower--which is how I imagine you have it right now.   Though, that mod is not going to be as 'tonally pure' as simply reducing the gain the JFETs are producing.  Also, you can tinker with putting lower gain JFETs (like MPF102) in there too for other tone/gain options--you never know, you might like that more....or not.   

Hope this helps lead you to some discoveries...and to others, please chime in if I'm giving incorrect info as I've been working with the mini-booster circuit a little bit, so I'm starting to understand it, but am still very sophomoric in my understanding.   

[jakehop]

John, Jered and liquids, thanks for your replies!

I know it might seem like a "noob"-kind of question, and it would probably be best to just use one of the more common buffer designs, but there's just something to that mini-booster sound that the other buffers I've tried doesn't have. And it's not that it's louder - I've been very anal as to not have my ears cheated by higher volume.

I'll try to experiment a little with the circuit, and get back with whatever I find. Any ideas are very welcome indeed! The two-transistor designs speak to me in a way. When I've rigged up my HV supply, I'm gonna do some tube experiments. The datasheet of the ECC81 really seems perfect for a buffer!

Kind regards, Jake

[Jered]

  If all your trying to achieve is unity gain, then do this. Remove the volume pot and add a 100k resister to ground after the output cap. Then remove the source resistor and bypass cap of the bottom FET and add a 50k pot to ground at the source. Then just dial the pot in until you get unity. Measure the pot resistance and replace it with a resistor of equal value.

[John Lyons]

We need some more information here.
Why are you looking for a buffer?
If you like the sound of the mini booster but with less gain, then do as Jered decribes above.
It still will not be a buffer though.

My point above is that The mini booster is as named BOOST.
A buffer theoretically does not have a sound in and of itself so this is why I am asking what it is you are after.

john

[MikeH]

Many single-transistor boosts can be reconfigured quite easily to become buffers instead of boosts; change some values, tap the output from the source instead of the drain, yadda yadda- boom.  Buffer.  But the mini booster is a Mu-amp configuration, which is a different animal all together.  Sounds like you're just looking to have it hard-wired at unity gain, so yeah- Jered has it right.

[jakehop]

My definition of a buffer is a circuit that provides electrical impedance transformation from the input to the output. The thing about the mu in this application is, that one transistor will be the current buffer and one the voltage buffer.

I don't want "transparent" as much as I want "musical" - I think those mojo words have been thrown around way too much. Anything you place in between your guitar and your amplifier is going to change the way the guitars signal sound through the amplifier - the goal here is to find something that changes it for the better! I'll post my results later tonight.

Kind regards, Jake

[analogmike]

I understand what Jake wants. I have heard the same thing, volume pots can harm the tone. So fixing it at the output for unity gain, as others suggested, will not satisfy you.

I wish I had an answer. I think looking into reducing the gain of the FETS is your best bet, BUT the Fets running at high gain MAY be giving you the tone you like...

Good luck!

[earthtonesaudio]

If you like the tone of the Minibooster, don't change it.  Every part contributes to that tone. 
If you want it to perform better as a buffer, add a buffer on the end.

My definition of a buffer is a circuit that provides electrical impedance transformation from the input to the output. The thing about the mu in this application is, that one transistor will be the current buffer and one the voltage buffer.

Your buffer definition I agree with, but the Minibooster/Mu-Amp doesn't function like an ideal buffer.  The input impedance is high, but the output impedance is not very low.  Adding a buffer on the end would fix that without altering the tone.

[DougH]

There are lot of semantic issues in this thread. But to cut to the chase I think he's just looking for a lower gain (unity gain?) minibooster. Try putting a resistor between the cap going to the upper gate and the source/drain connection. You will have to experiment with the value to get the volume reduction you want.