To boost or to buffer? - That's the Question!

[Blues_Boy_4096]

Hi all!

I'm building an A/B box, and I'm sort of in doubt about what I want.

First of all, I want to be able to control the volume of each send independently.

But I don't know if I want it buffered, so that I have unity gain when the volume pot is at max, or if I should have it boosted so that I have unity gain at the middle of the pots travel and about 6dB gain with the pot max.

Any suggestions?

Oh, BTW, I'm good with the buffer, but I don't know which booster to use to get only about those 6dB of gain. What do you guys recommend?

Thanks

[Crevil]

Hi Blues Boy

I think I would choose the boost, it is nice to have, and you don't loose the buffer part.

A simple way of doing it is using an op-amp

With an non-inverting input, and make it boost 2x.
If you any thing about using op-amp you should get it.

I have just made this fast sketch of a way to do it:

[Ben N]

Simplest, and maybe best: AMZ Mosfet Boost, with the option to switch between boosted output off the source and buffered off the drain. Just use separate jacks and a DPDT, wired to switch each jack tip (hot) to either its own ground or the respective output (buffered/A or boosted/B) of the circuit, or 3PDT for an LED. Adjust the value of the source resistor get the maximum gain down to the level you want. Voila.

(I know this only gives you level control on one channel, but really, that is enough, no? Alternatively, you could just stick a 25k pot on the end of the buffered output and have both.)

http://www.muzique.com/schem/mosfet.htm

Alternatively, just do the dual opamp thing. Whether you want your minimum setting to be gain of 0 or 1 determines whether you go inverting or non-inverting. Then just check the appropriate article at Geofex for the standard configuration.

hth,
Ben

[Pushtone]

IMHO, I would make the boost foot-switchable (for soloing) with a buffered bypass.
You could use switching jacks to add buffered and un-buffered outputs.

A buffered tuner output would be cool too.
The buffer will prevent noise generated by the tuner from getting into your amp sound.
And you could use it for a third send.

You might also consider detented pots so you can find unity gain on a boost fast.

Booster have so much character built into them. Each booster has it's own personality.
I can't imagine ALWAYS having the AMZ MosFet Booster sound.
The buffer would be more transparent than a booster.

I have been adding a simple 1M pot and 100K resistor to ground to the
GGG IC buffer input to add loading and therefore warm-up the brittle high freq. the buffer creates.
Works great!

I also can recommend the Stratoblaster as a unity gain preamp. It's has a full-range with little imposed character.
I'm using the Stratoblaster as the buffered bypass circuit in a Fetzer Booster.

[dachshund]

I'm building the AMZ Mosfet Boost now. What's the difference between "boost" and "buffer", in general and with this particular circuit? I'm guessing that boost means higher gain, and buffer is... unity gain or an impedance correction. I'm using the variation with max gain and the output off the drain into a volume pot. I'm interested to know more about this, if anyone would care to elaborate.
Thanks in advance.

[Pushtone]

What's the difference between "boost" and "buffer", in general and with this particular circuit? I'm guessing that boost means higher gain, and buffer is... unity gain or an impedance correction.

All yes'es to the above.

Generally a buffer (unity gain) will have a high input impedance and a low output impedance.
So do most (well designed) boosters so the major difference would be the gain.
I also think most boosters are designed to HAVE a sound while most buffers are
designed to be as transparent as possible, although nothing is 100% transparent.

[CoolJohnny]

bluesboy;

i'm considering almost the same type of project right now. i want an a/b capability but don't want to go totally passive, hoping that i can have some additional level correction ability in this thing, depending on what amps i run it to. i try not to have more than four or five pedals on my board at any time so im not sure i really need a buffer in there. but as i tend to overengineer just about everything i build, who knows. try let us know what you decide and i'll steal your ideas for my own personal use...ha ha ha. if i get anything together soon i'll post my results.

[Ben N]

My impression about the Mosfet boost is that, yes, it does seem to color the sound a bit, but the perceived coloring is mainly a function of (a) the big input impedance restoring high end that was otherwise lost, (b) saturation of the device as the gain goes up, and (c) whatever shelving effect the source bypass cap has. The input Z is easily lowered, if that is what you want, and (b) & (c) are irrelevant in buffer mode. So, while I haven't tried it, I would suspect that as a buffer this circuit would have very little effect on tone, and running it at a higher votage to increase headroom would also reduce the distortion in boost mode, although I doubt it would matter much at the modest gain levels you are proposing.

OTOH, as RG and others have repeated here countless times, if you want absolutely minimal coloration, for buffer or boost, go opamp, and run that at a higher votage, too. (Check out Barber Electronics's site for the description of the Launchpad, which can be used in the way you are suggesting, and is an opamp based device running at 24v, with some addtional features.)