Need a Second Opinion

[Paul Marossy]

Here is a A/B box that I am going to make for myself. Does anyone see any problems with what I am thinking of doing?

http://home.att.net/~u2p0j0m4/A-B_Box.pdf

[Ansil]

looks good to me eyes........... mate

[Peter Snowberg]

That looks great to me.

You'll probably need to ajdust one of the resistors to make the brightness equal because of the different forward voltages, but that's getting into really picky details.

Take care,
-Peter

[Mark Hammer]

'Twer mine, I'd stick in a simple high input-impedance buffer, and use a lower value pot.  My reasoning is that the box assumes a given impedance from whatever is feeding it, and in the grand scheme of things you can't really predict that with 100% accuracy.  My own inclination is to stick in a simple low-noise high input-Z dual opamp set for unity gain and stick a 10k-100k pot in place of the 1M.  That will let you use it with just about anything on the input or output.  You have the battery in there already.  Just my 2 cents.

[Paul Marossy]

Mark,

Yeah, that is what I was wondering about was the size of the pots. I was thinking bigger pots would increase the impedance some, but I was also thinking that maybe they ought to be smaller...
I think the buffer with smaller pots is a good idea. I already built it the way I have it drawn with smaller value pots than shown on there, I just haven't done some testing on it yet, gotta baby boy coming any minute now...

Thanks all for the suggestions/comments.

[BD13UK]

http://www.fulltone.com/qaframe.html
Hi Paul,
You might care to look at the above if you haven't seen it, looks like it might be helpful.

[Mark Hammer]

If you have another child on the way to complement the adorable one in the pictures, then I can understand your desire for a simpler build.

Higher value pots will retain more treble (or rather lose less) and 1megs are traditionally the preferred pot for volume pedals for that very reason.  As a passive device, though, they will interact with the pot values of the preceding device in line, particularly if you use true bypass mechanical switching (which you tend to do given your homebrew pedals).  Think of it this way: how much sheen could you expect to retain if you stuck three 100k pots set up as voltage dividers in a row, without anything between them?  Best to isolate pots from each other with buffer stages.

Ummm, when you say "any minute now", are you (in the plural sense) between contractions or do you mean "any day now"?  Either way, congrats.

[Paul Marossy]

Mark,

I was getting myself a little confused... I was initially thinking 10K or 50K pots and then I starting thinking that higher impedance would be better, but then that didn't quite make sense either because the guitar volume pot already is 500K. That's why I wanted the second opinion...  :wink:

Well, I'm not exactly down to contractions yet, but it will be any time that we'll get started... her due date is 12/11/03.

[Mark Hammer]

If your wife is overdue, ask me for a dinner invitation.  We're 5 for 5 now on people who we invited over that went into labour the evening of the invite.  I don't even bother cooking anymore, and quite frankly, it doesn't even matter that you're 3/4 of a continent away.

[Paul Marossy]

Where do you live? Canada? Maybe you could come to Vegas and play the slots after dinner...  :lol:

[Paul Marossy]

Just an update-

I decided to put a basic jfet buffer on the output. I have a 1M gate resistor connected to Vref (4.5V) instead of directly to ground for more headroom, and used a 2N3819. It's really quiet, and seems to work just fine.

I think that I might have to put a pull down resistor on the input/output, I may have a bit of popping when switching, but it's hard to tell with just headphones. I'll try it with an amp sometime in the next few days and see how it fares.

I also decided that 50K pots seem to work the best. I'm sure that a 100K would work fine, too. A 250K pot is just a little bit on the big side, it loses a little bit of treble, but is barely perceptible.

I'm also going to test it out with an acoustic guitar that has a piezo pickup at the bridge and see how well that works. That's really why I wanted to build the box in the first place- to switch between an acoustic and an electric, and use the same amp.

Still no baby, but today is the "official" due date, so we'll see...